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High‐calcium pyroxene as an indicator of igneous differentiation in asteroids and meteorites
Abstract
Our analyses of high quality spectra of several S-type asteroids (17 Thetis, 847 Agnia, 808 Merxia, and members of the Agnia and Merxia families) reveal that they include both low-and high-calcium pyroxene with minor amounts of olivine (<20%). In addition, we find that these asteroids have ratios of high-calcium pyroxene to total pyroxene of >~0.4. High-calcium pyroxene is a spectrally detectable and petrologically important indicator of igneous history and may prove critical in future studies aimed at understanding the history of asteroidal bodies. The silicate mineralogy inferred for Thetis and the Merxia and Agnia family members requires that these asteroids experienced igneous differentiation, producing broadly basaltic surface lithologies. Together with 4 Vesta (and its smaller "Vestoid" family members) and the main-belt asteroid 1489 Magnya, these new asteroids provide strong evidence for igneous differentiation of at least five asteroid parent bodies. Based on this analysis of a small subset of the near-infrared asteroid spectra taken to date with SpeX at the NASA IRTF, we expect that the number of known differentiated asteroids will increase, consistent with the large number of parent bodies inferred from studies of iron meteorites.
... Spectra collected by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) 13 show that these six bright boulders contain pyroxene, and not olivine, as indicated by a second absorption near 2 μm (Fig. 2c and Extended Data Fig. 1a). Pyroxene is a major rock-forming mineral in planetary materials, and numerous studies have quantitatively linked pyroxene compositions with spectral signatures at visible and near infrared wavelengths [14][15][16][17] . Pyroxenes can crystallize in different systems (monoclinic clinopyroxenes and orthorhombic orthopyroxenes) and with differing calcium cation chemistry. ...
... We find HCP% values that range from 45 to 55%, indicating that the pyroxene identified on Bennu came from a body large enough to support igneous processes ( Fig. 3b and Extended Data Fig. 1c). These values are not consistent with chondritic material, either from Bennu's parent body or from contamination by ordinary chondrites 17,19 . This composition, combined with the overall carbonaceous chondrite-like nature of Bennu, indicates that the observed pyroxene is exogenic. ...
... b, HCP% versus the ratio of the LCP to the HCP band strengths for the pyroxene-bearing boulders on Bennu, as determined by applying the MGM to their OVIRS spectra. The ranges for meteorites, including eucrites, ordinary chondrites and lodranites, are shown for context 17 . Error bars signify the standard deviation from the Monte Carlo fitting by the MGM (see Methods). ...
When rubble-pile asteroid 2008 TC3 impacted Earth on 7 October 2008, the recovered rock fragments indicated that such asteroids can contain exogenic material1,2. However, spacecraft missions to date have only observed exogenous contamination on large, monolithic asteroids that are impervious to collisional disruption3,4. Here, we report the presence of metre-scale exogenic boulders on the surface of near-Earth asteroid (101955) Bennu—the 0.5-km-diameter, rubble-pile target of the OSIRIS-REx mission⁵ that has been spectroscopically linked to the CM carbonaceous chondrite meteorites⁶. Hyperspectral data indicate that the exogenic boulders have the same distinctive pyroxene composition as the howardite–eucrite–diogenite (HED) meteorites that come from (4) Vesta, a 525-km-diameter asteroid that has undergone differentiation and extensive igneous processing7–9. Delivery scenarios include the infall of Vesta fragments directly onto Bennu or indirectly onto Bennu’s parent body, where the latter’s disruption created Bennu from a mixture of endogenous and exogenic debris. Our findings demonstrate that rubble-pile asteroids can preserve evidence of inter-asteroid mixing that took place at macroscopic scales well after planetesimal formation ended. Accordingly, the presence of HED-like material on the surface of Bennu provides previously unrecognized constraints on the collisional and dynamical evolution of the inner main belt.
... The near-IR (0.8-2.5 μm) spectral region is crucial for inferring asteroid mineralogies because both olivine (ol) and pyroxene (pyx) have strong absorption bands in this region, as detailed in Gaffey et al. (2002) and Burbine et al. (2002b). The NASA Infrared Telescope Facility (IRTF) SpeX instrument (Rayner et al. 2003), located on Mauna Kea, can observe 17th-18th magnitude objects, allowing high-quality data (e.g., Binzel et al. 2001;Sunshine et al. 2004;Hardersen et al. 2005Hardersen et al. , 2006 to be obtained for objects of sizes comparable to those available in the visible region. Much fainter objects can be observed using larger telescopes, of course, but such telescopes have not often been brought to bear in asteroidal studies, and the majority of asteroid data are obtained at 3-4 m-class observatories. ...
... MGM has been used to fi t the spectral features of minerals (e.g., Sunshine et al. 1990Sunshine et al. , 2000Sunshine andPieters 1993, 1998) and meteorites (e.g., Schade and Wäsch 1999;Schade et al. 2004;Mayne et al. 2006). A number of asteroid spectra have also been fi t using MGM for mineralogical analyses (e.g., Hiroi et al. 1996a;Ueda et al. 2002b;Sunshine et al. 2004Sunshine et al. , 2007de León et al. 2004de León et al. , 2006McFadden et al. 2005) or to determine a ratio of band areas (e.g., Rivkin et al. 2004a). This ratio of band areas is used for objects that do not have full spectral coverage of both Bands I and II, which does not allow for a Band Area Ratio to be determined but does allow for MGM to fi t the absorption features. ...
... There is evidence for partially differentiated S-complex asteroids (e.g., Gaffey et al. 1993a;Hardersen et al. 2006;Gaffey 2006), but it is still unclear whether chondritic or partially differentiated objects predominate among the S-types. Sunshine et al. (2004) suggest that some members of the Merxia and Agnia families might be differentiated. However, the location of these two "differentiated" families in the main belt (around 2.74 AU and 2.78 AU, respectively) does not seem to support the kind of heating gradient in the asteroid belt that Bell et al. (1989) suggested. ...
... Pyroxene and olivine are common rock-forming minerals of mafic/ultramafic terrestrial bodies. The presence of olivine in any asteroid or meteorite can signify its origin and evolution tracing the igneous or nebular history (Sunshine et al. 2007), while high-calcium pyroxenes (HCPs) can be used to trace the degree of melting and differentiation of the body (Sunshine et al. 2004). The visible and near-infrared wavelength region (0.4-2.5 µm) is widely applied to detect and analyze these mineralogical compositions. ...
... ii , nor in HEDs Mittlefehldt et al. , 2012 We follow Sunshine et al. (2004) and Klima et al. (2011) in eucrites with that of the synthetic pyroxenes investigated by Klima et al. (2011) and Sunshine et al. (2004) reveals that a small number of syntheitic pyroxenes plots outside the compositional variation of diogenites and eucrites (indicated by the gray box Bunch et al. 2010;Wittke et al. 2011;Ammannito et al. 2013a;. ...
... ii , nor in HEDs Mittlefehldt et al. , 2012 We follow Sunshine et al. (2004) and Klima et al. (2011) in eucrites with that of the synthetic pyroxenes investigated by Klima et al. (2011) and Sunshine et al. (2004) reveals that a small number of syntheitic pyroxenes plots outside the compositional variation of diogenites and eucrites (indicated by the gray box Bunch et al. 2010;Wittke et al. 2011;Ammannito et al. 2013a;. ...
This thesis presents insights into the surface compositional heterogeneity and geology of asteroid Vesta. Lithologic mapping using Framing Camera onboard the NASA Dawn spacecraft revealed that the majority of the surface is howarditic in composition (Thangjam et al. 2013). An important outcome of this thesis is identification of olivine-rich exposures for the first time using FC color data (Thangjam et al., 2014). Another significant contribution from this thesis is an innovative three-dimensional approach of spectral analysis that could be used to study surface compositional heterogeneity of Vesta (Thangjam et al., 2016). The three band parameter space with defined polyhedrons is more advanced and robust compared to the generally used two-dimensional spectral analysis i.e., two band parameter space. A revised petrologic evolution model is presented to explain the ongoing problem of missing olivine-rich mantle of Vesta and also the petrogenesis of HED meteorites (howardite, eucrite, diogenite clan of stony achondrites).
... These authors also point out that the dominant spectral classes of the background asteroids in the Agnia vicinity are C or X, very different from family dominating Sq-type asteroids. Sunshine et al. (2004) claim that Agnia family members, which have a high-calcium pyroxene component in their spectra (and possibly minor amounts of olivine), may have experienced igneous differentiation. A close examination of the family, shows that the members have nearly identical spectra. ...
... A close examination of the family, shows that the members have nearly identical spectra. Sunshine et al. (2004) claim this homogeneity may mean that Agnia is a secondary family produced by the breakup of a basaltic fragment from a primary asteroid parent body. Note that the Agnia parent body is only thought to have had a diameter D ∼ 50 km (Durda et al., 2006, in preparation), which would be consistent with such a scenario. ...
... Two asteroids, (15675) Goloseevo and (46611) 1993 TH4, fall outside this 90% confidence level threshold, indicating they are only marginally linked to the family. These bodies could be outliers or instead may support a possible larger spectral diversity in the family (see Sunshine et al. 2004); in the a − H plane, these two asteroids lie at the border of the family with a ≃ 2.773 AU. Note the PC 1 = 0.3 spectral slope is the dividing line between the S-complex (for which PC 1 > 0.3) and C-complex (for which PC 1 < 0.3; e.g. ...
We present a brief historical overview of non-gravitational phenomena observed
in the Solar System (Section 2.1). It is followed by a state-of-the-art review
of the non-gravitational forces acting on small bodies (Section 2.2);
hereinafter, we focus on the Yarkovsky/YORP effect. The mathematical
formulation of the Yarkovsky/YORP effect and the analytical and numerical
methods of its calculation are recalled in Section 2.3.
The major part of the thesis is a detailed description of models we developed
to describe the long-term evolution of meteoroids and main-belt asteroids,
which reside in both unstable and stable regions. We study transport mechanisms
of putative meteoroids from (6) Hebe, (170) Maria and (8) Flora parent bodies
to Mars- and Earth-crossing orbits (Section 2.3.4), the case of the asteroid
(2953) Vysheslavia (Section 4), the asteroidal population in the 2/1 mean
motion resonance with Jupiter (Section 4.5) and the Eos and Agnia asteroid
families (Sections 6 and 7).
Finally, we describe a by-product of our research: an on-line catalogue of
proper orbital elements (Section 9.1), we present numerical tests of a
new integrator (Section 9.2) and we reprint a "lost" publication by I.O.
Yarkovsky (Section 9.3).
... Pyroxenes have received increasing attention from mineralogists and petrologists since the late 1960s [1]. The word "pyroxene" comes from two Greek words, namely, fire (πυρ) and stranger (ξένος), because of its presence in volcanic lavas [2]. The general composition of AMZ 2 O 6 structure where A denotes mono-or divalent alkali or alkaline earth metal, M is the member of the transition metal group, and Z represents either germanium, silicon, or aluminium. ...
... Pyroxenes have many abundances in the upper mantle and Earth's crust. Nearly 80 high-quality, three-dimensional structures of extraterrestrial existence have been published within the last 15 years, specifically on the Moon and asteroids [2]. Any pyroxene belongs to either the monoclinic or the orthorhombic crystal system. ...
Ca-based pyroxene structures have gained significant attention due to the higher abundances of calcium in the earth's crust and relatively easy synthesis process. Here, we have analyzed the structural, mechanical, elastic, thermal, and optoelectronic properties of CaMSi2O6 (M = Co, Fe, Mn) pyroxene structures in the monoclinic phase using the first-principles density functional theory (DFT) approach using CASTEP code. The lattice constants of the simulated structures in ferromagnetic (FM) orientations using GGA-WC and GGA-PBSOL were consistent with the available experimental data. Using the Born stability criteria, these structures are considered mechanically stable. These are elastically anisotropic and brittle compounds. The melting temperatures are in the order of 10³ K which symbolizes these as potential candidates for high-temperature applications. The band structure alongside the electronic density of states at the Fermi level reveals half metallicity of these structures. CaMnSi2O6 structure has a maximum half-metallic gap of around 4.60 eV. The remaining two structures have an indirect band gap in the visible photon range given as 3.14 eV, and 2.65 eV for CaCoSi2O6, and CaMnSi2O6, respectively. Observing optical and electronic properties shows that the compound holds a promising future to be utilized in the optoelectronic and plasmonic fields.
... A few years later, Gaffey and Gilbert (1998) proposed asteroid (6) Hebe as the probable parent body of H-type ordinary chondrites and, since then, many authors have relied on this association in their studies (e.g., Akridge et al. 1998, Ghosh et al. 2003, Bottke et al. 2010abstract, Henke et al. 2012. In the early 2000s, the idea that several S-type asteroids have high fractions of high-Ca pyroxene on their surfaces, which is an indication that these bodies have undergone either melting or partial melting and that they can thus not be linked to OCs, was brought up again by Sunshine et al. (2004), Hardersen et al. (2006) and Abell et al. (2007) in the case of Itokawa. ...
... S-type asteroids are a diverse group of objects and one cannot exclude that some meteorites other than the ordinary chondrites could also originate from them (e.g., Burbine et al., 2003). Possibilities include differentiated meteorites, as proposed by Sunshine et al. (2004), such as pallasites, brachinites, ureilites, lodranites, winonaites, IAB irons, and mesosiderites (Gaffey et al., 1993). ...
Ordinary chondrites (OCs) are by far the most abundant meteorites (80% of all falls). Their origin has long been the matter of a heated debate. About thirty years ago (e.g., Pellas 1988), it was proposed that OCs should originate from S-type bodies (the most abundant asteroid spectral types in the inner part of the asteroid belt), but the apparent discrepancy between S-type asteroid and OC reflectance spectra generated what was known as the S-type--OC conundrum. This paradox has gradually been resolved over the years. It is now understood that space weathering processes are responsible for the spectral mismatch between S-type bodies and OCs. Furthermore, both telescopic observations and the first asteroid sample return mission (Hayabusa) indicate that most S-type bodies have mineralogies similar to those of OCs. Importantly, the S-type/OC link, which has remained sterile for more than 30 years, has been delivering fundamental constraints on the formation and evolution of planetesimals over the recent years.
... One of the arguments for a rubble pile is a high calculated porosity and this could, in part, be due to water in the interior of these asteroids Some researchers have suggested that the interior of all asteroids resemble the water-rich CM chondrites (Huang et al. 1996;Akridge and Sears 1998;Sears 2005), while yet others have suggested that the interiors of the of the S asteroids may be differentiated with a core and mantle, but with a chondritic crust (Elkins-Tanton et al. 2011). It has been suggested that most of the S asteroids have surfaces containing melts or partial melts (Sunshine et al. 2004), which would suggest another kind of interior and perhaps metal-rich interiors. ...
... Those that could be ordinary chondrite look-alikes are the so-called S(IV) asteroids. Most of the remaining S classes appear to contain clinopyroxene as their dominant pyroxene and are assumed to be melts or partial melts sincere terrestrially clinopyroxene is associated with igneous rocks (Sunshine et al. 2004). But this conclusion ignores the fact that the most "primitive" (i.e. ...
Interest in asteroids is currently high in view of their scientific importance, the impact hazard, and the in situ resource opportunities they offer. They are also a case study of the intimate relationship between science and exploration. A detailed review of the twelve asteroids that have been visited by eight robotic spacecraft is presented here. While the twelve explored asteroids have many features in common, like their heavily cratered and regolith covered surfaces, they are a remarkably diverse group. Some have low-eccentricity orbits in the main belt, while some are potentially hazardous objects. They range from dwarf planets to primary planetesimals to fragments of larger precursor objects to tiny shards. One has a moon. Their surface compositions range from basaltic to various chondrite-like compositions. Here their properties are reviewed and what was confirmed and what was newly learned is discussed, and additionally the explored asteroids are compared with comets and meteorites. Several topics are developed. These topics are the internal structure of asteroids, water distribution in the inner solar system and its role in shaping surfaces, and the meteoritic links.
... Lyons et al. (2002) used this relationship to derive mineral abundances from measured BARs of asteroid spectra. However, because the original regression was based on simple mixtures of olivine and orthopyroxene, the presence of more than one pyroxene (or other additional phases) complicated spectral interpretations of asteroids made using this calibration (Gaffey et al. 1993; Sunshine et al. 2004). In an attempt to develop BAR calibrations that were more useful in asteroid spectroscopy, Burbine et al. (2003) used normative abundances of the ordinary chondrites (abundances calculated from measured bulk chemistry) to derive a calibration for the Stype asteroids. ...
... Analyses of other S-complex asteroids suggest that the MGM can constrain the relative abundance of high-Ca to low- Ca pyroxene. For example, Sunshine et al. (2004) found high-Ca pyroxene abundances >40% for the large Main Belt asteroids (17) Thetis, (847) Agnia, and (808) Merxia, thus implying that these objects experienced high degrees of partial melting consistent with a history of igneous differentiation. The three overlapping 1 µm olivine bands, as seen in the spectra of A-type asteroids, are particularly well suited to MGM analysis, despite their sensitivity to space weathering (e.g. ...
Methods to constrain the surface mineralogy of asteroids have seen
considerable development during the last decade with advancement in laboratory
spectral calibrations and validation of our interpretive methodologies by
spacecraft rendezvous missions. This has enabled the accurate identification of
several meteorite parent bodies in the main asteroid belt and helped constrain
the mineral chemistries and abundances in ordinary chondrites and basaltic
achondrites. With better quantification of spectral effects due to temperature,
phase angle, and grain size, systematic discrepancies due to non-compositional
factors can now be virtually eliminated for mafic silicate-bearing asteroids.
Interpretation of spectrally featureless asteroids remains a challenge. This
paper presents a review of all mineralogical interpretive tools currently in
use and outlines procedures for their application.
... Pyroxene and olivine are common rock-forming minerals of mafic/ultramafic terrestrial bodies. The presence of olivine in any asteroid or meteorite can signify its origin and evolution tracing the igneous or nebular history (Sunshine et al. 2007), while high-calcium pyroxenes (HCPs) can be used to trace the degree of melting and differentiation of the body (Sunshine et al. 2004). The visible and near-infrared wavelength region (0.4-2.5 µm) is widely applied to detect and analyze these mineralogical compositions. ...
... iv) The HED samples (EET90020 eucrite (MP-TXH-076-A/CAMP76, TB-RPB-020/C1TB20/LATB20, Cachari eucrite ( This Figure (open circles/plus markers) is based on the compilation of pyroxene compositions in eucrites given by Mayne et al. (2009). Comparing the compositional variations of pyroxenes in eucrites with that of the synthetic pyroxenes investigated by Klima et al. (2011) and Sunshine et al. (2004) reveals that a small number of syntheitic pyroxenes plots outside the compositional variation of diogenites and eucrites (indicated by the gray box Where, | | is the standard deviation, and | | is the value of the band parameters or reflectance at their respective wavelength. The uncertainties of the band parameters for laboratory spectra (BT-1.4%, ...
We present an analysis of the olivine-rich exposures at Bellicia and Arruntia
craters using Dawn Framing Camera (FC) color data. Our results confirm the
existence of olivine-rich materials at these localities as described by
Ammannito et al. (2013a) using Visual Infrared Spectrometer (VIR) data.
Analyzing laboratory spectra of various Howardite-Eucrite-Diogenite meteorites,
high-Ca pyroxenes, olivines and olivine-orthopyroxene mixtures, we derive three
FC spectral band parameters that are indicators of olivine-rich materials.
Combining the three band parameters allows us, for the first time, to reliably
identify sites showing modal olivine contents >40%. The olivine-rich exposures
at Bellicia and Arruntia are mapped using higher spatial resolution FC data.
The exposures are located on the slopes of outer/inner crater walls, on the
floor of Arruntia, in the ejecta, as well as in nearby fresh small impact
craters. The spatial extent of the exposures ranges from a few hundred meters
to few kilometers. The olivine-rich exposures are in accordance with both the
magma ocean and the serial magmatism model (e.g., Righter and Drake 1997;
Yamaguchi et al. 1997). However, it remains unsolved why the olivine-rich
materials are mainly concentrated in the northern hemisphere (~36-42{\deg} N,
46-74{\deg} E) and are almost absent in the Rheasilvia basin.
... With the relatively recent introduction of the current generation of low-resolution, near-infrared spectrographs (TNG: Baffa et al., 2001;Subaru: Motohara et al., 2002; IRTF/SpeX: Rayner et al., 2003Rayner et al., , 2004, the number of VNIR asteroid observations extending to ∼2.5 µm is increasing, which is likely to continue in the coming years (Goldader et al., 1991;Granahan et al., 1994;Bus et al., 2001;Burbine and Binzel, 2002;Bus and Binzel, 2002;Dotto et al., 2003;Izenberg et al., 2003;Kelley et al., 2003;Binzel et al., 2004;Birlan et al., 2004;Clark et al., 2004;Duffard et al., 2004;Hardersen et al., 2004;Rivkin et al., 2004;Sasaki et al., 2004;Sunshine et al., 2004;Abell et al., 2005aAbell et al., , 2005bHardersen et al., 2005;Reddy et al., 2005). This trend suggests that our understanding of asteroids may evolve at a faster rate in the future with more workers studying the large variety of scientific questions that relate to asteroids and the early Solar System. ...
... S-asteroids with relatively well-constrained mineralogic and geologic interpretations can be found in Gaffey (1984), Burbine et al. (1992), Gaffey et al. (1993), Reed et al. (1997), Gaffey and Gilbert (1998), Izenberg et al. (2003), and Sunshine et al. (2004). Based on these results, ∼49 (∼18%) of the first ∼280 S-asteroids (as defined by Tholen, 1984;Tholen and Barucci, 1989) have received more than cursory observations and interpretations. ...
Near-infrared reflectance spectra of M-asteroids 135 Hertha, 224 Oceana, 516 Amherstia, and 872 Holda display spectral and mineralogical diversity that is becoming the norm for the M-asteroid taxonomic group. 224 Oceana exhibits a traditionally featureless M-asteroid spectrum, moderately reddish slope, and an IRAS albedo of 17%. Canonical interpretations include a NiFe metal-rich surface or a surface similar to that of the enstatite chondrites. 516 Amherstia, with an IRAS albedo of 16%, displays absorption features centered at 0.93- and 1.92-microns superimposed on an overall reddish slope. This suggests a surface with a single mafic silicate (i.e., pyroxene, Wo10±4Fs31±5) and NiFe metal. The Band I and II absorptions are 5% and 2% deep, respectively. Amherstia's features are similar to, but somewhat more intense and Fe-rich than, other M-asteroid spectra as discussed in Hardersen et al. (2005). 872 Holda displays a generally featureless NIR reflectance spectrum with the exception of a very broad and weak ( 2% depth) absorption that ranges from 0.5 to 1.2-microns. 872 Holda's overall spectrum is similar to that of synthetic troilite, but differs from spectra of meteoritic troilite as discussed in Cloutis and Burbine (1999). If troilite is present on Holda's surface, then a potential interpretation is a mixture of NiFe metal and troilite as seen in iron meteorites. The NIR reflectance spectrum for 135 Hertha will also be discussed and compared to this asteroid's 52-color survey spectrum, which indicates the presence of a weak 0.9-micron absorption feature. Of the 20 M-asteroid spectra that have been examined by this team thus far, 10 M-asteroids display the presence of low-Fe pyroxene absorption features at 0.9 microns, 3 suggest the presence of spinel, 2 indicate the presence of olivine, one suggests the possible presence of troilite, and 4 display featureless spectra that suggest a variety of potential interpretations.
... Thus, planetesimals that reached these high temperatures would have completely melted and could have differentiated into an "onion layer" structure with an Fe-Ni-S core, an olivine-rich mantle and a plagioclase/pyroxene-rich basaltic crust (Ruzicka et al. 1997). The subsequent solidification of the silicate crust and mantle would follow a crystallization sequence with olivine condensing first, followed by low-calcium pyroxene, then highcalcium pyroxene and finally plagioclase (Taylor 1992;Sunshine et al. 2004). Fully differentiated bodies were the most altered planetesimals, other bodies experienced less extreme thermal histories. ...
... Numerous authors have employed this modified Gaussian model (MGM) to make statements about the surface mineralogy of a wide range of asteroid types (e.g. Vernazza et al. 2008;Mothé-Diniz et al. 2008;Sunshine et al. 2004;Hiroi et al. 1995). ...
The differentiation of small proto-planetary bodies into metallic cores, silicate mantles and basaltic crusts was a common occurrence in the first few million years of Solar System history. In this thesis, observational and theoretical methods are employed to investigate this process. Particular focus is given to the basaltic, crustal remnants of those differentiated parent bodies. A visible-wavelength spectroscopic survey was designed and performed to constrain the population of basaltic asteroids in the Main Belt. The results of this survey were used to provide statistical constraints on the orbital and size-frequency distributions of these objects. These distributions imply that basaltic material is rare in the Main Belt (particularly beyond the 3:1 mean motion resonance at 2.5 AU), however relic fragments of crust from multiple differentiated parent bodies are likely present. To provide insight on the mineralogical diversity of basaltic asteroids in the Main Belt, we performed a series of near-infrared spectroscopic observations. We find that V-type asteroids in the inner belt have spectroscopic properties consistent with an origin from a single parent body, most likely the asteroid Vesta. Spectroscopic differences (namely band area ratio) between these asteroids and basaltic meteorites here on Earth are best explained by space weathering of the asteroid surfaces. We also report the discovery of unusual spectral properties for asteroid 10537 (1991 RY16), a V-type asteroid in the outer Main Belt that has an ambiguous mineralogical interpretation. We conclude this thesis with a theoretical investigation of the relevant stages in the process of differentiation. We show that if partial silicate melting occurs within the interior of a planetesimal then both core and crust formation could have happened on sub-million year (Myr) time scales. However, it is shown that the high temperatures necessary to facilitate these processes may have been affected by the migration of molten silicates within these planetesimals and by chemical interactions between liquid water and silicate rock. Finally, a 1-dimensional model of heat conduction is used to explore whether differentiation would have occurred for planetesimals across a range of sizes (4-250 km) and times of accretion (0-3 Myr).
... Following normal data reduction procedures of flat-fielding, sky subtraction, spectrum extraction, and wavelength calibration, each spectrum was fitted with the ATRAN atmospheric model for telluric absorption features (Lord, 1992;Bus et al., 2003;Sunshine et al., 2004). This procedure required an initial estimate of precip- (a) Rotationally averaged spectra of K-, Sk-and K-candidate (S and Xc) asteroids. ...
... In Fig. 7 we show a break-down of the meteorite and asteroid classes in terms of band center near 1 μm. The 1 μm band is sensitive to the olivine-pyroxene mineralogy of the material, and is perhaps the main mineralogically diagnostic spectral parameter we have measured (Cloutis et al., 1986;Sunshine et al., 2004). Although we do not have enough representatives from the CV, CH, or CI meteorites classes to suggest any correlations or trends, we note that K (including Sk and K-candidates) and Xk asteroids span the band center range of 0.9 to 1.2 microns, and that most meteorite classes (except for the CM-class) fit within this range. ...
B-type asteroids have a negative slope from ˜0.5 to ˜1.1 mum and beyond. What causes this? Visible to near-infrared reflectance spectra (0.4-2.5 mum) are assembled for 22 B-type asteroids. The spectra fall naturally into three groups: (1) those with negative (blue) spectral shapes like 2 Pallas (7 objects), (2) those with concave curve shapes like 24 Themis (11 objects), and (3) everything else (4 objects). The asteroid spectra are compared to mineral and meteorite spectra from the Reflectance Experiment Laboratory library of 15,000 samples, in a least squares search for particulate analogs, constrained by spectral brightness. The Pallas group objects show a trend of analogs from the CV, CO, and CK meteorite groups. Only three of the seven Pallas-like objects are determined to be dynamically related (2, 1508, and 6411). The Themis group objects show a trend of analogs from the CI, CM, CR, CI-Unusual, and CM-Unusual meteorites (as expected from the work of Hiroi et al. (1996)). Seven of the 11 Themis-like objects are dynamically related (24, 62, 222, 316, 379, 383, and 431). Allowing for reasonable uncertainties in the spectral matches, we find no need to invoke mineralogies that do not exist in the meteorite collection to explain B-type spectra or their negative slopes. Our Themis group results are as expected and are consistent with previous work, but our Pallas group results are new and, in some cases, in conflict with previous work.
... It will be challenging to distinguish Ibitira from some basaltic eucrites from vestan orbit. (Sunshine et al. 2004). One candidate for this opaque material is metal, so these asteroids are therefore potential crust fragments of the mesosiderite parent asteroid (Sunshine et al. 2004). ...
... (Sunshine et al. 2004). One candidate for this opaque material is metal, so these asteroids are therefore potential crust fragments of the mesosiderite parent asteroid (Sunshine et al. 2004). ...
Abstract— I have done a detailed petrologic study of Ibitira, a meteorite that has been classified as a basaltic eucrite since 1957. The mean Fe/Mn ratio of pyroxenes in Ibitira with <10 mole% wollastonite component is 36.4 ± 0.4; this value is well resolved from those of similar pyroxenes in five basaltic eucrites studied for comparison, which range from 31.2 to 32.2. Data for the latter five eucrites completely overlap. Ibitira pyroxenes have lower Fe/Mg than the basaltic eucrite pyroxenes; thus, the higher Fe/Mn ratio does not reflect a simple difference in oxidation state. Ibitira also has an oxygen isotopic composition, alkali element contents, and a Ti/Hf ratio that distinguish it from basaltic eucrites. These differences support derivation from a distinct parent asteroid. Thus, Ibitira is the first recognized representative of the fifth known asteroidal basaltic crust, the others being the HED, mesosiderite, angrite, and NWA 011 parent asteroids. 4 Vesta is generally assumed to be the HED parent asteroid. The Dawn mission will orbit 4 Vesta and will perform detailed mapping and mineralogical, compositional, and geophysical studies of the asteroid. Ibitira is only subtly different from eucritic basalts. A challenge for the Dawn mission will be to distinguish different basalt types on the surface and to attempt to determine whether 4 Vesta is indeed the HED parent asteroid.
... Meteorites offer an opportunity to study rocks from other planetary bodies. They are classified by petrologic type, metamorphic grade, and origin (e.g., Wasson, 1985;Papike, 1989;Lauretta & McSween, 2006), and their spectral properties have been measured and analyzed in an effort to connect meteorite classes with types of asteroids (e.g., Chapman & Salisbury, 1973;Salisbury & Hunt, 1974;Gaffey, 1976;McFadden et al., 1980;Salisbury et al., 1991;Moroz et al., 2000;Johnson & Hörz, 2003;Sunshine et al., 2004;Sunshine et al., 2007;Cloutis et al., 2010aCloutis et al., , 2011aCloutis et al., ,b, 2012aRuesch et al., 2015). This is complicated by space weathering on asteroid surfaces (Chapter 13) and terrestrial weathering of meteorites. ...
Remote Compositional Analysis - edited by Janice L. Bishop November 2019
... Analysis of the two major absorption bands found in S-type asteroids -band I (0.7-1.4μm) and band II (1.4-2.4μm) -can be used to identify and distinguish between achondrites and ordinary chondrites (Sunshine et al., 2004). Band area ratios are also able to subclassify asteroids of the ordinary chondrite type into their respective subgroups (LL, L and H, Gaffey et al., 2002;Dunn et al., 2010;). ...
Micrometeorites are millimetre-scale cosmic dust grains, derived from asteroids and comets. They represent the largest flux of extraterrestrial material currently falling to Earth, with an estimated contribution of 20,000-60,000 tons per year. In this thesis, the geological history, parent body properties and atmospheric entry of fine-grained micrometeorites are investigated through micro-analysis and spectroscopic techniques.
The degree of aqueous alteration within fine-grained micrometeorites was investigated using criteria initially developed for CM chondrites (Chapt.3). This revealed that most particles are intensely altered, with petrologic subtypes <CM2.3. Textural and geochemical evidence of aqueous alteration is seen in the form of hydrated CAIs, hydrated sulfides, pseudomorphic chondrules and complex intergrown and cross-cut assemblages of phyllosilicate, which attest to extended periods in contact with liquid water. Likewise, the apparent overabundance of CM-like matrix and the relative paucity of C2 chondrule material among fine-grained micrometeorites suggest that the parent bodies of fine-grained micrometeorites are predominantly intensely aqueously altered bodies.
This study also identified the first evidence for shock deformation in fine-grained micrometeorites (Chapt.6). Weak, pervasive petrofabrics, formed by aligned phyllosilicates and inferred from dehydration crack orientations were observed in the majority of micrometeorites studied (21). This requires relatively low peak pressures (<5GPa) and is most likely achieved by successive low-intensity impact events. The presence of a single micrometeorite containing brittle deformation cataclasis fabrics also provides evidence for brittle deformation shock processing of micrometeorites.
The first near-IR spectra of micrometeorites were collected and directly compared against the NIR spectra of young C-type asteroids (Chapt.8). Although these comparisons proved inconclusive, owing to limitations in the quality of the micrometeorite spectra, this study identified the first evidence of hydroxyl-group absorption bands at NIR wavelengths in Veritas family asteroids, suggesting the presence of intact phyllosilicates on their surfaces and thereby adding support to the genetic link between fine-grained micrometeorites and C-type asteroids.
Mid-IR spectroscopy revealed how micrometeorite mineralogy evolves during flash heating in the upper atmosphere, demonstrating that solid state recrystallization preserves pre-atmospheric textures, despite major changes in the mineralogy (Chapt.4). Spatially resolved Raman spectroscopy was used to investigate thermal gradients within micrometeorites during atmospheric entry and revealed that most micrometeorite cores preserve low-temperature (<300°C) carbonaceous phases inherited from their parent asteroid (Chapt.5). The development of secondary interconnected porosity was described for the first time, detailing how the growth and expansion of dehydration cracks driven by the out-gassing of volatiles leads to the formation of branching and sinuous channels (Chapt.7). These channels play an important role in the efficient heating of micrometeorite cores resulting in partial melting as scoriaceous micrometeorites are formed. In addition, the development of secondary porosity significantly lowers the mechanical strength of micrometeoroids, promoting their disruption in the atmosphere.
Finally, a small-scale study, attempting to retrieve fine-grained micrometeorites preserved in ancient sedimentary rocks was trailed (Chapt.9). This led to the recovery of a new collection of fossil micrometeorites derived from Cretaceous chalk. Although no unmelted micrometeorites were discovered, the preserved cosmic spherules are found to have experienced complete diagenetic alteration, resulting in preserved micro-textures and replaced terrestrial mineralogies. A repeat study at a different time period and location also found cosmic spherules with identical preservation styles, suggesting that diagenetically altered micrometeorites most likely represent the most common form of cosmic dust on Earth.
... It also uses an ATRAN model of the atmosphere (Lord 1992) to estimate the amount of precipitable water at the time of observation for each asteroid and star combination and remove it. This process has been used in several projects using SpeX data in the 0.8-2.5 and 2-4 µm regions (Clark et al. 2004, Sunshine et al. 2004, Rivkin et al. 2006, Rivkin et al. 2015b. Following this step, a weighted average of the spectrum for each corrected asteroid-star pair was created for each asteroid, leading to a final asteroid spectrum. ...
Low-albedo, hydrated objects dominate the list of the largest asteroids. These objects have varied spectral shapes in the 3-$\mu$m region, where diagnostic absorptions due to volatile species are found. Dawn's visit to Ceres has extended the view shaped by ground-based observing, and shown that world to be a complex one, potentially still experiencing geological activity. We present 33 observations from 2.2-4.0 $\mu$m of eight large (greater than 200 km diameter) asteroids from the C spectral complex, with spectra inconsistent with the hydrated minerals we see in meteorites. We characterize their absorption band characteristics via polynomial and Gaussian fits to test their spectral similarity to Ceres, the asteroid 24 Themis (thought to be covered in ice frost), and the asteroid 51 Nemausa (spectrally similar to the CM meteorites). We confirm most of the observations are inconsistent with what is seen in meteorites and require additional absorbers. We find clusters in band centers that correspond to Ceres- and Themis-like spectra, but no hiatus in the distribution suitable for use to simply distinguish between them. We also find a range of band centers in the spectra that approaches what is seen on Comet 67P. Finally, variation is seen between observations for some objects, with the variation on 324 Bamberga consistent with hemispheric-level difference in composition. Given the ubiquity of objects with 3-$\mu$m spectra unlike what we see in meteorites, and the similarity of those spectra to the published spectra of Ceres and Themis, these objects appear much more to be archetypes than outliers.
... However, a small number of eucrites are isotopically distinct from the other HEDs and may come from five other differentiated bodies ). This is consistent with the fact that there are a small number of V-type asteroids, unrelated to Vesta, that are distributed throughout the asteroid belt (Lazzaro et al. 2000;Sunshine et al. 2004;Moskovitz et al. 2008;Duffard and Roig 2009), the most distant having semi-major axes of ∼ 3.1 AU. ...
... The photometric colors and albedos of the ∼100 known asteroid families (Nesvorn´yNesvorn´y et al. 2015;Milani et al. 2014) tend to be very homogeneous ( Parker et al. 2008;Masiero et al. 2011). Through spectroscopic investiga- tion, however, a number of families have been identified as remnants of differentiated parent bodies: Vesta ( McCord et al. 1970;Consolmagno and Drake 1977;Binzel and Xu 1993), Merxia, Agnia ( Sunshine et al. 2004;Vernazza et al. 2014), Maria (Fieber- Beyer et al. 2011) and Hungaria ( Gaffey et al. 1992;Kelley and Gaffey 2002). Two families have been identi- fied as candidates for at least partial differentiation: Eos and Eu- nomia. ...
Differentiated asteroids are rare in the main asteroid belt despite evidence for ~100 distinct differentiated bodies in the meteorite record. We have sought to understand why so few main-belt asteroids differentiated and where those differentiated bodies or fragments reside. Using the Sloan Digital Sky Survey (SDSS) to search for a needle in a haystack we identify spectral A-type asteroid candidates, olivine-dominated asteroids that may represent mantle material of differentiated bodies. We have performed a near-infrared spectral survey with SpeX on the NASA IRTF and FIRE on the Magellan Telescope. We report results from having doubled the number of known A-type asteroids. We deduce a new estimate for the overall abundance and distribution of this class of olivine-dominated asteroids. We find A-type asteroids account for less than 0.16% of all main-belt objects larger than 2 km and estimate there are a total of ~600 A-type asteroids above that size. They are found rather evenly distributed throughout the main belt, are even detected at the distance of the Cybele region, and have no statistically significant concentration in any asteroid family. We conclude the most likely implication is the few fragments of olivine-dominated material in the main belt did not form locally, but instead were implanted as collisional fragments of bodies that formed elsewhere.
... Rather, the spectra of olivine-free orthopyroxenite and harzburgites containing 10 and 25 vol.% olivine were indistinguishable, consistent with other work on binary mixtures of olivine and pyroxene [Cloutis et al., 1986;Sunshine et al., 2004]. A greater proportion of olivine should be spectrally identifiable on Vesta, but it would have to occur at an abundance >25% over the scale of a VIR footprint. ...
Combined analyses of the surface elemental composition and mineralogy of Vesta and Ceres provide insights into their interior evolution, crustal formation, and regolith processes. Compositional data acquired by Dawn's Visible to Infrared Mapping Spectrometer (VIR) and Gamma Ray and Neutron Detector (GRaND) are sensitive to different depths and spatial scales. To compare these data sets, high-resolution maps of absorption band strengths from VIR are degraded to the broad spatial scales sampled by GRaND using a physics-based smoothing algorithm that accounts for the shape and topography of Vesta and Ceres. On Vesta, the distributions of elemental hydrogen and hydroxyl are similar, which implies that hydrogen is primarily in the form of hydroxyl, likely as phyllosilicates delivered by the infall of carbonaceous chondrite impactors. Small differences in the spatial patterns of hydroxyl and hydrogen imply that hydrogen is layered in some locations. In Vesta's dark hemisphere, hydrogen deposits are more extensive than hydroxyl, which indicates higher concentrations of hydrated minerals at depth. In contrast, the distributions of elemental hydrogen and hydrogen-bearing species (OH and NH_4^+) on Ceres are dissimilar. High concentrations of hydrogen in the Ceres’ polar regions (approaching 30 wt.% equivalent H_2O) indicate the presence of subsurface ice as predicted by ice stability theory. The concentration of iron follows a water-dilution trend when plotted as a function of regolith hydrogen content, consistent with the presence of subsurface water ice. The VIR and GRaND data jointly constrain aspects of Ceres’ surface chemistry and evolution. GRaND iron measurements place a firm upper bound on magnetite content, which supports graphitized carbon as an alternative to magnetite as a darkening agent. Lower-bounds on the concentration of carbon in carbonates implied by VIR, together with the ratio of carbonates to organics in carbonaceous chondrite meteorite analogs suggest high concentrations of carbon within Ceres’ regolith. GRaND neutron measurements permit elevated carbon concentrations, equal to or in excess of that found in CI chondrites (greater than a few wt.%). Organic matter, detected by VIR at Ernutet crater, might be widespread and may have been converted to graphite, e.g. via UV exposure, elsewhere on the surface. Furthermore, elevated concentrations of carbonaceous material can explain the difference between iron and hydrogen concentrations measured by GRaND and the CI carbonaceous chondrites, which are representative of the materials from which Ceres accreted. The elemental measurements indicate that ice and rock fractionated during Ceres’ evolution producing a crust that differs in composition from the whole body.
... However, a small number of eucrites are isotopically distinct from the other HEDs and may come from five other differentiated bodies ). This is consistent with the fact that there are a small number of V-type asteroids, unrelated to Vesta, that are distributed throughout the asteroid belt (Lazzaro et al. 2000;Sunshine et al. 2004;Moskovitz et al. 2008;Duffard and Roig 2009), the most distant having semi-major axes of ∼ 3.1 AU. ...
Asteroids and comets are the remnants of the swarm of planetesimals from which the planets ultimately formed, and they retain records of processes that operated prior to and during planet formation. They are also likely the sources of most of the water and other volatiles accreted by Earth. In this review, we discuss the nature and probable origins of asteroids and comets based on data from remote observations, in situ measurements by spacecraft, and laboratory analyses of meteorites derived from asteroids. The asteroidal parent bodies of meteorites formed \(\leq 4\) Ma after Solar System formation while there was still a gas disk present. It seems increasingly likely that the parent bodies of meteorites spectroscopically linked with the E-, S-, M- and V-type asteroids formed sunward of Jupiter’s orbit, while those associated with C- and, possibly, D-type asteroids formed further out, beyond Jupiter but probably not beyond Saturn’s orbit. Comets formed further from the Sun than any of the meteorite parent bodies, and retain much higher abundances of interstellar material. CI and CM group meteorites are probably related to the most common C-type asteroids, and based on isotopic evidence they, rather than comets, are the most likely sources of the H and N accreted by the terrestrial planets. However, comets may have been major sources of the noble gases accreted by Earth and Venus. Possible constraints that these observations can place on models of giant planet formation and migration are explored.
... However, a small number of eucrites are isotopically distinct from the other HEDs and may come from five other differentiated bodies ( ). This is consistent with the fact that there are a small number of V-type asteroids, unrelated to Vesta, that are distributed throughout the asteroid belt ( Lazzaro et al. 2000;Sunshine et al. 2004;Moskovitz et al. 2008;Duffard and Roig 2009), the most distant having semi-major axes of ∼ 3.1 AU. The signature feature of the CI and CM chondrites is that they have undergone extensive aqueous alteration and contain abundant phyllosilicates. ...
Of the potential volatile sources for the terrestrial planets, the CI and CM carbonaceous chondrites are closest to the planets' bulk H and N isotopic compositions. For the Earth, the addition of approximately 2–4 wt% of CI/CM material to a volatile-depleted proto-Earth can explain the abundances of many of the most volatile elements, although some solar-like material is also required. Two dynamical models of terrestrial planet formation predict that the carbonaceous chondrites formed either in the asteroid belt (‘classical’ model) or in the outer Solar System (5–15 AU in the Grand Tack model). To test these models, at present the H isotopes of water are the most promising indicators of formation location because they should have become increasingly D-rich with distance from the Sun. The estimated initial H isotopic compositions of water accreted by the CI, CM, CR and Tagish Lake carbonaceous chondrites were much more D-poor than measured outer Solar System objects. A similar pattern is seen for N isotopes. The D-poor compositions reflect incomplete re-equilibration with H 2 in the inner Solar System, which is also consistent with the O isotopes of chondritic water. On balance, it seems that the carbonaceous chondrites and their water did not form very far out in the disc, almost certainly not beyond the orbit of Saturn when its moons formed (approx. 3–7 AU in the Grand Tack model) and possibly close to where they are found today.
This article is part of the themed issue ‘The origin, history and role of water in the evolution of the inner Solar System’.
... Olivine has a complex absorption approximately 1 μm. It moves toward longer wavelengths with an increasing amount of Fe 2 + ( Sunshine et al., 2004 ). The right wing of the absorption shifts towards longer wavelengths as the FeO content in the olivine increases ( Poulet et al., 2007 ). ...
... We plot the central positions of deconvolved 1 and 2 mm bands from HCP and LCP components based on the data of Adams 27 and Cloutis and Gaffey 28 (Fig. 3c). Here we define the HCP as wollastonite (Wo)430 and LCP as Woo30, keeping with previous work by Sunshine et al. 31 and Klima et al. 30 . By comparison, the compositional features of LCP of the four soils are similar (Fig. 3c) and very Fe-rich, relative to orthopyroxene examined by Adams 27 and Cloutis and Gaffey 28 . ...
The chemical compositions of relatively young mare lava flows have implications for the late volcanism on the Moon. Here we report the composition of soil along the rim of a 450-m diameter fresh crater at the Chang′e-3 (CE-3) landing site, investigated by the Yutu rover with in situ APXS (Active Particle-induced X-ray Spectrometer) and VNIS (Visible and Near-infrared Imaging Spectrometer) measurements. Results indicate that this region's composition differs from other mare sample-return sites and is a new type of mare basalt not previously sampled, but consistent with remote sensing. The CE-3 regolith derived from olivine-normative basaltic rocks with high FeO/(FeO+MgO). Deconvolution of the VNIS data indicates abundant high-Ca ferropyroxene (augite and pigeonite) plus Fe-rich olivine. We infer from the regolith composition that the basaltic source rocks formed during late-stage magma-ocean differentiation when dense ferropyroxene-ilmenite cumulates sank and mixed with deeper, relatively ferroan olivine and orthopyroxene in a hybridized mantle source.
... Jitka is a member of the Agnia family of asteroids, named after the asteroid 847 Agnia. This group most likely formed from the breakup of a basaltic object, which in turn was spawned from a larger parent body that underwent igneous differentiation (Milani et al., 2014;Sunshine et al., 2004). According to the WISE satellite infrared radiometry, the diameter is 10.909 ± 0.155 km based on an absolute magnitude H = 11.7 (Masiero et al., 2011). ...
Analysis of photometric observations of the main-belt asteroid 3395 Jitka performed by the authors in February 2015 revealed a bimodal lightcurve with a synodic rotation period of 18.293 ± 0.006 hours as the most likely solution.
... Rather, the spectra of olivine-free orthopyroxenite and harzburgites containing 10 and 25 vol.% olivine were indistinguishable, consistent with other work on binary mixtures of olivine and pyroxene [Cloutis et al., 1986;Sunshine et al., 2004]. A greater proportion of olivine should be spectrally identifiable on Vesta, but it would have to occur at an abundance >25% over the scale of a VIR footprint. ...
We present and describe the maps of spectral parameters such as pyroxene band centers and depths, reflectance at 1.4 μm and 2.8μm band depth in the Rheasilvia quadrangle. We found a broad anti-correlation between pyroxene band centers and depths while the reflectance is not correlated with the pyroxene spectral parameters. In addition, we found that the Rheasilvia quadrangle is free of OH absorption signatures. We also derived lithological maps with improvements in the spatial resolution with respect to previous lithological maps of the same region. We confirm that the central mound is dominated by eucritic/howarditic pyroxene while diogenitic lithology has been found mainly in a region delineated by Tarpeia, Severina and Mariamne craters. We found small scale variations in the composition of pyroxene. These variations identify lithological units that extend for tens of km, although small units of less than 1 km have also been found. We consider this fact as an indication of a high level of compositional heterogeneity within the Vestan crust.
... The near-infrared provides wavelength coverage that allows better characterization of silicate features (Burbine & Binzel 2002, Gaffey et al. 1993. While no single large observing campaign has been initiated for main belt asteroids in the near-infrared, many small programs have been carried out that targeted specific asteroid families (Sunshine et al. 2004;Mothe-Diniz et al. 2005;Vernazza et al. 2006;Mothe-Diniz et al. 2008a;Mothe-Diniz & Nesvorný 2008b;Mothe-Diniz & Nesvorný 2008c;Willman et al. 2008;Harris et al. 2009;Reddy et al. 2009;de Sanctis et al. 2011;Reddy et al. 2011;Ziffer et al. 2011;de Leon et al. 2012). A compilation of spectral taxonomic classifications of asteroids is given in the Planetary Data System by Neese (2010) and is periodically updated. ...
An asteroid family is typically formed when a larger parent body undergoes a
catastrophic collisional disruption, and as such family members are expected to
show physical properties that closely trace the composition and mineralogical
evolution of the parent. Recently a number of new datasets have been released
that probe the physical properties of a large number of asteroids, many of
which are members of identified families. We review these data sets and the
composite properties of asteroid families derived from this plethora of new
data. We also discuss the limitations of the current data, and the open
questions in the field.
... Table 1 gives a summary of the asteroids that were observed in the near-infrared and radar for this program and Table 2 gives the observing circumstances. Following normal data reduction procedures of flat fielding, sky subtraction, spectrum extraction, and wavelength calibration, each spectrum was fitted with the ATRAN atmospheric model for telluric absorption features (Lord, 1992;Bus et al., 2003;Sunshine et al., 2004). This procedure required an initial estimate of precipitable water in the atmospheric optical path using the zenith angle for the observation and the known s-values (average atmospheric water) for Mauna Kea. ...
... Pyroxene has been identified in several S-type asteroid families (Chapman et al. 1975). Sunshine et al. (2004) applied the Modified Gaussian Model (MGM) to VNIR asteroid spectra collected by the NASA Infrared Telescope Facility (IRTF), to determine the relative amounts of low-and high-calcium pyroxene. They determined that the Vestoids and well as the S-type asteroid families Merxia and Agnia have a high ratio of high-calcium pyroxene to total pyroxene, indicating that they are derived from differentiated bodies. ...
We have determined the mid-IR optical constants of one alkali feldspar and four pyroxene compositions in the range of 250–4000
cm−1. Measured reflectance spectra of oriented single crystals were iteratively fit to modeled spectra derived from classical
dispersion analysis. We present the real and imaginary indices of refraction (n and k) along with the oscillator parameters with which they were modeled. While materials of orthorhombic symmetry and higher are
well covered by the current literature, optical constants have been derived for only a handful of geologically relevant monoclinic
materials, including gypsum and orthoclase. Two input parameters that go into radiative transfer models, the scattering phase
function and the single scattering albedo, are functions of a material’s optical constants. Pyroxene is a common rock-forming
mineral group in terrestrial bodies as well as meteorites and is also detected in cosmic dust. Hence, having a set of pyroxene
optical constants will provide additional details about the composition of Solar System bodies and circumstellar materials.
We follow the method of Mayerhöfer et al. (2010), which is based on the Berreman 4 × 4 matrix formulation. This approach provides a consistent way to calculate the reflectance
coefficients in low-symmetry cases. Additionally, while many models assume normal incidence to simplify the dispersion relations,
this more general model applies to reflectance spectra collected at non-normal incidence.
... Fig. 5). If we take $20% as the typical maximum reflectance at 0.555 lm, and assume that the surface of Vesta is fine grained, as seems likely (Hiroi et al., 1995), this suggests that the regions we are examining are not optically equivalent to powdered HEDs measured in the laboratory, which are much brighter (Hiroi et al., 1995(Hiroi et al., , 2001Sunshine et al., 2004;Duffard et al., 2005;Burbine et al., 2007;Donaldson Hanna and Sprague, 2009;Moskovitz et al., 2010;Mayne et al., 2010Mayne et al., , 2011Beck et al., 2011;Reddy et al., 2012a). If we use the full suite of Millbillillie + Murchison intimate mixtures as a guide to the DM terrains, the brightest regions ($20% reflectance at 0.653 lm) are most similar to the 80/20 Fig. 13. ...
... For decades, Vesta was thought to be spectrally unique-the only known candidate for the HED parent body. More recent discoveries of a limited number of other Main Belt asteroids with spectral characteristics similar to those of eucrites (Lazarro et al. 2000;Hardersen et al. 2004;Sunshine et al. 2004;Roig et al. 2008) now allow the possibility of other sources for some HEDs. ...
The Dawn mission has provided new evidence strengthening the
identification of asteroid Vesta as the parent body of the howardite,
eucrite, and diogenite (HED) meteorites. The evidence includes Vesta's
petrologic complexity, detailed spectroscopic characteristics, unique
space weathering, diagnostic geochemical abundances and neutron
absorption characteristics, chronology of surface units and impact
history, occurrence of exogenous carbonaceous chondritic materials in
the regolith, and dimensions of the core, all of which are consistent
with HED observations and constraints. Global mapping of the
distributions of HED lithologies by Dawn cameras and spectrometers
provides the missing geologic context for these meteorites, thereby
allowing tests of petrogenetic models and increasing their scientific
value.
... The use of a straight continuum follows the procedure used to derive the most commonly used interpretive calibrations from laboratory spectra of meteorites (e.g., Adams 1974Adams , 1975Cloutis et al., 1986;Cloutis and Gaffey, 1991;Gaffey et al., 1993Gaffey et al., , 2002Gaffey and Gilbert, 1998). Curve fitting techniques such as MGMFIT (e.g., Sunshine et al., 1998Sunshine et al., , 2004 treat the continuum as a two-term variable. Each approach has its advantages and disadvantages; neither is obviously superior. ...
Results of our analysis indicate 1036 Ganymed is an S (VI) asteroid with
a surface silicate assemblage consisting of opx and cpx,
[Fs23(+/-5)Wo3(+/-3) average of both phases] possibly with a metal
component.
... The use of a straight continuum follows the procedure used to derive the most commonly used interpretive calibrations from laboratory spectra of meteorites (e.g., Adams 1974 Adams , 1975 Cloutis et al., 1986; Cloutis and Gaffey, 1991; Gaffey et al., 1993 Gaffey et al., , 2002 Gaffey and Gilbert, 1998 ). Curve fitting techniques such as MGMFIT (e.g., Sunshine et al., 1998 Sunshine et al., , 2004) treat the continuum as a two-term variable. Each approach has its advantages and disadvantages; neither is obviously superior. ...
... The SpeX instrument (Rayner et al., 2003) is a medium-resolution cryogenic spectrograph located at the NASA Infrared Telescope Facility (IRTF) on Mauna Kea. SpeX can easily observe objects as faint as 17-18th magnitude, allowing highquality near-infrared (0.8-2.5 lm) data (e.g., Binzel et al., 2001;Sunshine et al., 2004;Hardersen et al., 2004Hardersen et al., , 2005Hardersen et al., , 2006Reddy et al., 2012a) to be obtained for relatively faint objects such as NEAs. Although the SpeX instrument is capable of producing reliable data down to 0.75 lm, SpeX data was collected with the dichoric filter in, which prevents acquisition of spectra at wavelengths shorter than 0.8 lm. ...
Near-Earth Asteroids (NEAs) offer insight into a size range of objects that are not easily observed in the main asteroid belt. Previous studies on the diversity of the NEA population have relied primarily on modeling and statistical analysis to determine asteroid compositions. Olivine and pyroxene, the dominant minerals in most asteroids, have characteristic absorption features in the visible and near-infrared (VISNIR) wavelengths that can be used to determine their compositions and abundances. However, formulas previously used for deriving compositions do not work very well for ordinary chondrite assemblages. Because two-thirds of NEAs have ordinary chondrite-like spectral parameters, it is essential to determine accurate mineralogies. Here we determine the band area ratios and Band I centers of 72 NEAs with visible and near-infrared spectra and use new calibrations to derive the mineralogies 47 of these NEAs with ordinary chondrite-like spectral parameters. Our results indicate that the majority of NEAs have LL-chondrite mineralogies. This is consistent with results from previous studies but continues to be in conflict with the population of recovered ordinary chondrites, of which H chondrites are the most abundant. To look for potential correlations between asteroid size, composition, and source region, we use a dynamical model to determine the most probable source region of each NEA. Model results indicate that NEAs with LL chondrite mineralogies appear to be preferentially derived from the ν6 secular resonance. This supports the hypothesis that the Flora family, which lies near the ν6 resonance, is the source of the LL chondrites. With the exception of basaltic achondrites, NEAs with non-chondrite spectral parameters are slightly less likely to be derived from the ν6 resonance than NEAs with chondrite-like mineralogies. The population of NEAs with H, L, and LL chondrite mineralogies does not appear to be influenced by size, which would suggest that ordinary chondrites are not preferentially sourced from meter-sized objects due to Yarkovsky effect.
... The use of a straight continuum follows the procedure used to derive the most commonly used interpretive calibrations from laboratory spectra of meteorites (e.g., Adams 1974 Adams , 1975 Cloutis et al., 1986; Cloutis and Gaffey, 1991; Gaffey et al., 1993 Gaffey et al., , 2002 Gaffey and Gilbert, 1998 ). Curve fitting techniques such as MGMFIT (e.g., Sunshine et al., 1998 Sunshine et al., , 2004) treat the continuum as a two-term variable. Each approach has its advantages and disadvantages; neither is obviously superior. ...
We present a mineralogical assessment of near-Earth Asteroid, (1036) Ganymed, using data obtained May 18, 2006 UT combined with 24 Color Asteroid Survey data to cover the spectral interval of 0.3-2.45 mum. Results of the analysis indicate (1036) Ganymed is an S (VI) asteroid with a surface silicate assemblage consisting primarily of orthopyroxene, (Fs23(±5)Wo3(±3)), consistent with calculated band centers and band area ratios (BAR). (1036) Ganymed appears to be once part of a large mesosiderite containing howardite, eucrite, and diogenite (HED) pyroxenes mixed with metal that was broken apart and dispersed. The calculated composition of the average pyroxenes in the surface material of (1036) Ganymed is consistent with mesosiderite pyroxenes, in particular the diogenites. A second possibility could be (1036) Ganymed is not yet represented in the meteorite collection. Our investigation has confirmed Ganymed is not a parent body of the ordinary chondrites and is not genetically related to (433) Eros.
... For both of these objects, their collective spectra were aligned and co-added to achieve an average spectrum for each. These average spectra were each fitted with the ATRAN atmospheric model for telluric absorption features (Lord et al., 1992;Bus et al., 2003;Sunshine et al., 2004). Telluric features were removed from the spectra, to within the level of the scatter in the data across the whole spectrum. ...
We present reflectance spectra from 0.4 to 2.4μm of Asteroid (101955) 1999 RQ36, the target of the OSIRIS-REx spacecraft mission. The visible spectral data were obtained at the McDonald Observatory 2.1-m telescope with the ES2 spectrograph. The infrared spectral data were obtained at the NASA Infrared Telescope Facility using the SpeX instrument. The average visible spectrum is combined with the average near-infrared wavelength spectrum to form a composite spectrum. We use three methods to constrain the compositional information in the composite spectrum of Asteroid (101955) 1999 RQ36 (hereafter RQ36). First, we perform a least-squares search for meteorite spectral analogs using 15,000 spectra from the RELAB database. Three most likely meteorite analogs are proposed based on the least-squares search. Next, six spectral parameters are measured for RQ36 and their values are compared with the ranges in parameter values of the carbonaceous chondrite meteorite classes. A most likely meteorite analog group is proposed based on the depth of overlap in parameter values. The results of the least-squares search and the parametric comparisons point to CIs and/or CMs as the most likely meteorite analogs for RQ36, and COs and CHs as the least likely. RQ36 has a spectrally “blue” continuum slope that is also observed in carbonaceous chondrites containing magnetite. We speculate that RQ36 is composed of a “CM1”-like material. Finally, we compare RQ36 to other B-type asteroids measured by Clark et al. (Clark, B.E. et al. [2010]. J. Geophys. Res. 115, E06005). The results of this comparison are inconclusive. RQ36 is comparable to Themis spectral properties in terms of its albedo, visible spectrum, and near-infrared spectrum from 1.1 to 1.45μm. However, RQ36 is more similar to Pallas in terms of its near-infrared spectrum from 1.6 to 2.3μm. Thus it is possible that B-type asteroids form a spectral continuum and that RQ36 is a transitional object, spectrally intermediate between the two end-members. This is particularly interesting because Asteroid 24 Themis was recently discovered to have H2O ice on the surface (Rivkin, A., Emery, J. [2010]. Nature 464, 1322–1323; Campins, H. et al. [2010a]. Nature 464, 1320–1321).
... These implications persuaded other authors of the value of MGM to study pyroxene absorptions in related assemblages (Schade et al., 2004). The MGM has also been used to study pyroxene-dominated Martian meteorites (McFadden and Cline, 2005;Schade and Wäsch, 1999;Sunshine et al., 2004aSunshine et al., , 2004b. MGM has been applied to model absorptions of fosterite-fayalite olivine series (Sunshine and Pieters, 1998), McFadden and Cline (2005) explored MGM identification of olivine bands in olivine-dominated meteorites. ...
This study advances curve-fitting modeling of absorption bands of reflectance spectra and applies this new model to spectra of Martian meteorites ALH 84001 and EETA 79001 and data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). This study also details a recently introduced automated parameter initialization technique. We assess the performance of this automated procedure by comparing it to the currently available initialization method and perform a sensitivity analysis of the fit results to variation in initial guesses. We explore the issues related to the removal of the continuum, offer guidelines for continuum removal when modeling the absorptions and explore different continuum-removal techniques. We further evaluate the suitability of curve fitting techniques using Gaussians/Modified Gaussians to decompose spectra into individual end-member bands. We show that nonlinear least squares techniques such as the Levenberg–Marquardt algorithm achieve comparable results to the MGM model (Sunshine and Pieters, 1993; Sunshine et al., 1990) for meteorite spectra. Finally we use Gaussian modeling to fit CRISM spectra of pyroxene and olivine-rich terrains on Mars. Analysis of CRISM spectra of two regions show that the pyroxene-dominated rock spectra measured at Juventae Chasma were modeled well with low Ca pyroxene, while the pyroxene-rich spectra acquired at Libya Montes required both low-Ca and high-Ca pyroxene for a good fit.
... The spectroscopic properties of the HED clan have been the subject of a number of studies (e.g., Hiroi et al., 1995Hiroi et al., , 2001Sunshine et al., 2004;Duffard et al., 2005;Burbine et al., 2007;Donaldson Hanna and Sprague, 2009;Moskovitz et al., 2010;Mayne et al., 2010Mayne et al., , 2011Beck et al., 2011;Reddy et al., 2012b). In the present study we examine the spectral reflectance properties of intimate and areal mixtures of the Millbillillie eucrite and the Murchison CM2 carbonaceous chondrite, as well as multiple grain size powders and slabs of the CM2-bearing howardite PRA 04401, and a mineralogically diverse suite of HEDs, most of which have not been previously characterized. ...
We have studied the spectral reflectance properties of the CM2 xenolith-bearing howardite PRA 04401, intimate and areal mixtures of a eucrite (Millbillillie) and a CM2 chondrite (Murchison), and a mineralogically diverse suite of 12 HED meteorites. The main goal is to ascertain whether the presence of CM2-type material can be discriminated from grain size variations in HEDs. This project was motivated by the presence of CM2 material in a number of howardites. This study is of high relevance to Vesta because of the detection of low-albedo features on its surface by the Framing Camera (FC) aboard NASA’s Dawn spacecraft. The addition of CM2 material and increasing HED grain size both lead to decreasing overall albedo. However these two processes can be recognized by how they affect pyroxene band depths and shapes and, to a lesser extent, band widths. Pyroxene band depths and widths decrease with increasing CM2 abundance and increase with increasing HED grain size, as do various FC reflectance ratios. HED pyroxene absorption bands appear to reach saturation when grain size is in the region of 60–170 μm, with band I saturating at smaller grain sizes than band II. Band I centers are largely insensitive to CM2 abundances or HED grain size variations, while Band II center positions increase by up to 40 nm with increasing CM2 abundance, and vary non-systematically by up to 22 nm as a function of grain size. The variation with CM2 abundance is probably due to the red-sloped nature of the CM2 spectrum and the broad and shallow nature of the pyroxene band II absorption. Band area ratios show less consistent behavior, likely due to CM2-induced slope changes in the mixtures and band saturation effects in the HEDs. Because HED pyroxene is so strongly featured, CM2 abundances must be well in excess of 80 wt.% to allow for the appearance of their much weaker CM2 phyllosilicate absorption bands. CM2 material may also cause a reddening of spectral slope and a shifting of pyroxene band minima to shorter wavelengths, although CM2 chondrites can exhibit a range of spectral slopes. The presence of submicron opaques (specifically chromite) in HED pyroxenes can lead to large variations in all spectral parameters. Discriminating the spectrum-altering effects of CM2 material from HED grain size variations is possible, with the confidence in the interpretation increasing as a larger range of spectral parameters are applied to the analysis.
... Rather, the spectra of olivine-free orthopyroxenite and harzburgites containing 10 and 25 vol.% olivine were indistinguishable, consistent with other work on binary mixtures of olivine and pyroxene [Cloutis et al., 1986;Sunshine et al., 2004]. A greater proportion of olivine should be spectrally identifiable on Vesta, but it would have to occur at an abundance >25% over the scale of a VIR footprint. ...
The estimated excavation depth of the huge Rheasilvia impact basin is
nearly twice the likely thickness of the Vestan basaltic crust, so the
mantle should be exposed. Spectral mapping by the Dawn spacecraft
reveals orthopyroxene-rich materials, similar to diogenite meteorites,
in the deepest parts of the basin and within its walls. Significant
amounts of olivine are predicted for the mantles of bulk-chondritic
bodies like Vesta, and its occurrence is demonstrated by some diogenites
that are harzburgite and dunite. However, olivine has so far escaped
detection by Dawn's instruments. Spectral detection of olivine in the
presence of orthopyroxene is difficult in samples with <25% olivine,
and olivine in Rheasilvia might have been diluted during impact mixing
or covered by the collapse of basin walls. The distribution of diogenite
inferred from its exposures in and around Rheasilvia provides a geologic
context for the formation of these meteorites, but does not clearly
distinguish between a magmatic cumulate versus partial melting restite
origin for diogenites. The former is favored by geochemical arguments,
and crystallization in either a magma ocean or multiple plutons emplaced
near the crust-mantle boundary is permitted by Dawn observations.
... A minor amount of olivine (∼10 vol% and less) is enough to detect the 630-nm absorption band in the spectra of pyroxene-olivine mixture, whereas the 950-nm band and reflectance maximum near 720 nm are still not affected by the presence of olivine . Previously, Sunshine et al. (2004) using MGM modeling also found that olivine in olivine-pyroxene mixtures does not appear to noticeably affect the Band I (near 950 nm) and Band II (near 1900 nm) in the spectrum for abundance of olivine less than 20%. Thus we believe the isolated absorption band centered longward of 620 nm in the Type-II vestoid spectra is the olivine feature. ...
Faint absorption bands in the visible range of the smoothed vestoid spectra have been found. The bands centered near 505, 530, and 550 nm
are attributed to ferrous iron in low-calcium pyroxene and are typical for pyroxene-bearing vestoid surfaces. In accordance with characteristics of
the faint absorption bands around 600 and 650 nm the studied vestoid spectra can be sorted into five types. Since the same absorptions are also
seen in the laboratory spectra of the minerals and meteorites, which appear to be similar to vestoid material, spectral types of the vestoids can
be related to their surface compositions. Regolith of the Type-I vestoids consists of pure low-calcium pyroxenes. Minor amount of olivine along
with pyroxene appear to be on the Type-II vestoids whereas the mixtures of low-calcium pyroxene with minor chromite define the Type-III and
-IV. The causes for the fifth spectral type in terms of minor mineral phases are unclear now. Simulation of the spectra of vestoids was employed
to estimate content of olivine (∼6–12 vol%) and chromite (∼12–30 vol%) on their surfaces.
... Several features of these best-fit MGM models suggest they are unreasonable. First, the band parameters, though within the ranges measured for eucrites (Mayne et al., 2010; Sunshine et al., 2004), are in many cases at the limits of those ranges. For example, the central wavelengths of the 1 lm LCP and HCP bands, 2 lm LCP band and M1 band are all more than one standard deviation away from the mean band centers for eucrites (Mayne et al., 2010). ...
Phase angle and temperature are two important parameters that affect the photometric and spectral behavior of planetary surfaces in telescopic and spacecraft data. We have derived photometric and spectral phase functions for the Asteroid 4 Vesta, the first target of the Dawn mission, using ground-based telescopes operating at visible and near-infrared wavelengths (0.4–2.5μm). Photometric lightcurve observations of Vesta were conducted on 15 nights at a phase angle range of 3.8–25.7° using duplicates of the seven narrowband Dawn Framing Camera filters (0.4–1.0μm). Rotationally resolved visible (0.4–0.7μm) and near-IR spectral observations (0.7–2.5μm) were obtained on four nights over a similar phase angle range. Our Vesta photometric observations suggest the phase slope is between 0.019 and 0.029mag/deg. The G parameter ranges from 0.22 to 0.37 consistent with previous results (e.g., Lagerkvist, C.-I., Magnusson, P., Williams, I.P., Buontempo, M.E., Argyle, R.W., Morrison, L.V. [1992]. Astron. Astrophys. Suppl. Ser. 94, 43–71; Piironen, J., Magnusson, P., Lagerkvist, C.-I., Williams, I.P., Buontempo, M.E., Morrison, L.V. [1997]. Astron. Astrophys. Suppl. Ser. 121, 489–497; Hasegawa, S. et al. [2009]. Lunar Planet. Sci. 40. ID 1503) within the uncertainty. We found that in the phase angle range of 0°
... As with the visiblewavelength spectra, solar analogs were observed to correct for telluric absorption and to remove the solar spectrum from the measured reflectance ( Table 1). Reduction of the SpeX data followed Sunshine et al. (2004), reduction of the FIRE data employed an IDL package designed for the instrument and based on the Spextool pipeline (Cushing et al., 2004). ...
... All objects were observed using SpeX, a mediumresolution near-infrared spectrograph (Rayner et al. 2003) on the NASA Infrared Telescope Facility (IRTF) located on Mauna Kea. Observing procedures using SpeX are discussed in Binzel et al. (2004a) and Sunshine et al. (2004). Appropriate solar analog standard stars were used to produce the final reflectance spectra, normalized to unity at 0.55 µm. ...
Abstract— We have calculated pyroxene mineralogies of seven near-Earth asteroids (NEAs) with reflectance spectra similar to HEDs (howardites, eucrites, and diogenites). Two different sets of formulas (Gaffey et al. 2002; Burbine et al. 2007) are used to calculate the pyroxene mineralogies of the NEAs from their Band I and II centers. The band centers have been adjusted to compensate for the low temperatures on the asteroid surfaces. All of the derived mineralogies from the Gaffey et al. (2002) formulas and the Burbine et al. (2007) formulas overlap. The derived wollastonite (Wo) contents are very similar with differences being only approximately 1 mol%. The derived ferrosilite (Fs) contents differ by only 3 to 8 mol%. The determined pyroxene mineralogies for all seven near-Earth vestoids are consistent with eucrites or howardites. None of the objects have pyroxene mineralogies consistent with diogenites. The absence of near-Earth vestoids with pyroxene mineralogies similar to diogenites may indicate that it is difficult to produce sizeable (km-sized or larger) bodies that are predominantly composed of diogenitic material, suggesting these objects are rubble piles of mixed ejecta.
... [4] and [5] have independently developed a set of equations that ingest spectral band parameters such as Band I and II centers, and Band Area Ratio of pyroxene absorption bands and output mean pyroxene chemistry (Fs, Wo and Mg#). These equations coupled with Gaussian analysis tools like MGM [6] can be used with Dawn VIR spectrometer data to constrain the mineralogy and chemistry of Vesta's surface. We have developed similar equations that ingest Dawn FC data (pseudo Band I minima) and output mean pyroxene chemistry. ...
The Dawn mission will rendezvous with asteroid (4) Vesta in July 2011. We have developed a set of equations for extracting mean pyroxene chemistry (Ferrosilite and Wollastonite) for classifying terrains on Vesta by using the Dawn Framing Camera (FC) multi-color bands. The Automated Spectral System (ASS) utilizes pseudo-Band I minima to estimate the mean pyroxene chemistry of diogenites, and basaltic eucrites. The mean pyroxene chemistries of cumulate eucrites, and howardites overlap each other on the pyroxene quadrilateral and hence are harder to distinguish. We expect our ASS to carry a bulk of the terrain classification and mineralogy workload utilizing these equations and complement the work of DawnKey (Le Corre et al., 2011, DPS/EPSC 2011). The system will also provide surface mineral chemistry layers that can be used for mapping Vesta's surface.
Although pyroxene has been detected remotely across the Solar System, limited information is available from infrared remote sensing about the Mg‐Fe composition of pyroxene, and distinguishing between augite (20 < CaSiO3 < 45) and diopside‐hedenbergite (CaSiO3 > 45) remains challenging. The characteristics of pyroxene in the intermediate infrared range (4–8 μm), meanwhile, have not been documented. Using reflectance spectra of 72 samples ranging across the pyroxene quadrilateral, we investigate the effect of variations in Mg# (Mg/[Mg + Fe] × 100) and Ca‐content on the positions of strong and well‐defined spectral bands at ∼5.1 and ∼5.3 μm in high‐Ca pyroxene and ∼5.2 in low‐Ca pyroxene. We find that the 5.1, 5.2, and 5.3 μm bands move to shorter wavelengths as Mg# increases, whereas Ca‐content does not significantly affect the positions of these bands, enabling the determination of pyroxene Mg# directly from band positions alone. We also find that the ∼5.1 μm band is significantly more distinctive in diopside‐hedenbergite and the ∼5.3 μm band significantly more so in augite. Therefore, the 5.1, 5.2, and 5.3 μm spectral bands enable discrimination among diopside‐hedenbergite, low‐Ca pyroxene, and augite. Additionally, the 5.1, 5.2, and 5.3 μm bands enable direct determination of Mg# of diopside‐hedenbergite, low‐Ca pyroxene, and augite within ±23, ±10, and ±29 mol% Mg‐Fe, respectively.
Radiative transfer models of remotely acquired infrared spectra result in quantitative identification of minerals on planetary surfaces. Optical constants, or the real (n) and imaginary (k) indices of refraction are necessary inputs in such models. Pyroxenes are ubiquitous on the surfaces of terrestrial bodies within our solar system and can be readily used as thermo‐barometers to interpret magmatic histories. However, optical constants for intermediate pyroxene compositions are undetermined. Here, we have determined the optical constants of two natural orthopyroxenes both in the visible/near‐infrared (VNIR) and mid‐infrared (MIR) regions. VNIR reflectance spectra were measured using powdered samples and modeled using a combination of Hapke theory and Kramers‐Kronig analysis. MIR reflectance spectra were measured on oriented single crystal samples with non‐normal incidence and modeled using Lorentz‐Lorenz dispersion theory. The optical constants derived here are available to the scientific community at https://doi.org/10.5281/zenodo.4758045 to be used in the interpretation of remote sensing data.
The Allan Hills 76005 polymict eucrite pairing group consists of 15 paired masses recovered during six different field seasons in the Transantarctic Mountains. Although this group has been well studied in general, most of the meteorites contain a significant portion of dark clasts that have not been well characterized. The Dawn mission to Vesta discovered dark materials that provide insight into its evolution. The ALH dark clasts are thus of great interest to understanding the evolution of Vesta. Here, 45 different dark clasts from 15 different thin sections from the pairing group are characterized in detail to better understand their nature and origin. Five different textural types of dark clasts are recognized among this group—skeletal, vitrophyric, pilotaxitic, fan spherulitic, and troilite‐silica‐plagioclase‐rich clasts with aphyric or blobby textures. Mineralogy of the clasts is dominated by plagioclase and pyroxene, with minor troilite, silica, ilmenite, chromite, and rare Fe‐Ni metal. All of the textures can be produced by rapid cooling rates on the order of 60–2500°C h⁻¹. Bulk compositions of the clasts are demonstrably eucritic, and not chondritic, howarditic, or diogenitic. The combination of mineralogy, composition, and textures strongly suggests that the dark clasts are eucritic impact melts. Several craters on Vesta have associated orange deposits that have been proposed as impact melt breccias. The ALH pairing group may thus represent material that originated near Oppia or Octavia craters.
Low‐albedo, hydrated objects dominate the list of the largest asteroids. These objects have varied spectral shapes in the 3‐μm region, where diagnostic absorptions due to volatile species are found. Dawn's visit to Ceres has extended the view shaped by ground‐based observing and shown that world to be a complex one, potentially still experiencing geological activity. We present 33 observations from 2.2 to 4.0 μm of eight large (D > 200 km) asteroids from the C spectral complex, with spectra inconsistent with the hydrated minerals we see in meteorites. We characterize their absorption band characteristics via polynomial and Gaussian fits to test their spectral similarity to Ceres, the asteroid 24 Themis (thought to be covered in ice frost), and the asteroid 51 Nemausa (spectrally similar to the CM meteorites). We confirm most of the observations are inconsistent with what is seen in meteorites and require additional absorbers. We find clusters in band centers that correspond to Ceres‐ and Themis‐like spectra, but no hiatus in the distribution suitable for use to simply distinguish between them. We also find a range of band centers in the spectra that approaches what is seen on Comet 67P. Finally, variation is seen between observations for some objects, with the variation on 324 Bamberga consistent with hemispheric‐level difference in composition. Given the ubiquity of objects with 3‐μm spectra unlike what we see in meteorites, and the similarity of those spectra to the published spectra of Ceres and Themis, these objects appear much more to be archetypes than outliers.
Although petrologic, chemical and isotopic studies of ordinary chondrites and
meteorites in general have largely helped establish a chronology of the
earliest events of planetesimal formation and their evolution, there are
several questions that cannot be resolved via laboratory measurements and/or
experiments only. Here we propose rationale for several new constraints on the
formation and evolution of ordinary chondrite parent bodies (and by extension
most planetesimals) from newly available spectral measurements and
mineralogical analysis of main belt S-type asteroids (83 objects) and
unequilibrated ordinary chondrite meteorites (53 samples). Based on the latter,
we suggest spectral data may be used to distinguish whether an ordinary
chondrite was formed near the surface or in the interior of its parent body. If
these constraints are correct, the suggested implications include that: i)
large groups of compositionally similar asteroids are a natural outcome of
planetesimal formation and, consequently, meteorites within a given class can
originate from multiple parent bodies; ii) the surfaces of large (up to ~200km)
S-type main-belt asteroids expose mostly the interiors of the primordial
bodies, a likely consequence of impacts by small asteroids (D<10km) in the
early solar system (Ciesla et al. 2013); iii) the duration of accretion of the
H chondrite parent bodies was likely short (instantaneous or in less then ~10^5
yr but certainly not as long as 1 Myr); iv) LL-like bodies formed closer to the
Sun than H-like bodies, a possible consequence of radial mixing and size
sorting of chondrules in the protoplanetary disk prior to accretion.
Hundreds of thousands of asteroids have been discovered in the asteroid belt and in near-Earth space. Oxygen is an abundant
element in meteorites and presumably in most asteroids. Spectral reflectance measurements of asteroids in the visible and
near-infrared can identify oxygen-bearing minerals such as those found in the olivine, pyroxene, and serpentine groups due
to their distinctive absorption features. Interpretation of the mineralogy of asteroids is complicated by the effects of space
weathering, which tends to redden and darken the surfaces of asteroids. Asteroids are primarily classified into a number of
taxonomic classes and subclasses according to their spectral properties in the visible wavelength region. However, asteroids
with similar spectral properties in the visible may have different spectral properties in the near-infrared and, therefore,
different interpreted mineralogies. Definite trends in the abundances of different taxonomic classes versus heliocentric distance
are apparent. These trends appear to be a function of both composition differences and degree of heating. However, dynamical
processes have significantly affected these trends. Close observation of asteroids by spacecraft, such as NEAR-Shoemaker to
433 Eros and Hayabusa to 25143 Itokawa, are currently the best way to discern the mineralogies of individual asteroids.
High signal-to-noise near-infrared spectrometer (NIS) spectra acquired during the low phase flyby of the near-Earth asteroid rendezvous (NEAR) mission to 433 Eros are analyzed to determine mineral chemistry and proportions of mafic silicates across the asteroid's surface at 2.68 x 5.50 km spatial resolution. Spectral band parameters are derived, and compared with those of laboratory samples of known mineral composition, grain size distribution and terrestrial, meteoritic and lunar pyroxene spectral properties. The NIS derived band parameters are consistent with ordinary chondrite meteorites. We invoke the presence of a clinopyroxene component in the spectra, which is consistent with ordinary chondrite mineralogy and/or some degree of partial melting of ordinary chondritic material. Spectra measured across the surface of Eros can reveal small but real spectral variations. Most relative spectra are uniform to within 1-2%. Some areas suggest compositional variations of a few percent. Spectral slope variations of a few percent are seen indicating a nonuniform distribution of materials affecting the slope parameter but with no resolved absorption bands. We find no correlation of slope with viewing geometry or compositional variation. The band parameter values do not consistently indicate a specific ordinary chondrite class but Eros is definitely undifferentiated with possible compositional variations of no more than 1-2%.
The past decade has seen a significant expansion both in the
interpretive methodologies used to extract mineralogical information
from asteroid spectra and other remote-sensing data and in the number of
asteroids for which mineralogical characterizations exist. Robust
mineralogical characterizations now exist for more than 40 asteroids, an
order of magnitude increase since Asteroids II was published. Such
characterizations have allowed significant progress to be made in the
identification of meteorite parent bodies. Although considerable
progress has been made, most asteroid spectra have still only been
analyzed by relatively ambiguous curvematching techniques. Where
appropriate and feasible, such data should be subjected to a
quantitative analysis based on diagnostic mineralogical spectral
features. The present paper reviews the recent advances in interpretive
methodologies and outlines procedures for their application.
A systematic spectral analysis has been Carried out on a large subset (39 of 144) of the S-type asteroid population. The S-asteroid class includes a number of distinct compositional subtypes [designated S(I)-S(VII)] which exhibit surface silicate assemblages ranging from pure olivine (dunites) through olivine-pyroxene mixtures to pure pyroxene or pyroxene-feldspar mixtures (basalts). S-asteroid absorption bands are weaker than expected for pure mafic silicate assemblages, indicating the presence of an additional phase, most probably FeNi metal, although the abundance of metallic or feldspar components is not well constrained, The diversity within the S-class probably arises from several sources, including the coexistence of undifferentiated, partially differentiated, and fully differentiated bodies within the general S-asteroid population and the exposure of compositionally distinct units from within metamorphosed and partially and fully differentiated parent bodies. Partial differentiation within planetesimals appears to be an important source of this diversity. The surface assemblages of these subtypes include both analogues to known meteorite classes (e.g., pallasites, mesosiderites, ureilites, lodranites, brachinites, winonaites) and materials not sampled in our present meteorite collections. No specific ordinary chondrite parent bodies have been identified within the S-class, but silicate mineralogy provides a strong test for possible ordinary chondritic affinities. This test is failed by 75% of the S-asteroids. Only the S(IV)-subtype objects have silicates consistent with ordinary chondrites (OC). This subtype provides the only viable OC parent body candidates among the large main-belt S-asteroid population, although the individual objects remain to be evaluated. The S(IV) objects are concentrated near the 3:1 Kirkwood gap at 2.5 AU, and their ejecta can be readily injected into the associated chaotic region and rapidly converted into Earth-crossing orbits which may contribute to the high abundance of OC meteorites. S-asteroid absorption band depth correlates with asteroid diameter. It is relatively constant for objects larger than 100 km and increases steeply toward smaller sizes. This suggests that some equilibrium has been attained in the optical surfaces of S-asteroids larger than 100 km but not on smaller bodies.
Over time, exposure of airless bodies to the space environment results in optical changes to their surfaces. These optical changes are functions of the porosity, grain size distribution, and composition of the surface, and they depend on the relative rates of surface modification processes. Collectively, surface modification processes (such as impacts, solar wind ion implantation, sputtering, and micrometeorite bombardment) and their resulting optical effects have come to be known as "space weathering." Studies of lunar rocks and soils are the most important foundation we have on which to build an understanding of space weathering on asteroids. We cannot directly measure asteroid surfaces in a laboratory environment; therefore, we describe the lunar case, and compare it with the evidence for asteroids. In this chapter we review the evidence for space weathering on asteroids, including spectroscopy of optical effects, microscopy of physical effects, simulations of processes, lunar soils, meteorite breccias, spacecraft observations, and theoretical modeling. An understanding of space weathering is important to all remote-sensing studies of asteroid surfaces.
The variety of evidence bearing on the nature of space weathering is reviewed. The effects of space weathering include spectral darkening, reddening and subdued absorption bands, and the distinctive magnetic electron spin resonance caused by single-domain metallic iron particles. Ever since the Apollo missions, two paradigms have dominated the thinking of the planetary science community concerning space weathering: (1) the optical effects are caused by impact-vitrified glass in agglutinates, and (2) the submicroscopic metallic iron results from the reduction of ferrous iron by the impact melting of minerals whose surfaces have been saturated with hydrogen from the solar wind. However, studies carried out since the Apollo program showed that both of these paradigms are invalid. A hypothesis first suggested by the author and his colleagues 26 years ago, but not generally accepted at that time, now appears to be essentially correct: Both the optical and magnetic effects are caused by metallic iron particles smaller than the wavelength in ubiquitous vapor-deposited coatings on soil particle surfaces and inside agglutinates. The vapor is generated by both solar wind sputtering and micrometeorite impact vaporization and injected preferentially downward into the porous regolith. The iron is reduced by a physical process, the selective loss of oxygen that occurs during deposition of the vapor, and does not require heating, melting, or a reducing environment. A mathematical theory that describes the optical effects of the submicroscopic iron quantitatively is derived and applied to the regoliths of the Moon, Mercury and an S asteroid.
The normative mineralogies of 94 ordinary chondrites have been calculated from high-quality bulk chemical analyses. Relative proportions of olivine, orthopyroxene, and metal are more variable within the H, L, and LL classes than has been recognized previously. Mineralogy within each chondrite class varies systematically with oxidation state. These data have important implications for the design of reflectance spectroscopy studies that match asteroid and meteorite spectral features and for quantitative spectral interpretations of asteroids. They also provide a tool with which to predict mineral abundances in unsampled asteroids that formed under different redox conditions.
Troilite (stoichiometric FeS) is a common mineral in most meteorites,
but meteorite spectroscopists have neglected to measure its spectral
properties and consider its possible role in interpretation of asteroid
spectra. Ordinary chondrites are typically 5-6 wt% troilite and this
mineral is present in almost all iron meteorites in amounts up to 60%.
Troilite's occurrence in meteorites is typically as 0.1-1.0 mm-sized
blebs in stony meteorites and cm-sized nodules in iron and stony-iron
meteorites [1]. Meteoritical and theoretical evidence strongly suggests
that there should be troilite-rich zones in the interiors of
differentiated asteroids. The cores of differentiated asteroids probably
contained a few wt% S which was largely concentrated in the final 5-10
vol% of eutectic liquid. This liquid crystallized as troilite (90 vol%)
and metal. An asteroid derived from a metal core would probably display
sections of crystallized eutectic liquid [2] provided that the asteroid
is not covered with regolith. The distribution of troilite on the
surface of metallic asteroids may therefore provide information about
the crystallization history of the core. A fraction of the S may,
however, become trapped in the dendrites during crystallization [3].
This would account for the abundance of troilite nodules in iron
meteorites. The troilite distribution in the core may also affect the
way the core breaks up during impacts. Fractures will preferentially
propagate through the much more friable troilite and large core
fragments may therefore have dendritic shapes. Regolith present on metal
cores may also be enriched in the more friable troilite. Material of the
expected eutectic composition would be very fragile, and collisional
and/or atmospheric disruption may account for its absence among
meteorites. Measurements: The bidirectional reflectance spectrum of
troilite was measured from a sample of the Mundrabilla iron meteorite
held in the collection of the University of Hawaii. This meteorite has
an unusually high sulfur content (8 wt%) and total troilite content is
estimated at 25-35 vol%. Average troilite composition in weight % is as
follows: 63% Fe, 0.5% Cr, 0.3% Zn, and 36.2% S [4]. The sample was
crushed in a clean ceramic mortar and pestle to a bulk powder and dry
sieved to a particle size of <250 micrometers. Six additional
particle size separates were dry sieved from this bulk sample. Shown in
Figure 1 are the spectra of the bulk sample and the particle size
separates of Mundrabilla troilite. The spectrum of the bulk material is
dark, always less than 10% reflective, and strongly red sloped. The
rapid increase in reflectance at the green and red wavelengths (0.4-0.5
microns) is probably responsible for the overall bronze color of hand
sample troilite. Since Mundrabilla is a find, the depth of the
UV-visible absorption may have been increased by small amounts of Fe3+
from terrestrial rust. Additional samples of troilite from fresh fall
need to be measured to confirm this result. The bulk sample has a
reflectance between the smallest and largest particle size separates
suggesting that its reflectance is dominated by small particles coating
larger grains. Previous work with spectral mixture modelling shows that
small particle size troilite and metal can dominate the spectra of
ordinary chondrite meteorites, producing a dark, subdued and reddened
spectrum similar to some dark asteroids [5]. Implications for Asteroids:
The strong red slopes and low reflectances of the troilite spectra are
similar to the spectral characteristics of the T and possibly some
M-class asteroids. Shown in Figure 2 are the spectra of bulk troilite
(solid lines) and four T-class asteroids (boxes and error bars). The IR
spectra of 96 Aegle, 114 Kassandra, and 233 Asterope are strongly
similar to the spectrum of bulk troilite. The deeper W absorption in
troilite may be due to terrestrial rust. The spectrum of 308 Polyxo is
substantially different, but Polyxo is also the only T-class asteroid
that has been shown to have strong water of hydration features at 3.0
micrometers. The heliocentric distribution of T-asteroids are similar to
that of the M-class, but their albedos are substantially less with an
average IRAS albedo for T-asteroids of 0.066. T-class asteroids may
represent a stage in the collisional evolution of the metallic cores of
differentiated asteroids during which the upper, troilite-rich zones of
the core are exposed and the friable troilite dominates the regolith.
The abundant troilite would produce the dark, red- sloped reflectance
spectrum characteristic of T-asteroids. Later stages of collisional
evolution may also produce troilite-rich regoliths if cores fracture
along zones of dendrite formation. Some M-class asteroids are almost
certainly rich in FeNi metal, and troilite is almost certainly present
in their regoliths. Variations in troilite abundance and particle size
may account for some of the variations in slope and albedo seen in this
class. Finally, the S-class asteroids may contain significant amounts of
troilite which may account for some of the red slope common in that
class. References: [1] Dodd R.T. (1981) Meteorites. [2] Haack H. and
Scott E.R.D. (1992) Asteroid core crystallization by inwards dendritic
growth. Submitted to JGR (Planets). [3] Kracher et al. (1977) Geochem.
J. 11, 207-217. [4] Buchwald V.F. (1975) Iron Meteorites. [5] Britt D.T.
(1991) Ph.D. Thesis, Brown University; Britt D.T. and Pieters C.M.
(1990) LPS XXI, 127-128. [6] Bell J.F. et al., (1990) Preprint. Figure
1, which in the hard copy appears here, shows bidirectional reflectance
spectra of powdered samples of troilite from the Mundrabilla iron
meteorite. Included are spectra bulk troilite and six particle size
separates of the same sample. The particle size of each spectrum is
listed in order in the lower right of the figure. Figure 2, which in the
hard copy appears here, shows spectra of Mundrabilla troilite and four
T-class asteroids. All spectra are scaled to unity at 0.55 micrometers.
Asteroid data are from [6].
1] Asteroid 4 Vesta is thought to be a differentiated object with intact internal structure due to its achondritic surface. In this paper, we report K-L band spectroscopic observations of Vesta. We have detected the presence of a 3-mm absorption feature at about the 1% level on the surface of Vesta at longitudes between 155° and 195°. This result indicates that OH and/or H 2 O-bearing minerals are present in this region of Vesta; the sources are plausibly fragments of carbonaceous chondrite impactors or solar wind implantation. The detection of 3-mm absorption features from Vesta, regarded as the smallest terrestrial planet, may provide clues to the origin of volatile materials on terrestrial planets.
For the purpose of simulating the surface alteration process called "space weathering", experiments of pulse laser irradiation, proton implantation, and laser irradiation to proton implanted samples were performed and reflectance spectra of altered materials were measured. To simulate the impact heating by micrometeorite bombardments, we made a new apparatus using a pulse laser whose pulse duration is 6–8 nanoseconds, comparable with a timescale of micrometeorite impacts. We find that the degree of space weathering, i.e., change of reflectance spectrum should depend on mineral composition. Laser irradiation onto olivine produces the largest reduction of albedo and the highest reddening of reflectance spectrum. In general, variation of olivine spectra is much larger than that of pyroxenes. Depths of absorption bands do not change in the scaled spectra. The olivine spectrum after the laser irradiation can match spectra of some olivine asteroids within a subtype of S-type asteroids. Comparison of Vesta spectrum with altered pyroxene spectra suggests that Vesta surface would be relatively older than olivine asteroids. We also investigate the influence of solar wind proton and pyroxene FeO content. The proton implantation causes small changes in olivine and enstatite spectra. Implanted protons do not influence spectral change by the laser irradiation: the laser irradiation and the proton implantation do not produce multiplicative but additive changes on the reflectance spectra. FeO content of pyroxenes does not relate to the degree of reflectance change.
Algorithmic modifications to the MELTS software package are presented in order that calculations of heterogeneous phase equilibria can be performed in the subsolidus. Methods
are presented for: (1) selecting an "initial guess assemblage" that satisfies the bulk composition constraints; (2) detecting saturation of new phases (including liquid) in an assemblage; (3) adding and removing phases from the assemblage without adjusting the system bulk composition; and (4) constraining the assemblage to a fixed f_(O2). These methods have O2 been added to MELTS, allowing it to calculate heterogeneous phase equilibria with or without liquid, closed or open to O, and with fixed intensive variables (P,T), (P,S), (P,H), or (V,T). Applications include fractional melting calculations, metamorphic phase equilibria, and geophysical models of subsolidus regions of the Earth.
Abstract— Based on recent progress in simulating space weathering on asteroids using pulse-laser irradiation onto olivine and orthopyroxene samples, detailed analyses of two of the A and R type asteroid reflectance spectra have been performed using reflectance spectra of laser-treated samples. The visible-near-infrared spectrum of olivine is more altered than that of pyroxene at the same pulse-laser energy, suggesting that olivine weathers more rapidly than orthopyroxene in space. The same trend can be detected from reflectance spectra of the asteroids, where the more olivine an asteroid has, the redder its 1 μm band continuum can become. Comparison of the 1 μm band continuum slope and the 2/1 μm band area ratio between the asteroids and olivine and pyroxene samples (including the laser-treated ones) suggests that asteroids may be limited in the degree of space weathering they can exhibit, possibly due to the short life of their surface regolith. Their pyroxenes may also have a limited chemical composition range. Fitting the visible continuum shape and other parts of the spectra (especially the 2μm part) has been impossible with any combination of common rock-forming minerals such as silicates and metallic irons. However, this study shows, for the first time, excellent fits of reflectance spectra of an A asteroid (Aeternitas) and an R asteroid (Dembowska), including their visible spectral curves, band depths and shapes, and overall continuum shapes. Our results provide estimates that Aeternitas consists of 2% fresh olivine, 93% space-weathered olivine, 1% space-weathered orthopyroxene, and 4% chromite, and that Dembowska consists of 1% fresh olivine, 55% space-weathered olivine, and 44% space-weathered orthopyroxene. These results suggest that space weathering effects maybe important to the interpretation of asteroid reflectance spectra, even those with deep silicate absorption bands. Modified Gaussian model deconvolutions of the laser-irradiated olivine samples show that their identity as olivine remained. The most recent submicroscopic mineralogical analyses have revealed that the laser-irradiated olivine samples contain nanophase iron particles similar to those in space-weathered lunar samples.
A revised regular solution-type thermodynamic model for twelve-component silicate liquids in the system SiO2-TiO2-Al2O3-Fe2O3-Cr2O3-FeO-MgO-CaO-Na2O-K2O-P2O5-H2O is calibrated. The model is referenced to previously published standard state thermodynamic properties and is derived from a set of internally consistent thermodynamic models for solid solutions of the igneous rock forming minerals, including: (Mg,Fe2+,Ca)-olivines, (Na,Mg,Fe2+,Ca)M2 (Mg,Fe2+, Ti, Fe3+, Al)M1 (Fe3+, Al,Si)2
TETO6-pyroxenes, (Na,Ca,K)-feldspars, (Mg,Fe2+) (Fe3+, Al, Cr)2O4-(Mg,Fe2+)2 TiO4 spinels and (Fe2+, Mg, Mn2+)TiO3-Fe2O3 rhombohedral oxides. The calibration utilizes over 2,500 experimentally determined compositions of silicate liquids coexisting at known temperatures, pressures and oxygen fugacities with apatite feldspar leucite olivine pyroxene quartz rhombohedral oxides spinel whitlockite water. The model is applicable to natural magmatic compositions (both hydrous and anhydrous), ranging from potash ankaratrites to rhyolites, over the temperature (T) range 900–1700C and pressures (P) up to 4 GPa. The model is implemented as a software package (MELTS) which may be used to simulate igneous processes such as (1) equilibrium or fractional crystallization, (2) isothermal, isenthalpic or isochoric assimilation, and (3) degassing of volatiles. Phase equilibria are predicted using the MELTS package by specifying bulk composition of the system and either (1) T and P, (2) enthalpy (H) and P, (3) entropy (S) and P, or (4) T and volume (V). Phase relations in systems open to oxygen are determined by directly specifying the f
o
2 or the T-P-f
o
2 (or equivalently H-P-f
o
2, S-P-f
o
2, T-V-f
o
2) evolution path. Calculations are performed by constrained minimization of the appropriate thermodynamic potential. Compositions and proportions of solids and liquids in the equilibrium assemblage are computed.
The recent discovery of a relatively small basaltic asteroid in the outer main belt with no apparent link to (4) Vesta raised several hypotheses on its origin. We present the results of a dynamical and mineralogical study of the region near (1459) Magnya intended to establish its origin. The dynamical analysis shows that the region is filled with high-order two-body and three-body mean motion resonances and nonlinear secular resonances, which can lead to slow chaotic diffusion. The mineralogical analysis has not identified any other asteroid with a composition similar to Magnya, nor the presence of fragments that could be securely related to the catastrophic disruption of a differentiated parent body. The various scenarios for the origin of Magnya are also discussed in the face of both the results presented here and recently published results.
The Acapulco meteorite fell in August, 1976, at El Quemado, near Acapulco, Mexico. It is a unique object with chondritic composition but achondritic texture.High degree of recrystallisation and mineral chemical data indicate formation of the meteorite under redox conditions intermediate between those of H- and E-chondrites at ~ 1100°C, from which it cooled at a rate > 10°C/Myr.The major element composition is within the range of H-chondrites. Troilite and metal, and associated trace elements, are inhomogeneously distributed. Chromium is a factor of two higher than in H-chondrites. Enrichments of P and U indicate high phosphate content. Limited extent of partial melting may explain the light REE enrichment. However other incompatible elements have normal H-chondritic abundances or are even depleted like K and Rb. Moderately volatile or volatile elements (e.g. Mn, Ga, Ge, Zn) are enriched nearly to the level of C1-chondrites. Planetary noble gases are also significantly higher than in equilibrated ordinary chondrites. High temperature recrystallisation has not affected volatile element abundances. Compared to H-chondrites Acapulco is enriched in refractory siderophile elements.The distribution of W and other siderophile elements between metal and silicate phases are indicative of the higher temperature and lower oxygen fugacity of the assemblage. However, contrary to previous claims, the distribution of W cannot be used to calculate the equilibration temperature.Low K and high U contents are also reflected in the anomalous amounts of 40Ar and 4He. The old K-Ar age (4.7 ± 0.3 Gyr) and high 244Pu track densities indicate mobilisation of U and Pu-rich phases shortly after formation of the parent material. This and other evidence suggests that Acapulco may represent a rock formed in the early stages of incipient melting of a chondritic parent body. However, since compositional differences between Acapulco and H-chondrites cannot be explained by fractionation processes on the Acapulco parent body. Acapulco must have originated from a different parent body.Lack of depletion of volatile elements, absence of chondrules and reduced mineral composition indicate some relationship of Acapulco to silicate inclusions in iron meteorites and to other unusual meteorites. Oxygen isotopes and chemical data suggest that there are at least three different groups of reduced chondritic meteorites: 1.(a) Acapulco, Lodran, and probably Allan Hills A 77081;2.(b) Pontlyfni, Mount Morris, Winona and silicate inclusions in IAB iron meteorites, and3.(c) Kakangari.An exposure age of 5 × 106 yr is deduced from spallogenic rare gas data.
We present the design, construction, and performance of SpeX, a medium-resolution 0.8-5.5 mum cryogenic spectrograph and imager, now in operation at the 3.0 m NASA Infrared Telescope Facility (IRTF) on Mauna Kea. The design uses prism cross-dispersers and gratings to provide resolving powers up to R~2000 simultaneously across 0.8-2.4, 1.9-4.2, or 2.4-5.5 mum, with a 15" long slit. A high-throughput low-resolution R~200 prism mode is also provided for faint-object and occultation spectroscopy. Single-order 60" long-slit modes with resolving powers up to R~2000 are available for extended objects. The spectrograph employs an Aladdin 3 1024×1024 InSb array and uses narrow slits and a spatial scale of 0.15" pixel-1 for optimum sensitivity on point sources. An autonomous infrared slit viewer is used for object acquisition, infrared guiding, and scientific imaging in the wavelength range 0.8-5.5 mum. The imager employs an Aladdin 2 512×512 InSb array that covers a 60''×60'' field of view at 0.12" pixel-1. SpeX was successfully commissioned on IRTF during 2000 May, June, and July. Astronomical observations are presented to illustrate performance.
Reflectance spectra of medium and high spectral resolution of 19 minerals from the tremolite-actinolite solid series are used to characterize systematic variations in absorption band parameters as a function of composition. The systematic changes in the energy, width, intensity, and number of OH overtone bands are easily quantified using the modified Gaussian model, and the results are highly correlated with the Fe/Mg ratios of the samples. These results indicate that the modified Gaussian model can be used to quantify objectively the bulk chemistry of the minerals of the tremolite-actinolite solid solution series using high-resolution reflectance spectra of the OH overtone bands. -from Author
Spectra of asteroid 4 Vesta and 21 small (estimated diameters less than 10 km) asteroids with Vesta-like spectral properties (Vestoids) were measured at visible and near-infrared wavelengths (similar to0.44 to similar to1.65 mum). All of the measured small asteroids (except for 2579 Spartacus) have reflectance spectra consistent with surface compositions similar to eucrites and howardites and consistent with all being derived from Vesta. None of the observed asteroids have spectra similar to diogenites. We find no spectral distinction between the 15 objects tabulated as members of the Vesta dynamical family and 6 of the 7 sampled "non-family" members that reside just outside the semi-major axis (a), eccentricity (e), and inclination (i) region of the family. The spectral consistency and close orbital (a-e-i) match of these "non-family" objects to Vesta and the Vesta family imply that the true bounds of the family extend beyond the subjective cut-off for membership. Asteroid 2579 Spartacus has a spectrum consistent with a mixture of eucritic material and olivine. Spartacus could contain olivine-rich material from Vesta's mantle or may be unrelated to Vesta altogether. Laboratory measurements of the spectra of eucrites show that samples having nearly identical compositions can display a wide range of spectral slopes. Finer particle sizes lead to an increase in the slope, which is usually referred to as reddening. This range of spectral variation for the best-known meteoritic analogs to the Vestoids, regardless of whether they are actually related to each other, suggests that the extremely red spectral slopes for some Vestoids can be explained by very fine-grained eucritic material on their surfaces.
An improved approach to spectral deconvolution is presented here that represents absorption bands as discrete mathematical distributions and resolves composite absorption features into individual absorptions bands. The frequently used Gaussian model of absorption bands is first evaluated and shown to be inappropriate for the Fe 2+ electronic transition absorptions in pyroxene spectra. Subsequently, a modified Gaussian model is derived using a power law relationship of energy to average bond length. -from Authors
The Bouvante eucrite, meteorite found on 30-07-1978, is a monomict recrystallized basaltic breccia. The mineralogy consists of zoned plagioclase and pyroxenes. These have rather constant Mg-contents (10-11 % MgO) while Ca and Fe contents show considerable variations. Tridymite, quartz, chromite, ilmenite, zircon, merrillite, iron and troilite are also found.
Chemical analyses show that this basalt which belongs to the Fe-rich end of the eucrite range is richer in incompatible trace-elements than all similar rocks analysed so far. It may represent a very low degree of partial melting of the source-rocks ; it would have flown superficially before being covered by a thick layer of debris and it would have come again on the surface of the parent body when impacted.
EETA 79001, a shergottite meteorite, includes a pair of nonbrecciated basaltic lithologies joined along a planar igneous contact. These two lithologies, A and B, are examples of the type of basaltic contacts likely to occur on the surfaces of many planetary bodies. As such, EETA 79001 offers a unique opportunity to compare analysis techniques available in situ with those available remotely. For comparison to petrologic and geochemical studies, reflectance spectra of Lithologies A and B are obtained and analyzed with the Modified Gaussian Model (MGM). This modeling leads to the unambiguous identification of two pyroxene phases in each lithology. All spectral features, including an absorption near 1.2 μm, often attributed to plagioclase, are found to be due solely to the presence of two pyroxenes. This ambiguity between pyroxene and plagioclase absorptions suggest that caution be used in interpretation of remote spectra. Results from the MGM analysis of spectra of EETA 79001 are also used to provide estimates on the major element composition of the pyroxene phases in the two lithologies. In addition, based solely on spectral analyses, Lithology B is shown to have 11-17% more high-calcium pyroxene relative to low-calcium pyroxene than Lithology A. Each of the results derived independently with the MGM are shown to compare favorably (within 5-10%) to those obtained from more traditional geochemical and petrographic methods. The compositional inferences that were derived from modeling the spectra of EETA 79001 with the MGM are examples of the type of information available from remotely acquired spectral data. By providing a method for determining changes in relative composition and abundance of pyroxenes in neighboring units, the MGM offers an opportunity to add compositional information to geologic and morphologic data and thus greatly enhance the science return on studies of surfaces throughout the Solar System.
The spectral characteristics of asteroids having a strong 1 um absorption feature are attributed to the presence of pyroxene, olivine, or both. Distinguishing between pyroxene and olivine requires measuring the strength of an additional absorption band centered near 2 um. Accurate measurements of both bands are now possible using SpeX, a new generation low- to medium-resolution infrared spectrograph in operation as a facility instrument at the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. Visible-wavelength surveys such as SMASSII (Bus 1999; Ph. D. thesis) show a continuum of properties in terms of spectral slope and the depth of the 1 um band, as measured over the wavelength interval 0.4-1.0 um. With SpeX, we are now examining whether this apparent continuum is also evident within spectra extending out to 2.5 um. Taking advantage of our ability to infer mineralogy based on near-infrared spectral information, we are also seeking to understand the extent to which this spectral continuum is rooted in a smooth gradient in olivine-pyroxene mineralogy. We are estimating olivine-pyroxene ratios using the Modified Gaussian Model (MGM) of Sunshine et al. (1990, JGR 95, 6955).
The spectral properties of troilite-pyrrhotite minerals (FeS to Fe(1-x)S) were examined. It was found that spectral properties are related to variations in structure and/or composition and that it may be identifiable in the spectra of asteroids.
This book provides an updated look at crystal field theory, one of the simplest models of chemical bonding, and its applications. After an introductory section chapters go on to describe: an outline of crystal field theory; energy level diagrams and crystal field spectra of transition metal ions; measurements of absorption spectra of minerals; crystal field spectra of transition metal ions in minerals; crystal chemistry of transition metal-bearing minerals; thermodynamic properties influenced by crystal field energies; trace element geochemistry in the crust and mantle; remote sensing compositions of planetary surfaces; and covalent bonding of transition metals. -A.W.Hall
Spin-allowed Fe z+ absorption bands occur in the visible and near-infrared diffuse reflectance spectra of most pyroxenes. The wavelengths of the bands centered near 1 #m and 2 #m vary as functions of pyroxene composition, making possible mineralogical and chemical deductions based on spectral reflectance curves. Typically, pyroxene bands are well developed in relation to absorption features in the spectra of other rock-forming minerals and glass; thus pyroxene often dominates the spectral curves of pyroxenebearing rocks. Telescopic spectra of the moon and some other solar system objects contain absorption features that can be interpreted in terms of pyroxenes mixed with other minerals and/or glass.
Extensive collection efforts in Antarctica and the Sahara in the past 10 years have greatly increased the number of known meteorites. Groupings of meteorites according to petrologic, mineralogical, bulk- chemical, and isotopic properties suggest the existence of 100-150 distinct parent bodies. Dynamical studies imply that most meteorites have their source bodies in the main belt and not among the near-Earth asteroids. Spectral observations of asteroids are currently the primary way of determining asteroid mineralogies. Linkages between ordinary chondrites and S asteroids, CM chondrites and C-type asteroids, the HEDs and 4 Vesta, and iron meteorites, enstatite chondrites, and M asteroids are discussed. However, it is difficult to conclusively link most asteroids with particular meteorite groups due to the number of asteroids with similar spectral properties and the uncertainties in the optical, chemical, and physical properties of the asteroid regolith.
The Modified Gaussian Model (MGM) is argued to be a powerful tool for analyzing mixture spectra of asteroids if additional physically based constraints are applied to the modeling. Using constraints based on the range of observed variations in absorptions in olivine across its solid solution series makes it possible to analyze and estimate the chemical composition of the olivine-rich asteroid 246 Asporina. The GM is successfully applied to quantify compositional variations of olivine spectra and pyroxene mixture spectra, providing a solid basis for interpretation of unknown spectra.
We present visible-wavelength (0.435–0.925 μm) spectroscopic measurements for 1341 main-belt asteroids observed during the second phase of the Small Main-belt Asteroid Spectroscopic Survey (SMASSII). The purpose of this survey is to provide a new basis for studying the compositional structure of the asteroid belt. Through the large sample size and the relatively high spectral resolution (R∼100) of the SMASSII data, we find that values of the spectral parameters describing these data form more of a continuum than previously realized. Objects with intermediate spectral characteristics are bridging gaps that once separated distinct spectral classes. In some cases, newly revealed small-scale spectral features may be indicative of previously unrecognized mineral constituents. Here we present the data and principal component analyses that describe the SMASSII spectra. A companion paper utilizes these principal component scores, along with other measures of the spectral features, to develop a new taxonomy that takes advantage of the information contained within charge-coupled device spectra.
Orthopyroxene from the Johnstown diogenite contains exsolution products. It shows anomalous kinetics for disordering Fe-Mg distribution, and the application of kinetic constants extracted from the disordering experiments carried out on Johnstown orthopyroxene leads to aberrant cooling rates without physical meaning. Crystals of this orthopyroxene were studied by several transmission electron microscopy (TEM) techniques in order to elucidate the nature of the exsolution products, their topology, chemistry and probable relationship to kinetics. For comparison this study also included a terrestrial granulitic orthopyroxene with known 'normal' disordering kinetics, and a Ca-rich orthopyroxene of volcanic origin which was supposed to be free of exsolution products due to fast quenching. The TEM results show that all studied samples contain very fine clinopyroxene exsolution lamellae (several unit-cell wide). Additionally, only Johnstown orthopyroxenes contain single unit-cell wide Guinier-Preston zones. These represent a chemically distinct pyroxene and their distribution in Johnstown is pervasive. They are still present after annealing experiments at the temperatures of kinetic studies. This high density of defects is proposed to be responsible for the anomalous kinetic behaviour.
The majority of the carbonaceous chondrite clasts found in howardites, eucrites and diogenites are CM2 material, a lesser proportion is CR2 material, and other rare types are present. A single clast that was found on the Moon and called the Bench Crater meteorite is apparently shocked CM1 material. The CM2 clasts are matrix supported mixtures of olivine‐pyroxene‐phyllosilicate‐sulfide bearing aggregates, loose olivines and pyroxenes, sulfides, carbonates, and sinuous spinel‐phyllosilicate‐diopside calcium‐aluminum‐rich inclusions (CAIs). Magnetite and metal are rare. Some aggregates have fine‐grained rims of material resembling matrix. The opaque, fine‐grained matrix consists predominantly of serpentine of extremely variable composition and sulfides; tochilinite is occasionally present. The trace element data for one Jodzie clast from this study and the average of similar clasts from Kapoeta support a CM classification; volatiles are depleted relative to CI and enriched relative to CR material.
The CR2 clasts are found (in small numbers) in only four howardites: Bholghati, Jodzie, Kapoeta and Y793497. Petrographically, they are matrix‐supported mixtures of olivine aggregates (sometimes containing sulfides), loose olivines, pyrrhotite, pentlandite, low‐Ca pyroxene (minor), hedenbergite (rare), kamacite (rare and only found within olivine), Ca‐carbonates and abundant magnetite framboids and plaquets. Phyllosilicates are fine‐grained and largely confined to matrix; they are mixtures of serpentine and saponite. The matrix of CR2 clasts also contains pyrrhotite, pentlandite, chromite and a significant fraction of poorly‐crystalline material with the same bulk composition as matrix phyllosilicate.
There is evidence of heating in a substantial number of clasts, both CM2 and CR2, including: (1) corrugated serpentine flakes, (2) pseudomorphs of anhydrous ferromagnesian material after flaky phyllosilicates, and (3) hedenbergite rims on calcite. While the timing of the hedenbergite rims is debatable, the destruction of phyllosilicates clearly occurred at a late stage, plausibly during impact onto the HED asteroid(s) and Moon, and required peak heating temperatures on the order of 400 °C.
We note that in general, CM2 material was the most common carbonaceous chondrite lithology impacting the HED asteroids (with howardites and eucrites taken together), as it is for the Earth today. A total of 61 out of 75 carbonaceous chondrite clasts from HED meteorites belong to the CM clan, petrologic grade 2. This is also supported by published siderophile and volatile element data on howardites, eucrites and diogenites that are taken to indicate that CM‐like materials were the most common impactors on the HED asteroid(s). The ratio of CR/CM clasts in HED asteroids is essentially the same as for modern falls at Earth. This may indicate that the ratio of disaggregated CM2 to CR2 asteroidal material has been approximately constant through the history of the solar system. Finally, our results are also compatible with type‐2 carbonaceous chondrites being equivalent to or from the same source as the material that originally accreted to form the HED asteroid.
A compilation of the chemical analysis of 241 stony and 36 iron
meteorites is presented; 196 analyses were published previously, 81 are
new. This compilation includes analyses of new falls, new finds,
previously analyzed meteorites, previously analyzed meteorites with
suspect values, analyses of separates and inclusions, and analyses of 53
stony and 29 iron meteorites from Antarctica, including one of the
'lunar' type. Mean compositions of chondrite falls, finds, and Antarctic
chondrites are compared. References are listed for earlier published
analyses and an appendix provides an outline of the sampling procedures,
sample preparation, and the analytical methods.
Abstract— Spectra of asteroid 4 Vesta and 21 small (estimated diameters less than 10 km) asteroids with Vesta-like spectral properties (Vestoids) were measured at visible and near-infrared wavelengths (∼0.44 to ∼1.65 μm). All of the measured small asteroids (except for 2579 Spartacus) have reflectance spectra consistent with surface compositions similar to eucrites and howardites and consistent with all being derived from Vesta. None of the observed asteroids have spectra similar to diogenites. We find no spectral distinction between the 15 objects tabulated as members of the Vesta dynamical family and 6 of the 7 sampled “non-family” members that reside just outside the semi-major axis (a), eccentricity (e), and inclination (i) region of the family. The spectral consistency and close orbital (a-e-i) match of these “non-family” objects to Vesta and the Vesta family imply that the true bounds of the family extend beyond the subjective cut-off for membership. Asteroid 2579 Spartacus has a spectrum consistent with a mixture of eucritic material and olivine. Spartacus could contain olivine-rich material from Vesta's mantle or may be unrelated to Vesta altogether. Laboratory measurements of the spectra of eucrites show that samples having nearly identical compositions can display a wide range of spectral slopes. Finer particle sizes lead to an increase in the slope, which is usually referred to as reddening. This range of spectral variation for the best-known meteoritic analogs to the Vestoids, regardless of whether they are actually related to each other, suggests that the extremely red spectral slopes for some Vestoids can be explained by very fine-grained eucritic material on their surfaces.
Reflectance spectra of olivines spanning the forsterite-fayalite solid-solution series have been analyzed with the modified Gaussian model (MGM). The compositional variability of the three primary absorption bands that constitute the diagnostic 1.0-gm olivine feature has been quantified by examining the centers, widths, and relative strengths in 18 spectra ranging in composition from Mg-rich forsterite to Fe-rich fayalite. These analyses have also revealed several interrelationships among the three absorption bands that prpvide new insights into their crystallographic origins. The primary olivine absorptions near 1.0-gin are well behaved and provide a means to remotely identify and estimate the composition of olivine from reflectance spectra. However, the spectral resolution and signal-to-noise of current remotely acquired data, combined with the significant overlap in the 1.0-gm region, provide enough uncertainty to allow models based on simple least squares minimization to reach solutions that are mathematically satisfying yet physically unrealistic. More sophisticated models using inverse theory that incorporate constraints among the absorption bands as determined from these laboratory MGM analysis are shown to yield meaningful results which can be confidently used to estimate composition in remote data. The MGM and inverse theory are used to analyze the spectrum of the olivine-rich asteroid 246 Asporina and to quantitatively show that the olivine component on Asporina is magnesium-rich (i.e., forsteritic). In contrast to the systematic behavior of the three primary olivine absorptions, absorption features short of 0.7 gm are found to exhibit no obvious relationship to composition and as such are not recommended for use in remote applications.
1] We have utilized telescopic near-infrared spectra and multispectral images of the Moon provided by the Galileo and Clementine missions to determine the distribution and modes of occurrence of pure anorthosite. Anorthosites have now been identified in all portions of the nearside, including the site of the putative Procellarum basin. Anorthosite is associated with the rings of Orientale, Grimaldi, Humorum, Nectaris, Nubium, Mutus-Vlaq, and Balmer basins. Major portions of the inner rings of Grimaldi, Humorum, Crisium, Orientale, and Nectaris are composed of pure anorthosite. The large spatial extent of these anorthosites appears to rule out an origin in the upper portions of discrete differentiated Mg-suite plutons. In every instance, the anorthosites were exposed from beneath a shallower near-surface layer of more pyroxene-rich material. More mafic material also occurs beneath the pure anorthosite unit. Large expanses of the northern farside exhibit very low FeO values. This region contains abundant anorthosite and stands in stark contrast to the mafic composition exhibited by the interior of the South Pole-Aitken basin (SPA). The distribution of compositional units on large portions of the lunar farside as well as the southern portion of the lunar nearside appears to be largely attributable to the SPA impact event. The distribution and modes of occurrence of anorthosites clearly indicate that a thick, global layer of anorthosite is present at various depths beneath most portions of the lunar surface. This anorthosite layer dominated the upper portion of the primordial crust and was produced by plagioclase flotation in the global magma ocean.
Abstract— High signal-to-noise near-infrared spectrometer (NIS) spectra acquired during the low phase flyby of the near-Earth asteroid rendezvous (NEAR) mission to 433 Eros are analyzed to determine mineral chemistry and proportions of mafic silicates across the asteroid's surface at 2.68 × 5.50 km spatial resolution. Spectral band parameters are derived, and compared with those of laboratory samples of known mineral composition, grain size distribution and terrestrial, meteoritic and lunar pyroxene spectral properties. The NIS derived band parameters are consistent with ordinary chondrite meteorites. We invoke the presence of a clinopyroxene component in the spectra, which is consistent with ordinary chondrite mineralogy and/or some degree of partial melting of ordinary chondritic material. Spectra measured across the surface of Eros can reveal small but real spectral variations. Most relative spectra are uniform to within 1–2%. Some areas suggest compositional variations of a few percent. Spectral slope variations of a few percent are seen indicating a non-uniform distribution of materials affecting the slope parameter but with no resolved absorption bands. We find no correlation of slope with viewing geometry or compositional variation. The band parameter values do not consistently indicate a specific ordinary chondrite class but Eros is definitely undifferentiated with possible compositional variations of no more than 1–2%.
Abstract— Using new techniques to examine the products of space weathering of lunar soils, we demonstrate that nanophase reduced iron (npFe0) is produced on the surface of grains by a combination of vapor deposition and irradiation effects. The optical properties of soils (both measured and modeled) are shown to be highly dependent on the cumulative amount of npFe0, which varies with different starting materials and the energetics of different parts of the solar system. The measured properties of intermediate albedo asteroids, the abundant S-type asteroids in particular, are shown to directly mimic the effects predicted for small amounts of npFe0 on grains of an ordinary chondrite regolith. This measurement and characterization of space weathering products seems to remove a final obstacle hindering a link between the abundant ordinary chondrite meteorites and common asteroids.
Spectral reflectance measurements of powdered olivine–orthopyroxene mixtures and two ordinary chondrites have been performed in the temperature range between 293 and 80 K. The decrease in temperature produced a number of significant spectral effects. The composite absorption feature due to olivine and pyroxene in the 1-μm region (band I) resolves into distinct bands, provided the opx/(opx+ol) ratio is less than 0.5. Since such a splitting makes it possible to better constrain the mineral abundances, we suggest that spectral data of future space missions acquired at large insolation angles (low surface temperatures) may be useful for the compositional interpretation. The pyroxene absorption band near 2 μm (band II) moves to shorter wavelength, as expected, and splits into two distinct bands even if the calcic pyroxene abundance is very low. Therefore, a possible detection of a clinopyroxene band beyond 2 μm in asteroid spectra does not indicate the presence of an abundant calcic pyroxene component. The increase in reflectivity of the interband peak near 1.5 μm with decreasing temperature suggests that the puzzling turnover of the infrared continuum slope of S-type asteroids might be caused, at least in part, by their lower surface temperatures. The position of a composite 1-μm absorption band (band I) is subject to an offset with decreasing temperature. The direction of the wavelength shift depends on the opx/(opx+ol) ratio. The band II/band I area ratio increases moderately. For relatively olivine-rich assemblages these effects offset the data points on the plot of band I center wavelength position versus band area ratio. The upward displacement of S(II) and possibly S(III) asteroid subgroups with respect to the olivine–orthopyroxene mixing line on the plot does not necessarily indicate high contents of calcic clinopyroxene on their surfaces.
We report five-color photometry, near-infrared spectra (0.8–2.5 μm), and thermal observations of the Earth-approaching asteroids (3551) 1983 RD, (3908) 1980 PA, and (4055) 1985 DO2. We derive their diameters as 1.2, 1.0, and 3.4 km, respectively. Apart from greater absorption band depths, the spectra of these three small asteroids are nearly identical to that of Vesta, which is known to have a differentiated basaltic surface. The deeper pyroxene bands (0.9 and 1.9 μm) relative to Vesta are consistent with more pyroxene-rich surfaces, larger mineral grain sizes in their optically immature regoliths, more bare rock surfaces, or all of these factors. These small Earth-approaching asteroids have similar orbits, with perihelia near Earth's orbit in July August. They are probably not fragments of Vesta, but may be fragments of one or more Vesta-like parent bodies. The spectra of Vesta and the three Earth-approachers resemble those of the basaltic meteorites, i.e., eucrites, howardites, and diogenites (HED meteorites). We find that HED fall times show a weak peak in July/August, suggesting that 3551, 3908, and 4055 may be fragments of the source bodies of the HED meteorites, or perhaps of a single common HED parent body. The kilometer-size asteroids have regoliths with significant insulating properties, but these regoliths differ from that of the Moon in that the particle sizes are larger than on the Moon, and lunar-like glasses and agglutinates are largely absent. The causes of these differences are related to impact velocities, impact shaking, and debris retention on small bodies. Differentiated basalt or mantle-like asteroids may have impacted Earth and left discontinuities in the geologic/climatic record without leaving the chemical signatures associated with the KT boundary event.
We have conducted petrologic, chemical, and isotopic studies of acapulcoites (Acapulco, Monument Draw, Yamato 74063, ALH A77081, ALH A81261, ALH A81315, ALH 78230, ALH A81187 and ALH 84190) in an attempt to constrain their genesis. Acapulcoites have distinctly different oxygen isotopic compositions than silicate inclusions in IAB and IIICD irons, winonaites and ureilites and, thus, formed on a different parent body. Oxygen isotopic compositions, which are slightly heterogeneous within the group, overlap with lodranites, indicating a likely origin on a common parent body. These groups can be distinguished on the basis of mafic silicate grain size. All acapulcoites have mafic silicate compositions intermediate between E and H chondrites, roughly chondritic mineralogies, achondritic, equigranular textures, micrometer to centimeter sized veins of Fe,NiFeS which cross-cut silicate phases, rapid metallographic cooling rates at ∼600−400°C (10³–10⁵°C/Myr) and trapped noble gas abundances comparable to type 3–4 ordinary chondrites. They exhibit variable mafic silicate zoning, abundance of Fe,NiFeS veins, REE abundances and patterns and, possibly, cosmic ray exposure ages (∼5–7 Ma). Momument Draw and Yamato 74063 retain rare relict chondrules. Phosphates are associated with Fe,NiFeS veins or form separate veins in Monument Draw and Acapulco. Heating and cooling of acapulcoites occurred very early in the history of the Solar System, as evidenced by the ³⁹Ar⁴⁰Ar ages of ∼4.51 Ga. These ages appear distinctly younger than the likely formation time for Acapulco of 4.557 Ga, but are older than analogous ³⁹Ar⁴⁰Ar ages for most chondrites.
The near-IR spectral properties of minerals, meteorites, and lunar soil vary with temperature. The manner in which these materials vary is diagnostic of aspects of their composition. We quantify the spectral dependence on temperature by reporting the change in relative reflectance with temperature as a function of wavelength. We call this quantity, ΔR/ΔT (in units of K−1), as a function of temperature the “thermo-reflectance spectrum.” The thermo-reflectance spectra of olivine and pyroxene are distinct, and most of the observable structure in thermo-reflectance spectra of the ordinary and carbonaceous chondrites can be understood in terms of a mixture of the thermo-reflectance spectra of olivine and pyroxene. The magnitude of thermo-reflectance spectra of meteorites and lunar soils is much less than that of pure minerals. Lunar soils are particularly subdued. While conventional analysis of remotely obtained spectra of the Moon can neglect temperature effects, spatially resolved measurements of the surface of the asteroid Vesta will likely have a strong temperature-dependent component based on measurements of a eucrite and a howardite.
Detailed near-infrared spectral observations of Asteroid 1459 Magnya reveal an asteroid that is primarily composed of pyroxene and plagioclase feldspar, confirming earlier suggestions that Magnya has a basaltic composition. The average Magnya spectrum for March 23, 2002 has a Band I center of 0.926 μm and a Band II center of 1.938 μm. Observations over hours show little variation in band center positions. The feldspar-to-pyroxene ratio is ∼0.6 on Magnya's surface. Comparing Magnya with the spectral parameters from 4 Vesta shows discordant pyroxene chemistries; Magnya's pyroxenes contain ∼10 mol% less Fs than Vesta's pyroxenes. This suggests that Magnya originated from a parent body other than 4 Vesta and that its progenitor formed in a more chemically reduced region of the solar nebula within the asteroid belt.
We present results of visible wavelength spectroscopic measurements for 48 near-Earth objects (NEOs) obtained with the 5-m telescope at Palomar Mountain Observatory during 1998, 1999, and early 2000. The compositional interpretations for 15 of these objects have been enhanced by the addition of near-infrared spectra obtained with the NASA Infrared Telescope Facility. One-third of our sampled objects fall in the Sq and Q classes and resemble ordinary chondrite meteorites. Overall our sample shows a clear transition between S-type and Q-type compositional classes over visible and near-infrared wavelengths. Taken together these results point toward an abundance of near-Earth asteroids capable of providing sources for ordinary chondrite meteorites. Our sampling strategy favors targeting the smallest observable objects and we report results for the 15-m diameter object 1998 BT13, the smallest spectroscopically measured NEO to date. NEOs show a greater spectral diversity than main-belt asteroids, and our small sample includes objects falling in the rare categories of K, L, O, and V classes. The K-class object 1999 JD6 is found to match CV chondrite meteorites. Potential spacecraft targets received top priority for observation, with the ordinary chondrite-like composition of (9969) Braille being reported prior to the Deep Space-1 encounter. The relatively accessible asteroid (10302) 1989 ML displays a neutral spectrum that may be interpreted as a shock-darkened ordinary chondrite.
The spectral characterization of small asteroids is important for understanding the evolution of their compositional and mineralogical properties. We report the results of a CCD spectroscopic survey of small main-belt asteroids which we call the Small Main-belt Asteroid Spectroscopic Survey (SMASS). Spectra of 316 asteroids were obtained, with wavelength coverage ranging from 4000 to 10000 Å (0.4 to 1 μm). More than half of the objects in our survey have diameters less than 20 km. Survey results include the identification of the first object resembling ordinary chondrite meteorites among the main-belt asteroids (Binzel, R. P., S. Xu, S. J. Bus, M. F. Skrutskie, M. R. Meyer, P. Knezek, and E. S. Barker 1993a. Science 262, 1541-1543) and observations of more than 20 asteroids showing basaltic achondrite spectral absorption features that strongly link Vesta as the parent body for the basaltic achondrite meteorites (Binzel, R. P., and S. Xu 1993. Science 260, 186-191). A potential Mars-crossing asteroid analog to ordinary chondrite meteorites (H chondrites), 2078 Nanking, is reported here. Through a principal component analysis, we have assigned classifications to the members of our sample. The majority of the small main-belt asteroids belong to S and C classes, similar to large asteroids. Our analysis shows that two new classes are justified which we label as J and O. Small asteroids display more diversity in spectral absorption features than the larger ones, which may indicate a greater variation of compositions in the small asteroid population. We found a few candidates for olivine-rich asteroids within the S class. Although the total number of olivine-rich candidates is relatively small, we present evidence suggesting that such objects are more prevalent at smaller sizes.
We present results of laboratory near-infrared reflectance studies of a set of calcic pyroxenes with comparable calcium contents (Wo45–50) but variable iron content and oxidation states. This new dataset complements earlier published data (Cloutis and Gaffey, 1991, J. Geophys. Res. 96, 22809–28826, and references therein). In particular, our new spectra extend the scarce available spectral data on chemically analyzed Fe-rich high-Ca clinopyroxenes. We attempted to interpret the spectral behavior of our samples in terms of chemistry and coordination site occupancies. Tentatively, we conclude that Fe-rich calcic pyroxenes have very low contents of Fe2+ in the M2 sites and belong to the spectral type A lacking the 2-μm band. This may be due to high Ca and Mn contents in these pyroxenes. Fe-poor high-Ca pyroxenes are more spectrally variable. In general, they tend to belong to the spectral type B with two major bands near 1 and 2 μm, unless the samples have high Fe3+/Fe2+ ratios or are rich in Mn and Ca. Some of them (including unusual meteorite Angra dos Reis) are of type B despite very high Ca contents. We applied the Modified Gaussian Model (MGM) to characterize three major Fe2+ absorption bands in the 1-μm region of the spectra of Ca-rich pyroxenes. Only the band due to Fe2+ in the M1 coordination site near 1.15 μm may be potentially useful to estimate the Fe content in calcic pyroxenes on remotely-sensed surfaces of Solar System bodies. The spectral variability of basaltic meteorites (angrites) that are rich in calcic pyroxenes is also discussed. The presence of spectral type A calcic pyroxenes in these meteorites complicates unambiguous identification of olivine in asteroid spectra.
This report describes a new software tool, ATRAN, which computes the transmittance of Earth's atmosphere at near- and far-infrared wavelengths. We compare the capabilities of this program with others currently available and demonstrate its utility for observational data calibration and reduction. The program employs current water-vapor and ozone models to produce fast and accurate transmittance spectra for wavelengths ranging from 0.8 microns to 10 mm.