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Age spot or youthful marking: Origin of Reiner Gamma
Geophysical Research Letters
Abstract
The highly magnetic (field magnitudes of 50 nT at 18 km altitude) Reiner Gamma albedo feature on the near side of the moon has been explained in terms of differential space weathering of an old feature, or a recent cometary impact. We investigated this feature using magnetometer data from Lunar Prospector. The minimum magnetization necessary to explain the magnetic field observations varies from 100 A/m for a 10 m thick layer, to 1 A/m for a 1 km thick layer. Magnetic sources appear to lie within a few km of the surface, and be magnetized in a north-south direction. The strength of the magnetization appears spatially related to the albedo of the feature. These constraints point towards an ancient origin for the magnetic field signal (possibly due to basin impact ejecta), and the origin of the albedo feature as a consequence of retarded ageing under the umbrella of the Reiner Gamma mini-magnetosphere.
... The largest concentration of strong anomalies was mapped antipodal to the young basins of the similar age such as Crisium, Serenitatis, Imbrium, and Orientale basins (Lin et al. 1998b;Richmond and Hood 2008). In addition, other strong isolated anomalies were mapped with more details such as Reiner Gamma (Hood and Schubert 1980;Hood et al. 2001;Nicholas et al. 2007), Descartes formations Richmond et al. 2003), Airy crater (Richmond and Hood 2008), and Rimma Sirsalis Halekas et al. 2001). Some geologic correlations were then established between high albedo and magnetic anomalies Richmond et al. 2005;Nicholas et al. 2007). ...
... In addition, other strong isolated anomalies were mapped with more details such as Reiner Gamma (Hood and Schubert 1980;Hood et al. 2001;Nicholas et al. 2007), Descartes formations Richmond et al. 2003), Airy crater (Richmond and Hood 2008), and Rimma Sirsalis Halekas et al. 2001). Some geologic correlations were then established between high albedo and magnetic anomalies Richmond et al. 2005;Nicholas et al. 2007). The LP-ER data were used to generate the highresolution global map of the nearside Mitchell et al. 2008). ...
... This method is efficient to filter the external field from each component and track by track effect was then reduced to the minimum. Our method reported here for filtering long period external field component differs from the method developed by Nicholas et al. (2007) and Purucker et al. (2006) based on a simple modeling that assume the external field is uniform and so determined by least square. Considering uniform field does not take into account the temporally varying of external fields in the east component, and their use was limited to the radial and north components. ...
We present a new study of the lunar magnetic fields using advanced methods and algorithms to select and process Lunar Prospector Magnetometer data (LP-MAG). These methods and algorithms have allowed us to derive a global model of the internal lunar magnetic field from the LP-MAG data acquired at low altitude, over 1999. To validate the results, we have developed a method based on properties of the potential fields, where the horizontal North and East components are derived from the vertical component, only. The global model estimated at spacecraft altitude is then downward continued at the Moon's surface using an inverse method approach. The obtained results confirm the crustal origin of the sources and show that the strongest anomalies are associated with formations of high albedo and/or are antipodal to large young basins (Orientale, Serenitatis, Imbrium, and Crisium) of age about 3.9 Ga. These basins are characterized by very weak anomalies. The modeling of some of the strongest anomalies shows clustered paleomagnetic pole positions within a radius of 35 degrees centered at (30S, 225E). These results support the hypothesis of a currently extincted paleo-lunar dynamo, which magnetized the lunar crust.
... Theories involving cometary and micrometeoroid impacts (Bell & Hawke, 1987;Pinet et al., 2000;Schultz et al., 1980;Starukhina & Shkuratov, 2004;Syal & Schultz, 2015) and lunar dust transport (Garrick-Bethell et al., 2011) attempt to explain the high albedo regions in swirls while also providing a mechanism by which lunar fines are preferentially deposited in the on-swirl locations, explaining the anomalous scattering properties of swirls. A comet impact, however, does not easily explain the association between lunar swirls and LMAs (Nicholas et al., 2007;Richmond et al., 2005), and the addition of lunar fines described by both a comet impact and lunar dust transport is predicted to result in evening surface temperatures much lower than the off-swirl regolith, a phonomenom that was not observed in measurements taken by the Diviner lunar radiometer onboard the LRO (Glotch et al., 2015). ...
... G. Shkuratov & Opanasenko, 1992). This anomaly has contributed to the resiliency of theories involving cometary and micrometeroid impacts or lunar dust transport, despite the inability of the former to provide a satisfactory answer to the correlation between lunar swirls and LMAs (Nicholas et al., 2007;Richmond et al., 2005), and both predicting reduced on-swirl surface temperatures during the lunar night, which has not been observed in the Diviner lunar radiometer data received onboard the LRO (Glotch et al., 2015). These surface roughness calculations, however, assume an isotropically oriented dust distribution (Chevrel et al., 2006;Hapke, 1984;KenKnight et al., 1967), which our simulations suggest might not exist. ...
The scattering of light off the bright regions of lunar swirls suggests a relatively higher fraction of fine‐grained dust compared to dust with similar optical maturity values found elsewhere on the lunar surface when assuming an isotropically oriented dust distribution, without preferred grain orientation. Here we propose a mechanism by which the ferromagnetic lunar fines, lofted by electrostatic effects or by meteoroid impacts, may rotate in a lunar magnetic anomaly and, upon landing, produce patches of organized alignments, generating anisotropic surface structures. We simulate these rotations for Reiner Gamma and provide a proof of concept for such a dust rotation model. While the magnetic forces remain negligibly small to influence the trajectories of the lofted particles, the magnetic torques have a significant effect on the rotation of magnetized dust, and can generate an organized landing pattern for elongated lunar fines, which offers a possible explanation for the lunar albedo patterns.
... The source body radius is not well constrained, but the 15 km we chose here seems reasonable given the depth of the magnetic anomaly. Furthermore, even shallow magnetic anomalies such as Reiner Gamma (Nicholas et al., 2007) are very well modeled by single dipoles (e.g., Kurata et al., 2005). Finally, we will show that our inversions yield good agreement with a dipole source. ...
... A final complication in our analysis above is that the cooling timescales may be smaller than the values calculated above if the spatial scale of the anomalies is smaller than a sphere that extends nearly to the surface. For example, if the true magnetizations of the anomalies are 10 A/m, as suggested for some of the Moon's anomalies (Nicholas et al., 2007), the spherical radii of the Crisium anomalies would be less than 10 km in radius ( Figure S3). This is smaller than the values assumed above. ...
Using Lunar Prospector magnetic field data, we identify four isolated anomalies (CNA, CSA1, CSA1, and CWA) within the Crisium basin. We assume the sources of the anomalies to be buried point dipoles and find that the estimated depths and radial positions are well contained within Crisium's melt pool (∼250‐km radii and ∼100‐km depth). This implies that the anomalies recorded a thermoremanent magnetization in a dynamo field at ∼3.9 Ga. We also find that the anomalies can be classified into two groups (Groups 1 and 2) according to source depths and paleopoles. The sources of Group 1 (CNA and CSA1) are buried at ∼30 km. Estimated paleopoles are located at ∼45°N for CNA and near the equator (∼12°N) CSA1, with remanence acquisition timescale of ∼5 Myr. The sources of Group 2 (CSA2 and CWA) are more deeply buried at ∼55–70 km. Their paleopole positions are close to the present south pole, with a remanence acquisition timescale of ∼15–23 Myr. These variable paleopoles imply that the local field orientation shifted during formation of the anomalies. That is, the Moon may have experienced a ∼14‐Myr‐epoch of complex field geometries. These changes may have arisen from true polar wander driven by impact or internal density changes, or from equatorial dynamo orientations produced by anomalous core heat flux conditions. The long cooling timescales of the anomalies also suggest that they were not magnetized by transient fields produced by impact‐induced stirring of the core.
... In the case of the Orientale basin, the impact ejecta layers are horizontal deposits, and their thickness in flat regions measures less than about 1000-m (Fassett et al., 2011). Nicholas et al. (2007) determined that an ejecta layer would require magnetizations of 1 A/m if it measured 1000-m in thickness, or 10 A/m if it had a thickness of 100-m. Since the maximum remanence intensities of returned lunar materials measure up to 1 A/m, it is important to determine if the ejecta layer has a thickness of 1000-m to constrain whether the remanence intensities of Apollo-returned materials are usual. ...
... This result corresponds to previous findings based on a shadow analysis, where the probable maximum thickness of the ejecta is 100-m (Hood, 1980). Assuming that the thickness of the magnetized crust is 100-m, Nicholas et al. (2007) predicted the remanence intensity for the Reiner Gamma ejecta to be about 10 A/m, being one-order-of-magnitude stronger than the intensity for any Apollo-returned lunar breccia samples of about 1 A/m (Strangway et al., 1973a,b). On the other hand, we cannot also exclude the other possibility of an isolated magnetic source with a depth deeper than 1000-m for Reiner Gamma, due to the limitations associated with the LRS penetration depth. ...
Reiner Gamma is a sinuous feature in Oceanus Procellarum; it has a higher reflectance of the visible wavelength than the surrounding flat mare basalt, and displays a high crustal magnetic field. Previous studies relating to the origin of Reiner Gamma have provided contradictory depths of magnetic source bodies in the lunar crust as either shallow or deep. If a shallow ejecta layer existed beneath the Reiner Gamma formation, a subsurface lithological boundary between the denser mare basalt and the less dense ejecta blanket would be expected. This study examines subsurface stratifications using the Lunar Radar Sounder (LRS) onboard the Kaguya spacecraft. Taking into account the LRS-determined dielectric constants, the influence of surface clutter, and the energy loss of the LRS radar pulses in the high frequency band (5 MHz), no evidence was found of subsurface boundaries down to a depth of 1000-m at Reiner Gamma. Given the LRS range resolution of 75-m, the source of the magnetic anomaly is considered to be either strongly magnetized thin breccia layers at depths shallower than 75-m, or less magnetized thick layers at depths deeper than 1000-m.
... No completely satisfactory model for swirl formation has emerged (Lucey et al., 2006). One class of hypotheses suggests recent (<100 Myr) impacts (Pinet et al., 2000;Schultz and Srnka, 1980;Starukhina and Shkuratov, 2004;Shkuratov et al., 2010) scoured away dark, mature material to reveal fresh unweathered soil, but these models do not easily explain the swirl association with magnetic anomalies (Richmond et al., 2005;Nicholas et al., 2007). Alternatively, Hood and Schubert (1980) proposed that local magnetic fields may form mini-magnetospheres that stand off the solar wind, thereby preventing maturation of the underlying soil. ...
... Larger diameter disks require higher magnetizations. These magnetizations are in reasonable agreement with minimum magnetizations of 1 and 0.1 A/m for layer thicknesses of 1 and 10 km, respectively, derived from an ideal bodies approach (Nicholas et al., 2007). Magnetizations of 0.1-1 A/m would be among the most magnetic samples measured. ...
Lunar swirls are albedo anomalies associated with strong crustal magnetic fields. Swirls exhibit distinctive spectral properties at both highland and mare locations that are plausibly explained by fine-grained dust sorting. The sorting may result from two processes that are fairly well established on the Moon, but have not been previously considered together. The first process is the vertical electrostatic lofting of charged fine dust. The second process is the development of electrostatic potentials at magnetic anomalies as solar wind protons penetrate more deeply into the magnetic field than electrons. The electrostatic potential can attract or repel charged fine-grained dust that has been lofted. Since the finest fraction of the lunar soil is bright and contributes significantly to the spectral properties of the lunar regolith, the horizontal accumulation or removal of fine dust can change a surface's spectral properties. This mechanism can explain some of the spectral properties of swirls, accommodates their association with magnetic fields, and permits aspects of weathering by micrometeoroids and the solar wind.
... However, the conditions taken (NRM= B/µ 0 , NRM/M rs = 2.5%) are optimistic and it is more realistic to use a M rs =10 − 2 Am 2 /kg threshold, i.e. 2.5 times higher. For Rainer Gamma satellite anomaly (50 nT at 18 km), Nicholas et al. (2007; see also Hood, 1980) modeling indicates a rather thin surface layer of minimum NRM = 1 A/m (for a 1 km thick layer) which again translates into a minimum M rs of the order 10 − 2 Am 2 /kg. This translates into a minimum logM rs of the order of 1 (using M rs in 10 − 3 Am 2 /kg) . ...
... This strongly suggests that mare basalts at depth are unable to carry significant magnetization in agreement with the rarity of measurable magnetic anomalies at satellite altitude ( Fig. 1) in lunar maria. The lithologies more likely to generate the observed anomalies are impact related materials in agreement with the thin sheet model of Nicholas et al. (2007). Accordingly, the strongest and densest magnetic anomaly group is located at the edge of the South Pole-Aitken basin, the largest one on the Moon ( Fig. 1; Richmond and Hood, 2008). ...
We present the first comprehensive study of the magnetic properties of lunar meteorites and compare them with measurements from Apollo and Luna returned samples. 37 unpaired lunar meteorites were studied, while new susceptibility measurements were performed on 88 Luna soil and rock samples, to complement published Luna and Apollo data. New magnetic data were also obtained on 4 Apollo mare basalt samples. Magnetic susceptibility and saturation remanence appear mainly controlled by the amount of metallic iron added by the regolith-forming processes and meteoritic contamination, as shown by a positive correlation with Ni and Ir content, a decrease with depth in regolith core profiles, and a decrease with increasing soil size fraction. The three sources of lunar materials provide coherent range of magnetic properties, although the much larger abundance of anorthositic highland samples in the meteorite collection allows one to better describe the properties of this major lunar lithology. The observed range of saturation remanence implies that mare basalts cannot contribute significantly to the patchy lunar crustal magnetizations, which must be attributed to superficial impact processed feldspathic or mafic lithologies.
... Recent global maps of the internal magnetic field include those of Richmond and Hood (2008) using the vector fluxgate magnetometer , and Mitchell et al. (2008) using the electron reflectometer . Magnetic maps such as these are of use in understanding the sources of the internal field (Hood and Schubert, 1980; Nicholas et al., 2007 ) and in modeling the interaction of the ...
... Details of the external field model are identical with that of Nicholas et al. (2007), and are summarized here. The external field was represented as a uniform field over each satellite half-orbit, and the half orbits extend from pole to pole. ...
A preliminary model of the internal magnetic field of the Moon is developed using a novel, correlative technique on the low-altitude Lunar Prospector magnetic field observations. Subsequent to the removal of a simple model of the external field, an internal dipole model is developed for each pole-to-pole half-orbit. This internal dipole model exploits Lunar Prospector's orbit geometry and incorporates radial and theta vector component data from immediately adjacent passes into the model. These adjacent passes are closely separated in space and time and are thus characteristic of a particular lunar regime (wake, solar wind, magnetotail, magnetosheath) or regimes. Each dipole model thus represents the correlative parts of three adjacent passes, and provides an analytic means of continuing the data to a constant surface of 30 km above the mean lunar radius. The altitude-normalized radial field from the wake and tail regimes is used to build a model in which 99.2% of the 360 by 360 bins covering the lunar surface are filled. This global model of the radial magnetic field is used to construct a degree 178 spherical harmonic model of the field via the Driscoll and Healy sampling theorem. Terms below about degree 150 are robust, and polar regions are considered to be the least reliable. The model resolves additional detail in the low magnetic field regions of the Imbrium and Orientale basins, and also in the four anomaly clusters antipodal to the large lunar basins. The model will be of use in understanding the sources of the internal field, and as a first step in modeling the interaction of the internal field with the solar wind.
... The origin of this magnetic anomaly is still under debate. Suggested scenarios include a thin near-surface layer of impact ejecta (Hood et al., 2001;Hood, Torres, et al., 2021;Nicholas et al., 2007), an intrusive magmatic dike (Hemingway & Tikoo, 2018), and a buried crater that was later filled by volcanic material (Kelley & Garrick-Bethell, 2020). ...
Spacecraft measurements show that the crust of the Moon is heterogeneously magnetized. The sources of these magnetic anomalies are yet not fully understood, with most not being related to known geological structures or processes. Here, we use an inversion methodology that relies on the assumption of unidirectional magnetization, commonly referred to as Parker's method, to elucidate the origin of the magnetic sources by constraining the location and geometry of the underlying magnetization. This method has been used previously to infer the direction of the underlying magnetization but it has not been tested as to whether it can infer the geometry of the source. The performance of the method is here assessed by conducting a variety of tests, using synthetic magnetized bodies of different geometries mimicking the main geological structures potentially magnetized within the lunar crust. Results from our tests show that this method successfully localizes and delineates the two‐dimensional surface projection of subsurface three‐dimensional magnetized bodies, provided their magnetization is close to unidirectional and the magnetic field data are of sufficient spatial resolution and reasonable signal‐to‐noise ratio. We applied this inversion method to two different lunar magnetic anomalies, the Mendel‐Rydberg impact basin and the Reiner Gamma swirl. For Mendel‐Rydberg, our analysis shows that the strongest magnetic sources are located within the basin's inner ring, whereas for Reiner Gamma, the strongest magnetic sources form a narrow dike‐like body that emanates from the center of the Marius Hills volcanic complex.
... The magnetic vector structure is shown in Figures 8 and 9a-c, which shows that the highest magnetization magnitude is 0.5 A/m, which, compared to the maximum magnetization intensity of the Earth's crust, is 12 times smaller [55]. Nicholas et al. [56] studied the magnetic field in the Reiner Gamma region and found that a magnetization intensity of 1 A/m is required if the ejecta is 1 km thick, and Carley et al. [57] calculated a maximum magnetization intensity of 0.2 A/m on the Moon but constructed a model with a minimum layer thickness of 10 km. In our results, the magnetic sources are buried at depths between 4-9 km and a thickness of 6 km, so the magnetization intensity of 0.5 A/m is possible. ...
The three-dimensional magnetic vector structure (magnetization intensity and direction) of the planet can be effectively used to analyze the characteristics of its formation and operation. However, the quick acquisition of a large region of the magnetic vector structure of the planet with bigger observation surfaces undulation is hard and indispensable. We firstly proposed a fast magnetization vector inversion method for the inversion of a magnetic anomaly with the undulating observation surfaces in the spherical coordinate system, which first transforms the data to a plane when the data are distributed on a surface. Then, it uses a block-Toeplitz-Toeplitz-block (BTTB)-FFT to achieve fast inversion with the constraint that the magnetization intensities of the grids between the transformed observation surfaces and the terrain are zero. In addition, Gramian constraint term is used to reduce the ambiguity of the magnetic vector inversion. The theoretical model tests show that the proposed method can effectively improve the computational efficiency by 23 times in the 60 × 60 × 10 grid division compared to the conventional inversion method, and the accuracy of the two computation methods is comparable. The root-mean-square error of the magnetization intensity is only 0.017, and the angle error is within 1°. The magnetization vector structure shows that the largest crater diameter does not exceed 340 km in the Mare Australe region, the amplitude of the magnetic anomaly is much higher than the current meteorite impact simulation results, and the depth of the magnetic source is less than 10 km, which cannot be explained by the impact simulation experiments. In addition, the magnetization directions of adjacent sources differ by 122° (or 238°), and the high-frequency dynamics of the Moon as well as the short-lived dynamics may be responsible for this phenomenon. The magnetization directions of the three adjacent sources in the Mare Crisium region are close to each other and differ in depth with different cooling times, making it difficult to record the transient fields produced by meteorite impacts. In addition to the above characteristics, the magnetization direction of the magnetic sources in both regions is uniformly distributed without reflecting the dispersion of the magnetization direction of the meteorite impact magnetic field. Therefore, it can be inferred that the magnetic anomalies in these two regions are related to the generator hypothesis.
... Several estimates of the properties of this body have been made. Overall, models approximate the shape of the body as an ellipsoid several tens of kilometers in width, located within a few kilometers below the surface, with a magnetization in the range of 1-100 A/m (e.g., Garrick-Bethell & Kelley, 2019;Hemingway & Garrick-Bethell, 2012;Nicholas et al., 2007). It might be possible that the actual horizontal extent of this body is greater than these estimates, so that it reaches the location of the Reiner K crater, and that it is regionally located at a shallower depth. ...
In a database of lunar fractured boulders (Rüsch & Bickel, 2023, https://doi.org/10.3847/psj/acd1ef), we found boulders with reflectance features dissimilar to previously known morphologies. We performed a photo‐geologic investigation and determined that the features correspond to a dust mantling on top of boulders with a unique photometric behavior. We next performed a photometric model inversion on the dust mantling using Bayesian inference sampling. Modeling indicates that the dust photometric anomaly is most likely due to a reduced opposition effect, whereas the single scattering albedo is not significantly different from that of the nearby background regolith. This implies a different structure of the dust mantling relative to the normal regolith. We identified and discussed several potential processes to explain the development of such soil. None of these mechanisms can entirely explain the multitude of observational constraints unless evoking anomalous boulder properties. Further study of these boulders can shed light on the workings of a natural dust sorting process potentially involving dust dynamics, a magnetic field, and electrostatic dust transport. The presence of these boulders appears to be limited to the Reiner K crater near the Reiner Gamma magnetic and photometric anomaly. This close spatial relationship further highlights that poorly understood processes occur in this specific region of the Moon.
... In one model (Hemingway and Tikoo 2018), the magnetic source bodies need to be both shallow (<1-5 km deep) and narrow. These restrictions on the geometry and volume of the magnetic sources implies that the source magnetization must exceed ~0.5 A m -1 and is likely less than 6 A m -1 (see also Nicholas et al. 2007). In another model, edge effects from a shallow but considerably wider (~13 km) magnetized disk give rise to a similar magnetic field morphology, with predicted magnetizations up to ~70 A m -1 (Garrick-Bethell and Kelley 2019). ...
... Multiple studies have aimed to determine the age of individual MAs by constraining the geologic context and potential source geometry, towards understanding when the MA may have been emplaced in the crust and the method of magnetization (e.g., Nicholas et al., 2007;Weiss & Tikoo, 2014;Wieczorek, 2018;Lee et al., 2019;Baek et al., 2019;Kelley & Garrick-Bethell, 2020). Two primary magnetization mechanisms have been proposed: 1) thermoremanent magnetization (TRM) during geologic processes (e.g., impacts and crystallization) within a period of active dynamo (e.g., Purucker et al., 2012;Wieczorek et al., 2012;Arkani-Hamed & Boutin, 2014;Hood & Spudis, 2016;Hemingway & Tikoo, 2018;Wakita et al., 2021), and 2) shock remanent magnetization (SRM) or pressure remanent magnetization (PRM) during impact or collision events that generated transient fields or amplified external fields in the immediate locality of the impact (e.g., Schultz and Srnka, 1980;Crawford & Schultz, 1999;Gattacceca et al., 2010;Bruck Syal and Schultz, 2015). ...
With the wealth of missions selected to visit the lunar surface in the decade ahead, preparatory investigations into surface conditions are underway to explore potential challenges and science returns during these missions. One such mission, Lunar Vertex, is slated to explore a much-anticipated region–the lunar swirl and magnetic anomaly known as Reiner Gamma. Lunar swirls are unique natural laboratories for exploring solar wind interactions with partially magnetized rocky bodies, and possess characteristics that have not yet been observed on any other body in the Solar System. This work aims to combine current magnetic mapping of Reiner Gamma with ultraviolet wavelength datasets, towards further understanding the sensitivities of ultraviolet measurements in regions that may be partially magnetically shielded from solar wind weathering and magnetospheric plasma populations. Observations and models herein are collected and derived from orbital sources and will be used for comparison to future orbital and surface observations of Reiner Gamma by Lunar Vertex.
... For instance, this effect has been suggested to explain the lack of "apparent" magnetic signatures of Hellas and Argyre large impact basins on Mars (Acuna et al. 1998), even if the absence of a dynamo during the cooling stage after these events is a better explanation (Langlais et al. 2004;Langlais and Th ebault 2011). This may also be suggested for most of the magnetic signatures of lunar impact structures: the lowest satellite magnetic field measurements were performed at minimum 30 km of altitude, excluding de facto the mapping of small-wavelength magnetized contrasts (Nicholas et al. 2007;Hemingway and Tikoo 2018), while a wealth of gravimetric details were observed by the GRAIL mission (Zuber et al. 2016). On Earth, few large impact structures-including Chicxulub (Gulick et al. 2013)-exhibit a gravimetric anomaly that can be observed by satellite measurements. ...
In 2011, the discovery of shatter cones confirmed the 28 km diameter Tunnunik complex impact structure, Northwest Territories, Canada. This study presents the first results of ground‐based electromagnetic, gravimetric, and magnetic surveys over this impact structure. Its central area is characterized by a ~10 km wide negative gravity anomaly of about 3 mGal amplitude, roughly corresponding to the area of shatter cones, and associated with a positive magnetic field anomaly of ~120 nT amplitude and 3 km wavelength. The latter correlates well with the location of the deepest uplifted strata, an impact‐tilted Proterozoic dolomite layer of the Shaler Supergroup exposed near the center of the structure and intruded by dolerite dykes. Locally, electromagnetic field data unveil a conductive superficial formation which corresponds to an 80–100 m thick sand layer covering the impact structure. Based on the measurements of magnetic properties of rock samples, we model the source of the magnetic anomaly as the magnetic sediments of the Shaler Supergroup combined with a core of uplifted crystalline basement with enhanced magnetization. More classically, the low gravity signature is attributed to a reduction in density measured on the brecciated target rocks and to the isolated sand formations. However, the present‐day fractured zone does not extend deeper than ~1 km in our model, indicating a possible 1.5 km of erosion since the time of impact, about 430 Ma ago.
... Its rim is now buried; thus,~1.5 km is the minimum burial depth of the magnetized disk, plus whatever the floor depth of the elliptical crater that formed the source (see section 3.1.2). A shallow (approximately kilometers) burial depth is consistent with the results of Nicholas et al. (2007). For a total burial depth of, say 2 km, the defining plasma-field interaction region is between the surface and~3 km above it. ...
The geologic origin of the Moon's crustal magnetic anomalies is unknown. Reiner Gamma is one of the most studied anomalies, and it is correlated with symmetric bright albedo markings known as swirls. Here we propose that its source magnetization arises from a uniformly magnetized elliptical disk, resulting from the melt sheet or floor deposits of an oblique impact crater. The magnetization was likely acquired in a dynamo field and may be as high as ~70 A/m, perhaps due to incorporation of impactor materials. The disk produces vertical fields at its edges that may channel solar wind flux to the surface, producing an elliptical dark region, while neighboring regions remain shielded and bright. Interestingly, the disk appears to be magnetized along its semiminor axis. Measurements of the low altitude magnetic field at Reiner Gamma would test these predictions and help answer additional questions about its interaction with the solar wind.
... Reiner Gamma ( Fig. 2; Hood et al. 1980, Schultz andSrnka 1980;Bell and Hawke 1981;Pinet et al. 2000;Nicholas et al. 2007;Kramer et al. 2011b), Mare Ingenii (complex, multiple swirl belt) ( Fig. 3; Kramer et al. 2011a, b), Gerasimovich (highland swirl) (Kramer et al. 2011b), Descartes ( Fig. 4; Richmond et al. 2003;Blewett et al. 2005). ...
... [44] The suggested 30 km thick magnetized layer is consistent with the spectral thickness calculated, and represents a global compromise of the fit to the data and RMS magnetization. For example, thinner layers provided a better fit in the Reiner Gamma region, compatible with theories that the magnetized source in this region is a thin near-surface layer of impact ejecta Nicholas et al., 2007] (though the layer would likely be thinner than the thinnest modeled here). Models with layer thicknesses >10 km require magnetization magnitudes that are comparable to those measured in the Apollo samples, demonstrating that minimum magnetization models from satellite data provide useful bounds on the magnetization of the lunar crust, and suggest that the magnetic mineralogies of the Apollo samples are representative of the lunar crust as a whole. ...
Magnetic fields measured by the satellite Lunar Prospector show large
scale features resulting from remanently magnetized crust. Vector data
synthesized at satellite altitude from a spherical harmonic model of the
lunar crustal field, and the radial component of the magnetometer data,
have been used to produce spatially continuous global magnetization
models for the lunar crust. The magnetization is expressed in terms of
localized basis functions, with a magnetization solution selected having
the smallest root-mean square magnetization for a given fit to the data,
controlled by a damping parameter. Suites of magnetization models for
layers with thicknesses between 10 and 50 km are able to reproduce much
of the input data, with global misfits of less than 0.5 nT (within the
uncertainties of the data), and some surface field estimates. The
magnetization distributions show robust magnitudes for a range of model
thicknesses and damping parameters, however the magnetization direction
is unconstrained. These global models suggest that magnetized sources of
the lunar crust can be represented by a 30 km thick magnetized layer.
Average magnetization values in magnetized regions are 30-40 mA/m,
similar to the measured magnetizations of the Apollo samples and
significantly weaker than crustal magnetizations for Mars and the Earth.
These are the first global magnetization models for the Moon, providing
lower bounds on the magnitude of lunar crustal magnetization in the
absence of multiple sample returns, and can be used to predict the
crustal contribution to the lunar magnetic field at a particular
location.
... As before, we utilized a preconditioned conjugate gradient approach [Purucker et al., 1996], and iterated each solution six times. Each solution, of which there were in excess of 5000 low-altitude ones, was used to calculate the radial magnetic field at an altitude of 30 km above the mean lunar radius.Figure 2 shows the radial magnetic field profiles and grids over the Reiner Gamma swirl [Nicholas et al., 2007], one of the largest isolated magnetic features on the Moon.Figure 3 shows the radial magnetic profiles and grids over the northwestern corner of the South Pole-Aitken basin, the location of the most extensive set of magnetic features on the Moon. Profiles and maps before and after the major processing steps are shown, and emphasize the clarification of the features which occurs after each processing step. ...
Three new models of the global internal magnetic field of the Moon based on Lunar Prospector (LP) fluxgate magnetometer observations are developed for use in understanding the origin of the Moon's crustal magnetic field and for modeling its interaction with the solar wind. The models are at spherical harmonic degree 170, corresponding to 64 km wavelength resolution, from 30 km mean altitude LP observations. Coverage is complete except for a few areas near each pole. Original signal amplitudes are best preserved in the sequential approach map, whereas feature recognition is superior in the coestimation and harmonic wave number correlation maps. Spherical harmonic degrees less than 15, corresponding to 666 km wavelength, are largely absent from the Moon's internal magnetic field. We interpret this bound in terms of the Moon's impact history. A derived magnetization map suggests magnetizations may locally exceed 0.2 A/m in the lunar crust at the survey resolution if the magnetic crust is as thick as 40 km.
... Therefore, the surface fields of other bodies are sourced from purely remanent magnetization in the planetary crust. On the Moon, a small number of thin (likely <1 km thick) crustal sources generate isolated magnetic field anomalies (≤10 nT at 40 km altitude) (Nicholas et al. 2007), with implied magnetizations of 1 ampere per meter (A/m). On the other hand, nearly half of the surface area of Mars today generates strong magnetic anomalies equivalent in strength to the Earth's total surface field (tens of µT or more) (Langlais et al. 2004) and sourced from deep crustal magnetization with intensities of ~10 A/m. ...
Extraterrestrial materials contain a diversity of ferromagnetic phases, ranging from common terrestrial oxides to exotic metal
alloys and silicides. Because of their great age and remote provenance, meteorites provide a unique window on early solar
system magnetic fields and the differentiation of other bodies. Interpreting the records of meteorites is complicated by their
poorly understood rock magnetic properties and unfamiliar secondary processing by shock and low-temperature phase transformations.
Here we review our current understanding of the mineral magnetism of meteorites and the implications for magnetic fields on
their parent bodies.
... If the crustal anomalies are able to deflect the solar wind, then the high albedo may be due to the anomalies shielding the regolith [Hood and Schubert, 1980;Hood and Williams, 1989;Richmond et al., 2003], which may indicate that solar wind ion bombardment is a necessary and sufficient process behind at least part of the optical maturation of the lunar regolith. Kurata et al. [2005] and Nicholas et al. [2007] have argued for the existence of a minimagnetosphere at Reiner Gamma, which provides further evidence in support of the magnetic shielding hypothesis. ...
Previous processing of the Lunar Prospector magnetometer (LP-MAG) data has yielded ~40% coverage of the Moon. Here, new mapping of the low-altitude LP-MAG data is reported with the goal of producing the first global vector map of the lunar crustal magnetic field. By considering all data regardless of the external plasma environment and using less restrictive editing criteria, 2360 partial and complete passes have been identified that can be used to investigate the lunar crustal magnetic anomalies. The cleanest global coverage is provided using 329 low-altitude nightside and terminator passes. An inverse power method has been used to continue the final mapping data to constant altitude. Using the 329 optimal passes, global maps of the lunar crustal magnetic field are constructed at 30 and 40 km. Consistent with previous studies: (1) the largest concentrations of anomalies are mapped antipodal to the Crisium, Serenitatis, Imbrium, and Orientale basins and (2) isolated anomalies at Reiner Gamma, Rima Sirsalis, Descartes, and Airy are mapped. Anomalies previously unmapped by the LP-MAG experiment include (1) isolated anomalies near the craters Abel and Hartwig, (2) weak magnetization within the Nectarian-aged Crisium and Moscoviense basins, and (3) a relatively weak anomaly in an area dominated by crater chains associated with the formation of Nectaris. Future work with the new low-altitude data set is discussed and will include determining whether the lunar anomalies are capable of deflecting the solar wind and investigating directions of magnetization to evaluate a possible former core dynamo.
... A conservative lower bound on the source magnetization intensity can be obtained from a relation derived by Parker (2003) (see also Nicholas et al. (2007)) between the minimum intensity, M min , the observed field amplitude jBj at the spacecraft altitude, h, and the layer thickness, t: ...
The nature of strong martian crustal field sources is investigated by mapping and modeling of Mars Global Surveyor magnetometer data near Apollinaris Patera, a previously proposed volcanic source, supplemented by large-scale correlative studies. Regional mapping yields evidence for positive correlations of orbital anomalies with both Apollinaris Patera and Lucus Planum, a nearby probable extrusive pyroclastic flow deposit that is mapped as part of the Medusae Fossae Formation. Iterative forward modeling of the Apollinaris Patera magnetic anomaly assuming a source model consisting of one or more uniformly magnetized near-surface disks indicates that the source is centered approximately on the construct with a scale size several times larger and comparable to that of the Apollinaris Patera free-air gravity anomaly. A significantly lower rms deviation is obtained using a two-disk model that favors a concentration of magnetization near the construct itself. Estimates for the dipole moment per unit area of the Lucus Planum source together with maximum thicknesses of ∼3km based on topographic and radar sounding data lead to an estimated minimum magnetization intensity of ∼50A/m within the pyroclastic deposits. Intensities of this magnitude are similar to those obtained experimentally for Fe-rich Mars analog basalts that cooled in an oxidizing (high fO2) environment in the presence of a strong (⩾10μT) surface field. Further evidence for the need for an oxidizing environment is provided by a broad spatial correlation of the locations of phyllosilicate exposures identified to date using Mars Express OMEGA data with areas containing strong crustal magnetic fields and valley networks in the Noachian-aged southern highlands. This indicates that the presence of liquid water, which is a major crustal oxidant, was an important factor in the formation of strong magnetic sources. The evidence discussed here for magnetic sources associated with relatively young volcanic units suggests that a martian dynamo existed during the late Noachian/early Hesperian, after the last major basin-forming impacts and the formation of the northern lowlands.
... Since Mercury apparently lacks swirls, the hypotheses for lunar swirls that invoke comet-related impacts are not favored. An additional objection to the comet-impact hypotheses is that the thin surface layer of regolith modified by the comet interaction would need an unrealistically high degree of magnetization in order to produce the magnetic fields observed (e.g., over the Reiner Gamma formation, Nicholas et al., 2007). Hypotheses for lunar swirls that require direct interaction between the solar wind and a local crustal magnetic field are therefore preferred, and this recognition has implications regarding the phenomena responsible for ''space weathering, " the physical, chemical, and optical changes that result when regolith is exposed to the space environment on an airless body (e.g., Hapke, 2001). ...
Images returned by the MESSENGER spacecraft from the Mercury flybys have been examined to search for anomalous high-albedo markings similar to lunar swirls. Several features suggested to be swirls on the basis of Mariner 10 imaging (in the craters Handel and Lermontov) are seen in higher-resolution MESSENGER images to lack the characteristic morphology of lunar swirls. Although antipodes of large impact basins on the Moon are correlated with swirls, the antipodes of the large impact basins on Mercury appear to lack unusual albedo markings. The antipodes of Mercury’s Rembrandt, Beethoven, and Tolstoj basins do not have surface textures similar to the “hilly and lineated” terrain found at the Caloris antipode, possibly because these three impacts were too small to produce obvious surface disturbances at their antipodes. Mercury does have a class of unusual high-reflectance features, the bright crater-floor deposits (BCFDs). However, the BCFDs are spectral outliers, not simply optically immature material, which implies the presence of material with an unusual composition or physical state. The BCFDs are thus not analogs to the lunar swirls. We suggest that the lack of lunar-type swirls on Mercury supports models for the formation of lunar swirls that invoke interaction between the solar wind and crustal magnetic anomalies (i.e., the solar-wind standoff model and the electrostatic dust-transport model) rather than those models of swirl formation that relate to cometary impact phenomena. If the solar-wind standoff hypothesis for lunar swirls is correct, it implies that the primary agent responsible for the optical effects of space weathering on the Moon is solar-wind ion bombardment rather than micrometeoroid impact.
The Moon does not currently possess a dynamo, but its crust contains numerous magnetic anomalies detected from orbit. The geologic origins of these anomalies are still unknown, including the archetypal Reiner Gamma magnetic anomaly. To gain insight, we study a small magnetic anomaly, herein called the octopus, which is possibly associated with Reiner Gamma. The octopus has curving bright albedo patterns characteristic of features known as swirls. We use high cadence 9 Hz Lunar Prospector magnetometer data, along with constraints provided by this swirl's albedo pattern, to perform inversions for the swirl's magnetic source body characteristics. We use three different inversion methods, and they all return similar results. We also estimate the depth of magnetization from characteristics of the horizontal component of the magnetic field and the albedo pattern. We find that performing inversion for source body properties at small swirls has advantages compared to larger anomalies, or anomalies without albedo markings. We find the octopus is magnetized in the same direction as the main Reiner Gamma anomaly (within 1σ uncertainties), suggesting they formed contemporaneously. The large spatial distance between these coeval anomalies and their inferred shallow source body depths are compatible with formation in a hot ejecta deposit that cooled in the presence of a dynamo field, as suggested by Hood et al. (2001). However, a key remaining enigma is why the northeastern Reiner Gamma “tail” formation has a magnetization direction ∼60° different from the main body and octopus.
During the Apollo era of lunar exploration, mysterious albedo patterns, called swirls, captured the imagination of the scientific community. A key aspect of this interest was due to the discovery that the swirls are associated with localized relatively strong remnant magnetic fields. Analysis of returned soil samples revealed that solar wind, galactic cosmic rays, and micrometeorite impacts change the albedo of surface soil grains, a process known as space weathering that reduces regolith reflectance over time. Thus it was natural to invoke local magnetic structures as shields that retard space weathering resulting in relatively less space weathering of the regolith in regions with the strongest localized magnetic fields. However, the origins of the magnetic anomalies and associated swirls remain enigmatic to this day. We designed the Swirl CubeSat to determine the nature of remnant lunar magnetic fields and investigate their role in moderating space weathering of the regolith and assess their suitability for radiation protection of surface assets. Swirl has one focused observational objective: characterize the magnetic fieeld associated with the Reiner Gamma Swirl (RGS) at sub-kilometer spatial sampling, with 0.5 nTesla accuracy and 100 m spatial precision. In the Swirl mission concept, the Swirl spacecraft, a 6U CubeSat, would deploy as a secondary payload from a vehicle on a deep space trajectory. A series of maneuvers would then place the spacecraft in a low orbit that would then be modified to have a periapse of 5-10 km for thirty orbits passing over RGS. Prime Swirl observations were designed during these low-altitude passes and consist of high-resolution vector magnetic field measurements and monochrome navigation imaging.
Spacecraft observations show that weak magnetic fields of crustal origin are ubiquitous across the surface of the Moon. To investigate the origin of these magnetic anomalies, a model was developed for the magnetic power spectrum that consists of ensembles of randomly magnetized sills or prisms. Localized spectrum analyses constrained how the parameters of this model vary with position, including the size of the sources, a quantity proportional to their mean-squared dipole moment, and the depth to the top and bottom of the magnetized region. The depth to the top of the magnetized region varies from the surface to about 25 km. The magnetic carriers in the deep crust likely formed at the same time as the crust itself, implying that a core-generated dynamo field must have existed when the crust was cooling during the first 100 million years of lunar evolution. The parameter related to the strength of magnetization shows the existence of a prominent region on the nearside hemisphere that is largely unmagnetized and that correlates with a region of extremely low surface field strengths. This region lies entirely within a geological province that is highly enriched in heat-producing elements (the Procellarum KREEP Terrane), suggesting that this region escaped being magnetized because of prolonged high crustal temperatures. The nearside magnetic low may be representative of the size of that portion of the crust that is highly enriched in heat producing elements, which is almost one third the size of the Procellarum KREEP Terrane based on surface thorium abundances.
A number of magnetic anomalies are present along the northern edge of the lunar South Pole-Aitken (SPA) basin. A variety of hypotheses for their formation have been proposed, but an in-depth study of their properties has not been performed. Here we use two different methods to invert for their source body characteristics: one that completely searches a small parameter space of less than ten uniform-strength dipoles per anomaly, and another that uses grids of hundreds of dipoles with variable magnetization strengths. Both methods assume uniform magnetization directions at each anomaly and with one exception, produce nearly the same results. We introduce new Monte Carlo methods to quantify errors in our inversions arising from Gaussian time-dependent changes in the external field and the uncertain geometry of the source bodies. We find the errors from uncertainty in source body geometry are almost always higher. We also find a diverse set of magnetization directions around SPA, which we combine with other physical arguments to conclude that the source bodies were likely magnetized in a dynamo field. Igneous intrusions are a reasonable explanation (Purucker et al., 2012) for the directional variability, since they could be intruded over different magnetic epochs. However, the directional variability also implies either surprisingly large amounts of true polar wander or a dynamo not aligned with the lunar spin axis. We also explore the possibility that true polar wander caused by the SPA impact could allow iron-rich SPA ejecta to record a diverse set of magnetic field directions. Some of this material may have also become sesquinary ejecta and re-impacted across the Moon on 10⁴-10⁶ year timescales to capture such changes. No completely satisfactory answer emerges, except that the dipole-axis of the lunar dynamo may have been variable in direction.
Remanent magnetization has long been known to exist in the lunar crust, yet both the detailed topology and ultimate origin(s) of these fields remains uncertain. Some crustal magnetic fields coincide with surface albedo anomalies, known as lunar swirls, which are thought to be formed by differential surface weathering of the regolith underlying crustal fields due to deflection of incident solar wind protons. Here, we present results from a three-dimensional, self-consistent, plasma hybrid model of the solar wind interaction with two different possible source magnetizations for the Reiner Gamma anomaly. We characterize the plasma interaction with these fields and the resulting spatial distribution of charged-particle weathering of the surface and compare these results to optical albedo measurements of Reiner Gamma. The model results constrain the proposed source magnetizations for Reiner Gamma and suggest that vertical crustal magnetic fields are required to produce the observed "dark lanes.".
Relatively recent cometary impacts at the Moon could leave unique traces of their origins: high impact velocities and volatile abundances, combined with the presence of a dust- and ice-laden coma, may thermally and mechanically process the lunar surface in ways distinct from the impact of an asteroid. Here we analytically and numerically assess the consequences of a cometary impact at the Moon by considering the combined effects of a collision by the nucleus and inner coma. Our results show that cometary impacts entrain the finest fraction of lunar soil grains (<10 μm) over regional scales (∼100–1000 km), produce large masses of vaporized material, and likely generate transient magnetic fields that could exceed the Earth’s surface field strength by a factor of . This combination of processes is consistent with a mechanism to generate lunar swirls: the diffuse, meandering disturbances in brightness and regolith texture that curl across much of the lunar far-side and are also commonly (but not exclusively) associated with magnetic anomalies. Previous observations of swirl features indicate that bright regions also possess a peculiar, altered regolith structure, which can be produced by the removal of fine soil grains. Regional scouring by an impacting comet explains both the structure and albedo variations: large dynamic pressures entrain the smallest grains within a near-surface flow of dusty plasma, disrupting the backscattering, “fairy-castle” structure of lunar soils in equilibrium with the airless environment. The resulting surface is brightened by compaction of the previously open, porous macrostructure. Darker lanes observed within swirl regions are interpreted as possible melt and/or vapor deposits. Finally, the intense magnetic fields generated during high-speed cometary impacts provide an explanation for correlations between swirl locations and magnetic anomalies.
We have provided preliminary global maps of three components of the lunar magnetic anomaly on the surface applying the surface vector mapping (SVM) method. The data used in the present study consist of about 5 million observations of the lunar magnetic field at 10–45 km altitudes by Kaguya and Lunar Prospector. The lunar magnetic anomalies were mapped at 0.2° equi-distance points on the surface by the SVM method, showing the highest intensity of 718 nT in the Crisium antipodal region. Overall features on the SVM maps indicate that elongating magnetic anomalies are likely to be dominant on the Moon except for the young large basins with the impact demagnetization. Remarkable demagnetization features suggested by previous studies are also recognized at Hertzsprung and Kolorev craters on the farside. These features indicate that demagnetized areas extend to about 1–2 radii of the basins/craters. There are well-isolated central magnetic anomalies at four craters: Leibnitz, Aitken, Jules Verne, and Grimaldi craters. Their magnetic poles through the dipole source approximation suggest occurrence of the polar wander prior to 3.3–3.5 Ga. When compared with high-albedo markings at several magnetic anomalies such as the Reiner Gamma anomalies, three-dimensional structures of the magnetic field on/near the surface are well correlated with high-albedo areas. These results indicate that the global SVM maps are useful for the study of the lunar magnetic anomalies in comparison with various geological and geophysical data.
Palaeomagnetic measurements suggest that an active core dynamo operated on the Moon from 4.2 to 3.56 billion years ago(1-3). Since the Apollo era, many magnetic anomalies have been observed on the Moon. The magnetization of the lunar crust in some of these regions could preserve the signature of an early dipolar magnetic field generated by a core dynamo. Thus, the magnetic anomalies may yield information about the position of the palaeomagnetic pole during the time that the dynamo operated. Here we present a comprehensive survey of magnetic anomalies on the lunar surface using magnetometer data(4,5) obtained by the Lunar Prospector and Kaguya lunar orbiters. We extract magnetization vectors from 24 magnetic anomalies using an iterative inversion method and derive the palaeomagnetic poles. We find that the north poles, as well as the antipodal south poles, cluster in two distinct locations: one near the present rotation axis and the other at mid-latitude. The clustering is consistent with a dipole-dominated magnetic field generated in the lunar core by a dynamo that was reversing, much like that of Earth. Furthermore, the two pole clusters imply that the Moon experienced a polar wander event during its ancient history due to the reorientation of the Moon with respect to its spin axis by 45 degrees-60 degrees.
We have developed a new method for regional mapping of the lunar magnetic anomalies as the vector field at the surface using the satellite observation, that is the surface vector mapping (SVM). The SVM is based on the inverse boundary value problem with a spherical boundary surface. There are two main procedures for reducing effects of bias and noise on mapping: (1) preprocessing the data to provide first derivatives along the pass, and (2) the Bayesian statistical procedure in the inversion using Akaike’s Bayesian Information Criterion. The SVM was applied to two regions: the northwest region of the South Pole-Aitken basin as a strong magnetic anomaly region, and the southeast region of the lunar near side as a weak magnetic anomaly region. Since the results from the different datasets of the Kaguya and Lunar Prospector observations show good consistency, characteristic features of the lunar magnetic anomalies at the surface are considered to be well estimated except for components of wavelength shorter than about 1°. From the results by the SVM, both of the regions show elongation patterns of the lunar magnetic anomalies, suggesting lineated structures of the magnetic anomaly sources.
The existence of magnetization signatures and landform modification
antipodal to young lunar impact basins is investigated further by (a)
producing more detailed regional crustal magnetic field maps at low
altitudes using Lunar Prospector magnetometer data; and (b) examining
Lunar Reconnaissance Orbiter Wide Angle Camera imagery. Of the eight
youngest lunar basins, five are found to have concentrations of
relatively strong magnetic anomalies centered within 10° of their
antipodes. This includes the polar Schrödinger basin, which is one
of the three youngest basins and has not previously been investigated in
this context. Unusual terrain is also extensively present near the
antipodes of the two largest basins (Orientale and Imbrium) while less
pronounced manifestations of this terrain may be present near the
antipodes of Serenitatis and Schrödinger. The area near the Imbrium
antipode is characterized by enhanced surface thorium abundances, which
may be a consequence of antipodal deposition of ejecta from Imbrium. The
remaining three basins either have antipodal regions that have been
heavily modified by later events (Hertzsprung and Bailly) or are not
clearly recognized to be a true basin (Sikorsky-Rittenhouse). The most
probable source of the Descartes anomaly, which is the strongest
isolated magnetic anomaly, is the hilly and furrowed Descartes terrain
near the Apollo 16 landing site, which has been inferred to consist of
basin ejecta, probably from Imbrium according to one recent sample
study. A model for the origin of both the modified landforms and the
magnetization signatures near lunar basin antipodes involving shock
effects of converging ejecta impacts is discussed.
Many of the Moon's crustal magnetic anomalies are accompanied by high
albedo features known as swirls. A leading hypothesis suggests that
swirls are formed where crustal magnetic anomalies, acting as mini
magnetospheres, shield portions of the surface from the darkening
effects of solar wind ion bombardment, thereby leaving patches that
appear bright compared with their surroundings. If this hypothesis is
correct, then magnetic field direction should influence swirl
morphology. Using Lunar Prospector magnetometer data and Clementine
reflectance mosaics, we find evidence that bright regions correspond
with dominantly horizontal magnetic fields at Reiner Gamma and that
vertical magnetic fields are associated with the intraswirl dark lane at
Airy. We use a genetic search algorithm to model the distributions of
magnetic source material at both anomalies, and we show that source
models constrained by the observed albedo pattern (i.e., strongly
horizontal surface fields in bright areas, vertical surface fields in
dark lanes) produce fields that are consistent with the Lunar Prospector
magnetometer measurements. These findings support the solar wind
deflection hypothesis and may help to explain not only the general form
of swirls, but also the finer aspects of their morphology. Our source
models may also be used to make quantitative predictions of the near
surface magnetic field, which must ultimately be tested with very low
altitude spacecraft measurements. If our predictions are correct, our
models could have implications for the structure of the underlying
magnetic material and the nature of the magnetizing field.
A re-examination of all available low-altitude LP magnetometer data confirms that magnetic anomalies are present in at least four Nectarian-aged lunar basins: Moscoviense, Mendel–Rydberg, Humboldtianum, and Crisium. In three of the four cases, a single main anomaly is present near the basin center while, in the case of Crisium, anomalies are distributed in a semi-circular arc about the basin center. These distributions, together with a lack of other anomalies near the basins, indicate that the sources of the anomalies are genetically associated with the respective basin-forming events. These central basin anomalies are difficult to attribute to shock remanent magnetization of a shocked central uplift and most probably imply thermoremanent magnetization of impact melt rocks in a steady magnetizing field. Iterative forward modeling of the single strongest and most isolated anomaly, the northern Crisium anomaly, yields a paleomagnetic pole position at 81° ± 19°N, 143° ± 31°E, not far from the present rotational pole. Assuming no significant true polar wander since the Crisium impact, this position is consistent with that expected for a core dynamo magnetizing field. Further iterative forward modeling demonstrates that the remaining Crisium anomalies can be approximately simulated assuming a multiple source model with a single magnetization direction equal to that inferred for the northernmost anomaly. This result is most consistent with a steady, large-scale magnetizing field. The inferred mean magnetization intensity within the strongest basin sources is ∼1 A/m assuming a 1-km thickness for the source layer. Future low-altitude orbital and surface magnetometer measurements will more strongly constrain the depth and/or thicknesses of the sources.
Analysis of spectra from the Clementine ultraviolet-visible and near-infrared cameras of small, immature craters and surface soils both on and adjacent to the lunar swirls at Mare Ingenii has yielded the following conclusions about space weathering at a magnetic anomaly. (1) Despite having spectral characteristics of immaturity, the lunar swirls are not freshly exposed surfaces. (2) The swirl surfaces are regions of retarded weathering, while immediately adjacent regions experience accelerated weathering. (3) Weathering in the off-swirl regions darkens and flattens the spectrum with little to no reddening, which suggests that the production of larger (>40 nm) nanophase iron dominates in these locations as a result of charged particle sorting by the magnetic field. Preliminary analysis of two other lunar swirl regions, Reiner Gamma and Mare Marginis, is consistent with our observations at Mare Ingenii. Our results indicate that sputtering/vapor deposition, implanted solar wind hydrogen, and agglutination share responsibility for creating the range in npFe0 particle sizes responsible for the spectral effects of space weathering.
The purpose of this research is to investigate the internal origin hypothesis by assuming that the crustal lunar magnetic field was generated by a paleo-dynamo process. The study is focused on four comparatively high intensity magnetic anomalies associated with high marked swirl albedo. These four formations: Reiner Gamma, Descartes Formation, Mare Marginis and Mare Ingenii, all having a similar Imbrian age, can also be fairly well modeled using simple magnetized disks at depth. Using these simple assumptions, the paleomagnetic pole positions have been determined. The modeling of these anomalies shows a cluster of paleomagnetic pole positions within a radius of about 35 degrees centered at (30S, 215E). These preliminary results are consistent with the hypothesis of a now extinct paleo-dynamo being responsible for magnetization of lunar crust. However, a more statistical analysis remains to be done over regions of weaker magnetic anomalies to be fully conclusive.
We have used multispectral images from Clementine and data from Lunar Prospector's magnetometer to conduct a survey of lunar crustal magnetic anomalies, prominent lunar swirls, and lesser known swirl markings to provide new information on the nature of swirls and their association with magnetic anomalies. We find that all swirls and swirl-like albedo patterns are associated with areas of magnetized crust, but not all areas of magnetized crust are colocated with swirl-like albedo anomalies. All observed swirls exhibit spectral characteristics similar to immature material and generally have slightly lower FeO values compared with their surroundings as determined with a multispectral iron-mapping method. We discuss these results in relation to the various hypotheses for swirl formation. The comet impact hypothesis for lunar swirls would not predict a difference in the spectrally determined FeO content between swirls and nearby ordinary surfaces. The compositional difference could be explained as a consequence of (1) magnetic shielding of the surface from the solar wind, which could produce anomalous space weathering (little darkening with limited reddening) and potentially alter the predictions of the multispectral iron-mapping algorithm while the compositional contrast could be enhanced by delivery of lower-FeO ejecta from outside the swirl; and (2) accumulation of fine plagioclase-rich dust moving under the influence of electric fields induced by solar wind interactions with a magnetic anomaly. Therefore, we cannot at present clearly distinguish between the solar wind shielding and electrostatic dust accumulation models for swirl formation. We describe future measurements that could contribute to solution of the puzzle of swirl origin.
We present Lunar Prospector magnetometer data showing observational evidence of a mini-magnetosphere at a lunar magnetic anomaly near the crater Airy, and discuss the association between the anomaly source properties and surface albedo.
1] We describe a strong crustal magnetic anomaly, recently identified in Lunar Prospector magnetometer data, that is associated with a previously unreported albedo feature near the crater Airy in the lunar nearside highlands. Other workers have demonstrated a correlation between magnetic anomalies and the enigmatic bright markings known as lunar swirls. We have used Earth-based telescopic spectra and Clementine multispectral images to investigate the compositional and optical maturity characteristics of the Airy swirl. The Airy albedo feature does not exhibit the complex sinuous structure of well-known swirls such as the Reiner Gamma Formation, but does possess a bright loop and central dark lane. Another strong magnetic anomaly, in the Apollo 16/Descartes region, corresponds to a simple diffuse bright albedo spot. On this basis we suggest that a continuum of swirl morphologies exists on the Moon, with the Airy feature representing an intermediate or incipient swirl form. Citation: Blewett, D. T., B. R. Hawke, N. C. Richmond, and C. G. Hughes (2007), A magnetic anomaly associated with an albedo feature near Airy crater in the lunar nearside highlands, Geophys. Res. Lett., 34, L24206, doi:10.1029/ 2007GL031670.
The Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) mission is a spin-off from NASA’s Medium-class Explorer (MIDEX) mission THEMIS, a five identical micro-satellite (hereafter termed “probe”) constellation in high altitude Earth-orbit since 17 February 2007. By repositioning two of the five THEMIS probes (P1 and P2) in coordinated, lunar equatorial orbits, at distances of ∼55–65 R
E
geocentric (∼1.1–12 R
L
selenocentric), ARTEMIS will perform the first systematic, two-point observations of the distant magnetotail, the solar wind, and the lunar space and planetary environment. The primary heliophysics science objectives of the mission are to study from such unprecedented vantage points and inter-probe separations how particles are accelerated at reconnection sites and shocks, and how turbulence develops and evolves in Earth’s magnetotail and in the solar wind. Additionally, the mission will determine the structure, formation, refilling, and downstream evolution of the lunar wake and explore particle acceleration processes within it. ARTEMIS’s orbits and instrumentation will also address key lunar planetary science objectives: the evolution of lunar exospheric and sputtered ions, the origin of electric fields contributing to dust charging and circulation, the structure of the lunar interior as inferred by electromagnetic sounding, and the lunar surface properties as revealed by studies of crustal magnetism. ARTEMIS is synergistic with concurrent NASA missions LRO and LADEE and the anticipated deployment of the International Lunar Network. It is expected to be a key element in the NASA Heliophysics Great Observatory and to play an important role in international plans for lunar exploration.
MESSENGER and Mariner 10 observations of Mercury's magnetic field suggest that small-scale crustal magnetic fields, if they exist, are at the limit of resolution. Large-scale crustal magnetic fields have also been suggested to exist at Mercury, originating from a relic of an internal dipole whose symmetry has been broken by latitudinal and longitudinal variations in surface temperature. If this large-scale magnetization is confined to a layer averaging 50 km in thickness, it must be magnetized with an intensity of at least 2.9 A/m. Fits to models constrained by such large-scale insolation variations do not reveal the predicted signal, and the absence of small-scale features attributable to remanence further weakens the case for large-scale magnetization. Our tests are hindered by the limited coverage to date and difficulty in isolating the internal magnetic field. We conclude that the case for large- and small-scale remanence on Mercury is weak, but further measurements by MESSENGER can decide the issue unequivocally. Across the terrestrial planets and the Moon, magnetization contrast and iron abundance in the crust show a positive correlation. This correlation suggests that crustal iron content plays a determining role in the strength of crustal magnetization.
We show presence of a mini-magnetosphere above the Reiner Gamma magnetic anomaly (RGA) region in the solar wind, using Lunar Prospector magnetometer (MAG) measurement data. RGA is one of the strongest magnetic anomalies on the Moon. Two magnetic anomalies are found from six MAG datasets at 17–40 km altitudes in the lunar wake or the geomagnetic tail lobe and are well explained by a two-dipole model. When RGA was exposed to the solar wind plasma, two MAG datasets were obtained at 27–29 km altitudes. Although the magnetic anomalies survived against the plasma pressure, they were heavily distorted in comparison with the magnetic field of the two-dipole model. Flow directions and dynamic pressures of the solar wind plasma at those periods indicate that the distortions were caused by forming a mini-magnetosphere over the RGA region in the solar wind.
Lunar Prospector (LP) electron reflectometer measurements show that
surface fields are generally weak in the large mare basalt filled impact
basins on the near side but are stronger over highland terranes,
especially those lying antipodal to young large impact basins. Between
the Imbrium and Nectaris basins, many anomalies correlate with the
Cayley and Descartes Formations. Statistical analyses show that the most
strongly magnetic nearside terranes are Cayley-type light plains, terra
materials, and pre-Imbrian craters. Light plains and terrae include
basin impact ejecta as a major component, suggesting that magnetization
effects from basin-forming impacts were involved in their formation. The
magnetization of pre-Imbrian craters, however, may be evidence of early
thermal remanence. Relatively strong, small-scale magnetic anomalies are
present over the Reiner Gamma feature on western Oceanus Procellarum and
over the Rima Sirsalis rille on the southwestern border of Procellarum.
Both Apollo subsatellite and LP data show that the latter anomaly is
nearly aligned with the rille, though LP magnetometer and reflectometer
data show that the anomaly peak is actually centered over a light plains
unit. This anomaly and the Reiner Gamma anomaly are approximately
radially aligned with the center of Imbrium, suggesting an association
with ejecta from this basin.
Unusual swirl patterns of bright and dark material on the Moon and Mercury are proposed to be remnants of collisions with gas/dust-rich regions within a cometary coma. This interpretation provides important new clues for understanding cometary fine structure, impact effects of low-density material, and the origin of certain pronounced magnetic anomalies.
Previous work has shown that the strongest concentrations of lunar
crustal magnetic anomalies are located antipodal to four large,
similarly aged impact basins (Orientale, Serenitatis, Imbrium, and
Crisium). Here, we report results of a correlation study between
magnetic anomaly clusters and geology in areas antipodal to Imbrium,
Orientale, and Crisium. Unusual geologic terranes, interpreted to be of
seismic or ejecta origin associated with the antipodal basins, have been
mapped antipodal to both Orientale and Imbrium. All three antipode
regions have many high-albedo swirl markings. Results indicate that both
of the unusual antipode terranes and Mare Ingenii (antipodal to Imbrium)
show a correlation with high-magnitude crustal magnetic anomalies. A
statistical correlation between all geologic units and regions of medium
to high magnetization when high-albedo features are present (antipodal
to Orientale) may suggest a deep, possibly seismic origin to the
anomalies. However, previous studies have provided strong evidence that
basin ejecta units are the most likely sources of lunar crustal
anomalies, and there is currently insufficient evidence to differentiate
between an ejecta or seismic origin for the antipodal anomalies. Results
indicate a strong correlation between the high-albedo markings and
regions of high magnetization for the Imbrium, Orientale, and Crisium
antipodes. Combined with growing evidence for an Imbrian age to the
magnetic anomalies, this supports a solar wind deflection origin for the
lunar swirls.
"A comprehensive review of lunar science and evolution from the
viewpoint of historical geology, based on data from both photogeologic
observations and lunar-sample analysis."
Long, arcuate magnetic field features whose origin may lie deep within the Moon's crust have been isolated from Lunar Prospector magnetic field observations from the South Pole-Aitken (SPA) basin region. These features may reflect compositional variations or tectonic responses to the SPA impact.
Unusual swirl patterns of bright and dark material on the moon and Mercury are proposed to be remnants of collisions with gas/dust-rich regions within a cometary coma. This interpretation provides important new clues for understanding cometary fine structure, impact effects of low-density material, and the origin of certain pronounced magnetic anomalies.
Conjugate gradient and sparse matrix techniques are utilized in the
solution of a geomagnetic inverse problem. Global crustal data sets
collected from low-earth orbit are quickly inverted (using a design
matrix approach) or continued to a common altitude (using a normal
matrix approach) even when using parameterizations of 10,000 or more
dipoles. The sparsity results from the rapid decay of the magnetic field
with distance from the dipole. Iterative techniques such as the
conjugate gradient save computer time and space when compared to more
direct approaches using the Householder transformation, thus allowing
problems that were intractable to all but the largest supercomputers to
be performed on workstations of only moderate power.
The high-amplitude magnetic anomalies observed by the Mars Global
Surveyor imply the presence of a large intensity of magnetization in the
Martian crust. We investigate the mathematical question of determining
the distribution of magnetization that has the smallest possible
intensity, without any assumptions about the direction of magnetization.
The greatest lower bound on intensity found in this way depends on an
assumed layer thickness. An analytical expression is discovered for the
optimal magnetization, and numerical methods are described for solving
the equations that determine the distribution. Some relatively small
scale numerical calculations illustrate the theory. These calculations
enable us to conclude, for example, that if the magnetization of Mars is
confined to a 50-km thick layer, it must be magnetized with an intensity
of at least 4.76 A/m.
Equivalent point source inversion in the rectangular coordinate system has been widely used to reduce satellite magnetic data collected at different altitudes to a common elevation over small areas. This method is based on the expression of the magnetic anomaly caused by a magnetic dipole. Such an expression derived in a spherical coordinate system by von Frese et al. [1981] is found erroneous. We point out the errors in von Frese et al.'s [1981] formulas and present the correct expression for the magnetic field of a magnetic dipole in a spherical coordinate system.
Maps of relatively strong crustal magnetic field anomalies detected at low altitudes with the magnetometer instrument on Lunar Prospector are presented. On the lunar nearside, relatively strong anomalies are mapped over the Reiner Gamma Formation on western Oceanus Procellarum and over the Rima Sirsalis rille on the southwestern border of Oceanus Procellarum. The main Rima Sirsalis anomaly does not correlate well with the rille itself but is centered over an Imbrian-aged smooth plains unit interpreted as primary or secondary basin ejecta. The stronger Reiner Gamma anomalies correlate with the locations of both the main Reiner Gamma albedo marking and its northeastward extension. Both the Rima Sirsalis and the Reiner Gamma anomalies are extended in directions approximately radial to the center of the Imbrium basin. This alignment suggests that Imbrium basin ejecta materials (lying in many cases beneath the visible mare surface) are the sources of the nearside anomalies. If so, then the albedo markings associated with the stronger Reiner Gamma anomalies may be consistent with a model involving magnetic shielding of freshly exposed mare materials from the solar wind ion bombardment. Two regions of extensive magnetic anomalies are mapped in regions centered on the Ingenii basin on the south central farside and near the crater Gerasimovic on the southeastern farside. These regions are approximately antipodal to the Imbrium and Crisium basins, respectively. The Imbrium antipode anomaly group is the most areally extensive on the Moon, while the largest anomaly in the Crisium antipode group is the strongest detected by the Lunar Prospector magnetometer. A consideration of the expected antipodal effects of basin-forming impacts as well as a combination of sample data and orbital measurements on the nearside leads to the conclusion that the most probable sources of magnetic anomalies in these two regions are ejecta materials from the respective impacts. In both regions the strongest individual anomalies correlate with swirl-like albedo markings of the Reiner Gamma class visible on available orbital photography.
We show that plowing of the lunar and mercurian regoliths by dense meteoroid swarms (the remnants of degassed comet nuclei) can be considered as the most probable mechanism of swirl formation. Frequently discussed mechanical and thermal effects of coma gas in cometary encounters with the Moon or Mercury are shown to be negligible as compared to those of the impact of a compact cometary nucleus. The result of such an impact does not differ substantially from that of denser impactors, so impacts of comets with compact nuclei can hardly be the mechanism of swirl formation. On the other hand, the projectile swarm consisting of numerous fragments of previously disrupted cometary nucleus produces many small craters and ejecta in a large area. The particles of the ejecta go through numerous collisions with each other. This may result in formation of the characteristic swirl pattern and dust component of the regolith. This can also decrease surface micro-roughness, which is consistent with photometric observations. Regolith plowing to depths up to a few meters excavates the immature regolith to the surface but cannot noticeably change the initial chemical composition of the upper layers in the area of swarm fall. This is generally in agreement with the results obtained from Clementine spectral data. Swirls are expected to be more numerous on Mercury due to more frequent swarm encounters and more dense clouds of debris in the vicinity of the Sun.
The rock magnetism and paleomagnetism of the Apollo samples is reviewed and evidence is presented for an era of strong lunar magnetic fields between 3.9 and 3.6 By. The most plausible model for these fields is a short lived lunar dynamo, which may have been driven by compositional convection associated with the freezing of a lunar core.
The results of data based modeling of the magnetospheric configuration and its response to changes in the solar wind dynamical pressure and the interplanetary magnetic field (IMF) are described. Previous models did not have a pre-defined magnetopause and were calibrated by the Kp index, and therefore did not correctly model the solar wind effects. Models are presented which include the effects of the solar wind-controlled magnetopause, the region 1 and 2 Birkeland currents, and the interconnection of the magnetospheric and solar wind fields at the boundary. They predict the most significant effects of solar wind variation on the global magnetospheric structure. On the dayside, the region 1 Birkeland currents cause major changes of the magnetic configuration as the IMF turns southward. The tail current intensity is controlled mainly by the pressure of the solar wind, although the influence of the southward IMF is clearly seen. The effects of the IMF-induced interconnection field were found to be significant.
In the reported investigation, bulk magnetization properties within two lunar surface geologic units have been inferred using low-altitude, high-resolution Apollo 16 subsatellite magnetometer data. On the basis of correlations of mapped anomalies with relatively surficial units on the central near side, a surface plate model with thickness much less than the subsatellite altitude was adopted and was used to represent the sources of largest anomalies. The results strongly suggest that directional coherence of the surface density of magnetization can occur over horizontal scales up to 100 km. Tentative evidence for a lack of directional coherence on scales greater than 100 km was found in the case of the Fra Mauro Formation.
For typical solar wind conditions, lunar magnetic anomalies with dipole moments m >> 5 x 1013 gauss-cubic centimeters will strongly deflect the solar wind, producing local plasma voids at the lunar surface. The correlation
of the largest observed anomalies (m ∼ 1016 gauss-cubic centimeters) with unusual, relatively high albedo surface features may therefore imply that solar wind ion bombardment
is an important determinant of the optical properties of the lunar surface.
Mapping of lunar crustal magnetic fields using lunar prospector electron reflectometer data
- J. S. Halekas
- D. L. Mitchell
- R. P. Lin
- S. Frey
- M. H. Acuña
- A. B. Binder
- J. S. Halekas
- D. L. Mitchell
- R. P. Lin
- S. Frey
- M. H. Acuña
- A. B. Binder
The Lunar magnetic field environment: Interpretation of new maps of the internal and external fields, Lunar Planet
- M Purucker
- T Sabaka
- N Tsyganenko
- N Olsen
- J Halekas
- M Acuña
The Lunar magnetic field environment: interpretation of new maps of the internal and external fields
- M Purucker
- T Sabaka
- N Tsyganenko
- N Olsen
- J Halekas
- M Acuña
Purucker, M., T. Sabaka, N. Tsyganenko, N. Olsen, J. Halekas,
and M. Acuña (2006), The Lunar magnetic field environment:
interpretation of new maps of the internal and external fields,
in Lunar and Planetary Science XXXVII, Abstract #1933,
Lunar and Planetary Institute.
Mapping of lunar crustal magnetic fields using lunar prospector electron reflectometer data
- Halekas
- Wilhelms