Article

The Temperature-Dependent Spectra of α and β Nitrogen Ice with Application to Triton

September 1993
Icarus 105(1):254-258

September 1993
105(1):254-258

DOI:10.1006/icar.1993.1122

Authors:

Bernard Schmitt

Université Grenoble Alpes

Eric Quirico

University Grenoble Alpes, France

We present measurements of the temperature dependence of the near-infrared spectra of α and β nitrogen ice, measured in transmission in 1-cm-thick samples. Our 0.9-cm-1 resolution spectra of β ice reveal that the 4650 cm-1 (2.15 μm) hand is a complex, temperature-dependent absorption, composed of at least two components. For lower temperature α ice, our spectra show a complex, temperature-dependent pattern involving several weak features in addition to the strong, narrow band we attribute to a double phonon transition. We apply our results in modeling telescopic spectra of Triton and conclude that the spectral data are consistent with a surface composed predominantly of β nitrogen ice at temperatures between 35.6 and 41 K, in the form of grains of order 1 cm or larger in size or in the form of a surface glaze with a depth of at least 6 cm.

A New Two-molecule Combination Band as a Diagnostic of Carbon Monoxide Diluted in Nitrogen Ice on Triton

Article

Jun 2019
ASTRON J

A combination band due to a mechanism whereby a photon excites two or more vibrational modes (e.g., a bend and a stretch) of an individual molecule is commonly seen in laboratory and astronomical spectroscopy. Here, we present evidence of a much less commonly seen combination band—one where a photon simultaneously excites two adjacent molecules in an ice. In particular, we present near-infrared spectra of laboratory CO/N 2 ice samples where we identify a band at 4467.5 cm ⁻¹ (2.239 μ m) that results from single photons exciting adjacent pairs of CO and N 2 molecules. We also present a near-infrared spectrum of Neptune’s largest satellite Triton taken with the Gemini-South 8.1 m telescope and the Immersion Grating Infrared Spectrograph that shows this 4467.5 cm ⁻¹ (2.239 μ m) CO–N 2 combination band. The existence of the band in a spectrum of Triton indicates that CO and N 2 molecules are intimately mixed in the ice rather than existing as separate regions of pure CO and pure N 2 deposits. Our finding is important because CO and N 2 are the most volatile species on Triton and so dominate seasonal volatile transport across its surface. Our result will place constraints on the interaction between the surface and atmosphere of Triton.

The surface compositions of Pluto and Charon

Article

Dec 2014

The surface of Pluto as it is understood on the eve of the encounter of the New Horizons spacecraft (mid-2015) consists of a spatially heterogeneous mix of solid N2, CH4, CO, C2H6, and an additional component that imparts color, and may not be an ice. The known molecular ices are detected by near-infrared spectroscopy. The N2 ice occurs in the hexagonal crystalline β-phase, stable at T > 35.6 K. Spectroscopic evidence for wavelength shifts in the CH4 bands attests to the complex mixing of CH4 and N2 in the solid state, in accordance with the phase diagram for N2 + CH4. Spectra obtained at several aspects of Pluto's surface as the planet rotates over its 6.4-day period show variability in the distribution of CH4 and N2 ices, with stronger CH4 absorption bands associated with regions of higher albedo, in correlation with the visible rotational light curve. CO and N2 ice absorptions are also strongly modulated by the rotation period; the bands are strongest on the anti-Charon hemisphere of Pluto. Longer term changes in the strengths of Pluto's absorption bands occur as the viewing geometry changes on seasonal time-scales, although a complete cycle has not been observed. The non-ice component of Pluto's surface may be a relatively refractory material produced by the UV and cosmic-ray irradiation of the surface ices and gases in the atmosphere, although UV does not generally penetrate the atmospheric CH4 to interact with the surface. Laboratory simulations indicate that a rich chemistry ensues by the irradiation of mixtures of the ices known to occur on Pluto, but specific compounds have not yet been identified in spectra of the planet. Charon's surface is characterized by spectral bands of crystalline H2O ice, and a band attributed to one or more hydrates of NH3. Amorphous H2O ice may also be present; the balance between the amorphization and crystallization processes on Charon remains to be clarified. The albedo of Charon and its generally spatially uniform neutral color indicate that a component, not yet identified, is mixed in some way with the H2O and NH3·nH2O ices. Among the many known small bodies in the transneptunian region, several share characteristics with Pluto and Charon, including the presence of CH4, N2, C2H6, H2O ices, as well as components that yield a wide variety of surface albedo and color. The New Horizons investigation of the Pluto-Charon system will generate new insight into the physical properties of the broader transneptunian population, and eventually to the corresponding bodies expected in the numerous planetary systems currently being discovered elsewhere in the Galaxy.

Distribution and Evolution of CH 4, N 2, and CO Ices on Pluto's Surface: 1995 to 1998

Article

Oct 2001

We present new near-infrared spectra of the planet Pluto obtained at Lowell Observatory on 83 nights during 1995–1998. The dense temporal sampling of our observations enables us to measure with unprecedented detail cyclical changes in the depths of methane, carbon monoxide, and nitrogen ice absorption bands, modulated by Pluto's diurnal rotation. We show that CO, N2, and weak CH4 absorption band depths exhibit rotational patterns very different from those of Pluto's visible lightcurve, unlike the strong CH4 absorption bands which are closely correlated with the visible lightcurve. Our observations are used to constrain the longitudinal distributions of the three ice species on Pluto's surface. The data also reveal a subtle, longer term strengthening of Pluto's strong near-infrared CH4 bands, which is used to constrain the latitudinal distribution of CH4 ice. We simulate the observed diurnal and seasonal spectral and photometric behavior of Pluto by means of model distributions of three terrain types. We see no evidence for changes in the distributions of Pluto's surface ices during the 1995–1998 interval.

Investigating the Condensation of Benzene (C6H6) in Titan's South Polar Cloud System with a Combination of Laboratory, Observational and Modeling Tools

Article

Full-text available

Jul 2021

We have combined laboratory, modeling, and observational efforts to investigate the chemical and microphysical processes leading to the formation of the cloud system that formed at an unusually high altitude (>250 km) over Titan's south pole after the northern spring equinox. We present here a study focused on the formation of C6H6 ice clouds at 87°S. As the first step of our synergistic approach, we have measured, for the first time, the equilibrium vapor pressure of pure crystalline C6H6 at low temperatures (134–158 K) representative of Titan's atmosphere. Our laboratory data indicate that the experimental vapor pressure values are larger than those predicted by extrapolations found in the literature calculated from higher-temperature laboratory measurements. We have used our experimental results along with temperature profiles and C6H6 mixing ratios derived from observational data acquired by the Cassini Composite Infrared Spectrometer (CIRS) as input parameters in the coupled microphysics radiative transfer Community Aerosol and Radiation Model for Atmospheres (CARMA). CARMA simulations constrained by these input parameters were conducted to derive C6H6 ice particle size distribution, gas volume mixing ratios, gas relative humidity, and cloud altitudes. The impact of the vapor pressure on the CIRS data analysis and in the CARMA simulations was investigated and resulted in both cases in benzene condensation occurring at lower altitude in the stratosphere than previously thought. In addition, the stratospheric C6H6 gas abundances predicted with the new saturation relationship are ~1000× higher than previous calculations between 150–200 km, which results in larger particle sizes.

Physical state and distribution of materials at the surface of Pluto from New Horizons LEISA imaging spectrometer

Article

Dec 2016
ICARUS

From Earth based observations Pluto is known to be the host of N2, CH4 and CO ices and also a dark red material. Very limited spatial distribution information is available from rotational visible and near-infrared spectral curves obtained from hemispheric measurements. In July 2015 the New Horizons spacecraft reached Pluto and its satellite system and recorded a large set of data. The LEISA spectro-imager of the RALPH instruments are dedicated to the study of the composition and physical state of the materials composing the surface. In this paper we report a study of the distribution and physical state of the ices and non-ice materials on Pluto's illuminated surface and their mode and degree of mixing. Principal Component analysis as well as various specific spectral indicators and correlation plots are used on the first set of 2 high resolution spectro-images from the LEISA instrument covering the whole illuminated face of Pluto at the time of the New Horizons encounter. Qualitative distribution maps have been obtained for the 4 main condensed molecules, N2, CH4, CO, H2O as well as for the visible-dark red material. Based on specific spectral indicators, using either the strength or the position of absorption bands, these 4 molecules are found to indicate the presence of 3 different types of ices: N2-rich:CH4:CO ices, CH4-rich(:CO:N2?) ices and H2O ice. The mixing lines between these ices and with the dark red material are studied using scatter plots between the various spectral indicators. CH4 is mixed at the molecular level with N2, most probably also with CO, thus forming a ternary molecular mixture that follows its phase diagram with low solubility limits. The occurrence of a N2-rich – CH4-rich ices mixing line associated with a progressive decrease of the CO/CH4 ratio tells us that a fractionation sublimation sequence transforms one type of ice to the other forming either a N2-rich – CH4-rich binary mixture at the surface or an upper CH4-rich ice crust that may hide the N2-rich ice below. The strong CH4-rich – H2O mixing line witnesses the subsequent sublimation of the CH4-rich ice lag left behind by the N2:CO sublimation (N spring-summer), or a direct condensation of CH4 ice on the cold H2O ice (S autumn). The weak mixing line between CH4-containing ices and the dark red material and the very sharp spatial transitions between these ices and this non-volatile material are probably due to thermal incompatibility. Finally the occurrence of a H2O ice – red material mixing line advocates for a spatial mixing of the red material covering H2O ice, with possibly a small amount intimately mixed in water ice. From this analysis of the different materials distribution and their relative mixing lines, H2O ice appears to be the substratum on which other ices condense or non-volatile organic material is deposited from the atmosphere. N2-rich ices seem to evolve to CH4-dominated ices, possibly still containing traces of CO and N2, as N2 and CO sublimate away. The spatial distribution of these materials is very complex.

A Tentative Identification of HCN Ice on Triton

Article

Full-text available

Jul 2010

Spectra of Triton between 1.8 and 5.5 μm, obtained in 2007 May and 2009 November, have been analyzed to determine the global surface composition. The spectra were acquired with the grism and the prism of the Infrared Camera on board AKARI with spectral resolutions of 135 and 22, respectively. The data from 4 to 5 μm are shown in this Letter and compared to the spectra of N2, CO, and CO2, i.e., all the known ices on this moon that have distinct bands in this previously unexplored wavelength range. We report the detection of a 4σ absorption band at 4.76 μm (2101 cm–1), which we attribute tentatively to the presence of solid HCN. This is the sixth ice to be identified on Triton and an expected component of its surface because it is a precipitating photochemical product of Triton's thin N2 and CH4 atmosphere. It is also formed directly by irradiation of mixtures of N2 and CH4 ices. Here we consider only pure HCN, although it might be dissolved in N2 on the surface of Triton because of the evaporation and recondensation of N2 over its seasonal cycle. The AKARI spectrum of Triton also covers the wavelengths of the fundamental (1-0) band of β-phase N2 ice (4.296 μm, 2328 cm–1), which has never been detected in an astronomical body before, and whose presence is consistent with the overtone (2-0) band previously reported. Fundamental bands of CO and CO2 ices are also present.

Laboratory data on ices, refractory carbonaceous materials and minerals relevant to Transneptunian objects and Centaurs

Chapter

Full-text available

Apr 2008

Composition, Physical State, and Distribution of Ices at the Surface of Triton

Article

Jun 1999
ICARUS

This paper presents the analysis of near-infrared observations of the icy surface of Triton, recorded on 1995 September 7, with the cooled grating spectrometer CGS4 at the United Kingdom Infrared Telescope (Mauna Kea, HI). This analysis was performed in two steps. The first step consisted of identifying the molecules composing Triton's surface by comparing the observations with laboratory transmission spectra (direct spectral analysis); this also gives information on the physical state of the components. Most of the bands in Triton's spectrum were assigned to specific vibration bands of the CH4, N2, CO, and CO2 molecules previously discovered. A detailed comparison of the frequencies of the CH4 bands confidently indicated that this molecule exists in a diluted state in solid β-N2. Three new bands peaking at 5717, 5943, and 6480 cm−1 (1.749, 1.683, and 1.543 μm, respectively) were also observed. Laboratory experiments have shown that C2H6 isolated in solid N2 fits well the second band, but this would imply the appearance of unobserved bands and thus rules out this assignment. However, C2H6 may exist in another physical state, and more experiments are necessary. No plausible candidate was found for these three bands when comparing with the spectra of nine molecules (C2H2, C2H4, C3H8, NH3, SO2, HC3N, CH3OH, NO, NO2).

Constraints on the structure and seasonal variations of Triton’s atmosphere from the 5 October 2017 stellar occultation and previous observations

Article

Full-text available

Apr 2022

Context. A stellar occultation by Neptune’s main satellite, Triton, was observed on 5 October 2017 from Europe, North Africa, and the USA. We derived 90 light curves from this event, 42 of which yielded a central flash detection. Aims. We aimed at constraining Triton’s atmospheric structure and the seasonal variations of its atmospheric pressure since the Voyager 2 epoch (1989). We also derived the shape of the lower atmosphere from central flash analysis. Methods. We used Abel inversions and direct ray-tracing code to provide the density, pressure, and temperature profiles in the altitude range ~8 km to ~190 km, corresponding to pressure levels from 9 µbar down to a few nanobars. Results. (i) A pressure of 1.18 ± 0.03 µbar is found at a reference radius of 1400 km (47 km altitude). (ii) A new analysis of the Voyager 2 radio science occultation shows that this is consistent with an extrapolation of pressure down to the surface pressure obtained in 1989. (iii) A survey of occultations obtained between 1989 and 2017 suggests that an enhancement in surface pressure as reported during the 1990s might be real, but debatable, due to very few high S/N light curves and data accessible for reanalysis. The volatile transport model analysed supports a moderate increase in surface pressure, with a maximum value around 2005-2015 no higher than 23 µbar. The pressures observed in 1995-1997 and 2017 appear mutually inconsistent with the volatile transport model presented here. (iv) The central flash structure does not show evidence of an atmospheric distortion. We find an upper limit of 0.0011 for the apparent oblateness of the atmosphere near the 8 km altitude.

Constraints on the structure and seasonal variations of Triton's atmosphere from the 5 October 2017 stellar occultation and previous observations

Preprint

Full-text available

Jan 2022

A stellar occultation by Neptune's main satellite, Triton, was observed on 5 October 2017 from Europe, North Africa, and the USA. We derived 90 light curves from this event, 42 of which yielded a central flash detection. We aimed at constraining Triton's atmospheric structure and the seasonal variations of its atmospheric pressure since the Voyager 2 epoch (1989). We also derived the shape of the lower atmosphere from central flash analysis. We used Abel inversions and direct ray-tracing code to provide the density, pressure, and temperature profiles in the altitude range $\sim$8 km to $\sim$190 km, corresponding to pressure levels from 9 {\mu}bar down to a few nanobars. Results. (i) A pressure of 1.18$\pm$0.03 {\mu}bar is found at a reference radius of 1400 km (47 km altitude). (ii) A new analysis of the Voyager 2 radio science occultation shows that this is consistent with an extrapolation of pressure down to the surface pressure obtained in 1989. (iii) A survey of occultations obtained between 1989 and 2017 suggests that an enhancement in surface pressure as reported during the 1990s might be real, but debatable, due to very few high S/N light curves and data accessible for reanalysis. The volatile transport model analysed supports a moderate increase in surface pressure, with a maximum value around 2005-2015 no higher than 23 {\mu}bar. The pressures observed in 1995-1997 and 2017 appear mutually inconsistent with the volatile transport model presented here. (iv) The central flash structure does not show evidence of an atmospheric distortion. We find an upper limit of 0.0011 for the apparent oblateness of the atmosphere near the 8 km altitude.

Composition of the surface of Pluto by spectro-imagery. New Horizons mission

Thesis

Sep 2021

Leila Gabasova

In 2015 the New Horizons spacecraft reached the Pluto system and returned unprecedentedly detailed measurements of its surface properties. These measurements have already been integrated into global reflectance, topography and narrow-band multispectral surface composition maps. However, analysis of the hyperspectral data from the Ralph/LEISA infrared spectrometer, which lets us analyse the surface composition, has until now been confined to the high-resolution encounter hemisphere of Pluto, and has only been carried out in a qualitative fashion, without a detailed analysis of the proportions and mixture types of the various components.This thesis uses the technique of intensity-based registration, commonly used in medical imagery and newly applied in planetary science, to present the first global qualitative composition maps for the main materials of Pluto's surface. We then carry out a statistical analysis of these maps and compare them with the other Pluto surface datasets to make geological interpretations of the maps. This dataset has already been used for global atmospheric modeling of Pluto and promises to be a valuable asset for further descriptions and predictions of its climate and behaviour.We also present a metaheuristic, simulated-annealing-based method for spectral inversion, using the DISORT radiative transfer model. We use this method to develop the qualitative LEISA dataset into quantitative compositional maps, and show the first few results using this method. Finally, we discuss the computational costs and runtime needed to produce the global quantitative compositional map of Pluto, and suggest algorithmic improvements to make the spectral inversion more accurate and more efficient. As the computational costs are very high, we outline a path to developing these first results into a distributed computing project that would make completing the global compositional map feasible in a few years.

Pluto's Sputnik Planitia: Composition of geological units from infrared spectroscopy

Article

Jan 2021

We have compared spectroscopic data of Sputnik Planitia on Pluto, as acquired by New Horizons' Linear Etalon Imaging Spectral Array (LEISA) instrument, to the geomorphology as mapped by White et al. (2017) using visible and panchromatic imaging acquired by the LOng-Range Reconnaissance Imager (LORRI) and the Multi-spectral Visible Imaging Camera (MVIC). We have focused on 13 of the geologic units identified by White et al. (2017), which include the plains and mountain units contained within the Sputnik basin. We divided the map of Sputnik Planitia into 15 provinces, each containing one or more geologic units, and we use LEISA to calculate the average spectra of the units inside the 15 provinces. Hapke-based modeling was then applied to the average spectra of the units to infer their surface composition, and to determine if the composition resulting from the modeling of LEISA spectra reflects the geomorphologic analyses of LORRI data, and if areas classified as being the same geologically, but which are geographically separated, share a similar composition. We investigated the spatial distribution of the most abundant ices on Pluto's surface - CH4, N2, CO, H2O, and a non-ice component presumed to be a macromolecular carbon-rich material, termed a tholin, that imparts a positive spectral slope in the visible spectral region and a negative spectral slope longward of ~1.1 μm. Because the exact nature of the non-ice component is still debated and because the negative spectral slope of the available tholins in the near infrared does not perfectly match the Pluto data, for spectral modeling purposes we reference it generically as the negative spectral slope endmember (NSS endmember). We created maps of variations in the integrated band depth (from LEISA data) and areal mass fraction (from the modeling) of the components. The analysis of correlations between the occurrences of the endmembers in the geologic units led to the observation of an anomalous suppression of the strong CH4 absorption bands in units with compositions that are dominated by H2O ice and the NSS endmember. Exploring the mutual variation of the CH4 and N2 integrated band depths with the abundance of crystalline H2O and NSS endmember revealed that the NSS endmember is primarily responsible for the suppression of CH4 absorptions in mountainous units located along the western edge of Sputnik Planitia. Our spectroscopic analyses have provided additional insight into the geological processes that have shaped Sputnik Planitia. A general increase in volatile abundance from the north to the south of Sputnik Planitia is observed. Such an increase first observed and interpreted by Protopapa et al., 2017 and later confirmed by climate modeling (Bertrand et al., 2018) is expressed geomorphologically in the form of preferential deposition of N2 ice in the upland and mountainous regions bordering the plains of southern Sputnik Planitia. Relatively high amounts of pure CH4 are seen at the southern Tenzing Montes, which are a natural site for CH4 deposition owing to their great elevation and the lower insolation they are presently receiving. The NSS endmember correlates the existence of tholins within certain units, mostly those coating the low-latitude mountain ranges that are co-latitudinal with the tholin-covered Cthulhu Macula. The spectral analysis has also revealed compositional differences between the handful of occurrences of northern non-cellular plains and the surrounding cellular plains, all of which are located within the portion of Sputnik Planitia that is presently experiencing net sublimation of volatiles, and which do not therefore exhibit a surface layer of bright, freshly-deposited N2 ice. The compositional differences between the cellular and non-cellular plains here hint at the effectiveness of convection in entraining and trapping tholins within the body of the cellular plains, while preventing the spread of such tholins to abutting non-cellular plains.

Thermal Modeling for the Triton Hopper

Article

Aug 2020
CRYOGENICS

NASA is currently designing a conceptual vehicle to explore the surface and atmosphere of Triton, Neptune’s captured Kuiper Belt Object, under a NASA Innovative Advanced Concepts (NIAC) Phase 2 study. This tholin-rich moon is dynamic, with an atmosphere, active geysers, and a unique “cantaloupe” terrain. Based on observations from Voyager-2, the surface of Triton is at temperatures between 33 – 38K and it has a slight atmosphere that varies in pressure depending on location between equator and pole. A vehicle is being proposed which relies on scavenging solid and gaseous nitrogen and for use as a propellant. Under the current conceptual design, nitrogen is collected, liquefied, gasified, pressurized, and then fed into warm gas rocket nozzles to provide thrust to hop from one location to the next. Gaseous nitrogen is collected using cryopumping. This paper presents the design and analysis of the passive thermal system for the collection tank and active cryocooler system used to cryopump nitrogen from the rarified atmosphere to a tank. Models are developed for parasitic heat leak, multi-layer insulation heat leak, and structural heat leak to determine the required cryocooler power and mass as a function of multiple parameters.

Survey of Cryogenic Nitrogen Thermo‐Mechanical Property Data Relevant to Outer Solar System Bodies

Article

Full-text available

Sep 2020

Abstract The outer Solar System has many bodies of interest that have continued to captivate the planetary science community, recently Triton, a captured Kuiper belt object (KBO) of Neptune, and Pluto. Limited fly‐by observational data shows evidence that nitrogen is the dominant constituent on these two bodies, potentially also existing on other KBOs as well. Current simulations related to answering fundamental science questions and also to develop future mission science packages and vehicles require accurate, reliable thermodynamic, and mechanical property data of solid and gaseous nitrogen at relevant surface and atmospheric conditions. This paper thus presents an exhaustive review of all available experimental N2 property data dating back to 1887. Each historical study is systematically analyzed and summarized and then the consolidated database is assembled. Comments are made on the validity of data sets, with an emphasis on specific heat capacity at constant pressure and constant volume (CP, Cv), thermal conductivity (κ), volume thermal expansion (αV), density (ρ), equilibrium vapor pressure (Pvap), heat of sublimation (ΔHS), heat of transition (ΔHT), Gruneisen parameter (γG), adiabatic and isothermal compressibility (xS, xT) and moduli of elasticity (C11, C12, C13, C33, C44). Results here can be used directly and immediately to perform new simulations on N2‐based bodies as well as to determine gaps in the consolidated database for future experiments.

Pluto's global surface composition through pixel-by-pixel Hapke modeling of New Horizons Ralph/LEISA data

Article

Apr 2016
ICARUS

On July 14th 2015, NASA's New Horizons mission gave us a first view of the Pluto system. The complex compositional diversity of Pluto's encounter hemisphere was revealed by the Ralph/LEISA infrared spectrometer on board of New Horizons. We present compositional maps of Pluto defining the spatial distribution of the abundance and textural properties of the volatiles methane and nitrogen ices and non-volatiles water ice and tholin. These results are obtained by applying a pixel-by-pixel Hapke radiative transfer model to the LEISA scans. Our analysis focuses mainly on the large scale latitudinal variations of methane and nitrogen ices and aims at setting observational constraints to volatile transport models. Our findings are consistent with expectations of vigorous spring sublimation after a long polar winter. The latitudinal pattern is broken by Sputnik Planum, a large depository of volatiles, with nitrogen playing the most important role. The physical properties of methane and nitrogen in this region are suggestive of the presence of a cold trap or possible volatile stratification. Furthermore our modeling results point to a possible flow of nitrogen from the northwest edge of Sputnik Planum toward the south.

Longitudinal variability of ethane ice on the surface of Pluto

Article

Jun 2014

We present the results of an investigation using near-infrared spectra of Pluto taken on 72 separate nights using SpeX/IRTF. These data were obtained between 2001 and 2013 at various sub-observer longitudes. The aim of this work was to confirm the presence of ethane ice and to determine any longitudinal trends on the surface of Pluto. We computed models of the continuum near the 2.405 {\mu}m ethane band using Hapke theory and calculated an equivalent width of the ethane absorption feature for six evenly-spaced longitude bins and a grand average spectrum. Ethane on Pluto was confirmed at the 7.5-{\sigma} level from the grand average spectrum. Additionally, ethane was found to vary longitudinally with the highest abundance present in the N2-rich region and the lowest abundance found in the visibly dark tholin-rich region. We argue for ethane production in the atmosphere and present a theory of volatile transport to explain the observed longitudinal trend.

Optical Constants of NH3 and NH3:N2 Amorphous Ices in the Near-infrared and Mid-infrared Regions

Article

Full-text available

Nov 2013

Ammonia ice has been detected on different astrophysical media ranging from interstellar medium (ISM) particles to the surface of various icy bodies of our solar system, where nitrogen is also present. We have carried out a detailed study of amorphous NH3 ice and NH3:N2 ice mixtures, based on infrared (IR) spectra in the mid-IR (MIR) and near-IR (NIR) regions, supported by theoretical quantum chemical calculations. Spectra of varying ice thicknesses were obtained and optical constants were calculated for amorphous NH3 at 15 K and 30 K and for a NH3:N2 mixture at 15 K over a 500-7000 cm–1 spectral range. These spectra have improved accuracy over previous data, where available. Moreover, we also obtained absolute values for the band strengths of the more prominent IR features in both spectral regions. Our results indicate that the estimated NH3 concentration in ISM ices should be scaled upward by ~30%.

The Ices on Transneptunian Objects and Centaurs

Article

Jan 2013

Transneptunian objects (TNOs) and Centaurs are small bodies orbiting the Sun in the cold outer regions of the Solar System. TNOs include Pluto and its satellite Charon, and Neptune's large satellite Triton is thought to have been captured from the TNO population. Visible and near-infrared spectroscopy of a number of the brightest of these bodies shows surface ices of H2O, CH4, N2, CH3OH, C2H6, CO, CO2, NH3•nH2O, and possibly HCN, in various combinations; water ice is by far the most common. Silicate minerals and solid complex carbonaceous materials are thought to occur on these bodies, but their spectral signatures have not yet been positively identified. The pronounced red color of several TNOs and Centaurs is presumed to result from the presence of carbonaceous materials. In all, the TNOs and Centaurs are thought to be primitive bodies in the sense that they have undergone relatively little modification by heating and by the space environment since their condensation in the volatile-rich outer regions of the solar nebula. As such, they hold the potential to yield important information on the chemical and physical conditions of the solar nebula. Continued and expanded studies of TNOs and Centaurs require additional basic laboratory data on the physical and the optical properties of the ices already identified and those candidate materials that have not yet been confirmed. New sky surveys and large telescopes projected for operation in the near future will reveal many more objects in the outer Solar System for detailed study.

HEAT AND MASS TRANSPORT IN NITROGEN ICE WITH APPLICATION TO PLUTO AND TRITON. http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/22228/1/97-0696.pdf

Article

Full-text available

Jan 1997

Classical,and’’super”solid-state,greenhouses,have,been,suggested,asmech- anismsfor the solar energy,supply,to Tritonian geyser-like plumes,(Brown et al. 1990). In this work we evaluate solar and internal (owing to radioactive decay of U, 7’hand 401{in Pluto’s inferred core) heat,sources,and,their corresponding,mechanisms,for predicted,erup- tive activity on Pluto. For the internal energy supply, a model of conductive-convective heat,and,mass,transport,on Triton (Duxburyand,Brown,1997) is applied to Pluto’s solid N2 layer. Previous,models,of the solid-state convection,for different,celestial bodies,con- sidered,only simpllayer. Convection,in a seasonal,N2 layer on Triton and Pluto is plausible even without the solid-state greenhouse effect, because N2 on these bodies,is so close to its melting,temperature,of= 63.148 K(at zero pressure) that an upper,stagnant,layer does not form. Since d~r~ ~~ < 0.3pmfor the fresh transparent,N2

Optical Properties of Low Temperature Water Ice

Article

Jan 1996

The surfaces of many outer solar system satellites are predominantly composed of low temperature water ice. Spectra and phase curves of light reflected from their surfaces have previously been interpreted by means of multiple scattering models (\eg\ Buratti 1985, Verbiscer and Veverka 1990, Domingue et al. 1995). These radiative transfer models require as inputs the optical constants of H_2O ice. Optical constants compiled by Warren (1984) are frequently used, and these values are appropriate for temperatures within 60 K of the melting point. However, the optical constants of water ice vary with temperature, as shown by Fink and Larson (1975). We have measured the near-infrared spectral transmission of monocrystalline samples of water ice at cryogenic temperatures and will present the results of these measurements. Radiative transfer models of icy satellite surfaces also depend on a variety of simplifying assumptions about the optical and mechanical character of a satellite's surface. Using Monte-Carlo simulations of the scattering of photons within individual ice grains and in surfaces composed of multiple ice grains, we have been exploring the sensitivity of reflectance to some of the effects frequently ignored in radiative transfer models. These include effects due to physical contact between partially sintered grains, and effects due to the polarization of light. We will present results from our simulations, as well as from laboratory experiments involving well-characterized samples of granular ice, carried out to test the accuracy of our approach.

A Monte Carlo ray-tracing model for scattering and polarization by large particles with complex shapes

Article

Dec 2000

We present a new model based on Monte Carlo ray tracing which simulates scattering and linear polarization by particles with arbitrary shapes and sizes much larger than the wavelength. The model, called S-Scat, provides a powerful tool for exploring the relationship between actual physical properties of large particles and the single-particle parameters used in multiple-scattering radiative transfer models, and will be particularly valuable for studies of icy outer solar system surfaces. We describe the model algorithm and apply the model to examine absorption and scattering behavior of single, irregular particles as functions of particle size, shape, and optical constants. Single-scattering albedos are investigated first, with results used to test the validity of the widely used equivalent slab model. Single-scattering phase functions are examined next, along with possibilities of parameterization via simple analytic expressions. Finally, the behaviors of linear polarization functions are explored, along with internal path lengths and the spatial distribution of scattered light in close proximity to the particle.

The temperature-dependent near-infrared absorption spectrum of hexagonal H2O ice

Article

Full-text available

Oct 1998

Transmission spectra were measured between 1.0 and 2.7 mum for monocrystalline samples of hexagonal water ice at temperatures between 20 and 270 K. Samples were crystallized from liquid water within closed cells, with thicknesses ranging from 100 mum to 1.0 cm. The absorption spectrum of ice changes with temperature in several ways. With higher temperature, the shapes of absorption bands become more smoothed, the strengths of some absorption bands decrease, the absorption in continuum wavelengths increases, and the band centers of some absorption bands shift to shorter wavelengths. In this paper we present the new absorption coefficient spectra along with an examination of the different temperature effects. These data should prove extremely valuable for analysis of near-infrared reflectance spectra of low-temperature icy surfaces, such as those of outer solar system satellites, Kuiper Belt objects, Pluto and Charon, comet nuclei, the polar caps of Mars, and terrestrial snow- and ice-covered regions. The data may also be of value in simulating radiative transfer in clouds of ice particles in the atmospheres of planets.

New Horizons: NASA's Pluto-Kuiper Belt Mission

Article

Jan 2008

The New Horizons (NH) mission was selected by NASA in November 2001 to conduct the first in situ reconnaissance of Pluto and the Kuiper belt. The NH spacecraft was launched on January 19, 2006, received a gravity assist from Jupiter during closest approach on February 28, 2007, and is currently heading for a flyby encounter with the Pluto system. NH will study thePluto system for nearly seven months beginning in early 2015, with closest approach currently planned for mid-July 2015 at an altitude of ~12,500 km above Pluto's surface. If NASA approves an extended mission phase, the NH spacecraft will be targeted toward a flyby encounter with one or more small (~50 km diameter) Kuiper belt objects (KBOs) after the Pluto flyby. The NH spacecraft has a total dry mass of only 400 kg and was launched with 76.8 kg of hydrazine propellant to provide in-flight trajectory correction and spacecraft attitude control. The launch performance was virtually flawless, so less fuel was used for trajectory correction than originally budgeted, which means that more fuel should be available for targeting KBOs beyond the Pluto system. NH carries a sophisticated suite of seven scientific instruments, altogether weighing approximately 30 kg and drawing less than 30 W of power, which includes panchromatic and color imagers, ultraviolet and infrared spectral imagers, a radio science package, plasma and charged particle sensors, and a dust counting experiment. These instruments enable the first detailed exploration of a new class of solar system objects, the dwarf planets, which have exotic volatiles on their surfaces, escaping atmospheres, and satellite systems. NH will also provide the first dust density measurements beyond 18 AU and cratering records that document both the ancient and present-day collisional environment in the outer solar system down to sizes of tens of meters. In addition, NH is the first principal-investigator-led mission to be launched to the outer solar system, potentially opening the door to other nontraditional exploration of the outer solar system in the future.

Near-Infrared Spectral Monitoring of Pluto's Ices: Spatial Distribution and Secular Evolution

Article

Jan 2013

We report results from monitoring Pluto's 0.8 to 2.4 {\mu}m reflectance spectrum with IRTF/SpeX on 65 nights over the dozen years from 2001 to 2012. The spectra show vibrational absorption features of simple molecules CH4, CO, and N2 condensed as ices on Pluto's surface. These absorptions are modulated by the planet's 6.39 day rotation period, enabling us to constrain the longitudinal distributions of the three ices. Absorptions of CO and N2 are concentrated on Pluto's anti-Charon hemisphere, unlike absorptions of less volatile CH4 ice that are offset by roughly 90{\deg} from the longitude of maximum CO and N2 absorption. In addition to the diurnal variations, the spectra show longer term trends. On decadal timescales, Pluto's stronger CH4 absorption bands have been getting deeper, while the amplitude of their diurnal variation is diminishing, consistent with additional CH4 absorption at high northern latitudes rotating into view as the sub-Earth latitude moves north (as defined by the system's angular momentum vector). Unlike the CH4 absorptions, Pluto's CO and N2 absorptions appear to be declining over time, suggesting more equatorial or southerly distributions of those species. Comparisons of geometrically-matched pairs of observations favor geometric explanations for the observed secular changes in CO and N2 absorption, although seasonal volatile transport could be at least partly responsible. The case for a volatile transport contribution to the secular evolution looks strongest for CH4 ice, despite it being the least volatile of the three ices.

Near-infrared spectral monitoring of Triton with IRTF/SpeX I: Establishing a baseline for rotational variability

Article

Dec 2004
ICARUS

We present eight new 0.8 to 2.4 µm spectral observations of Neptune's satellite Triton, obtained at IRTF/SpeX during 2002 July 15– 22 UT. Our objective was to determine how Triton's near-infrared spectrum varies as Triton rotates, and to establish an accurate baseline for comparison with past and future observations. The most striking spectral change detected was in Triton's nitrogen ice absorption band at 2.15 µm; its strength varies by about a factor of two as Triton rotates. Maximum N 2 absorption approximately coincides with Triton's Neptune-facing hemisphere, which is also the longitude where the polar cap extends nearest Triton's equator. More subtle rotational variations are reported for Triton's CH 4 and H 2 O ice absorption bands. Unlike the other ices, Triton's CO 2 ice absorption bands remain nearly constant as Triton rotates. Triton's H 2 O ice is shown to be crystalline, rather than amorphous. Triton's N 2 ice is confirmed to be the warmer, hexagonal, β N 2 phase, and its CH 4 is confirmed to be highly diluted in N 2 ice.

Triton's surface ices: Distribution, temperature and mixing state from VLT/SINFONI observations

Article

Nov 2018

Triton, the largest Solar System satellite beyond Saturn, was probably captured from the Transneptunian population by Neptune. It is mainly covered by N2, CO, CO2, CH4 and H2O in solid state and, except for H2O and CO2, these species are also present in gas phase. Sublimation and recondensation of the volatile species may lead to geographical and temporal variation on the surface composition, and could participate to the formation of complex chemical compounds formed from photochemistry occurring in the atmosphere (Krasnopolsky and Cruikshank, 1999) or from irradiation of N2 : CH4 : CO layers (Moore and Hudson, 2003). We present new near-IR observations performed at the VLT-ESO with SINFONI in 2010, 2011 and 2013, from which band depths, areas and positions of the main ice features are determined at different longitudes. We also re-reduce and re-analyze earlier data obtained during the last 20 years. Models based on the Hapke theory (Hapke, 1993) are developed to constrain the abundance, grain size, temperature, and state of mixing of the different ices (N2, CH4, CO, CO2, H2O) as well as attempt to identify other species. For this purpose, we present and use new optical constants of CO2 measured at 35 and 54 K. Our analyses confirm the longitudinal variation of the N2 and CO surface abundances previously evidenced, and suggest additional latitudinal and/or temporal variability of these two species. We confirm the presence of deep N2 layers in which CO and CH4 are diluted. In particular, we demonstrate that CO is present in diluted (as opposed to pure ice) form. In contrast, our models support the presence of small and longitudinally variable amounts of pure CH4 ice, providing an explanation to the enhanced atmospheric CH4/N2 ratio over expectations based on an ideal N2-CH4-CO mixture. They also suggest significant smaller particles of H2O and CO2 than reported previously in Quirico et al. (1999), with CO2 being probably distributed over a large area of the surface. We find that the 2.40 µm band is primarily due to ¹³CO ice, with a telluric value of the ¹³CO/¹²CO ratio, and not to ethane. We infer a N2 ice temperature of 37.5 ± 1K, suggesting that the atmospheric pressure over 2010–2013 was similar to that during the Voyager 1989 epoch.

Solids and fluids at low temperatures

Chapter

Jul 2016

Spectroscopy from Space

Article

Dec 2013

This chapter reviews detection of materials on solid and liquid (lakes and ocean) surfaces in the solar system using ultraviolet to infrared spectroscopy from space, or near space (high altitude aircraft on the Earth), or in the case of remote objects, earth-based and earth-orbiting telescopes. Point spectrometers and imaging spectrometers have been probing the surfaces of our solar system for decades. Spacecraft carrying imaging spectrometers are currently in orbit around Mercury, Venus, Earth, Mars, and Saturn, and systems have recently visited Jupiter, comets, asteroids, and one spectrometer-carrying spacecraft is on its way to Pluto. Together these systems are providing a wealth of data that will enable a better understanding of the composition of condensed matter bodies in the solar system.Minerals, ices, liquids, and other materials have been detected and mapped on the Earth and all planets and/or their satellites where the surface can be observed from space, with the exception of Venus whose thick atmosphere limits surface observation. Basaltic minerals (e.g., pyroxene and olivine) have been detected with spectroscopy on the Earth, Moon, Mars and some asteroids. The greatest mineralogic diversity seen from space is observed on the Earth and Mars. The Earth, with oceans, active tectonic and hydrologic cycles, and biological processes, displays the greatest material diversity including the detection of amorphous and crystalline inorganic materials, organic compounds, water and water ice.Water ice is a very common mineral throughout the Solar System and has been unambiguously detected or inferred in every planet and/or their moon(s) where good spectroscopic data has been obtained.In addition to water ice, other molecular solids have been observed in the solar system using spectroscopic methods. Solid carbon dioxide is found on all systems beyond the Earth except Pluto, although CO2 sometimes appears to be trapped in other solids rather than as an ice on some objects. The largest deposits of carbon dioxide ice are found on Mars. Sulfur dioxide ice is found in the Jupiter system. Nitrogen and methane ices are common beyond the Uranian system.Saturn's moon Titan probably has the most complex active extra-terrestrial surface chemistry involving organic compounds. Some of the observed or inferred compounds include ices of benzene (C6H6), cyanoacetylene (HC3N), toluene (C7H8), cyanogen (C2N2), acetonitrile (CH3CN), water (H2O), carbon dioxide (CO2), and ammonia (NH3). Confirming compounds on Titan is hampered by its thick smoggy atmosphere, where in relative terms the atmospheric interferences that hamper surface characterization lie between that of Venus and Earth.In this chapter we exclude discussion of the planets Jupiter, Saturn, Uranus, and Neptune because their thick atmospheres preclude observing the surface, even if surfaces exist. However, we do discuss spectroscopic observations on a number of the extra-terrestrial satellite bodies. Ammonia was predicted on many icy moons but is notably absent among the definitively detected ices with possible exceptions on Charon and possible trace amounts on some of the Saturnian satellites. Comets, storehouses of many compounds that could exist as ices in their nuclei, have only had small amounts of water ice definitively detected on their surfaces from spectroscopy. Only two asteroids have had a direct detection of surface water ice, although its presence can be inferred in others.

Observed Ices in the Solar System

Article

Apr 2013

Ices have been detected and mapped on the Earth and all planets and/or their satellites further from the sun. Water ice is the most common frozen volatile observed and is also unambiguously detected or inferred in every planet and/or their moon(s) except Venus. Carbon dioxide is also extensively found in all systems beyond the Earth except Pluto although it sometimes appears to be trapped rather than as an ice on some objects. The largest deposits of carbon dioxide ice is on Mars. Sulfur dioxide ice is found in the Jupiter system. Nitrogen and methane ices are common beyond the Uranian system. Saturn's moon Titan probably has the most complex active chemistry involving ices, with benzene (C6H6) and many tentative or inferred compounds including ices of Cyanoacetylene (HC3N), Toluene (C7H8), Cyanogen (C2N2), Acetonitrile (CH3CN), H2O, CO2, and NH3. Confirming compounds on Titan is hampered by its thick smoggy atmosphere. Ammonia was predicted on many icy moons but is notably absent among the definitively detected ices with the possible exception of Enceladus. Comets, storehouses of many compounds that could exist as ices in their nuclei, have only had small amounts of water ice definitively detected on their surfaces. Only one asteroid has had a direct detection of surface water ice, although its presence can be inferred in others. This chapter reviews some of the properties of ices that lead to their detection, and surveys the ices that have been observed on solid surfaces throughout the Solar System.

Near-Infrared Absorption Coefficients of Solid Nitrogen as a Function of Temperature

Article

Aug 1995
ICARUS

We present the results of laboratory measurements characterizing the near infrared spectrum of solid nitrogen at temperatures between 35 K and 60 K. The measurements show that the appearance of the spectrum in the regions of both the fundamental vibrational transition and its first overtone is temperature dependent. The temperature dependence of the spectrum in the overtone region provides a means of determining the temperature of N-2 ice on Solar System objects using groundbased spectroscopy. (C) 1995 Academic Press, Inc.

Surface-atmosphere coupling on Triton and Pluto

Article

Jan 1994

John A. Stansberry

Sublimation of volatile ices and convection play important roles in determining the present and past climates of Neptune's large moon, Triton, and Pluto. The author has developed models of these two processes and used the distribution of albedo on the surfaces of these two bodies to study surface temperatures, distribution of volatile ices, and lower atmospheric structure. Initial studies focused on Triton, which was encountered by Voyager 2 in 1989. One of the surprising results is that Triton's South Polar Cap is considerably larger than predicted by the model. Another basic result is that the volatile N2 ice on Triton's surface has a low thermal emissivity (approximately equal to 0.7) relative to canonical emissivity values, which are near unity. Some ambiguity in the thermal structure of Triton's atmosphere resulted from the encounter. By modeling the convective transport of heat between the surface and atmosphere it is shown that the near-surface atmospheric temperature was close to the low end of the expected range; previous analyses of the occultation of a star by Pluto in 1988 may have erroneously concluded that Pluto's radius is approximately 1200 km. The current results, while not ruling out that conclusion, show that Pluto could be much smaller than 1200 km and the atmosphere could still have produced the observed occultation lightcurve. A smaller surface radius, combined with the occultation lightcurve, implies that Pluto possesses a troposphere, which has never been considered before. The remaining piece of the Pluto atmosphere puzzle is the somewhat anomalous atmospheric composition required to explain the temperature structure derived from the occultation results. By expanding earlier Triton work on the distribution of N2 ice to include the physics of simultaneous sublimation of N2 and CH4, the required 'anomalous' atmospheric composition is shown to be totally reasonable. Synthesizing these results with other recent work, a new and testable paradigm for Pluto's atmosphere is proposed. Bibtex entry for this abstract Preferred format for this abstract (see Preferences) Find Similar Abstracts: Use: Authors Title Keywords (in text query field) Abstract Text Return: Query Results Return items starting with number Query Form Database: Astronomy Physics arXiv e-prints

Outer planets and their major satellites: Atmospheric and surface properties

Article

Jan 1995

J. I. Lunine

The outer planets and their satellites constitute a broad area of investigation in planetary science, and one in which progress has been a result both of high visibility spacecraft explorations as well as novel opportunities and technological improvements in ground-based studies. The time period from 1991-1994 was dominated by detailed analyses of data from the Voyager flyby, radar and sensitive infrared techniques applied to the Galilean satellites and Titan, reanalysis and interpretation of stellar occultations of Titan and Pluto, and increasingly sensitive spectroscopic measurements from the ground of all these objects at a range of wavelengths. Meanwhile, the July 1994 impacts of Comet Shoemaker-Levy 9 fragments into Jupiter, as well as the arrival in 1995 of Galileo, has led to an increased interest in this largest planet.

A coupled microphysical/radiative transfer model of albedo and emissivity of planetary surfaces covered by volatile ices

Article

Dec 2003

A new model of albedo and emissivity of planetary surfaces covered by volatile ices in the form of porous slab-like deposits is described. In the model, a radiative transfer model is coupled with a microphysical model in order to link changes in albedo and emissivity to changes in porosity caused by ice metamorphism. Preliminary results for Triton, Pluto, and Io are presented (the martian CO2 caps will be the subject of a separate publication). The coupled model will aid in the interpretation of ground-based and spacecraft observations and should lead to advances in surface and atmospheric modeling.

Near-Infrared Spectroscopy of Simple Hydrocarbons and Carbon Oxides Diluted in Solid N 2and as Pure Ices: Implications for Triton and Pluto

Article

Jun 1997

In this paper are presented near-infrared laboratory spectra of pure ices of CH4, C2H4, C2H6, CO, and CO2, as well as a systematic study of changes in their spectral behavior when isolated in a matrix of nitrogen ice. These studies were prompted by recent low-noise and high-spectral-resolution infrared observations of the surfaces of Triton and Pluto (e.g., D. P. Cruikshanket al.,1993,Science261,742–745; T. C. Owenet al., Science261,745–748). The data in this paper permit a sophisticated analysis of the published Pluto and Triton spectra, and will be useful in interpreting future observations as well. Two different techniques were employed for preparing our ice samples: (1) condensation of thin films on a cold window, and (2) growth of crystals from the liquid phase in a closed cryogenic cell. An important result we obtained is that spectra strongly depend on the technique used. With a closed cell, samples are formed under conditions of thermodynamical equilibrium and experiments are perfectly reproducible. Additionally, the samples formed from mixtures of N2and CH4, in the closed cell, show characteristics consistent with the N2:CH4phase diagram obtained by A. I. Prokhvatilov and L. D. Yantsevich (1982,Sov. J. Low Temp. Phys.9,94–98). On the other hand, it appeared that this is not at all the case with thin films. Assuming that the surfaces of Triton and Pluto are in thermodynamical equilibrium, the closed cell technique is more appropriate. Finally, the measurements conducted with different closed cryogenic cells show that the peak frequencies of the bands of the CH4molecule isolated in solid N2are shifted with respect to pure CH4ice and are also dependent on the temperature and crystal phase of solid N2(α vs β). These dependencies have been precisely measured, and it is shown how they could be used to determine the CH4dilution state and the temperature at the surfaces of Triton and Pluto to higher precision than has been previously achieved.

Charon: More than Water Ice?

Article

Apr 1994

T. L. Roush

A significant non-H2O ice component may be present on the surface of Pluto's satellite Charon and yet remain undetected by existing observations. This suggestion arises from a comparison of calculated reflectance spectra with Charon's 1.5- to 2.5-μm reflectance spectrum. The calculated spectra rely upon descriptions of the interaction of light scattered from particulate surfaces and the optical constants of H2O, CH4, and CO2 ices. Calculated spectra of mixtures composed of H2O and CO2 ice remain consistent with the observed spectrum of Charon for high abundances of CO2 (≈50% relative mass fraction) in intimate mixtures, and for areal coverages of about 40% CO2 in spatial mixtures. Calculations for mixtures of H2O and CH4 ice indicate that ≥5% relative mass fraction of CH4 in intimate mixtures and ≥5-10% areal coverage of CH4 in spatial mixtures result in spectra that cannot reproduce the observed Charon spectrum. Calculated spectra of three-component intimate mixtures of H2O, CH4, and CO2 ices with similar grain sizes can fit the observed spectrum of Charon only for low abundances of CH4 (≤ 5%). If the CH4 ice grain size is much greater than the other components, then the spectrum of Charon can be modeled by calculated spectra containing up to ≈30% CH4 in the intimate mixtures. Calculated spectra for spatial mixtures of H2O, CH4, and CO2 ices indicate that ≤5-10% areal coverage of CH4 can be incorporated and remain consistent with the observational data. The suggestion of significant amounts of non-H2O components on Charon can be tested as Earth-based telescopic instrumentation improves. This suggestion should be considered during instrumental design for spacecraft destined for the Pluto-Charon system.

Spectroscopy of some ices of astrophysical interest: SO 2, N 2 and N 2: CH 4 mixtures

Article

Sep 1996
PLANET SPACE SCI

Near infrared spectroscopic observations of icy surfaces provide powerful keys to identify specific molecules, and to derive information about the physical and chemical states of the surface ices. In particular, the high spectral resolution recently achievable in astronomical spectra, opens a new insight, but also implies that the complete analysis of these spectra requires careful spectroscopic studies, i.e. clean and systematic laboratory experiments associated with a rigorous interpretation of the spectra. Spectroscopic interpretation is focused on, taking into consideration the specific physical aspects of some ices (molecular solids). It is shown how this analysis allows specific astrophysical problems to be solved. At first, some relevant fundamentals of physics and spectroscopy of molecular solids are presented. The spectroscopy of these solids largely belongs to molecular physics, but also involves solid state effects (Davydov splitting, LO-TO splitting, etc.) which have to be considered to correctly assign spectra, as well as to understand the behaviour of the spectral profile of the bands as a function of various physical parameters. The specific treatment needed to explain the structure of combination and overtone bands occurring in the whole infrared range is particularly focused on. Those theoretical considerations are applied to two different problems concerning surface ices: the first one deals with the identification of two narrow SO2 bands on Io (Schmitt et al., Icarus111, 79–105, 1994), the second one with the physical state of N2 ice on Triton and Pluto. In a second step, the first results are presented of a systematic spectroscopic study in the near infrared on the two-phase system N2: CH4 for CH4 concentrations ranging from 0.1 to 10%. This study was initiated with the view of investigating the question of the physical state of the surface of Pluto. It is shown that it is possible to investigate the N2: CH4 phase diagram using the spectral profile of both the ν1 + ν4 and the ν3 + ν4 bands of CH4. Finally, the physical parameters (temperature, crystalline phase, etc.) that are expected to be extracted from a detailed analysis of near infrared observations of icy planetary surfaces are briefly reviewed.

Cooperative two-phonon absorption in solid α-nitrogen in the 4600–4700 cm −1 region

Article

Jun 1996
CHEM PHYS

The spectrum of pure α-nitrogen was recorded with a Fourier transform spectrometer in the near infrared. Two bands at 4657.3 and 4618.7 cm−1, previously studied at low resolution by Grundy et al. (Icarus 105 (1993) 254.), were recorded at 5 K at high resolution and analyzed. The effect of the temperature, impurities and crystal defects was investigated at lower resolution. The first band, due to a simultaneous excitation of two vibrons, exhibits a peculiar shape with several singularities. The vibron energy and the induced dipole moments have been calculated as a function of the crystal wave vector and used in the standard two-phonon theory. By changing somewhat these coefficients from their ab initio calculated values, the shape and the intensity are fairly well reproduced. The second band, narrower and much weaker, is due to the simultaneous excitation of the molecules in the 14N15N,N2 couples.

The temperature of nitrogen on Pluto

Article

Jan 1994

With Hapke scattering theory and absorption coefficients derived from our laboratory measurements of solid N2 we have modeled the spectrum of Triton. By comparing a Hapke scattering model to the measured spectrum from Triton, we determined the temperature of the N2 on the satellite's surface to be 38 (+2, -1) K which is in accord with the measurements of Voyager 2. Applying this technique to Pluto we find that the temperature of N2 on that body is 40 +/- 2 K. Other aspects of this investigation are discussed.

The 2140 cm −1 (4.673 Microns) Solid CO Band: The Case for Interstellar O 2 and N 2 and the Photochemistry of Nonpolar Interstellar Ice Analogs

Article

Apr 1997

The infrared spectra of CO frozen in nonpolar ices containing N2, CO2, O2, and H2O and the UV photochemistry of these interstellar/precometary ice analogs are reported. The spectra are used to test the hypothesis that the narrow 2140 cm⁻¹ (4.673 μm) interstellar absorption feature attributed to solid CO might be produced by CO frozen in ices containing nonpolar species such as N2 and O2. It is shown that mixed molecular ices containing CO, N2, O2, and CO2 provide a good match to the interstellar band at all temperatures between 12 and 30 K both before and after photolysis. The optical constants (real and imaginary parts of the index of refraction) in the region of the solid CO feature are reported for several of these ices. The N2 and O2 absorptions at 2328 cm⁻¹ (4.296 μm) and 1549 cm⁻¹ (6.456 μm), respectively, are also shown. The best matches between the narrow interstellar band and the feature in the laboratory spectra of nonpolar ices are for samples which contain comparable amounts of N2, O2, CO2, and CO. Co-adding the CO band from an N2:O2:CO2:CO=1:5::1 ice with that of an H2O:CO = 20:1 ice provides an excellent fit across the entire interstellar CO feature. The four-component, nonpolar ice accounts for the narrow 2140 cm⁻¹ portion of the feature which is associated with quiescent regions of dense molecular clouds. Using this mixture, and applying the most recent cosmic abundance values, we derive that between 15% and 70% of the available interstellar N is in the form of frozen N2 along several lines of sight toward background stars. This is reduced to a range of 1%-30% for embedded objects with lines of sight more dominated by warmer grains. The cosmic abundance of O tied up in frozen O2 lies in the 10%-45% range toward background sources, and it is between 1% and 20% toward embedded objects. The amount of oxygen tied up in CO and CO2 frozen in nonpolar ices can be as much as 2%-10% toward background sources and on the order of 0.2%-5% for embedded objects. Similarly 3%-13% of the carbon is tied up in CO and CO2 frozen in nonpolar ices toward field stars, and 0.2%-6% toward embedded objects. These numbers imply that most of the N is in N2, and a significant fraction of the available O is in O2 in the most quiescent regions of dense clouds. Ultraviolet photolysis of these ices produces a variety of photoproducts including CO2, N2O, O3, CO3, HCO, H2CO, and possibly NO and NO2. XCN is not produced in these experiments, placing important constraints on the origin of the enigmatic interstellar XCN feature. N2O and CO3 have not been previously considered as interstellar ice components.

Nature of infrared-active phonon sidebands to internal vibrations: Spectroscopic studies of solid oxygen and nitrogen

Article

Sep 2002

The ir-active phonon sidebands to internal vibrations of oxygen and nitrogen were precisely investigated by Fourier transform infrared spectroscopy in the fundamental and first overtone spectral regions from 10 K to the boiling points at ambient pressure. We showed that an analysis of ir-active phonon sidebands yields important information on the internal vibrations of molecules in a condensed medium (solid or liquid), being complementary to Raman data on vibron frequencies. Analyzing the complete profile of these bands, we determined the band origin frequencies and explored their temperature behavior in all phases of both substances. We present unambiguous direct experimental proofs that this quality corresponds to the frequency of internal vibrations of single molecules. Considering solid oxygen and nitrogen as two limiting cases for simple molecular solids, we interpret this result as a strong evidence for a general fact that an ir-active phonon sideband possesses the same physical origin in pure molecular solids and in impurity centers. The key characteristics of the fundamental vibron energy zone (environmental and resonance frequency shifts) were deduced from the combined analysis of ir and Raman experimental data and their temperature behavior was explored in solid and liquid phases of oxygen and nitrogen at ambient pressure. The character of the short-range orientational order was established in the β-nitrogen based on our theoretical analysis consistent with the present experimental results. We also present the explanation of the origin of pressure-caused changes in the frequency of the Raman vibron mode of solid oxygen at low temperatures.

Orientational order in the molecular solids N_ {2}, CO, and O_ {2} investigated via combined excitations with FTIR

Article

Jan 2007
Phys Rev B

We present results of studies of very weak absorption bands in the mid infrared (IR), lying in the overtone region, of solid N2, CO, O2, and a N2-CO mixture. The IR activity is induced in a molecule when two neighboring molecules interact and vibrate out of phase simultaneously: (0-1)(0-1) excitation. From the temperature dependence of band intensities and bandwidths, we were able to evaluate quantitative measures of order/disorder phenomena, i.e., long range orientational ordering and/or short range correlation between molecules. The long range order parameter for ordered phases, which we determined by optical spectroscopy, agrees with values η(T) found by x-ray Bragg scattering or nuclear quadrupole resonance data. We were able to model the temperature dependence of the order parameter by assuming a specific interaction of neighboring molecules (dipole-dipole, quadrupole-quadrupole, magnetic) and/or correlation between molecules.

Optical Properties of Ices From UV to Infrared

Article

Full-text available

Jan 1998

Remote sensing of ices at the surfaces and in the atmospheres of system solar objects are the subject of increasing studies in the UV, visible and infrared ranges. The spectro-imagers and spectrophotometers aboard space probes will further expand these studies. One critical problem for the interpretation of the astronomical absorption and emission spectra is the availability of laboratory data on the optical properties of the relevant ices. After a discussion of the different types of observations and their specific spectral ranges, we review the different types of laboratory measurements of the optical properties of ices and discuss the problem of optical constant calculation in each case. The various physical parameters (i.e. phase, crystalline quality, temperature and thermal history, isotopes) that influence the spectra of pure ices are analyzed. Similarly, we discuss the optical properties of mixtures and their dependence on the type of mixture (solid solution, specific compound or multi-phase system) as well as on various physical parameters (temperature, composition, phase, thermodynamical state). A brief summary of the available optical properties of ices and mixtures of planetary interest is followed by an assessement of what is still unknown (or poorly known) in the field. Finally, we discuss the use of laboratory data in reflectance and emittance models.

Methane and Nitrogen Ices on Pluto and Triton: a Combined Laboratory and Telescope Investigation.

Article

Dec 1994

W. M. Grundy

In this dissertation, the surfaces of Pluto and Triton were investigated by means of spectroscopy over the visible and near infrared wavelength range from 0.5 to 2.5 mum. In this spectral region, the spectra of both bodies are dominated by solar continuum light reflected from their surfaces, interrupted by absorption bands due to the ices of methane, nitrogen, carbon monoxide, and carbon dioxide, in decreasing order of spectral significance. New spectroscopic observations of Pluto and Triton were made at the University of Arizona's 1.54 meter telescope on Mt. Bigelow and 2.3 meter telescope on Kitt Peak. These data were combined with earlier, unpublished observations and with data from the literature. In particular, the Pluto+Charon spectra span a dozen years from 1983 to 1994, the full 360^circ range of subsolar longitudes, and subsolar latitudes from -10 to +13^circ. Laboratory studies were undertaken to measure the spectral properties of nitrogen and methane ices at temperatures comparable to the surface temperatures of Pluto and Triton. These data were used as inputs to radiative transfer models in order to interpret the telescope spectra. The abundances of various ice species on Pluto and Triton were estimated under a wide range of model assumptions. The relative strengths of different methane absorption bands were examined, revealing an enhancement of weak bands relative to stronger bands. Various multiple scattering effects which might be responsible for the observed enhancement were simulated. The spectrum of Pluto's satellite Charon was shown to be consistent with a composition of dirty water ice. Finally, variations in the spectrum of Pluto with rotational phase were examined and used to constrain the geographic distributions of ices on Pluto's surface.

The Surface Compositions of Triton, Pluto, and Charon

Article

May 1995

Neptune's satellite Triton, and the planet-satellite binary Pluto and Charon, are the most distant planetary bodies on which ices have been directly detected. Triton and Pluto have very similar dimensions and mean densities, suggesting a similar or common origin. Through earth-based spectroscopic observations in the near-infrared, solid N2, CH4, and CO have been found on both bodies, with the additional molecule C02 on Triton. N2 dominates both surfaces, although the coverage is not spatially uniform. On Triton, the CH4 and CO are mostly or entirely frozen in the N2 matrix, while CO2 may be spatially segregated. On Pluto, some CH4 and the CO are frozen in the N2 matrix, but there is evidence for additional CH4 in a pure state, perhaps lying as a lag deposit on a subsurface layer of N2. Despite their compositional and dimensional similarities, Pluto and Triton are quite different from one another in detail. Additional hydrocarbons and other volatile ices have been sought spectroscopically but not yet have been detected. The only molecule identified on Pluto's satellite Charon is solid H2O, but the spectroscopic data are of low precision and admit the presence of other ices such as CH4.

Methane and Nitrogen Abundances On Pluto and Eris

Article

Full-text available

Nov 2010

We present spectra of Eris from the MMT 6.5 m Telescope and Red Channel Spectrograph (5700-9800 Å, 5 Å pixel–1) on Mt. Hopkins, AZ, and of Pluto from the Steward Observatory 2.3 m Telescope and Boller and Chivens Spectrograph (7100-9400 Å, 2 Å pixel–1) on Kitt Peak, AZ. In addition, we present laboratory transmission spectra of methane-nitrogen and methane-argon ice mixtures. By anchoring our analysis in methane and nitrogen solubilities in one another as expressed in the phase diagram of Prokhvatilov & Yantsevich, and comparing methane bands in our Eris and Pluto spectra and methane bands in our laboratory spectra of methane and nitrogen ice mixtures, we find Eris' bulk methane and nitrogen abundances are ~10% and ~90% and Pluto's bulk methane and nitrogen abundances are ~3% and ~97%. Such abundances for Pluto are consistent with values reported in the literature. It appears that the bulk volatile composition of Eris is similar to the bulk volatile composition of Pluto. Both objects appear to be dominated by nitrogen ice. Our analysis also suggests, unlike previous work reported in the literature, that the methane and nitrogen stoichiometry is constant with depth into the surface of Eris. Finally, we point out that our Eris spectrum is also consistent with a laboratory ice mixture consisting of 40% methane and 60% argon. Although we cannot rule out an argon-rich surface, it seems more likely that nitrogen is the dominant species on Eris because the nitrogen ice 2.15 μm band is seen in spectra of Pluto and Triton.

The Abundances of Solid N2 and Gaseous CO2 in Interstellar Dense Molecular Clouds

Article

Dec 2008

We present 2338-2322 cm-1 (4.277-4.307 μm) infrared spectra of a number of N2-containing mixed molecular ices and demonstrate that the strength of the infrared "forbidden" band due to the N≡N stretch near 2328 cm-1 (4.295 μm) is extremely sensitive to the composition of the ice. The strength of the 2328 cm-1 N2 fundamental is significantly enhanced relative to that of pure N2 ice when NH3, H2O, or CO2 are present, but it is largely unaffected by the presence of CO, CH4, or O2. We use the laboratory data in conjunction with Infrared Space Observatory (ISO) data that probe several lines of sight through dense molecular clouds to place limits on the abundance of interstellar solid phase N2 and the composition of the ices. Deriving upper limits is complicated by the presence of overlapping absorptions due to CO2 gas in the clouds and, in some cases, to photospheric CO in the background star. These upper limits are just beginning to be low enough to constrain interstellar grain models and the composition of possible N2-bearing interstellar ices. We outline the search criteria that will need to be met if solid interstellar N2 is to be detected in the future. We also discuss some of the implications of the presence of warm CO2 gas along the lines of sight to embedded protostars and demonstrate that its presence may help resolve certain puzzles associated with the previously derived gas/solid CO2 ratios and the relative abundances of polar and nonpolar ices toward these objects. Finally, we briefly comment on the possible implications of these results for the interpretation of N2 detections on outer solar system bodies.

New Horizons: Anticipated Scientific Investigations at the Pluto System

Chapter

Full-text available

Feb 2009

The New Horizons spacecraft will achieve a wide range of measurement objectives at the Pluto system, including color and panchromatic maps, 1.25–2.50 micron spectral images for studying surface compositions, and measurements of Pluto’s atmosphere (temperatures, composition, hazes, and the escape rate). Additional measurement objectives include topography, surface temperatures, and the solar wind interaction. The fulfillment of these measurement objectives will broaden our understanding of the Pluto system, such as the origin of the Pluto system, the processes operating on the surface, the volatile transport cycle, and the energetics and chemistry of the atmosphere. The mission, payload, and strawman observing sequences have been designed to achieve the NASA-specified measurement objectives and maximize the science return. The planned observations at the Pluto system will extend our knowledge of other objects formed by giant impact (such as the Earth–moon), other objects formed in the outer solar system (such as comets and other icy dwarf planets), other bodies with surfaces in vapor-pressure equilibrium (such as Triton and Mars), and other bodies with N2:CH4 atmospheres (such as Titan, Triton, and the early Earth).

Spectroscopy of Icy Moon Surface Materials

Article

Jun 2010

James B Dalton

Remote sensing of icy objects in the outer solar system relies upon availability of appropriate laboratory measurements. Surface deposits of specific substances often provide our most direct route to understanding interior composition, thereby informing theories of endogenic surface modification, exogenic surface processing and processes involving exchange of material with the interiors. Visible to near-infrared reflectance spectra of properly prepared compounds are required to enable retrieval of surface abundances through linear and nonlinear mixture analysis applied to spacecraft observations of icy bodies. This chapter describes the techniques, conditions and approaches necessary to provide reference spectra of use to theoretical models of icy satellite surface compositions, and summarizes the current state of knowledge represented in the published literature. KeywordsIce-Infrared spectroscopy-Remote sensing-Planetary science

Chemical Composition of Icy Satellite Surfaces

Article

Jun 2010

Much of our knowledge of planetary surface composition is derived from remote sensing over the ultraviolet through infrared wavelength ranges. Telescopic observations and, in the past few decades, spacecraft mission observations have led to the discovery of many surface materials, from rock-forming minerals to water ice to exotic volatiles and organic compounds. Identifying surface materials and mapping their distributions allows us to constrain interior processes such as cryovolcanism and aqueous geochemistry. The recent progress in understanding of icy satellite surface composition has been aided by the evolving capabilities of spacecraft missions, advances in detector technology, and laboratory studies of candidate surface compounds. Pioneers 10 and 11, Voyagers I and II, Galileo, Cassini and the New Horizons mission have all made significant contributions. Dalton (Space Sci. Rev., 2010, this issue) summarizes the major constituents found or inferred to exist on the surfaces of the icy satellites (cf. Table1 from Dalton, Space Sci. Rev., 2010, this issue), and the spectral coverage and resolution of many of the spacecraft instruments that have revolutionized our understanding (cf. Table2 from Dalton, Space Sci. Rev., 2010, this issue). While much has been gained from these missions, telescopic observations also continue to provide important constraints on surface compositions, especially for those bodies that have not yet been visited by spacecraft, such as Kuiper Belt Objects (KBOs), trans-Neptunian Objects (TNOs), Centaurs, the classical planet Pluto and its moon, Charon. In this chapter, we will discuss the major satellites of the outer solar system, the materials believed to make up their surfaces, and the history of some of these discoveries. Formation scenarios and subsequent evolution will be described, with particular attention to the processes that drive surface chemistry and exchange with interiors. Major similarities and differences between the satellites are discussed, with an eye toward elucidating processes operating throughout the outer solar system. Finally we discuss the outermost satellites and other bodies, and summarize knowledge of their composition. Much of this review is likely to change in the near future with ongoing and planned outer planet missions, adding to the sense of excitement and discovery associated with our exploration of our planetary neighborhood. KeywordsComposition-Icy satellites-Infrared spectroscopy

Synoptic CCD Spectrophotometry of Pluto Over the Past 15 Years

Article

Nov 1996
ICARUS

Over the 15 years from 1980 to 1994, the same spectrometer has been used to obtain spectrophotometric observations of Pluto+Charon over the wavelength range from 0.5 to 1.0 μm. This time period spanned Pluto's perihelion passage in 1989, as well as the Pluto–Charon mutual event season. The data set is presented and a search made for variations in Pluto's methane absorptions and continuum slope correlating with Pluto's 6.4-day lightcurve as well as possible longer term secular evolution.Four quantities derived from Pluto's spectrum are examined for variation with Pluto's rotational phase. Although the depths of the methane bands are generally deeper away from the lightcurve minimum, they are found not to correlate in a simple way with the lightcurve, in contrast with the behavior of the stronger methane bands at longer wavelengths. This behavior implies that Pluto's CH4is not exclusively associated with the brightest or the darkest terrain, and that weak and strong methane bands are sensitive to different methane reservoirs on Pluto's surface. The wavelengths of CH4bands also change, being longest at the rotational phases where the CH4bands are deepest. The wavelength shifts can be attributed to variable concentration of CH4dissolved in N2ice. No statistically significant trend of bandwidth with longitude is detected. It is confirmed that the reddest slope of the continuum coincides with the darkest surface terrain. Examining spectral data pairs taken at similar Pluto longitudes, a modest but consistent secular trend is seen, with the depth of the 0.73-μm CH4absorption diminishing relative to that of the stronger 0.89-μm band.The observations are interpreted by means of Hapke scattering models. While unique solutions are not possible, it is found that the observed spectra can be matched by geographically diverse models possessing both methane rich and methane poor terrains. Terrain types proposed are a high albedo, CH4poor, N2dominated surface, a dark, red,tholin-rich surface, and a transitional zone, rich in CH4, with an intermediate albedo. The slight secular weakening of the 0.73-μm CH4band can be interpreted as resulting from differences between Pluto's southern and northern hemispheres or from a temporal evolution in the geographic proportions of the CH4rich and CH4poor terrains.

Voyager photometry of Triton - Haze and surface photometric properties

Article

Full-text available

Nov 1991

The Voyager whole-disk observations of Triton at 0.41, 0.48, and 0.56 micron filter wavelengths are analyzed using a model which combines an improved version of Hapke's photometric equation with a thin atmospheric haze layer in the appropriate spherical geometry. The model is shown to describe accurately the phase curves over a range of phase angles and to agree with disk-resolved brightness scans along the photometric equator and mirror meridian. According to the model, the photometric parameters of Triton's regolith are reasonably typical of icy satellites, except for the extremely high (close to unity) single-scattering albedo.

The absorption coefficient of the liquid N2 2.15-micron band and application to Triton

Article

Sep 1991

The present measurements of the temperature dependence exhibited by the liquid N2 2.15-micron 2-0 collision-induced band's absorption coefficient and integrated absorption show the latter to be smaller than that of the N2 gas, and to decrease with decreasing temperature. Extrapolating this behavior to Triton's nominal surface temperature yields a new estimate of the N2-ice grain size on the Triton south polar cap; a mean N2 grain size of 0.7-3.0 cm is consistent with grain growth rate calculation results.

Optical constants of solid methane and ethane from 10,000 to 450 cm-1

Article

Sep 1991

We made thin film transmission measurements of solid CH4 (phases I and II) and C2H6 (phase II). From these, the complex indices of refraction at near- and mid-infrared wavelengths were determined by using a combined least squares and Kramers-Kronig analysis.

Vibrational absorption spectrum of solid CO in the first harmonic region. Two-phonon transition

Article

Feb 1982
CHEM PHYS

Infrared absorption spectrum of CO crystal at low temperature, recorded in the Δυ = 2 region, shows a strong Δυ = 2 zero phonon line, a broad phonon side band and a small line which is attributed to the simultaneous vibrational transitions between two molecules. The absorption coefficient and line frequency of this two-phonon line is calculated by a theory taking account both the induced dipole moment between molecular couples and the Fermi resonance between the ν = 2 level and the two-phonon level. It is shown that most of the two-phonon line intensity comes from the Fermi resonance effect. Good agreement with experiment is obtained. The linewidth is given by the convolution product of two 0 → 1 line shapes.

Low-temperature infrared absorption of gaseous N2 and N2 + H2 in the 2.0–2.5 μm region: Application to the atmospheres of Titan and Triton

Article

Aug 1989
ICARUS

A. R. W. McKellar

Collision-induced spectra of pure N2 and N2 + H2 mixtures in the 2.0–2.5 μm spectral region are observed in the laboratory at low temperatures appropriate to the atmospheres of Titan and Triton. The N2 first overtone (v = 2−0) band near 2.16 μm is studied at 97.5°K and found to have a peak absorption of 3.2 × 10−8 cm−1 amagat−2 at 4630 cm−1 and a full width at half-maximum of about 84 cm−1. The N2 enhancement of the fundamental (v = 1−0) band of H2 in the 2.05–2.45 μm region is studied at 77°K in both para- and normal H2. In the case of Titan, it is shown that the pure N2 and N2 + H2 absorptions are likely to be significant and of roughly comparable strength given the accepted H2 abundance. For Triton, where much less is currently known about the atmosphere, it is shown that the observed N2 absorption feature at 2.16 μm may be due to the gas rather than to liquid nitrogen as previously supposed. These results emphasize the value of long-path, low-temperature laboratory data for interpreting spectra of the outer planets and their satellites.

Triton's surface-atmosphere energy balance

Article

Oct 1992

We explore the energetics of Triton's surface-atmosphere system using a model that includes the turbulent transfer of sensible heat as well as insolation, reradiation, and latent heat transport. The model relies on a 1° by 1° resolution hemispheric bolometric albedo map of Triton for determining the atmospheric temperature, the N2 frost emissivity, and the temperatures of unfrosted portions of the surface consistent with a frost temperature of ≅38 K. For a physically plausible range of heat transfer coefficients, we find that the atmospheric temperature roughly 1 km above the surface is approximately 1 to 3 K hotter than the surface. Atmospheric temperatures of 48 K suggested by early analysis of radio occultation data cannot be obtained for plausible values of the heat transfer coefficients. Our calculations indicate that Triton's N2 frosts must have an emissivity well below unity in order to have a temperature of ≅38 K, consistent with previous results. We also find that convection over small hot spots does not significantly cool them off, so they may be able to act as continous sources of buoyancy for convective plumes, but have not explored whether the convection is vigorous enough to entrain particulate matter thereby forming a dust devil. Our elevated atmospheric temperatures make geyser driven plumes with initial upward velocities ≤10 m s−1 stagnate in the lower atmosphere. These “wimpy” plumes provide a possible explanation for Triton's “wind streaks.”

Detection of a CH4 atmosphere on Pluto

Article

Oct 1980

Using a low-resolution spectrograph and a CCD array, a spectrum of Pluto from 0.58 to 1.06 μm was obtained. The spectrum had a resolution of and a signal-to-noise ratio of ∼300. It showed CH4 absorption bands at 6200, 7200, 7900, 8400, 8600, 8900 and 10,000 Å. The strongest of these bands was at 8900 Å with an absorption depth of 0.23. This band was heavily saturated, compared to the weaker bands, providing proof for the gaseous origin of the observed absorptions. By applying CH4 band model parameters to our data, a total CH4 abundance of 80 ± 20 m-am was derived. This translates into a one-way abundance of 27 ± 7 m-am and a CH4 surface pressure of 1.5 × 10−4 atm. An upper limit to the total pressure of ∼0.05 atm could be set. First-order calculations on atmospheric escape showed that this methane atmosphere would be stable if the mass of Pluto is increased 50% over its current value and its radius is 1400 km. Alternatively a heavier gas mixed with the CH4 atmosphere would aid its stability. The relatively large amount of gaseous CH4 observed implies that the absorption bands recently reported at 1.7 and 2.3 μm are likely due to atmospheric CH4 absorptions rather than surface frost as interpreted earlier.

Nitrogen on Triton

Article

May 1984

The near-infrared spectrum of Triton is characterized by strong absorption bands of methane, probably in the solid state. An additional absorption band at 2.16 μm is tentatively identified as the density-induced (2-0) band of molecular nitrogen in the liquid state. The fundamental overtones of this band system cannot presently be observed because of limitations of the terrestrial atmosphere or spectral signal precision. Using the absorption coefficient for this band derived from laboratory observations and from the literature, it is calculated that Triton must have a layer of nitrogen at least tens of centimeters deep over much of its surface; this quantity is plausible in terms of the cosmic abundance of nitrogen and by comparison with Titan where a massive atmosphere of nitrogen exists. The Triton spectrum has been modeled with liquid nitrogen and solid methane, and it is found that the shape of the continuum in two spectral regions can be properly accounted for by adding a spectral component corresponding to fine-grained water frost. It is speculated that yet another component, a dark, solid, photochemical derivative of methane, may occur as a trace contaminant of the surface materials. If much of the surface of Triton is liquid, the radiometric observations of the satellite must be reinterpreted to derive the radius and surface albedo. If there is liquid nitrogen exposed on the surface, the atmosphere of Triton is probably dominated by nitrogen rather than methane because of the much higher vapor pressure of the former. At the calculated subsolar temperature of Triton, the vapor pressure of nitrogen implies a surface atmospheric pressure in the range 0.13 to 0.30 atm.

On the microphysical state of the surface of Triton

Article

Nov 1991

Janusz Eluszkiewicz

The microphysical processes involved in the pressureless sintering of particulate materials and the physical conditions likely to prevail on Triton are examined in order to investigate the processes leading to the frost metamorphism on Triton. It is argued that the presence of a well-annealed transparent nitrogen layer offers a natural explanation for most of what is seen on the surface of Triton; results of observations suggest that such a layer can form on Triton at 37 K on a seasonal time scale (about 100 earth years), provided the initial grain diameter is less than 1 micron. Grains up to 10 microns are allowed if grain growth does not hinder densification.

Triton's surface properties - A preliminary analysis from ground-based, Voyager photopolarimeter subsystem, and laboratory measurements

Article

Nov 1991

The surface properties of Triton were investigated using data from the ground-based and Voyager photopolarimeter subsystem (PPS) observations of Triton's phase curve. The results indicate that Triton has a high single-scattering albedo (0.96 +/-0.01 at 0.75 micron) and an unusually compacted surface, possibly similar to that of Europa. Results also suggest that Triton's single-particle phase function and the macroscopically rough character of its surface are similar to those of most other icy satellites.

Global color and albedo variations on Triton

Article

Oct 1990

Alfred S. Mcewen

Global multispectral mosaics of Triton have been produced from Voyager approach images; six spectral units are defined and mapped. The margin of the south polar cap (SPC) is scalloped and ranges in latitude from + 10 deg to -30 deg. A bright fringe is closely associated with the cap's margin; form it, diffuse bright rays extend north-northeast for hundreds of kilometers. Thus, the rays may consist of fringe materials that were redistributed by northward-going Coriolis-deflected winds. From 1977 to 1989, Triton's full-disk spectrum changed from markedly red and UV-dark to nearly neutral white and UV-bright. This spectral change can be explained by new deposition of nitrogen frost over both the northern hemisphere and parts of a formerly redder SPC. Frost deposition in the southern hemisphere during southern summer is possible over relatively high albedo areas of the cap (Stansberry et al., 1990), which helps to explain the apparent stability of the unexpectedly large SPC and the presence of the bright fringe.

Infrared Observations of the Neptunian System

Article

Jan 1990

The infrared interferometer spectrometer on Voyager 2 obtained thermal emission spectra of Neptune with a spectral resolution of 4.3 cm(-1). Measurements of reflected solar radiation were also obtained with a broadband radiometer sensitive in the visible and near infrared. Analysis of the strong C(2)H(2) emission feature at 729 cm(-1) suggests an acetylene mole fraction in the range between 9 x 10(-8) and 9 x 10(-7). Vertical temperature profiles were derived between 30 and 1000 millibars at 70 degrees and 42 degrees S and 30 degrees N. Temperature maps of the planet between 80 degrees S and 30 degrees N were obtained for two atmospheric layers, one in the lower stratosphere between 30 and 120 millibars and the other in the troposphere between 300 and 1000 millibars. Zonal mean temperatures obtained from these maps and from latitude scans indicate a relatively warm pole and equator with cooler mid-latitudes. This is qualitatively similar to the behavior found on Uranus even though the obliquities and internal heat fluxes of the two planets are markedly different. Comparison of winds derived from images with the vertical wind shear calculated from the temperature field indicates a general decay of wind speed with height, a phenomenon also observed on the other three giant planets. Strong, wavelike longitudinal thermal structure is found, some of which appears to be associated with the Great Dark Spot. An intense, localizd cold region is seen in the lower stratosphere, which does not appear to be correlated with any visible feature. A preliminary estimate of the effective temperature of the planet yields a value of 59.3 +/- 1.0 kelvins. Measurements of Triton provide an estimate of the daytime surface temperature of 38(+3)(-4) kelvins.

The Temperature-Dependent Spectra of α and β Nitrogen Ice with Application to Triton

Abstract

No full-text available

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