Water sorption on martian regolith analogs: Thermodynamics and near-infrared reflectance spectroscopy

The near-infrared reflectance spectra of the martian surface present strong absorption features attributed to hydration water present in the regolith. In order to characterize the relationships between this water and atmospheric vapor and decipher the physical state of water molecules in martian regolith analogs, we designed and built an experimental setup to measure near-IR reflectance spectra under martian atmospheric conditions. Six samples were studied that cover part of the diversity of Mars surface mineralogy: a hydrated ferric oxide (ferrihydrite), two igneous samples (volcanic tuff, and dunite sand), and three potential water rich soil materials (Mg-sulfate, smectite powder and a palagonitic soil, the JSC Mars-1 regolith stimulant). Sorption and desorption isotherms were measured at 243 K for water vapor pressure varying from 10⁻⁵ to ∼0.3 mbar (relative humidity: 10⁻⁴ to 75%). These measurements reveal a large diversity of behavior among the sample suite in terms of absolute amount of water adsorbed, shape of the isotherm and hysteresis between the adsorption and desorption branches. Simultaneous in situ spectroscopic observations permit a detailed analysis of the spectral signature of adsorbed water and also point to clear differences between the samples. Ferric (oxy)hydroxides like ferrihydrite or other phases present in palagonitic soils are very strong water adsorbent and may play an important role in the current martian water cycle by allowing large exchange of water between dust-covered regions and atmosphere at diurnal and seasonal scales.     

Deconvolution of lunar olivine reflectance spectra: Implications for remote compositional assessment

Lunar olivines typically contain inclusions of Cr-spinel (chromite) that influence their measured optical properties. These altered optical properties complicate modeled predictions of olivine composition from reflectance spectra. Approaches developed for inclusion-free terrestrial olivine spectra must be modified to be applied to chromite-bearing lunar olivine spectra. We present a revised approach for predicting the compositions of chromite-bearing lunar olivines using the Modified Gaussian Model (MGM). The results of this revised approach for chromite-bearing lunar olivines are consistent with previous results for terrestrial olivine reflectance spectra, and successfully predict the olivine’s composition. These results are an important step in compositional assessment of remotely-sensed olivine spectra, and are essential to ongoing investigations of that topic. Our results are based on a limited set of available lunar olivine separates, and would be strengthened by the inclusion of additional compositions.     

The volcanic history of central Elysium Planitia: Implications for martian magmatism

Central Elysium Planitia (CEP) is located south of Elysium Mons. Back to the era of the Viking orbiters, clues accumulated in favor of recent volcanism in relation with ground water release and the formation of long sub-parallel fissures. Four aqueous flood channel systems emanate from linear fissures. Recent eruptions of low viscosity lavas originate from these fissures and from low shield volcanoes. The objective of this paper is to constrain the volcanic history of this region, and to determine the chronological relationships with fluvial/erosional processes. New observations (e.g., new shield volcanoes and one new fluvial event) are summarized on a context map. Thirty-five surfaces have been dated from the count of about 15,000 impact craters. Ages have been cross-checked with relative stratigraphy when possible. A probabilistic approach has been introduced to compare similar ages and define periods of volcanic activity. Our results confirm that some volcanic features are extremely recent (∼2 My). Active periods are found at 2.5-3 My, 4.3 My, 13.5-16.2 My, 19 My, 21-32 My, 58 My, 71 My, 85-95 My, 134 My, 173 My and 234 My, not excluding the possibility that some of the gaps would be filled with additional crater counts. The volcanic activity thus extended for at least the last 250 My. The lava volumes have been estimated from the topographic modeling of the floor of depressions filled up by volcanic products, including the volumes of several large crater cavities buried under lavas (>20% of the total volume). Our new estimation of the total lava volume is 1.5 ± 0.2 × 10⁵ km³. This value corresponds to an average thickness of one hundred meters of lavas for the young volcanic plain. As a consequence, the total eruption rate at CEP, defined as the total volume of lava divided by the time of emplacement 1.4 × 10⁻²-1.8 × 10⁻² m³/s is lower than values typically estimated for terrestrial hot spots or large igneous provinces, suggesting longer inactive periods. The concept of mantle plumes responsible for terrestrial flood volcanism may not be applicable to the case of CEP and the mechanism proposed in Schumacher and Dreuer (2007) offers a plausible alternative to explain our observations.     

Thermal alteration of nontronite and montmorillonite: Implications for the martian surface

To test the effects of meteorite impacts on martian phyllosilicate deposits, we heated two smectites (nontronite and montmorillonite) to temperatures ranging from 350 °C to 1150 °C for durations of 4-24 h in two different atmospheres, under air and a steady flow of CO₂. Samples were analyzed using X-ray diffraction (XRD) and near-infrared (NIR) and mid-infrared (MIR) reflectance spectroscopy. Interlayer water was lost after heating to temperatures of ∼400 °C. Between 400 °C and 700 °C, nontronite converted to an intermediary phase which conserved the XRD pattern of untreated nontronite with the exception of the 0 0 1 peak. Nanocrystalline high-temperature phases formed for both smectites at temperatures between 700 °C and 1000 °C. Finally, after being heated to temperatures above ∼1100 °C, the samples melted and recrystallized into secondary phases. Secondary high-temperature phases included sillimanite and cristobalite for both smectites plus hematite for nontronite. NIR and MIR reflectance spectra significantly evolved with increasing temperature. NIR spectra of smectites showed that 1.4 and 1.9 μm bands decrease in intensity and disappear above 700 °C. Similarly, the 2.2-2.3 μm metal-OH band showed a decrease in intensity as well as an overall shift towards lower wavelengths (for nontronite) due to the thermal resistance of the Al-OH bond compared to the Fe-OH bond. NIR spectra of montmorillonite showed a gradual increase in band depth up to temperatures between 500 °C and 600 °C, then decreased with increasing temperature. In the MIR spectra of samples heated to temperatures above ∼1100 °C, newly formed bands confirmed the secondary phases identified by XRD. Similarities between the NIR spectra of our heated samples and the phyllosilicates in some martian craters imply that these phyllosilicates were altered by the impact-generated heat and thus were not formed post-impact. In addition, NIR reflectance spectra provide a proxy for shock temperatures of smectites up to 700 °C while MIR is optimum for identification of high-temperature phases of smectites above 700 °C.     

The ultraviolet reflectance of Enceladus: Implications for surface composition

The reflectance of Saturn’s moon Enceladus has been measured at far ultraviolet (FUV) wavelengths (115-190 nm) by Cassini’s Ultraviolet Imaging Spectrograph (UVIS). At visible and near infrared (VNIR) wavelengths Enceladus’ reflectance spectrum is very bright, consistent with a surface composed primarily of H₂O ice. At FUV wavelengths, however, Enceladus is surprisingly dark - darker than would be expected for pure water ice. Previous analyses have focused on the VNIR spectrum, comparing it to pure water ice (Cruikshank, D.P., Owen, T.C., Dalle Ore, C., Geballe, T.R., Roush, T.L., de Bergh, C., Sandford, S.A., Poulet, F., Benedix, G.K., Emery, J.P. [2005] Icarus, 175, 268-283) or pure water ice plus a small amount of NH₃ (Emery, J.P., Burr, D.M., Cruikshank, D.P., Brown, R.H., Dalton, J.B. [2005] Astron. Astrophys., 435, 353-362) or NH₃ hydrate (Verbiscer, A.J., Peterson, D.E., Skrutskie, M.F., Cushing, M., Helfenstein, P., Nelson, M.J., Smith, J.D., Wilson, J.C. [2006] Icarus, 182, 211-223). We compare Enceladus’ FUV spectrum to existing laboratory measurements of the reflectance spectra of candidate species, and to spectral models. We find that the low FUV reflectance of Enceladus can be explained by the presence of a small amount of NH₃ and a small amount of a tholin in addition to H₂O ice on the surface. The presence of these three species (H₂O, NH₃, and a tholin) appears to satisfy not only the low FUV reflectance and spectral shape, but also the middle-ultraviolet to visible wavelength brightness and spectral shape. We expect that ammonia in the Enceladus plume is transported across the surface to provide a global coating.     

Oxygen isotopes in the martian atmosphere: Implications for the evolution of volatiles

Non-thermal escape of oxygen by recombination of exospheric O₂⁺ combined with diffusive separation of gases at lower altitude provides a mechanism through which the Martian atmosphere may be enriched in ¹⁸O relative to ¹⁶O. Measurement of the abundance of ¹⁸O relative to ¹⁶O together with a determination of the turbopause may be used to develop important constraints on the history of Martian volatiles. Models for the interpretation of these data are developed and discussed in light of present information.     

Spectrum of the Sunyaev-Zel'dovich effect for high electron temperatures

We analyse in detail the spectral shape of the Sunyaev-Zel'dovich effect in the cosmic background radiation, taking into account the relativistic corrections for a hot electron gas. We calculate the displacement of the zero-signal frequency, which is especially informative in a new method for measuring the millimetric temperature of the background radiation; we also present a simple analytical expression, to be used to fit the experimental data in spectral measurements of the effect in very hot cluster gases.     

Recombination rates of O and CO on solid CO2: Implications for the composition of the martian atmosphere

An atomic oxygen flow system and a C¹⁴ radiochemical technique have been used to show that the reactions O + CO → CO₂ and O + O → O₂, are heterogeneously catalysed by solid CO₂ at 77 K.The O-CO recombination is first order in CO and inhibited by O, whereas the O-O recombination is first order in O and weakly inhibited by CO. Assuming simple first order kinetics, recombination coefficients γ_co = 1.3(±0.9) × 10^?5 and γ_O = 0.05± 0.02 are determined. A recombination mechanism involving an intermediate adsorbed CO₃ is proposed. If the kinetic results are assumed to apply under Martian surface conditions, then conversion of CO to CO₂ by reaction on the solid CO₂ at the polar caps occurs at ~10 times the total column recombination rates for homogeneous reactions previously proposed; night-side CO₂ ice clouds would also constitute an important recombination surface.     

Magnitude of global contraction on Mars from analysis of surface faults: Implications for martian thermal history

Faults provide a record of a planet’s crustal stress state and interior dynamics, including volumetric changes related to long-term cooling. Previous work has suggested that Mars experienced a pulse of large-scale global contraction during Hesperian time. Here we evaluate the evidence for martian global contraction using a recent compilation of thrust faults. Fault-related strains were calculated for wrinkle ridges and lobate scarps to provide lower and upper bounds, respectively, on the magnitude of global contraction from contractional structures observed on the surface of Mars. During the hypothesized pulse of global contraction, contractional strain of −0.007% to −0.13% is indicated by the structures, corresponding to decreases in planetary radius of 112 m to 2.24 km, respectively. By contrast, consideration of all recognized thrust faults regardless of age produces a globally averaged contractional strain of −0.011% to −0.22%, corresponding to a radius decrease of 188 m to 3.77 km since the Early Noachian. The amount of global contraction predicted by thermal models is larger than what is recorded by the faults at the surface, paralleling similar studies for Mercury and the Moon, which suggests that observations of fault populations at the surface may provide tighter bounds on planetary thermal evolution than models alone.     

Replacement of glass in the Nakhla meteorite by berthierine: Implications for understanding the origins of aluminum‐rich phyllosilicates on Mars

A scanning and transmission electron microscope study of aluminosilicate glasses within melt inclusions from the Martian meteorite Nakhla shows that they have been replaced by berthierine, an aluminum‐iron serpentine mineral. This alteration reaction was mediated by liquid water that gained access to the glasses along fractures within enclosing augite and olivine grains. Water/rock ratios were low, and the aqueous solutions were circumneutral and reducing. They introduced magnesium and iron that were sourced from the dissolution of olivine, and exported alkalis. Berthierine was identified using X‐ray microanalysis and electron diffraction. It is restricted in its occurrence to parts of the melt inclusions that were formerly glass, thus showing that under the ambient physico‐chemical conditions, the mobility of aluminum and silicon were low. This discovery of serpentine adds to the suite of postmagmatic hydrous silicates in Nakhla that include saponite and opal‐A. Such a variety of secondary silicates indicates that during aqueous alteration compositionally distinct microenvironments developed on sub‐millimeter length scales. The scarcity of berthierine in Nakhla is consistent with results from orbital remote sensing of the Martian crust showing very low abundances of aluminum‐rich phyllosilicates.     

A sensitive search for SO2 in the martian atmosphere: Implications for seepage and origin of methane

Mars was observed near the peak of the strongest SO₂ band at 1364-1373 cm⁻¹ with resolving power of 77,000 using the Texas Echelon Cross Echelle Spectrograph on the NASA Infrared Telescope Facility. The observation covered the Tharsis volcano region which may be preferable to search for SO₂. The spectrum shows absorption lines of three CO₂ isotopomers and three H₂O isotopomers. The water vapor abundance derived from the HDO lines assuming D/H = 5.5 times the terrestrial value is 12±1.0 pr. μm, in agreement with the simultaneous MGS/TES observations of 14 pr. μm at the latitudes (50° S to 10° N) of our observation. Summing of spectral intervals at the expected positions of sixteen SO₂ lines puts a 2σ upper limit on SO₂ of 1 ppb. SO₂ may be emitted into the martian atmosphere by seepage and is removed by three-body reactions with OH and O. The SO₂ lifetime, 2 years, is longer than the global mixing time 0.5 year, so SO₂ should be rather uniformly distributed across Mars. Seepage of SO₂ is less than 15,000 tons per year on Mars which is smaller than the volcanic production of SO₂ on the Earth by a factor of 700. Because CH₄/SO₂ is typically 10⁻⁴-10⁻³ in volcanic gases on the Earth, our results show seepage is unlikely to be the source of the recently discovered methane on Mars and therefore strengthen its biogenic origin.     

IRTF spectra for 17 asteroids from the C and X complexes: A discussion of continuum slopes and their relationships to C chondrites and phyllosilicates

In order to gain further insight into their surface compositions and relationships with meteorites, we have obtained spectra for 17 C and X complex asteroids using NASA’s Infrared Telescope Facility and SpeX infrared spectrometer. We augment these spectra with data in the visible region taken from the on-line databases. Only one of the 17 asteroids showed the three features usually associated with water, the UV slope, a 0.7 μm feature and a 3 μm feature, while five show no evidence for water and 11 had one or two of these features. According to DeMeo et al. (2009), whose asteroid classification scheme we use here, 88% of the variance in asteroid spectra is explained by continuum slope so that asteroids can also be characterized by the slopes of their continua. We thus plot the slope of the continuum between 1.8 and 2.5 μm against slope between 1.0 and 1.75 μm, the break at ∼1.8 μm chosen since phyllosilicates show numerous water-related features beyond this wavelength. On such plots, the C complex fields match those of phyllosilicates kaolinite and montmorillonite that have been heated to about 700 °C, while the X complex fields match the fields for phyllosilicates montmorillonite and serpentine that have been similarly heated. We thus suggest that the surface of the C complex asteroids consist of decomposition products of kaolinite or montmorillonite while for the X complex we suggest that surfaces consist of decomposition products of montmorillonite or serpentine. On the basis of overlapping in fields on the continuum plots we suggest that the CI chondrites are linked with the Cgh asteroids, individual CV and CR chondrites are linked with Xc asteroids, a CK chondrite is linked with the Ch or Cgh asteroids, a number of unusual CI/CM meteorites are linked with C asteroids, and the CM chondrites are linked with the Xk asteroids. The associations are in reasonable agreement with chondrite mineralogy and albedo data.     

Mineralogy and petrology of the Dar al Gani 476 martian meteorite: Implications for its cooling history and relationship to other shergottites

Abstract— Dar al Gani 476, the 13th martian meteorite, was recovered from the Sahara in 1998. It is a basaltic shergottitic rock composed of olivine megacrysts reaching 5 mm (24 vol%) set in a finegrained groundmass of pyroxene (59 vol%) and maskelynitized plagioclase (12 vol%) with minor amounts of accessory phases (spinel, merrillite, ilmenite). Dar al Gani 476 is similar to lithology A of Elephant Moraine A79001 (EETA79001) in petrography and mineralogy, but is distinct in several aspects. Low‐Ca pyroxenes in the Dar al Gani 476 groundmass are more magnesian (En₇₆Fs₂₁ Wo₃～En₅₈Fs₃₀Wo₁₂) than those in lithology A of EETA79001 (En₇₃Fs₂₂Wo₅～En₄₅Fs₄₃Wo₁₂), rather similar to pyroxenes in lherzolitic martian meteorites (En₇₆Fs₂₁ Wo₃～En₆₃Fs₂₂Wo₁₅). Dar al Gani 476 olivine is less magnesian and shows a narrower compositional range (Fo_76‐58) than EETA79001 olivine (Fo_81‐53), and is also similar to olivines in lherzolitic martian meteorites (Fo_74‐65). The orthopyroxene‐olivine‐chromite xenolith typical in the lithology A of EETA79001 is absent in Dar al Gani 476. It seems that Dar al Gani 476 crystallized from a slightly more primitive mafic magma than lithology A of EETA79001 and several phases (olivine, pyroxene, chromite, and ilmenite) in Dar al Gani 476 may have petrogenetic similarities to those of lherzolitic martian meteorites. Olivine megacrysts in Dar al Gani 476 are in disequilibrium with the bulk composition. The presence of fractured olivine grains in which the most Mg‐rich parts are in contact with the groundmass suggests that little diffusive modification of original olivine compositions occurred during cooling. This observation enabled us to estimate the cooling rates of Dar al Gani 476 and EETA79001 olivines, giving similar cooling rates of 0.03‐3 °C/h for Dar al Gani 476 and 0.05‐5 °C/h for EETA79001. This suggests that they were cooled near the surface (burial depth shallower than about 3 m at most), probably in lava flows during crystallization of groundmass. As is proposed for lithology A of EETA79001, it may be possible to consider that Dar al Gani 476 has an impact melt origin, a mixture of martian lherzolite and other martian rock (Queen Alexandra Range 94201, nakhlites?).     

Petrologic insights from the spectra of the unbrecciated eucrites: Implications for Vesta and basaltic asteroids

Abstract– We investigate the relationship between the petrology and visible–near infrared spectra of the unbrecciated eucrites and synthetic pyroxene–plagioclase mixtures to determine how spectra obtained by the Dawn mission could distinguish between several models that have been suggested for the petrogenesis of Vesta’s crust (e.g., partial melting and magma ocean). Here, we study the spectra of petrologically characterized unbrecciated eucrites to establish spectral observables, which can be used to yield mineral abundances and compositions consistent with petrologic observations. No information about plagioclase could be extracted from the eucrite spectra. In contrast, pyroxene dominates the spectra of the eucrites and absorption band modeling provides a good estimate of the relative proportions of low‐ and high‐Ca pyroxene present. Cr is a compatible element in eucrite pyroxene and is enriched in samples from primitive melts. An absorption at 0.6 μm resulting from Cr³⁺ in the pyroxene structure can be used to distinguish these primitive eucrites. The spectral differences present among the eucrites may allow Dawn to distinguish between the two main competing models proposed for the petrogenesis of Vesta (magma ocean and partial melting). These models predict different crustal structures and scales of heterogeneity, which can be observed spectrally. The formation of eucrite Allan Hills (ALH) A81001, which is primitive (Cr‐rich) and relatively unmetamorphosed, is hard to explain in the magma ocean model. It could only have been formed as a quench crust. If the magma ocean model is correct, then ALHA81001‐like material should be abundant on the surface of Vesta and the Vestoids.     

Searching for the source regions of martian meteorites using MGS TES: Integrating martian meteorites into the global distribution of igneous materials on Mars

Abstract— The objective of this study was to identify and map possible source regions for all 5 known martian meteorite lithologies (basalt, lherzolite, clinopyroxenite, orthopyroxenite, and dunite) using data from the Mars Global Surveyor Thermal Emission Spectrometer (MGS TES). We deconvolved the TES data set using laboratory spectra of 6 martian meteorites (Los Angeles, Zagami, ALH A77005, Nakhla, ALH 84001, and Chassigny) as end members, along with atmospheric and surface spectra previously derived from TES data. Global maps (16 pixels/degree) of the distribution of each meteorite end member show that meteorite‐like compositions are not present at or above TES detectability limits over most of the planet's dust‐free regions. However, we have confidently identified local‐scale (100s‐1000s km²) concentrations of olivine‐ and orthopyroxene‐bearing materials similar to ALH A77005, Chassigny, and ALH 84001 in Nili Fossae, in and near Ganges Chasma, in the Argyre and Hellas basin rims, and in Eos Chasma. Nakhla‐like materials are identified near the detection limit throughout the eastern Valles Marineris region and portions of Syrtis Major. Basaltic shergottites were not detected in any spatially coherent areas at the scale of this study. Martian meteorite‐like lithologies represent only a minor portion of the dust‐free surface and, thus, are not representative of the bulk composition of the ancient crust. Meteorite‐like spectral signatures identified above TES detectability limits in more spatially restricted areas (<tens of km) are targets of ongoing analysis.     

New reflectance spectra of 40 asteroids: A comparison with the previous results and an interpretation

This paper presents and discusses selected reflectance spectra of 40 Main Belt asteroids. The spectra have been obtained by the author in the Crimean Laboratory of the Sternberg Astronomical Institute (2003–2009). The aim is to search for new spectral features that characterize the composition of the asteroids’ material. The results are compared with earlier findings to reveal substantial irregularities in the distribution of the chemical-mineralogical compositions of the surface material of a number of minor planets (10 Hygiea, 13 Egeria, 14 Irene, 21 Lutetia, 45 Eugenia, 51 Nemausa, 55 Pandora, 64 Angelina, 69 Hesperia, 80 Sappho, 83 Beatrix, 92 Undina, 129 Antigone, 135 Hertha, and 785 Zwetana), which are manifest at different rotation phases. The vast majority of the analyzed high-temperature asteroids demonstrate subtle spectral features of an atypical hydrated and/or carbonaceous chondrite material (in the form of impurities or separate units), which are likely associated with the peculiarities of the formation of these bodies and the subsequent dynamic and impact processes, which lead, inter alia, to the delivery of atypical materials. Studies of 4 Vesta aboard NASA’s Dawn spacecraft have found that asteroids of similar types can form their own phyllosilicate generations provided that their surface material contains buried icy or hydrated fragments of impacting bodies. The first evidence has been obtained of a spectral phase effect (SPE) at small phase angles (≤4°) for 10 Hygiea, 21 Lutetia, and, possibly, 4 Vesta. The SPE manifests itself in an increasing spectral coefficient of brightness in the visible range with decreasing wavelength. This effect is present in the reflectance spectrum of CM2 carbonaceous material at a phase angle of 10° and absent at larger angles (Cloutis et al., 2011a). The shape of Hygeia’s reflectance spectra at low phase angles appears to be controlled by the SPE during the most part of its rotation period, which may indicate a predominantly carbonaceous chondrite composition on a part of the asteroid’s surface. For Vesta, the SPE may manifest itself in the flat or slightly concave shape of the asteorid’s reflectance spectra at some of the rotation phases, which is likely caused by the increased number of dark spots on corresponding parts of its surface.     

The survivability of phyllosilicates and carbonates impacting Stardust Al foils: Facilitating the search for cometary water

Comet 81P/Wild 2 samples returned by NASA's Stardust mission provide an unequalled opportunity to study the contents of, and hence conditions and processes operating on, comets. They can potentially validate contentious interpretations of cometary infrared spectra and in situ mass spectrometry data: specifically the identification of phyllosilicates and carbonates. However, Wild 2 dust was collected via impact into capture media at ~6 km s^?1, leading to uncertainty as to whether these minerals were captured intact, and, if subjected to alteration, whether they remain recognizable. We simulated Stardust Al foil capture conditions using a two‐stage light‐gas gun, and directly compared transmission electron microscope analyses of pre‐ and postimpact samples to investigate survivability of lizardite and cronstedtite (phyllosilicates) and calcite (carbonate). We find the phyllosilicates do not survive impact as intact crystalline materials but as moderately to highly vesiculated amorphous residues lining resultant impact craters, whose bulk cation to Si ratios remain close to that of the impacting grain. Closer inspection reveals variation in these elements on a submicron scale, where impact‐induced melting accompanied by reducing conditions (due to the production of oxygen scavenging molten Al from the target foils) has resulted in the production of native silicon and Fe‐ and Fe‐Si‐rich phases. In contrast, large areas of crystalline calcite are preserved within the calcite residue, with smaller regions of vesiculated, Al‐bearing calcic glass. Unambiguous identification of calcite impactors on Stardust Al foil is therefore possible, while phyllosilicate impactors may be inferred from vesiculated residues with appropriate bulk cation to Si ratios. Finally, we demonstrate that the characteristic textures and elemental distributions identifying phyllosilicates and carbonates by transmission electron microscopy can also be observed by state‐of‐the‐art scanning electron microscopy providing rapid, nondestructive initial mineral identifications in Stardust residues.