Thermal behavior of horizontally mixed surfaces on Mars

Current methods for deriving thermal inertia from spacecraft observations of planetary brightness temperature generally assume that surface properties are uniform for any given observation or co-located set of observations. As a result of this assumption and the nonlinear relationship between temperature and thermal inertia, sub-pixel horizontal heterogeneity may yield different apparent thermal inertia at different times of day or seasons. We examine the effects of horizontal heterogeneity on Mars by modeling the thermal behavior of various idealized mixed surfaces containing differing proportions of either dust, sand, duricrust, and rock or slope facets at different angles and azimuths. Latitudinal effects on mixed-surface thermal behavior are also investigated. We find large (several 100 J m⁻² K⁻¹ s^−1/2) diurnal and seasonal variations in apparent thermal inertia even for small (∼10%) admixtures of materials with moderately contrasting thermal properties or slope angles. Together with similar results for layered surfaces [Mellon, M.T., Putzig, N.E., 2007. Lunar Planet. Sci. XXXVIII. Abstract 2184], this work shows that the effects of heterogeneity on the thermal behavior of the martian surface are substantial and may be expected to result in large variations in apparent thermal inertia as derived from spacecraft instruments. While our results caution against the over-interpretation of thermal inertia taken from median or average maps or derived from single temperature measurements, they also suggest the possibility of using a suite of apparent thermal inertia values derived from single observations over a range of times of day and seasons to constrain the heterogeneity of the martian surface.     

Apparent thermal inertia and the surface heterogeneity of Mars

Thermal inertia derivation techniques generally assume that surface properties are uniform at horizontal scales below the footprint of the observing instrument and to depths of several decimeters. Consequently, surfaces with horizontal or vertical heterogeneity may yield apparent thermal inertia which varies with time of day and season. To investigate these temporal variations, we processed three Mars years of Mars Global Surveyor Thermal Emission Spectrometer observations and produced global nightside and dayside seasonal maps of apparent thermal inertia. These maps show broad regions with diurnal and seasonal differences up to 200 J m⁻² K⁻¹s^−1/2 at mid-latitudes (60° S to 60° N) and 600 J m⁻² K⁻¹s^−1/2 or greater in the polar regions. We compared the seasonal mapping results with modeled apparent thermal inertia and created new maps of surface heterogeneity at 5° resolution, delineating regions that have thermal characteristics consistent with horizontal mixtures or layers of two materials. The thermal behavior of most regions on Mars appears to be dominated by layering, with upper layers of higher thermal inertia (e.g., duricrusts or desert pavements over fines) prevailing in mid-latitudes and upper layers of lower thermal inertia (e.g., dust-covered rock, soils with an ice table at shallow depths) prevailing in polar regions. Less common are regions dominated by horizontal mixtures, such as those containing differing proportions of rocks, sand, dust, and duricrust or surfaces with divergent local slopes. Other regions show thermal behavior that is more complex and not well-represented by two-component surface models. These results have important implications for Mars surface geology, climate modeling, landing-site selection, and other endeavors that employ thermal inertia as a tool for characterizing surface properties.     

Observations of CO in the atmosphere of Mars with PFS onboard Mars Express

We have analyzed spectra of CO recorded with the instrument PFS onboard Mars Express in the (1-0) band. The dataset we used ranges in time from January until June 2004 (L_S=331^°.17 until L_S=51^°.61; end of Mars Year 26, beginning of Mars Year 27). The aim of this work was to determine the amplitude of the CO mixing ratio departures from the mean globally averaged value currently admitted (8±3×10^-4) [Kaplan, L.D., Connes, J., Connes, P., 1969. Carbon monoxide in the martian atmosphere. Astron. J. 157, L187-L192] as a function of season, local time and location on the planet. We therefore processed the data from 90 calibrated orbits. The globally averaged CO mixing ratio value we derive from our dataset, 11.1×10^-4, is compatible with the range found by Kaplan et al. [1969. Carbon monoxide in the martian atmosphere. Astron. J. 157, L187-L192], although somewhat higher than the “standard” value. However, the CO mixing ratio we retrieve exhibits large variations (roughly between 3×10^-4 and 18×10^-4). Such relative variations have been used on a statistical basis to derive main trends as a function of latitude for three L_S ranges: 331-360^°, 0-30^° and 30-52^°. For the first L_S range, we seem to have an enhancement of the CO mixing ratio towards the northern latitudes, probably linked to the CO₂ condensation in winter on the north polar cap. The situation for the two other L_S ranges is not so clear, mainly as we lack data on the southern hemisphere. We roughly agree with the work of Krasnopolsky [2007. Long-term spectroscopic observations of Mars using IRTF/CSHELL: mapping of O2 dayglow, CO and search for CH4. Icarus 190, 93-102] for L_S=331-360^°, thus confirming the effect of seasonal condensation of CO₂ on the north polar cap, but we have no agreement for other seasons.     

Investigation of water vapor on Mars with PFS/SW of Mars Express

New insight into the seasonal, diurnal and spatial distribution of water vapor on Mars has been obtained from analyzing the spectra of the short-wavelength channel (SW) of the Planetary Fourier Spectrometer (PFS) onboard Mars Express. The processed dataset, recorded between January 2004 and April 2005, covers the seasons from LS=331° of Mars Year 26 to LS=196° of the following year. In this period the mean column density around vernal equinox was 8.2 pr. μm. The maximum values during northern summer were about 65 pr. μm, located around 75° N latitude with a longitudinally inhomogeneous distribution. Regarding the atmospheric transport, the majority of polar water vapor remains in the north polar region while only about a quarter is transported southward. Geographically there are two water vapor maxima visible, over Arabia Terra and the Tharsis plateau, that are most likely caused both by atmosphere-ground interaction and by atmospheric circulation. A comparison with other instruments generally shows a good agreement, only the SPICAM results are systematically lower. Compared to the results from the PFS long-wavelength channel the results of this work are slightly higher. A strong discrepancy is visible northward of about 50° N during the northern summer that is possibly explained by a non-uniform vertical H₂O mixing. In particular, a confinement of the water to the lower few kilometers yields a much better agreement between the retrieved column densities of the two PFS channels.     

Raman spectroscopic detection of key biomarkers of cyanobacteria and lichen symbiosis in extreme Antarctic habitats: Evaluation for Mars Lander missions

With proposals that micro-miniaturised Raman spectrometers could soon be part of a suite of analytical instrumentation on the surface of Mars, it is critically important to examine the spectral information that could be forthcoming from attempts to determine key molecular biosignatures under the hostile conditions of extra-terrestrial planetary exploration. Current approaches include the analysis of genuine martian geological material in the form of the SNC class meteorites, the formulation of simulated martian regoliths and the examination of putative martian terrestrial analogues; the latter provide the basis of this paper in the form of Antarctic extremophile habitats. In particular, specimens of epilithic, chasmolithic and endolithic lichen and cyanobacterial colonies sampled along a progressively worsening transect towards a “limits of life” situation, beyond which survival of organisms becomes impossible, provide what is arguably the best terrestrial proving-ground for prototype Raman spectrometers for martian exploration. Here, we report the results of experiments on these extant Antarctic extremophile colonies using a range of Raman excitation wavelengths and experimental conditions and also include a compilation of molecular spectral biosignatures, which may be considered as a suitable database for recognition of bioorganic modification of geological strata.     

Global thermal inertia and surface properties of Mars from the MGS mapping mission

We present a new high-resolution map of thermal inertia derived from observations of planetary brightness temperature by the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) obtained during the entire MGS primary mapping mission. Complete seasonal coverage provides a nearly global view of Mars, including the polar regions, at a spatial resolution of approximately 3 km. Our map of nighttime thermal-bolometer-based thermal inertia covers approximately 60% of the surface between 80° S and 80° N latitudes. We confirm the global pattern of high and low thermal inertia seen in lower resolution mapping efforts and provide greater detail concerning a third surface unit with intermediate values of both thermal inertia and albedo first identified by Mellon et al. 2000, Icarus 148, 437-455. Several smaller regional units with distinct characteristics are observed. Most notably, a unit of low thermal inertia () and low-to-intermediate albedo (0.09-0.22) dominates the region polewards of 65° S. We consider possible causes for these characteristics and conclude that a low-density mantle formed by desiccation of a previously ice-rich near-surface layer is the most likely explanation for the observed thermophysical properties. Global comparison of thermal inertia and elevation shows that high and low thermal inertia values can be found over a broad range of elevation, with only low values (30-) occurring at the highest elevations and the highest values occurring only at lower elevations. However, the lowest values () are found only at lower elevations, implying that the distribution of low thermal inertia material is not solely controlled by atmospheric pressure and the trapping of fines at high elevations. A new estimate of thermal inertia for the Viking and Pathfinder landing sites helps establish an important link between surface characteristics observed in situ and those derived from remote-sensing data.     

Thermal analysis of fractures at Cerberus Fossae, Mars: Detection of air convection in the porous debris apron

This study investigates the cause of high nighttime temperatures within Cerberus Fossae, a system of fractures affecting the Central Elysium Planitia. The inner parts (walls and floor) of the fractures are up to 40 K warmer than the surrounding plains. However, several temperature profiles exhibit a local temperature minima occurring in the central part of the fractures. We examined first the influence of cooling efficiency at night in the case of a strong reduction of the sky proportion induced by the fracture’s geometry. However, the lack of correlation between temperature and sky proportion, calculated from extracted Mars Orbiter Laser Altimeter (MOLA) profiles argues against this hypothesis. Albedo variations were considered but appear to be limited within the fractures, and are generally not correlated with the temperatures. Variations of the thermal properties of bedrocks exposures, debris aprons and sand dunes inferred from high-resolution images do not either correlate with temperature variations within the fractures. As none of these factors taken alone, or combined, can satisfactorily explain the temperature variations within and near the fracture, we suggest that geothermal heat transported by air convection within the porous debris aprons may contribute to explain high temperatures at night and the local minima on the fracture floor. The conditions for the occurrence of the suggested phenomenon and the consequences on the surface temperature are numerically explored. A conservative geothermal gradient of 20 mW/m² was used in the simulations, this value being consistent with either inferred lithosphere elastic thicknesses below the shield volcanoes of the Tharsis dome or values predicted from numerical simulations of the thermal evolution of Mars. The model results indicate that temperature differences of 10-20 K between the central and upper parts of the fracture are explained in the case of high Darcy velocities which require high permeability values (5 × 10⁻⁶ m²). The presence of coarse material composing the debris aprons may explain why this key criteria was met in the context of Cerberus Fossae.     

Multiple techniques for mineral identification on Mars:: a study of hydrothermal rocks as potential analogues for astrobiology sites on Mars

Spectroscopic studies of Mars analog materials combining multiple spectral ranges and techniques are necessary in order to obtain ground truth information for interpretation of rocks and soils on Mars. Two hydrothermal rocks from Yellowstone National Park, Wyoming, were characterized here because they contain minerals requiring water for formation and they provide a possible niche for some of the earliest organisms on Earth. If related rocks formed in hydrothermal sites on Mars, identification of these would be important for understanding the geology of the planet and potential habitability for life. XRD, thermal properties, VNIR, mid-IR, and Raman spectroscopy were employed to identify the mineralogy of the samples in this study. The rocks studied here include a travertine from Mammoth Formation that contains primarily calcite with some aragonite and gypsum and a siliceous sinter from Octopus Spring that contains a variety of poorly crystalline to amorphous silicate minerals. Calcite was detected readily in the travertine rock using any one of the techniques studied. The small amount of gypsum was uniquely identified using XRD, VNIR, and mid-IR, while the aragonite was uniquely identified using XRD and Raman. The siliceous sinter sample was more difficult to characterize using each of these techniques and a combination of all techniques was more useful than any single technique. Although XRD is the historical standard for mineral identification, it presents some challenges for remote investigations. Thermal properties are most useful for minerals with discrete thermal transitions. Raman spectroscopy is most effective for detecting polarized species such as CO₃, OH, and CH, and exhibits sharp bands for most highly crystalline minerals when abundant. Mid-IR spectroscopy is most useful in characterizing Si-O (and metal-O) bonds and also has the advantage that remote information about sample texture (e.g., particle size) can be determined. Mid-IR spectroscopy is also sensitive to structural OH, CO₃, and SO₄ bonds when abundant. VNIR spectroscopy is best for characterizing metal excitational bands and water, and is also a good technique for identification of structural OH, CO₃, SO₄, or CH bonds. Combining multiple techniques provides the most comprehensive information about mineralogy because of the different selection rules and particle size sensitivities, in addition to maximum coverage of excitational and vibrational bands at all wavelengths. This study of hydrothermal rocks from Yellowstone provides insights on how to combine information from multiple instruments to identify mineralogy and hence evidence of water on Mars.     

Albedo and photometric study of Mars with the Planetary Fourier Spectrometer on-board the Mars Express mission

The Short Wavelength Channel of the Planetary Fourier Spectrometer (PFS) covers the 8333-1750 cm⁻¹ (1.2-5.7 μm) spectral range, that is well suited to study the reflectance properties of the martian soil. These properties vary with time due to the dust dynamics in the martian environment. Wind can blow off dust exposing soil and fresh rocks and can support grain mobility inducing local dust settling. We have analyzed PFS data from January 2004 to April 2005. A detailed photometric study of the radiance acquired from the planet has been performed in order to compare correctly measurements obtained at different viewing geometries and to produce a mosaic image of the planet. The results show good agreement with data from the Thermal Emission Spectrometer (on-board NASA Mars Global Surveyor orbiter), although some variations are observed. Some albedo changes could be due to small to medium scale dust storms. A very accurate estimation of the limb-darkening parameter has been computed from the analyzed data. The obtained values are compared with a surface roughness and a thermal inertia map in order to assess the relation between the limb-darkening parameter and the physical properties of surface.     

Characterization of halophiles in natural MgSO4 salts and laboratory enrichment samples: Astrobiological implications for Mars

The presence of sulfate salts and limited subsurface water (ice) on Mars suggests that any liquid water on Mars today will occur as (magnesium) sulfate-rich brines in regions containing sources of magnesium and sulfur. The Basque Lakes of British Columbia, Canada, represent a hypersaline terrestrial analogue site, which possesses chemical and physical properties similar to those observed on Mars. The Basque Lakes also contain diverse halophilic organisms representing all three Kingdoms of life, growing in surface and near-subsurface environments. Of interest from an astrobiological perspective, crushed magnesium sulfate samples that were analyzed using a modified Lowry protein assay contained biomass in every crystal inspected, with biomass values from 0.078 to 4.21 mg_biomass/g_salt; average=0.74±0.7 mg_biomass/g_salt. Bacteria and Archaea cells were easily observed even in low-biomass samples using light microscopy, and bacteria trapped within magnesium sulfate crystals were observed using confocal microscopy. Regions within the salt also contained bacterial pigments, e.g., carotenoids, which were separate from the cells, indicating that cell lysis might have occurred during entrapment within the salt matrix. These biosignatures, cells, and any ‘soluble’ organic constituents were primarily found trapped within fluid inclusions or fluid-filled void spaces between intergrown crystals. Diffuse reflectance infrared Fourier transform spectroscopy (reflectance IR) analysis of enrichment cultures, containing cyanobacteria, Archaea, or dissimilatory sulfate-reducing bacteria, highlighted molecular biosignature features between 550-1650 and 2400-3000 cm⁻¹. Spectra from natural salts demonstrated that we can detect biomass within salt crystals using the most sensitive biosignatures, which are the 1530-1570 cm⁻¹, C-N, N-H, -COOH absorptions and the 1030-1050 cm⁻¹ C-OH, C-N, PO₄³⁻ bond features. The lowest detection limit for a biosignature absorption feature using reflectance IR was with a natural sample that possessed 0.78 mg_biomass/g_salt. In a model cell, i.e., a 0.5 by 1 μm bacillus, this biomass value corresponds to approximately 7.8×10⁸ cells/g_salt. Based on its ability to detect biomass entrapped within natural sulfate salts, reflectance IR may make an effective remote-sensing tool for finding enrichments of organic carbon within outcrops and surficial sedimentary deposits on Mars.     

A sensitive search for nitric oxide in the lower atmospheres of Venus and Mars: Detection on Venus and upper limit for Mars

High-resolution spectra of Venus and Mars at the NO fundamental band at 5.3 μm with resolving power ν/δν=76,000 were acquired using the TEXES spectrograph at NASA IRTF on Mauna Kea, Hawaii. The observed spectrum of Venus covered three NO lines of the P-branch. One of the lines is strongly contaminated, and the other two lines reveal NO in the lower atmosphere at a detection level of 9 sigma. A simple photochemical model for NO and N at 50-112 km was coupled with a radiative transfer code to simulate the observed equivalent widths of the NO and some CO₂ lines. The derived NO mixing ratio is 5.5±1.5 ppb below 60 km and its flux is . Predissociation of NO at the (0-0) 191 nm and (1-0) 183 nm bands of the δ-system and the reaction with N are the only important loss processes for NO in the lower atmosphere of Venus. The photochemical impact of the measured NO abundance is significant and should be taken into account in photochemical modeling of the Venus atmosphere. Lightning is the only known source of NO in the lower atmosphere of Venus, and the detection of NO is a convincing and independent proof of lightning on Venus. The required flux of NO is corrected for the production of NO and N by the cosmic ray ionization and corresponds to the lightning energy deposition of . For a flash energy on Venus similar to that on the Earth (∼¹⁰⁹ J), the global flashing rate is ∼90 s⁻¹ and ∼6 km⁻² y⁻¹ which is in reasonable agreement with the existing optical observations. The observed spectrum of Mars covered three NO lines of the R-branch. Two of these lines are contaminated by CO₂ lines, and the line at 1900.076 cm⁻¹ is clean and shows some excess over the continuum. Some photochemical reactions may result in a significant excitation of NO (v=1) in the lowest 20 km on Mars. However, quenching of NO (v=1) by CO₂ is very effective below 40 km. Excitation of NO (v=1) in the collisions with atomic oxygen is weak because of the low temperature in the martian atmosphere, and we do not see any explanation of a possible emission of NO at 5.3 μm. Therefore the data are treated as the lack of absorption with a 2 sigma upper limit of 1.7 ppb to the NO abundance in the lower atmosphere of Mars. This limit is above the predictions of photochemical models by a factor of 3.     

Hydrogen peroxide on Mars: evidence for spatial and seasonal variations

Hydrogen peroxide (H₂O₂) has been suggested as a possible oxidizer of the martian surface. Photochemical models predict a mean column density in the range of 10¹⁵-10¹⁶ cm⁻². However, a stringent upper limit of the H₂O₂ abundance on Mars (9×10¹⁴ cm⁻²) was derived in February 2001 from ground-based infrared spectroscopy, at a time corresponding to a maximum water vapor abundance in the northern summer (30 pr. μm, Ls=112°). Here we report the detection of H₂O₂ on Mars in June 2003, and its mapping over the martian disk using the same technique, during the southern spring (Ls=206°) when the global water vapor abundance was ∼10 pr. μm. The spatial distribution of H₂O₂ shows a maximum in the morning around the sub-solar latitude. The mean H₂O₂ column density (6×10¹⁵ cm⁻²) is significantly greater than our previous upper limit, pointing to seasonal variations. Our new result is globally consistent with the predictions of photochemical models, and also with submillimeter ground-based measurements obtained in September 2003 (Ls=254°), averaged over the martian disk (Clancy et al., 2004, Icarus 168, 116-121).     

Detectability of minor constituents in the martian atmosphere by infrared and submillimeter spectroscopy

Spectroscopic remote sensing in the infrared and (sub)millimeter range is a powerful technique that is well suited for detecting minor species in planetary atmospheres (Planet Space Sci. 43(1995) 1485). Yet, only a handful of molecules in the Mars atmosphere (CO₂, CO and H₂O along with their isotopic species, O_3, and more recently H₂O₂ and CH₄) have been detected so far by this method. New high performance spectroscopic instruments will become available in the future in the infrared and (sub)millimeter range, for observations from the ground (infrared spectrometers on 8 m class telescopes, large millimeter and submillimeter interferometers) and from space, in particular the Planetary Fourier Spectrometer (PFS) aboard Mars Express (MEx), and the Heterodyne Instrument for the Far-Infrared (HIFI) aboard the Herschel Space Observatory (HSO). In this paper we will present results of a study that determines detectability of minor species in the atmosphere of Mars, taking into account the expected performance of the above spectroscopic instruments. In the near future, a new determination of the D/H value is expected with the PFS, especially during times of maximum H₂O abundance in the martian atmosphere. PFS is also expected to place constraints on the abundance of several minor species (H₂O₂,CH₄,CH₂O, SO₂, H₂S, OCS, HCl) above any local outgassing sources, the hot spots. It will be possible to obtain complementary information on some minor species (O₃,H₂O₂, CH₄) from ground-based infrared spectrometers on large telescopes. In the more distant future, HIFI will be ideally suited for measuring the isotopic ratios with unprecedented accuracy. Moreover, it should be able to observe O₂, which has not yet been detected spectroscopically in the IR/submm range, as well as H₂O₂. HIFI should also provide upper limits for several species that have not yet been detected (HCl, NH₃, PH₃) in the atmosphere of Mars. Some species (SO, SO₂,H₂S, OCS, CH₂O) that may be observable from the ground could be searched for with present single-dish antennae and arrays, and in the future with the Atacama Large Millimeter Array (ALMA) submillimeter interferometer.     

Constraints on the origin and evolution of the layered mound in Gale Crater, Mars using Mars Reconnaissance Orbiter data

Gale Crater contains a 5.2 km-high central mound of layered material that is largely sedimentary in origin and has been considered as a potential landing site for both the MER (Mars Exploration Rover) and MSL (Mars Science Laboratory) missions. We have analyzed recent data from Mars Reconnaissance Orbiter to help unravel the complex geologic history evidenced by these layered deposits and other landforms in the crater. Results from imaging data from the High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) confirm geomorphic evidence for fluvial activity and may indicate an early lacustrine phase. Analysis of spectral data from the CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) instrument shows clay-bearing units interstratified with sulfate-bearing strata in the lower member of the layered mound, again indicative of aqueous activity. The formation age of the layered mound, derived from crater counts and superposition relationships, is ∼3.6-3.8 Ga and straddles the Noachian-Hesperian time-stratigraphic boundary. Thus Gale provides a unique opportunity to investigate global environmental change on Mars during a period of transition from an environment that favored phyllosilicate deposition to a later one that was dominated by sulfate formation.     

Recent geological and hydrological activity on Mars: The Tharsis/Elysium corridor

The paradigm of an ancient warm, wet, and dynamically active Mars, which transitioned into a cold, dry, and internally dead planet, has persisted up until recently despite published Viking-based geologic maps that indicate geologic and hydrologic activity extending into the Late Amazonian epoch. This paradigm is shifting to a water-enriched planet, which may still exhibit internal activity, based on a collection of geologic, hydrologic, topographic, chemical, and elemental evidences obtained by the Viking, Mars Global Surveyor (MGS), Mars Odyssey (MO), Mars Exploration Rovers (MER), and Mars Express (MEx) missions. The evidence includes: (1) stratigraphically young rock materials such as pristine lava flows with few, if any, superposed impact craters; (2) tectonic features that cut stratigraphically young materials; (3) features with possible aqueous origin such as structurally controlled channels that dissect stratigraphically young materials and anastomosing-patterned slope streaks on hillslopes; (4) spatially varying elemental abundances for such elements as hydrogen (H) and chlorine (Cl) recorded in rock materials up to 0.33 m depth; and (5) regions of elevated atmospheric methane. This evidence is pronounced in parts of Tharsis, Elysium, and the region that straddles the two volcanic provinces, collectively referred to here as the Tharsis/Elysium corridor. Based in part on field investigations of Solfatara Crater, Italy, recommended as a suitable terrestrial analog, the Tharsis/Elysium corridor should be considered a prime target for Mars Reconnaissance Orbiter (MRO) investigations and future science-driven exploration to investigate whether Mars is internally and hydrologically active at the present time, and whether the persistence of this activity has resulted in biologic activity.     

Observations of atmospheric water vapor above the Tharsis volcanoes on Mars with the OMEGA/MEx imaging spectrometer

The OMEGA imaging spectrometer onboard the Mars Express spacecraft is particularly well suited to study in detail specific regions of Mars, thanks to its high spatial resolution and its high signal-to-noise ratio. We investigate the behavior of atmospheric water vapor over the four big volcanoes located on the Tharsis plateau (Olympus, Ascraeus, Pavonis and Arsia Mons) using the 2.6 μm band, which is the strongest and most sensitive H₂O band in the OMEGA spectral range. Our data sample covers the end of MY26 and the whole MY27, with gaps only in the late northern spring and in northern autumn. The most striking result of our retrievals is the increase of water vapor mixing ratio from the valley to the summit of volcanoes. Corresponding column density is often almost constant, despite a factor of ∼5 decrease in air mass from the bottom to the top. This peculiar water enrichment on the volcanoes is present in 75% of the orbits in our sample. The seasonal distribution of such enrichment hints at a seasonal dependence, with a minimum during the northern summer and a maximum around the northern spring equinox. The enrichment possibly also has a diurnal trend, being the orbits with a high degree of enrichment concentrated in the early morning. However, the season and the solar time of the observations, due to the motion of the spacecraft, are correlated, then the two dependences cannot be clearly disentangled. Several orbits exhibit also spatially localized enrichment structures, usually ring- or crescent-shaped. We retrieve also the height of the saturation level over the volcanoes. The results show a strong minimum around the aphelion season, due to the low temperatures, while it raises quickly before and after this period. The enrichment is possibly generated by the local circulation characteristic of the volcano region, which can transport upslope significant quantities of water vapor. The low altitude of the saturation level during the early summer can then hinder the transport of water during this season. The influence of the coupling between atmosphere and surface, due mainly to the action of the regoliths, can also contribute partially to the observed phenomenon.     

Composition and thermal inertia of the Mawrth Vallis region of Mars from TES and THEMIS data

Clay mineral-bearing deposits previously discovered on Mars with near infrared (λ=0.3-5 μm) remote sensing data are of major significance for understanding the aqueous history, geological evolution, and past habitability of Mars. In this study, we analyzed the thermal infrared (λ=6-35 μm) surface properties of the most extensive phyllosilicate deposit on Mars: the Mawrth Vallis area. Clay mineral-bearing units, which in visible images appear to be relatively light-toned, layered bedrock, have thermal inertia values ranging from 150 to 460 J m⁻² K⁻¹ s^−1/2. This suggests the deposits are composed of a mixture of rock with sand and dust at 100-meter scales. Dark-toned materials that mantle the clay-bearing surfaces have thermal inertia values ranging from 150 to 800, indicating variable degrees of rockiness or induration of this younger sedimentary or pyroclastic unit. Thermal Emission Spectrometer (TES) spectra of the light-toned rocks were analyzed with a number of techniques, but none of the results shows a large phyllosilicate component as has been detected in the same surfaces with near-infrared data. Instead, TES spectra of light-toned surfaces are best modeled by a combination of plagioclase feldspar, high-silica materials (similar to impure opaline silica or felsic glass), and zeolites. We propose three hypotheses for why the clay minerals are not apparent in thermal infrared data, including effects due to surface roughness, sub-pixel mixing of multiple surface temperatures, and low absolute mineral abundances combined with differences in spatial sampling between instruments. Zeolites modeled in TES spectra could be a previously unrecognized component of the alteration assemblage in the phyllosilicate-bearing rocks of the Mawrth Vallis area. TES spectral index mapping suggests that (Fe/Mg)-clays detected with near infrared data correspond to trioctahedral (Fe²⁺) clay minerals rather than nontronite-like clays. The average mineralogy and geologic context of these complex, interbedded deposits suggests they are either aqueous sedimentary rocks, altered pyroclastic deposits, or a combination of both.     

An Overview Of The Origin Of Life: The Case For Biological Prospecting On Mars

Studies of the Earth's earliest biosphere have suggested a close coupling between the evolution of early life forms and the physical and chemical evolution of the planetary surface. From a biological perspective there were many similarities between early Earth and early Mars. This has led to the idea that an origin of life event may have occurred on Mars, leading to the development of microbial life. Various theories have been advanced to explain the origin of life on Earth, and these are reviewed with relevance to Mars. If traces of past or present biogenic activity are to be found on Mars, then the most likely place to prospect is several kilometers below the surface where liquid water might be stable. Such prospecting may best lend itself to human exploration. This revised version was published online in July 2006 with corrections to the Cover Date.     

Properties of cryobrines on Mars

Brines, i.e. aqueous salty solutions, increasingly play a role in a better understanding of physics and chemistry (and eventually also putative biology) of the upper surface of Mars. Results of physico-chemical modeling and experimentally determined data to characterize properties of cryobrines of potential interest with respect to Mars are described. Eutectic diagrams, the related numerical eutectic values of composition and temperature, the water activity of Mars-relevant brines of sulfates, chlorides, perchlorides and carbonates, including related deliquescence relative humidity, are parameters and properties, which are described here in some detail. The results characterize conditions for liquid low-temperature brines (“cryobrines”) to evolve and to exist, at least temporarily, on present Mars.     

On the origin of a double, oblique impact on Mars

A double, oblique impact feature north of Olympus Mons provides a unique opportunity to investigate the event that formed it. The sizes of the craters, their ellipticity, shapes of ejecta blankets, separation from each other, and positions relative to each other, all give us information about the event. Coupling this information with an existing model of meteoritic flight through an atmosphere allows us to test several possible scenarios for the event (object type and origin, pre-entry trajectory, atmospheric trajectory, prevailing atmospheric density). We find it highly improbable that the impactor was simply an extra-martian asteroid or comet. We also find that it is unlikely to have been a double-asteroid or a tidally fractured one, but is more likely to have been a Mars-orbiting moonlet whose orbit tidally decayed, and that denser atmospheric conditions than today's may have prevailed when it impacted.