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Over the past decade, our knowledge of the geochemical and mineralogical diversity of Martian rocks has greatly increased. Rocks on the surface of Mars are older than most Martian meteorites and have a wider range in bulk chemistry (higher SiO 2 , higher total alkalis, and lower MgO contents). Recent Martian meteorite discoveries (NWA 7034, NWA 7635, and NWA 8159) are also significantly older than the shergottites and NWA 7034 contains clasts which have a similar range in chemistry of the surface basalts. In this invited review, I summarize what is known about the bulk chemistry (major elements) of Martian igneous rocks and use the chemistry to constrain the formation conditions in the interior, and how these conditions have changed through time. The basalts at Gale Crater are consistent with forming from a metasomatized mantle source which affected not only the chemistry of the basalts, but also the formation conditions. Basalts at Gusev Crater, Bounce Rock at Meridiani Planum, and those basalts not affected by metasomatism like those at Gale Crater have a mantle potential temperature of ~ 1450 °C for the Noachian. Conversely, Martian shergottites have a much higher mantle potential temperature (~ 1745 °C), which has always been problematic to explain for such young rocks. New formation conditions calculations are consistent with previous models of formation by a hot mantle plume, but solve the pressure of formation paradox from the previous model. Combining the calculations for mantle potential temperature with previous estimates for GRS measurements of surface volcanics from the Hesperian and Amazonian, I calculate a baseline cooling curve and mantle melting conditions for the Martian interior through time.

Ganges Chasma is a mineralogically diverse canyon in the far-eastern portion of the Valles Marineris system. It exposes bedrock outcrops of olivine, large dune fields enriched in olivine, a central Interior Layered Deposit with monohydrated sulfates, and Fe/Mg-phyllosilicates on the plateaus surrounding the canyon and in the canyon walls. The co-existence of hydrated minerals with unaltered olivine makes this an excellent location to study the timing of aqueous processes in this region. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and the High-Resolution Imaging Science Experiment (HiRISE), synthesizing them with previous data sets to produce a composite mineralogical map of Ganges Chasma. We then produce a timeline of mineralogic deposition for the Ganges area, relating it to regional geologic events.

The raw exospheric composition data of Mars for the period September 2014 to October 2015 has been retrieved and analysed using the observations carried out by Mars Exospheric Neutral Composition Analyser (MENCA) payload on-board the Mars Orbiter Mission (MOM) launched by India on 5 November 2013. The state parameters viz. latitude, longitude, altitude and solar zenith angle coverage of the partial pressures of different exospheric constituents are determined and assigned to enrich the usefulness of data with orbit-wise assimilation particularly between 250 and 500 km altitude range of main interest. Apart from getting the results on mean individual orbits partial pressure profiles during the northern winter, the variations of total as well as partial pressures are also studied with respect to the distribution of major atmospheric constituents and their dependence on solar activity. In particular, CO2 and O variations are considered together for any differential effects due to photo-dissociation and photo-ionisation. The results on gradual reduction in densities due to the decreasing daily mean sunspot numbers and strong response of CO2 and O pressures to solar energetic particle events like that of 24 December 2014 following a solar flare were discussed in this paper., 14 pages, 11 figures, 1 table. Presented in the 42nd COSPAR Scientific Assembly held at Pasadena, CA, USA during 14-22, July 2018

The rock alteration and rind formation in analog environments like Antarctica may provide clues to rock alteration and therefore paleoclimates on Mars. Clastic sedimentary rocks derived from basaltic sources have been studied in situ by martian rovers and are likely abundant on the surface of Mars. Moreover, how such rock types undergo alteration when exposed to different environmental conditions is poorly understood compared with alteration of intact basaltic flows. Here we characterize alteration in the chemically immature Carapace Sandstone from Antarctica, a terrestrial analog for martian sedimentary rocks. We employ a variety of measurements similar to those used on previous and current Mars missions. Laboratory techniques included bulk chemistry, powder X-ray diffraction (XRD), hyperspectral imaging and X-ray absorption spectroscopy. Through these methods we find that primary basaltic material in the Carapace Sandstone is pervasively altered to hydrated clay minerals and palagonite as a result of water–rock interaction. A thick orange rind is forming in current Antarctic conditions, superimposing this previous aqueous alteration signature. The rind exhibits a higher reflectance at visible-near infrared wavelengths than the rock interior, with an enhanced ferric absorption edge likely due to an increase in Fe3+ of existing phases or the formation of minor iron (oxy)hydroxides.more » This alteration sequence in the Carapace Sandstone results from decreased water–rock interaction over time, and weathering in a cold, dry environment, mimicking a similar transition early in martian history. This transition may be recorded in sedimentary rocks on Mars through a similar superimposition mechanism, capturing past climate changes at the hand sample scale. These results also suggest that basalt-derived sediments could have sourced significant volumes of hydrated minerals on early Mars due to their greater permeability compared with intact igneous rocks.« less

Spectral reflectance data from the MRO-CRISM (Mars Reconnaissance Orbiter-Compact Reconnaissance Imaging Spectrometer for Mars) of Capri Chasma, a large canyon within Valles Marineris on Mars, have been studied. Results of this analysis reveal the presence of minerals, such as, phyllosilicates (illite, smectite (montmorillonite)) and carbonates (ankerite and manganocalcite). These minerals hint of the aqueous history of Noachian time on Mars. Phyllosilicates are products of chemical weathering of igneous rocks, whereas carbonates could have formed from local aqueous alteration of olivine and other igneous minerals. Four different locations within the Capri Chasma region were studied for spectral reflectance based mineral detection. The study area also shows the spectral signatures of iron-bearing minerals, e.g. olivine with carbonate, indicating partial weathering of parent rocks primarily rich in ferrous mineral. The present study shows that the minerals of Capri Chasma are characterized by the presence of prominent spectral absorption features at 2.31 μm, 2.33 μm, 2.22 μm, 2.48 μm and 2.52 μm wavelength regions, indicating the existence of hydrous minerals, i.e., carbonates and phyllosilicates. The occurrence of carbonates and phyllosilicates in the study area suggests the presence of alkaline environment during the period of their formation. Results of the study are important to understand the formation processes of these mineral assemblages on Mars, which may help in understanding the evolutionary history of the planet.

An accurate assessment of the bulk chemical composition of Mars is fundamental to understanding planetary accretion, differentiation, mantle evolution, the nature of the igneous parent rocks that were altered to produce sediments on Mars, and the initial concentrations of volatiles such as H, Cl and S, important constituents of the Martian surface. This paper reviews the three main approaches that have been used to estimate the bulk chemical composition of Mars: geochemical/cosmochemical, isotopic, and geophysical. The standard model is one developed by Wanke and Dreibus in a series of papers, which is based on compositions of Martian meteorites. Since their groundbreaking work, substantial amounts of data have become available to allow a reassessment of the composition of Mars from elemental data, including tests of the basic assumptions in the geochemical models. The results adjust some of the concentrations in the Wanke–Dreibus model, but in general confirm its accuracy. Bulk silicate Mars has roughly uniform depletion of moderately volatile elements such as K (0.6 × CI), and strong depletion of highly volatile elements (e.g., Tl). The highly volatile elements are within uncertainties uniformly depleted at about 0.06 CI abundances. The highly volatile chalcophile elements are likewise roughly uniformly depleted, but with more scatter, with normalized abundances of 0.03 CI. Bulk planetary H 2 O is much higher than estimated previously: it appears to be slightly less than in Earth, but D / H is similar in Earth and Mars, indicating a common source of water-bearing material in the inner solar system. K/Th ranges from ∼3000 to ∼5000 among the terrestrial planets, a small range compared to CI chondrites (19,000). FeO varies throughout the inner solar system: ∼3 wt% in Mercury, 8 wt% in Earth and Venus, and 18 wt% in Mars. These differences can be produced by varying oxidation conditions, hence do not suggest the terrestrial planets were formed from fundamentally different materials. The broad chemical similarities among the terrestrial planets indicate substantial mixing throughout the inner solar system during planet formation, as suggested by dynamical models.

We use the total mass, possible core radius and the observed mean moment of inertia factor of Mars to constrain mineralogical and compositional structures of Mars. We adopt a liquid Fe–S system for the Martian core and construct density models of the interior of Mars for a series of mantle compositions, core compositions and temperature profiles. The moment of inertia factor of the planet is then calculated and compared to the observation to place constraints on Mars composition. Based on the independent constraints of total mass, possible core radius of 1630–1830 km, and the mean moment of inertia factor ( 0.3645 ± 0.0005 ) of Mars, we find that Fe content in the Martian mantle is between 9.9 and 11.9 mol%, Al content in the Martian mantle smaller than 1.5 mol%, S content in the Martian core between 10.6 and 14.9 wt%. The inferred Fe content in the bulk Mars lies between 27.3 and 32.0 wt%, and the inferred Fe/Si ratio in Mars between 1.55 and 1.95, within a range too broad to make a conclusion whether Mars has the same nonvolatile bulk composition as that of CI chondrite. We also conclude that no perovskite layer exists in the bottom of the Martian mantle. Based on the inferred density models, we estimate the flattening factor and J 2 gravitational potential related to the hydrostatic figure of the rotating Mars to be ( 5.0304 ± 0.0098 ) × 10 - 3 and ( 1.8151 ± 0.0065 ) × 10 - 3 , respectively. We also discuss implications of these compositional models to the understanding of formation and evolution of the planet.

Knowledge of the interior structure of Mars is of fundamental importance to the understanding of its past and present state as well as its future evolution. The most prominent interior structure properties are the state of the core, solid or liquid, its radius, and its composition in terms of light elements, the thickness of the mantle, its composition, the presence of a lower mantle, and the density of the crust. In the absence of seismic sounding only geodesy data allow reliably constraining the deep interior of Mars. Those data are the mass, moment of inertia, and tides. They are related to Mars’ composition, to its internal mass distribution, and to its deformational response to principally the tidal forcing of the Sun. Here we use the most recent estimates of the moment of inertia and tidal Love number k2 in order to infer knowledge about the interior structure of the Mars. We have built precise models of the interior structure of Mars that are parameterized by the crust density and thickness, the volume fractions of upper mantle mineral phases, the bulk mantle iron concentration, and the size and the sulfur concentration of the core. From the bulk mantle iron concentration and from the volume fractions of the upper mantle mineral phases, the depth dependent mineralogy is deduced by using experimentally determined phase diagrams. The thermoelastic properties at each depth inside the mantle are calculated by using equations of state. Since it is difficult to determine the temperature inside the mantle of Mars we here use two end-member temperature profiles that have been deduced from studies dedicated to the thermal evolution of Mars. We calculate the pressure and temperature dependent thermoelastic properties of the core constituents by using equations state and recent data about reference thermoelastic properties of liquid iron, liquid iron–sulfur, and solid iron. To determine the size of a possible inner core we use recent data on the melting temperature of iron–sulfur. Within our model assumptions the geodesy data imply that Mars has no solid inner core and that the liquid core contains a large fraction of sulfur. The absence of a solid inner is in agreement with the absence of a global magnetic field. We estimate the radius of the core to be 1794 ± 65 km and its core sulfur concentration to be 16 ± 2 wt%. We also show that it is possible for Mars to have a thin layer of perovskite at the bottom of the mantle if it has a hot mantle temperature. Moreover a chondritic Fe/Si ratio is shown to be consistent with the geodesy data, although significantly different value are also possible. Our results demonstrate that geodesy data alone, even if a mantle temperature is assumed, can almost not constrain the mineralogy of the mantle and the crust. In order to obtain stronger constraints on the mantle mineralogy bulk properties, like a fixed Fe/Si ratio, have to be assumed.

Understanding the evolution of Mars requires determining the composition of the surface and atmosphere of the planet. The European Space Agency’s ExoMars rover mission, which is expected to launch in 2016, is part of the Aurora programme. The instruments on the rover will search for evidence of life on Mars and will map a sub-section of the Martian surface, extracting compositional information. Currently our understanding of the bulk composition (and mineralogy) of Mars relies on orbital data from instruments on-board satellites such as 2001 Mars Odyssey, Mars Reconnaissance Orbiter and Mars Express, in addition to in-situ instrumentation on rovers such as Spirit and Opportunity. γ-ray spectroscopy can be used to determine the composition of Mars, but it has yet to be successfully carried out in-situ on Mars. This study describes some of the results obtained from the γ-ray spectrometer on 2001 Mars Odyssey during solar proton events and discusses whether the increased emissions are useful in γ-ray spectroscopy. The study shows that although increased γ-ray emissions were expected from the Martian surface during a solar proton event, they were not detected from orbit probably due to insufficient signal-to-background. However, this does not preclude the possibility of measuring changes in γ-ray flux corresponding to changes in solar activity on the surface of the planet.

The Mars Global Surveyor, Mars Exploration Rover, and Mars Express missions have stimulated considerable thinking about the surficial geochemical evolution of Mars. Among the major recent mission findings are the presence of jarosite (a ferric sulfate salt), which requires formation from an acid-sulfate brine, and the occurrence of hematite and goethite on Mars. Recent ferric iron models have largely focused on 25 °C, which is a major limitation for models exploring the geochemical history of cold bodies such as Mars. Until recently, our work on low-temperature iron-bearing brines involved ferrous but not ferric iron, also obviously a limitation. The objectives of this work were to (1) add ferric iron chemistry to an existing ferrous iron model (FREZCHEM), (2) extend this ferrous/ferric iron geochemical model to lower temperatures ( The FREZCHEM model is an equilibrium chemical thermodynamic model parameterized for concentrated electrolyte solutions using the Pitzer approach for the temperature range from 4 –NO 3 –OH–HCO 3 –CO 3 –CO 2 –O 2 –CH 4 –H 2 O system. The model was used to develop paragenetic sequences for Rio Tinto waters on Earth and a hypothetical Martian brine derived from acid weathering of basaltic minerals. In general, model simulations were in agreement with field evidence on Earth and Mars in predicting precipitation of stable iron minerals such as jarosites, goethite, and hematite. In addition, paragenetic simulations for Mars suggest that other iron minerals such as lepidocrocite, schwertmannite, ferricopiapite, copiapite, and bilinite may also be present on the surface of Mars. Evaporation or freezing of the Martian brine led to similar mineral precipitates. However, in freezing, compared to evaporation, the following key differences were found: (1) magnesium sulfates had higher hydration states; (2) there was greater total aqueous sulfate (SO 4T = SO 4 + HSO 4 ) removal; and (3) there was a significantly higher aqueous Cl/SO 4T ratio in the residual Na–Mg–Cl brine. Given the similarities of model results to observations, alternating dry/wet and freeze/thaw cycles and brine migration could have played major roles in vug formation, Cl stratification, and hematite concretion formation on Mars.

New data returned from the Mars Exploration Rover (MER) mission have revealed abundant evaporites in the sedimentary record at Meridiani Planum. A working hypothesis for Meridiani evaporite formation involves the evaporation of fluids derived from the weathering of martian basalt and subsequent diagenesis. On Earth, evaporite formation in exclusively basaltic settings is rare. However, models of the evaporation of fluids derived from experimentally weathering synthetic martian basalt provide insight into possible formation mechanisms. The thermodynamic database assembled for this investigation includes both Fe2+ and Fe3+ in Pitzer's ion interaction equations to evaluate Fe redox disequilibrium at Meridiani Planum. Modeling results suggest that evaporation of acidic fluids derived from weathering olivine-bearing basalt should produce Mg, Ca, and Fe-sulfates such as jarosite and melanterite. Calculations that model diagenesis by fluid recharge predict the eventual breakdown of jarosite to goethite as well as the preservation of much of the initial soluble evaporite component at modeled porosity values appropriate for relevant depositional environments (< 0.30). While only one of several possible formation scenarios, this simple model is consistent with much of the chemical and mineralogical data obtained on Meridiani Planum outcrop. B) 2005 Elsevier B.V. All rights reserved.

The Dry Valleys of Antarctica are an excellent analog of the environment at the surface of Mars. Soil formation histories involving slow processes of sublimation and migration of water-soluble ions in polar desert environments are characteristic of both Mars and the Dry Valleys. At the present time, the environment in the Dry Valleys is probably the most similar to that in the mid-latitudes on Mars although similar conditions may be found in areas of the polar regions during their respective Mars summers. It is thought that Mars is currently in an interglacial period, and that subsurface water ice is sublimating poleward. Because the Mars sublimation zones seem to be the most similar to the Antarctic Dry Valleys, the Dry Valleys-type Mars climate is migrating towards the poles. Mars has likely undergone drastic obliquity changes, which means that the Dry Valleys analog to Mars may be valid for large parts of Mars, including the polar regions, at different times in geologic history. Dry Valleys soils contain traces of silicate alteration products and secondary salts much like those found in Mars meteorites. A martian origin for some of the meteorite secondary phases has been verified previously; it can be based on the presence of shock effects and other features which could not have formed after the rocks were ejected from Mars, or demonstrable modification of a feature by the passage of the meteorite through Earth's atmosphere (proving the feature to be pre-terrestrial). The martian weathering products provide critical information for deciphering the near-surface history of Mars. Definite martian secondary phases include Ca-carbonate, Ca-sulfate, and Mg-sulfate. These salts are also found in soils from the Dry Valleys of Antarctica. Results of earlier Wright Valley work are consistent with what is now known about Mars based on meteorite and orbital data. Results from recent and current Mars missions support this inference. Aqueous processes are active even in permanently frozen Antarctic Dry Valleys soils, and similar processes are probably also occurring on Mars today, especially at the mid-latitudes. Both weathering products and life in Dry Valleys soils are distributed heterogeneously. Such variations should be taken into account in future studies of martian soils and also in the search for possible life on Mars.

The surface of Mars is covered by weathered material. Mars' rusty red colour in particular is commonly ascribed to ferric iron-bearing minerals. The planet's surface is generally iron rich. Mossbauer spectroscopy is a powerful tool for quantitative mineralogical analysis of Fe-bearing minerals. Consequently, the miniaturized Mossbauer spectrometer MIMOS II is part of the payload of NASA's twin Mars Exploration Rovers “Spirit” and “Opportunity”, and ESA's ill-fated Mars Express lander “Beagle 2”. Both Mars Exploration Rovers are currently conducting successful surface operations on Mars. In this paper, we give a brief insight into mission operations with respect to the reconstruction of local weathering scenarios at the landing sites, which in turn will help to illuminate the climatic history of the planet. Mossbauer spectra obtained in preparation of the mission from the SNC meteorites Nakhla, Dar al Gani 476, and Sayh al Uhaymir, show weathering and other alteration features. Preliminary results of laboratory weathering experiments on Fe-bearing minerals (olivine and pyroxene) show the importance of analysing individual minerals to understand weathering of more complex mineral assemblages like, e.g., basalt.

High phosphorus concentrations in the range of 0.5 wt% in rocks and soil have been measured on the martian surface, terrestrial P concentrations are far less uniform and generally lower. Reactions of terrestrial basalt and granite powders with phosphate solution result in an enrichment of phosphorus in both, with basalt having a far better reactivity than granite. The implications of these results for P on Mars are discussed.

The SNC meteorites Chassigny, ALH84001, Nakhla, and the newly discovered nakhlite NWA817 contain high concentrations of xenon isotopes produced by the fission of the extinct radionuclide 244 Pu ( t 1/2 =82 Ma). The fission gas is released at temperatures >900°C together with indigenous solar-type Xe which represents a Martian interior component. Both nakhlites are rich in U, rare earth elements, and fissiogenic 136 Xe*, suggesting that fissiogenic xenon has been enriched through magmatic differentiation in closed system conditions. In Chassigny and ALH84001, fission Xe concentrations are consistent with a chondritic initial abundance of 244 Pu in Mars. The ratios ( 129 Xe/ 136 Xe)* (where 129 Xe* was produced by the decay of extinct 129 I ( t 1/2 =16 Ma) and 136 Xe* by 244 Pu fission) observed in these meteorites at high temperature are systematically lower than the value that would be expected from decay in a closed mantle reservoir having a bulk Mars composition, requiring early differentiation of volatile iodine with respect to refractory plutonium. We develop a model in which iodine and xenon are degassed together during large-scale magmatic events (e.g., magma ocean episode) on early Mars. The results show that bulk 129 I/ 244 Pu fractionation must have occurred ≤35 Ma after the start of solar system formation and that degassing might have continued during the first 330 Ma for the nakhlite mantle source. After this period, the mantle sources of these meteorites did not experience significant degassing, suggesting that Mars has been a static planet for most of its history. The computed amount of mantle Xe released into the early Martian atmosphere is about three orders of magnitude higher than the Xe abundance observed in the present-day atmosphere. The amount of 129 Xe* produced by the decay of 129 I transferred to the Martian surface is also three orders of magnitude higher than the present-day atmospheric 129 Xe*, implying that loss of Martian atmospheric gases must have lasted over the decay interval of several tens of Ma.

SNC meteorites (Shergottites, Nakhlites and Chassigny) define a fractionation line in a delta(17)O/delta(18)O diagram as expected for rocks differentiated from a formerly homogenized parent body, which is intermediate between H ordinary chondrites and EH enstatite chondrites. The planet Mars is located between the Earth and the asteroid belts, the potential source of ordinary chondrites. Since oxygen is a major component of the terrestrial planets, our model assumes that Mars composition is a mixture of two chondritic sources whose proportions are calculated by mass balance based on oxygen isotopes only. Two possible model compositions can be derived: if the average isotopic composition of SNC is relaxed along its fractionation Line (Model 1) and if the end members are average H and EH chondrites, one obtains a 30:70 H:EH mixture. if the average isotopic composition of SNC is a robust feature, then an extreme composition of H chondrites must be selected which yields the proportion, 55:45 for the H:EH components. This composition carries the same oxygen isotopic composition as the iron inclusions in the IIE of the conjectured end member of the ordinary chondrite group. The proportions obtained this way enable to calculate two model compositions for all the refractory elements and oxygen. Model 1 can be discarded as it does not permit to fit reasonably the physical properties of the planet. Mass and composition of the core (Model 2) is easily derived (23% of Mars mass, containing 16% S); the remainder forming the bulk mantle composition. Comparison with recent estimates based on the composition of SNC meteorites reveals only minor differences, essentially for Si, Mg and Fe; this is because of our choice of non-CI chondritic composition, unlike previous models. Discussion of the assumptions made in previous models confirms that the new composition is in agreement with the SNC compositions. The model also permits to calculate adequate physical properties of the planet like its zero pressure mantle density, density profile with depth and dimensionless moment of inertia. The superiority of the present model resides in the minimal number of necessary hypotheses and the possibility to test it, using physical and chemical data about the planet: as far as we know of, all constraints can be satisfied within the errors of the measurements or uncertainty of the model. The Fe abundance and the low Mg/Si ratio imply that pyroxenes and garnet at depth will play a major role in the differentiation of the planet, a feature which differs markedly from the terrestrial mantle. (C) 1999 Elsevier Science B.V. All rights reserved.

The density profile of an SNC model Martian interior is calculated from the results of previous experimental work that determined the modal mineralogy of the model mantle up to Martian core-mantle boundary pressures. The moment-of-inertia factor of Mars is calculated as a function of core composition and crustal thickness using the SNC model mantle density profile as a constraint. Two sets of calculations are performed. In the first, the bulk composition of the planet is not constrained to a C1 chondrite composition. Assuming a Martian crust density of 2.7–3.0 g/cm3, a crust thickness of 25–150 km, and the core composition proposed by Dreibus and Wanke, Fe 14 wt% S, the calculated moment-of-inertia factor ranges from 0.367 to 0.357. These models include a perovskite-bearing zone in the Martian interior. Considering core compositions ranging from pure Fe to pure FeS changes the moment-of-inertia factor by only ±0.001, but at higher S abundances, core size increases, such that the depth of the core-mantle boundary is shallower than the depth of perovskite stability. In the second set of calculations, the bulk composition of the planet is constrained to a C1 composition requiring a crust thickness of 180–320 km, assuming a crust density of 2.7–3.0 g/cm3. The calculated moment-of-inertia factor is 0.354 and a perovskite-bearing layer is absent from the Martian interior. As it is unlikely that the thickness of the Martian crust is greater than 100 km, the bulk composition of Mars cannot be constrained to a C1 chondrite composition as proposed by the Dreibus and Wanke model (G. Dreibus, H. Wanke, Meteoritics 20 (1985) 367–382). In order to determine if the Martian mantle is more iron-rich than the Earth's mantle, we may need not only an improved estimate of the moment-of-inertia factor of Mars, but also tighter constraints on Martian crust thickness and density. The absolute degree of iron-enrichment, however, cannot be specified without also knowing the size of the Martian core. A moment-of-inertia factor of less than 0.342 is not geochemically feasible, because it requires that the mantle of Mars contains no iron.

We derive the bulk chemical composition, physical properties, and trace element abundances of Mars from two assumptions: (1) Mars is the parent body for the Shergottite–Nakhlite–Chassignite (SNC) meteorites, and (2) the oxygen isotopic composition of Mars was determined by the oxygen isotopic compositions of the different types of nebular material that accreted to form Mars. We use oxygen isotopes to constrain planetary bulk compositions because oxygen is generally the most abundant element in rock, and is either the first or second (after iron) most abundant element in any terrestrial planet, the Moon, other rocky satellites, and the asteroids. The oxygen isotopic composition of Mars, calculated from oxygen isotopic analyses of the SNC meteorites, corresponds to the accretion of about 85% H-, 11% CV-, and 4% CI-chondritic material. (Unless noted otherwise, mass percentages are used in this paper.) The bulk composition of Mars follows from mass balance calculations using mean compositions for these chondrite groups. We predict that silicates (mantle + crust) comprise about 80% of Mars. The composition of the silicate fraction represents the composition of the primordial martian mantle prior to crustal formation. The FeO content of the mantle is 17.2%. A metal–sulfide core, containing about 10.6% S, makes up the remaining 20% of the planet. Our bulk composition is similar to those from other models. We calculate the abundances of siderophile (“metal-loving”) and chalcophile (“sulfide-loving”) elements in the martian mantle from the bulk composition using (metal–sulfide)/silicate partition coefficients. Our results generally agree with predictions of the SNC meteorite model of Wanke and Dreibus for the composition of Mars. However, we predict higher abundances for the alkalis and halogens than those derived from SNC meteorite models for Mars. The apparent discrepancy indicates that the alkalis and halogens were lost from the martian mantle by hydrothermal leaching and/or vaporization during accretion. Geochemical arguments suggest that vaporization was only a minor loss process for these elements. On the other hand, aqueous transport of the alkalis and halogens to the surface is supported by the terrestrial geochemistry of these elements and the high K, Rb, Cl, and Br abundances found by the Viking XRF and Phobos gamma ray experiments on the surface of Mars.

The general scientific objectives of an in situ experiment employing a Mossbauer spectrometer on a Martian lander are, for both rock and soil samples, identification and relative abundance of iron-bearing minerals (including carbonates, phyllosilicates (clays), hydroxyoxides, phosphates, oxides, silicates, sulfides, sulfates), measurement of the ferric (Fe3−) to ferrous (Fe2+) ratio, determination of the properties of magnetic phases including the size distribution of magnetic particles (nanophase versus larger particles) in the Martian soil. These data provide information about the nature and extent of atmosphere-surface chemical and physical weathering processes involving Fe-bearing phases. These objectives are directly relevant to studying the evolution of volatiles and climate over time on Mars because surface materials are major volatile sinks. In fact one of the major problems associated with understanding the evolution of volatiles on Mars is understanding the processes in the past and/or present that are responsible for oxidizing the red planet. A miniaturized backscattering Mossbauer spectrometer, developed at the Technical University of Darmstadt, is reported on which is a flight prototype of an instrument that could be used for in situ analysis as part of a payload of a Martian lander. Its critical instrument parameters are

Simulations of the Gas Exchange Experiment (GEX), one of the Viking Lander Biology Experiments, were run using palagonite and Fe-rich montmorillonite as terrestrial analogs of the Martian soil. These terrestrial analogs were exposed to a nutrient solution of the same composition as that of the Viking Landers under humid (no contact with nutrient) and wet (intimate contact) conditions. The headspace gases in the GEX sample cell were sampled and then analyzed by gas chromatography under both humid and wet conditions. Five gases were monitored: CO 2 , N 2 , O 2 , Ar and Kr. It was determined that in order to simulate the CO 2 gas changes of the Viking GEX experiment, the mixture of soil analog mineral plus nutrient medium must be slightly (pH = 7.4) to moderately basic (pH = 8.7). This conclusion suggests constraints upon the composition of terrestrial analogs to the Mars soil; acidic components may be present, but the overall mixture must be basic in order to simulate the Viking GEX results.

Mixtures of a Hawaiian palagonite and an iron-rich, montmorillonite clay (15.8 ± 0.4 wt% Fe as Fe 2 O 3 ) were evaluated as Mars surface spectral analogs from their diffuse reflectance spectra. The presence of the 2.2-μm absorption band in the reflectance spectrum of clays and its absence in the Mars spectrum have been interpreted as indicating that highly crystalline aluminous hydroxylated clays cannot be a major mineral component of the soil on Mars. The palagonite sample used in this study does not show this absorption feature in its spectrum. In mixtures of palagonite and iron-rich montmorillonite, the 2.2-μm AlOH clay lattice band is not seen below 15 wt% montmorillonite. This suggests the possibility that iron-rich montmorillonite clay may be present in the soil of Mars at up to 15 wt% in combination with palagonite, and remain undetected in remotely sensed spectra of Mars.

With the assumption that Mars is indeed their parent body, the SNC-meteorites can be used to calculate the bulk composition of Mars. The resulting chemistry of mantle and core as well as the core mass fit well with geophysical data of Mars, especially its density and its moment of inertia factor. With the data for Mars, the compositional similarity of the lunar and terrestrial mantle becomes even more striking as they differ considerably from that of Mars. This can be taken as further strong evidence for a close genetic relation of Moon and Earth, most likely to be explained by the impact model for the formation of the Moon. The Moon is depleted relative to the Earth for all volatile and moderately volatile elements. These depletions can be understood by a second fractionation step during the accretion of the Moon in an orbit around the Earth. Increasing depletion of siderophile elements in the lunar mantle with increasing siderophility are easy explainable by a second segregation of a small amount of metal. On the other hand, relative to C 1-abundances the lunar mantle shows the same striking depletion of Mn, Cr, V, and P as the Earth's mantle. To the contrary, these elements have normal abundances in the Martian mantle. A further special feature of the Earth's mantle the high abundance of Ni and Co is not observed in the Martian mantle.

Habitable environments must sustain liquid water at least intermittently and also provide both chemical building blocks and useful sources of energy for life. Observations by Spirit rover indicate that conditions have probably been too dry to sustain life, at least since the emplacement of the extensive basalts that underlie the plains around the Columbia Memorial Station landing site. Local evidence of relatively minor aqueous alteration probably occurred under conditions where the activity of water was too low to sustain biological processes as we know them. In contrast, multiple bedrock units in West Spur and Husband Hill in the Columbia Wills have been extensively altered, probably by aqueous processes. The Fe in several of these units has been extensively oxidized, indicating that, in principle, any microbiota present during the aqueous alteration of these rocks could have obtained energy from Fe oxidation. Spirit discovered oliving-rich ultramafic rocks during her descent from Husband Hill southward into Inner Basin. Alteration of similar ultramafic rocks on Earth can yield H2 that can provide both energy and reducing power for microorganisms. Spirit s discovery of "salty" soil horizons rich in Fe and/or Mg is consistent with the aqueous dissolution and/or alteration of olivine. Such processes can oxidize Fe and also yield H2 under appropriate conditions. Very high S concentrations in these salty deposits indicate that soluble salts were mobilized by water and/or that S oxidation, a potential energy source for life, occurred. The Athena team has not yet established whether these salt components were deposited as large beds in ancient water bodies or, for example, were concentrated by more recent groundwater activity. Collectively these observations are consistent with the possibility that habitable environments existed at least intermittently in the distant geologic past.

Based on our new and previous determinations of halogens in SNC meteorites, the bulk concentrations of halogens in the SPB, which is thought to be Mars, are estimated. The two-component model for the formation of terrestrial planets as proposed byA. E. Ringwood (Geochem. J. 11, 111–135 (1977) andOn the Origin of the Earth and Moon, Springer-Verlag, New York, 1979) andH. Wa¨nke (Philos. Trans. Roy. Soc. London, Ser. A 303, 287–302 (1981) is further substantiated. It is argued that almost all of the H2O added to Mars during its homogeneous accretion was converted on reaction with metallic Fe to H2, which escaped. By comparing the solubilities of H2O and HCl in molten silicates, the amount of H2O left in the mantle of Mars at the end of accretion can be related to the abundance of Cl. In this way an H2O content in the Martian mantle of 36 ppm is obtained, corresponding to an ocean covering the whole planet to a depth of about 130 m. The huge quantities of H2 produced by the reaction of H2O with metallic iron should also have removed other volatile species by hydrodynamic escape. Thus it is postulated that the present atmospheres of Venus, Earth, and Mars were formed by degassing the interiors of the planets, after the production of H2 had ceased, i.e., after metallic iron was no longer available. It is also postulated that the large differences in the amounts of primordial rare gases in the atmospheres of Venus, Earth, and Mars are due mainly to different loss factors. Except for gaseous species, Mars is found to be richer in volatile (halogens) and moderately volatile elements than the Earth. The resulting low release factor of40Ar for Mars is attributed to a low degree of fractionation, leading to a relatively small crustal enrichment of even the most incompatible elements like K.