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The abundances of water and highly to moderately volatile elements in planets are considered critical to mantle convection, surface evolution processes, and habitability. From the first flyby space probes to the more recent “Perseverance” and “Tianwen-1” missions, “follow the water,” and, more broadly, “volatiles,” has been one of the key themes of martian exploration. Ratios of volatiles relative to refractory elements (e.g., K/Th, Rb/Sr) are consistent with a higher volatile content for Mars than for Earth, despite the contrasting present-day surface conditions of those bodies. This study presents K isotope data from a spectrum of martian lithologies as an isotopic tracer for comparing the inventories of highly and moderately volatile elements and compounds of planetary bodies. Here, we show that meteorites from Mars have systematically heavier K isotopic compositions than the bulk silicate Earth, implying a greater loss of K from Mars than from Earth. The average “bulk silicate” δ(41)K values of Earth, Moon, Mars, and the asteroid 4-Vesta correlate with surface gravity, the Mn/Na “volatility” ratio, and most notably, bulk planet H(2)O abundance. These relationships indicate that planetary volatile abundances result from variable volatile loss during accretionary growth in which larger mass bodies preferentially retain volatile elements over lower mass objects. There is likely a threshold on the size requirements of rocky (exo)planets to retain enough H(2)O to enable habitability and plate tectonics, with mass exceeding that of Mars. more...

The Mars Science Laboratory rover, Curiosity, is using a comprehensive scientific payload to explore rocks and soils in Gale crater, Mars. Recent investigations of the Bagnold Dune Field provided the first in situ assessment of an active dune on Mars. The Chemistry and Mineralogy (CheMin) X-ray diffraction instrument on Curiosity performed quantitative mineralogical analyses of the more...

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. more...

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. more...

We provide an overview of the composition, vertical structure, and variability of the nightside ionosphere of Mars as observed by Mars Atmosphere and Volatile EvolutioN (MAVEN)'s Neutral Gas and Ion Mass Spectrometer (NGIMS) through 19 months of the MAVEN mission. We show that O 2+ is the most abundant ion down to ∼130 km at all nightside solar zenith angles (SZA). However, below 130 km NO+ is the most abundant ion, and NO+ densities increase with decreasing altitude down to at least 120 km. We also show how the densities of the major ions decrease with SZA across the terminator. At lower altitudes the O 2+ and CO 2+ densities decrease more rapidly with SZA than the NO+ and HCO+ densities, which changes the composition of the ionosphere from being primarily O 2+ on the dayside to being a mixture of O 2+, NO+, and HCO+ on the nightside. These variations are in accord with the expected ion-neutral chemistry, because both NO+ and HCO+ have long chemical lifetimes. Additionally, we present median ion density profiles from three different nightside SZA ranges, including deep on the nightside at SZAs greater than 150° and discuss how they compare to particle precipitation models. Finally, we show that nightside ion densities can vary by nearly an order of magnitude over monthlong timescales. The largest nightside densities were observed at high northern latitudes during winter and coincided with a major solar energetic particle event. more...

Comparing compositional models of the terrestrial planets provides insights into physicochemical processes that produced planet-scale similarities and differences. The widely accepted compositional model for Mars assumes Mn and more refractory elements are in CI chondrite proportions in the planet, including Fe, Mg, and Si, which along with O make up $>$90\% of the mass of Mars. However, recent improvements in our understandings on the composition of the solar photosphere and meteorites challenge the use of CI chondrite as an analog of Mars. Here we present an alternative model composition for Mars that avoids such an assumption and is based on data from Martian meteorites and spacecraft observations. Our modeling method was previously applied to predict the Earth's composition. The model establishes the absolute abundances of refractory lithophile elements in the bulk silicate Mars (BSM) at 2.26 times higher than that in CI carbonaceous chondrites. Relative to this chondritic composition, Mars has a systematic depletion in moderately volatile lithophile elements as a function of their condensation temperatures. Given this finding, we constrain the abundances of siderophile and chalcophile elements in the bulk Mars and its core. The Martian volatility trend is consistent with $ \leq $7 wt\% S in its core, which is significantly lower than that assumed in most core models (i.e., $ > $10 wt\% S). Furthermore, the occurrence of ringwoodite at the Martian core-mantle boundary might have contributed to the partitioning of O and H into the Martian core., 81 pages, 30 figures more...

Spacecraft exploring Mars such as the Mars Exploration Rovers Spirit and Opportunity, as well as the Mars Science Laboratory or Curiosity rover, have accumulated evidence for wet and habitable conditions on early Mars more than 3 billion years ago. Current conditions, by contrast, are cold, extremely arid and seemingly inhospitable. To evaluate exactly how dry today's environment is, it is important to understand the ongoing current weathering processes. Here we present chemical weathering rates determined for Mars. We use the oxidation of iron in stony meteorites investigated by the Mars Exploration Rover Opportunity at Meridiani Planum. Their maximum exposure age is constrained by the formation of Victoria crater and their minimum age by erosion of the meteorites. The chemical weathering rates thus derived are ∼1 to 4 orders of magnitude slower than that of similar meteorites found in Antarctica where the slowest rates are observed on Earth., Little is known about the impacts of Mars' contemporary dryness on weathering processes. Here, using iron oxidation estimates from the Mars Rover Opportunity, the authors quantify chemical weathering rates for Mars, finding appreciably slower rates compared with the lowest values on Earth. more...

Over 100 martian gully sites were analyzed using orbital data collected by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO). Most gullies are spectrally indistinct from their surroundings, due to mantling by dust. Where spectral information on gully sediments was obtained, a variety of mineralogies were identified. Their relationship to the source rock suggests that gully-forming processes transported underlying material downslope. There is no evidence for specific compositions being more likely to be associated with gullies, or with the formation of hydrated minerals in situ as a result of recent liquid water activity. Seasonal CO2 and H2O frosts were observed in gullies at mid- to high latitudes, consistent with seasonal frost-driven processes playing important roles in the evolution of gullies. Our results do not clearly indicate a role for long-lived liquid water in gully formation and evolution. more...

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 more...

Modern Martian dust is similar in composition to the global soil unit and bulk basaltic Mars crust, but it is enriched in S and Cl. The Alpha Particle X-ray Spectrometer (APXS) on the Mars Science Laboratory Curiosity rover analyzed air fall dust on the science observation tray (o-tray) in Gale Crater to determine dust oxide compositions. The o-tray dust has the highest concentrations of SO3 and Cl measured in Mars dust (SO3 8.3%; Cl 1.1 wt %). The molar S/Cl in the dust (3.35 ± 0.34) is consistent with previous studies of Martian dust and soils (S/Cl = 3.7 ± 0.7). Fe is also elevated ~25% over average Mars soils and the bulk crust. These enrichments link air fall dust with the S-, Cl-, and Fe-rich X-ray amorphous component of Gale Crater soil. Dust and soil have the same S/Cl, constraining the surface concentrations of S and Cl on a global scale. more...

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 more...

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. more...

Direct analysis of the composition of Mars is possible through delivery of meteorites to Earth. Martian meteorites include ∼165 to 2400 Ma shergottites, originating from depleted to enriched mantle sources, and ∼1340 Ma nakhlites and chassignites, formed by low degree partial melting of a depleted mantle source. To date, no unified model has been proposed to explain the petrogenesis of these distinct rock types, despite their importance for understanding the formation and evolution of Mars. Here we report a coherent geochemical dataset for shergottites, nakhlites and chassignites revealing fundamental differences in sources. Shergottites have lower Nb/Y at a given Zr/Y than nakhlites or chassignites, a relationship nearly identical to terrestrial Hawaiian main shield and rejuvenated volcanism. Nakhlite and chassignite compositions are consistent with melting of hydrated and metasomatized depleted mantle lithosphere, whereas shergottite melts originate from deep mantle sources. Generation of martian magmas can be explained by temporally distinct melting episodes within and below dynamically supported and variably metasomatized lithosphere, by long-lived, static mantle plumes., A unified model for the formation of martian rock types is required to understand Mars’s formation and evolution. Here the authors show that nakhlite and chassignite meteorites originate from melting of metasomatized depleted mantle lithosphere, whereas shergottite melts originate from deep plume sources. more...

The ChemCam laser-induced breakdown spectrometer on the rover Curiosity has provided more than 200,000 spectra from over 5000 different locations on Mars. This instrument is the first chemical microprobe on Mars and has an analytical footprint 0.3–0.6 mm in diameter. ChemCam has observed a measure of hydration in all the sedimentary materials encountered along the rover traverse in Gale Crater, indicating the ubiquity of phyllosilicates as a constituent of the analyzed sandstones, mudstones, and conglomerates. Diagenetic features, including calcium sulfate veins, millimeter-thick magnesium-rich diagenetic ridges, and manganese-rich rock surfaces, provide clues to water–rock interactions. Float clasts of coarse-grained igneous rocks are rich in alkali feldspars and some are enriched in fluorine, indicating greater magmatic evolution than expected on Mars. The identification of individual soil components has contributed to our understanding of the evolution of Martian soil. These observations have broadened our understanding of Mars as an active and once habitable planet. more...

The science investigations enabled by Curiosity rover's instruments focus on identifying and exploring the habitability of the Martian environment. Measurements of noble gases, organic and inorganic compounds, and the isotopes of light elements permit the study of the physical and chemical processes that have transformed Mars throughout its history. Samples of the atmosphere, volatiles released from soils, and rocks from the floor of Gale Crater have provided a wealth of new data and a window into conditions on ancient Mars. more...

Studies of rocks in Gale Crater and clasts within the Martian meteorite breccia Northwest Africa (NWA) 7034 (and paired stones) have expanded our knowledge of the diversity of igneous rocks that make up the Martian crust beyond those compositions exhibited in the meteorite collection or analyzed at any other landing site. Therefore, the magmas that gave rise to these rocks may have been generated at significantly different conditions in the Martian mantle than those derived from previously studied rocks. Here we build upon our previous models of basalt formation based on rocks analyzed in Gusev Crater and Meridiani Planum to the new models of basalt formation for compositions from Gale Crater and a clast in meteorite NWA 7034. Estimates for the mantle potential temperature, TP based on Noachian age rock analyses in Gale Crater, Gusev Crater, and Bounce Rock in Meridiani Planum, and a vitrophyre clast in NWA 7034 are within error, which suggests that the calculated average TP of 1450 ± 70°C may represent an average global mantle temperature during the Noachian. The TP estimates for the Hesperian and Amazonian, based on orbital analyses of the chemistry of the crust, are lower in temperature than our estimates for the Noachian, which is consistent with simple convective cooling of the interior of Mars. However, the TP estimates from the young meteorites are significantly higher than the estimates based on surface chemistry and are consistent with localized “hot spot” melting and not heating up of the interior. more...

© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. Variability in the sulfur isotopic composition in sediments can reflect atmospheric, geologic and biological processes. Evidence for ancient fluvio-lacustrine environments at Gale crater on Mars and a lack of efficient crustal recycling mechanisms on the planet suggests a surface environment that was once warm enough to allow the presence of liquid water, at least for discrete periods of time, and implies a greenhouse effect that may have been influenced by sulfur-bearing volcanic gases. Here we report in situ analyses of the sulfur isotopic compositions of SO2volatilized from ten sediment samples acquired by NASA's Curiosity rover along a 13 km traverse of Gale crater. We find large variations in sulfur isotopic composition that exceed those measured for Martian meteorites and show both depletion and enrichment in34S. Measured values of Δ34S range from - 47 ± 14‰ to 28 ± 7‰, similar to the range typical of terrestrial environments. Although limited geochronological constraints on the stratigraphy traversed by Curiosity are available, we propose that the observed sulfur isotopic signatures at Gale crater can be explained by equilibrium fractionation between sulfate and sulfide in an impact-driven hydrothermal system and atmospheric processing of sulfur-bearing gases during transient warm periods. more...

The first four rocks examined by the Mars Science Laboratory Alpha Particle X-ray Spectrometer indicate that Curiosity landed in a lithologically diverse region of Mars. These rocks, collectively dubbed the Bradbury assemblage, were studied along an eastward traverse (sols 46–102). Compositions range from Na- and Al-rich mugearite Jake_Matijevic to Fe-, Mg-, and Zn-rich alkali-rich basalt/hawaiite Bathurst_Inlet and span nearly the entire range in FeO* and MnO of the data sets from previous Martian missions and Martian meteorites. The Bradbury assemblage is also enriched in K and moderately volatile metals (Zn and Ge). These elements do not correlate with Cl or S, suggesting that they are associated with the rocks themselves and not with salt-rich coatings. Three out of the four Bradbury rocks plot along a line in elemental variation diagrams, suggesting mixing between Al-rich and Fe-rich components. ChemCam analyses give insight to their degree of chemical heterogeneity and grain size. Variations in trace elements detected by ChemCam suggest chemical weathering (Li) and concentration in mineral phases (e.g., Rb and Sr in feldspars). We interpret the Bradbury assemblage to be broadly volcanic and/or volcaniclastic, derived either from near the Gale crater rim and transported by the Peace Vallis fan network, or from a local volcanic source within Gale Crater. High Fe and Fe/Mn in Et_Then likely reflect secondary precipitation of Fe^(3+) oxides as a cement or rind. The K-rich signature of the Bradbury assemblage, if igneous in origin, may have formed by small degrees of partial melting of metasomatized mantle. more...

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. more...

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. more...

The knowledge of Martian salts has gone through substantial changes during the past decades. In the 70th of last century, Viking landers have noticed the existence of salts on Mars. Several salt species have been suggested from then on, such as sulfates and chlorides. However, their origin was a mystery due to the lack of observations. The recent explorations and related studies at the beginning of this century revealed that the crustal composition of Mars is similar to that of Earth, and it was hypothesized that almost one third of Martian surface was covered by oceans and lakes in the early stage of Mars. The huge water bodies may have dissolved a large quantity of ions from Martian primary rocks during the whole Noachian and Hesperian epoch. After the enormous drought event happened during the late Hesperian and the early Amazonian, these dissolved ions have formed huge salts deposits and most of them were preserved on Mars until today. To date, carbonates, sulfates, chlorides have all been detected by orbital remote sensing and by landers and rovers. However, the salt mineral assemblages on Mars seems to have some differences from those on Earth, e.g., rich in sulfates and lack of massive carbonates. To explain this difference, we propose that most of the surface carbonates precipitated from the ancient oceans may have been dissolved by the later ubiquitous acidic fluids originated from the global volcanism in the Hesperian era, and formed the enormous sulfate deposits as detected, and this hypothesis seems to be supported by the evidence that most of the sulfate deposits distribute around the Tharsis volcanic province while the survived carbonates located far from it. This process can release most of the carbon on Mars to the atmosphere in the form of CO2 and then be erased by the late heavy bombardments, which might have profound influence on the climate change happened in the Hesperian age. The positive correlation between the GRS results of the potassium distributions and the distribution of chlorides on Mars, together with the high Br concentration measured from the evaporate sediments at two Mars exploration rover landing sites, indicate that the brines in the regions where the chlorides deposited may have reached the stage for potassium salts deposition, thus we propose for the first time that potassium salts deposits might be prevalent in these regions. more...

[1] A one-dimensional model of the Martian ionosphere is used to explore the importance of atomic and molecular hydrogen chemistry in the upper atmosphere and ionosphere. Neutral and ionized H and H2 undergo chemical reactions that lead to the production of the hydrogenated ions: H+, H2+, H3+, OH+, HCO+, ArH+, N2H+, HCO2+, and HOC+. Simulations are conducted for the cases of photochemistry only and photochemistry coupled with transport in order to asses the separate effects of plasma diffusion in the topside ionosphere. For both of these cases, the sensitivity of the ionosphere is tested for (1) molecular hydrogen abundance and (2) reaction rate, k1, for the charge exchange between H+ and H2. Results are reported for midday solar minimum conditions. We find that the ionospheric composition of Mars is sensitive to H2 abundance, but relatively insensitive to the reaction rate, k1. Depending on the conditions simulated, the topside ionosphere can contain appreciable amounts of hydrogenated species such as H3+, OH+, and HCO+. Comparisons are made with Viking ion density measurements as well as with results of other published Mars ionospheric models. Future comparisons with more extensive ion composition will be available when the Mars Atmosphere and Volatile Evolution mission arrives at Mars. more...

In the search for habitable environments on Mars, sites that have been altered by liquid water represent an important target. Quickly dissolving and precipitating carbonates, found in multiple locations on Mars, represent a key potential mineral indicator of locations with a history of aqueous alteration, and therefore possible habitability. Recent liquid surface water on Mars may differ significantly from the fluids, either sedimentary, hydrothermal, or pedogenic, that formed carbonate minerals on Mars. Interactions of carbonate rocks with such recent surface water may therefore cause changes in chemistry and mineralogy with depth that form weathering profiles recording the conditions of alteration. To test the prediction that martian carbonates may record a signature of aqueous alteration on Mars, we performed reactive transport modeling of putative carbonate outcrops on Mars using the reactive transport code CrunchFlow. Model inputs included two parent rocks chosen based on the carbonate-rich rock at Gusev Crater, Comanche, one consisting of 60% magnesite, 20% siderite, and 20% porosity, and one consisting of 50% magnesite, 20% siderite, 9% calcite, 1.6% rhodochrosite, and ~20% porosity. Reacting solutions consisted of four scenarios: acidic, sulfate-containing solutions, perhaps resulting from an acid vapor scenario; dilute solutions resulting from martian analog basalt dissolution; concentrated solutions resulting from modeled evaporation of the dilute solutions; and solutions based on the results of the Phoenix Lander. Modeling was performed under low oxidation and high oxidation conditions. Based on the modeling results, a carbonate rock would become enriched in siderite upon interaction with acidic water under low oxidation conditions; the same rock would become enriched in calcite after interaction with Ca-enriched solutions. A carbonate rock reacting under more oxidizing conditions would preferentially dissolve siderite, leaving a surface enriched in magnesite and ferric oxide. Therefore, the Mg-rich carbonates on Mars are consistent with possible interaction with liquid water under oxidizing conditions, similar to those measured at the Phoenix landing site. Future detailed chemical and mineralogical profiles through weathered rock into the unaltered interior of carbonate rocks may therefore allow the quantitative interpretation of the aqueous history and possible habitability of locations on Mars. more...

Iron is the most abundant multivalent element in planetary reservoirs, meaning its isotope composition (expressed as δ57Fe) may record signatures of processes that occurred during the formation and subsequent differentiation of the terrestrial planets. Chondritic meteorites, putative constituents of the planets and remnants of undifferentiated inner solar system bodies, have δ57Fe ≈ 0‰; an isotopic signature shared with the Martian Shergottite–Nakhlite–Chassignite (SNC) suite of meteorites. The silicate Earth and Moon, as represented by basaltic rocks, are distinctly heavier, δ57Fe≈+0.1‰. However, some authors have recently argued, on the basis of iron isotope measurements of abyssal peridotites, that the composition of the Earth’s mantle is δ57Fe = +0.04 ± 0.04‰, indistinguishable from the mean Martian value. To provide a more robust estimate for Mars, we present new high-precision iron isotope data on 17 SNC meteorites and 5 mineral separates. We find that the iron isotope compositions of Martian meteorites reflect igneous processes, with nakhlites and evolved shergottites displaying heavier δ57Fe(+0.05 ± 0.03‰), whereas MgO-rich rocks are lighter (δ57Fe≈−0.01 ±0.02‰). These systematics are controlled by the fractionation of olivine and pyroxene, attested to by the lighter isotope composition of pyroxene compared to whole rock nakhlites. Extrapolation of the δ57Fe SNC liquid line of descent to a putative Martian mantle yields a δ57Fe value lighter than its terrestrial counterpart, but indistinguishable from chondrites. Iron isotopes in planetary basalts of the inner solar system correlate positively with Fe/Mn and silicon isotopes. While Mars and IV-Vesta are undepleted in iron and accordingly have chondritic δ57Fe, the Earth experienced volatile depletion at low (1300 K) temperatures, likely at an early stage in the solar nebula, whereas additional post-nebular Fe loss is possible for the Moon and angrites. more...

Identification of Martian surface minerals can contribute to understand the Martian environmental change and geological evolution as well as explore the habitability of the Mars. The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter (MRO) is covers the visible to near infrared wavelengths along with enhanced spectral resolution, which provides ability to map the mineralogy on Mars. In this paper, based on the spectrum matching, mineral composition and geological evolution of Martian Jezero and Holden crater are analyzed using the MRO CRISM. The hydrated minerals are detected in the studied areas, including the carbonate, hydrated silicate and hydrated sulfate. These minerals suggested that the Holden and Jezero craters have experienced long time water-rock interactions. Also, the diverse alteration minerals found in these regions indicate the aqueous activities in multiple distinct environments. more...

Clays form on Earth by near-surface weathering, precipitation in water bodies within basins, hydrothermal alteration (volcanic- or impact-induced), diagenesis, metamorphism, and magmatic precipitation. Diverse clay minerals have been detected from orbital investigation of terrains on Mars and are globally distributed, indicating geographically widespread aqueous alteration. Clay assemblages within deep stratigraphic units in the Martian crust include Fe/Mg smectites, chlorites and higher temperature hydrated silicates. Sedimentary clay mineral assemblages include Fe/Mg smectites, kaolinite, and sulfate, carbonate, and chloride salts. Stratigraphic sequences with multiple clay-bearing units have an upper unit with Al-clays and a lower unit with Fe/Mg-clays. The typical restriction of clay minerals to the oldest, Noachian terrains indicates a distinctive set of processes involving water-rock interaction that was prevalent early in Mars history and may have profoundly influenced the evolution of Martian geochemical systems. Current analyses of orbital data have led to the proposition of multiple clay-formation mechanisms, varying in space and time in their relative importance. These include near-surface weathering, formation in ice-dominated near-surface groundwaters, and formation by subsurface hydrothermal fluids. Near-surface, open system formation of clays would lead to fractionation of Mars’ crustal reservoir into an altered crustal reservoir and a sedimentary reservoir, potentially involving changes in the composition of Mars’ atmosphere. In contrast, formation of clays in the subsurface by either aqueous alteration or magmatic cooling would result in comparatively little geochemical fractionation or interaction of Mars’ atmospheric, crustal, and magmatic reservoirs, with the exception of long-term sequestration of water. Formation of clays within ice would have geochemical consequences intermediate between these endmembers. We outline the future analyses of orbital data, in situ measurements acquired within clay-bearing terrains, and analyses of Mars samples that are needed to more fully elucidate the mechanisms of martian clay formation and to determine the consequences for the geochemical evolution of the planet. more...

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. more...

Martian mineral detection and mapping can provide important information and constraints on Martian aqueous history, which can be used to assess the potential habitability of Mars. The key parameters to Martian aqueous alteration are the depth and extent of the Martian hydrous mineral. Therefore, it is important to know detailed minerals and chemical induction of the existence of water on the Martian surface at past or present. The Jezero crater located in the Nili Fossae region of Mars is the once-flooded crater, which has rich fan-delta deposit clays. It is a good case to study the clays and mineral components at Jezero crater, so as to know the geogloical processes and evolution on Mars. The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter (MRO) is a visible and near infrared spectrometer with enhanced spectral resolution, which provides an opportunity to map detailed and large-area mineralogy on Mars. In this paper, CRISM nearinfrared spectral data are analyzed using the mixture tuned filtering (MTMF) along with spectral angle mapper (SAM), and mineral components at Martian Jezero region are recognized, including the phyllosilicate, carbonate, nitrates and tectosil. Some detailed characteristics and implications of minerals at Martian Jezero crater are further studied and discussed, including implications on Martian climate change and geological evolution. more...

The sulfur cycle is arguably the most important geochemical cycle on Mars because the transfer of sulfur places limits on Mars's differentiation processes, sedimentary, geomorphic and aqueous processes, past climate, and current and past habitability. The presence of sulfur-rich compositions on Mars is suggested by meteorite data, in situ bulk chemical and mineralogical analyses, remote sensing data from dust and surfaces, and geochemical models. The inferred sulfur-rich nature of Mars may have resulted in an Fe–(Ni–)S core that has been liquid throughout Mars's history. On the surface, Mg- and Ca-sulfates are widespread and Fe3+-sulfates are found locally. It is likely that these minerals occur in a variety of hydration states and host much of the mineral-bound hydrogen in the Martian subsurface. more...

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. more...

Geochemical models for Mars predict carbonate formation during aqueous alteration. Carbonate-bearing rocks had not previously been detected on Mars' surface, but Mars Reconnaissance Orbiter mapping reveals a regional rock layer with near-infrared spectral characteristics that are consistent with the presence of magnesium carbonate in the Nili Fossae region. The carbonate is closely associated with both phyllosilicate-bearing and olivine-rich rock units and probably formed during the Noachian or early Hesperian era from the alteration of olivine by either hydrothermal fluids or near-surface water. The presence of carbonate as well as accompanying clays suggests that waters were neutral to alkaline at the time of its formation and that acidic weathering, proposed to be characteristic of Hesperian Mars, did not destroy these carbonates and thus did not dominate all aqueous environments. more...

Mossbauer spectroscopy of surface rocks, soil, and dust on Mars from the Mars Exploration Rovers (MER) suggests that the mineral olivine is widespread on the surface. Detection of the mineral by near-IR optical spectroscopy from Martian orbit indicates that it is found in relatively small isolated outcrops concentrated in the floors and rims of craters distributed around the ancient cratered highlands of Mars. To shed light on this apparent paradox, we have performed a detailed Mossbauer and visible-near-IR (VNIR) investigation of dust generated from Icelandic olivine basalt, which is a good Mossbauer analogue to the igneous rocks at Gusev crater on Mars. The results show that the amount of olivine relative to pyroxene can be underestimated by almost an order of a magnitude in VNIR reflectance spectra, most probably because of the longer effective optical path length in pyroxene compared to olivine. more...

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. more...

The Mars Science Laboratory Curiosity rover landed in Gale crater in August 2012 to assess the habitability of sedimentary deposits that show orbital evidence for diverse ancient aqueous environments. Gale crater contains a 5 km high mound of layered sedimentary rocks in its center, informally named Mount Sharp. The lowermost rocks of Mount Sharp contain minerals that are consistent with a dramatic climate change during Mars' early history. During the rover's traverse across the Gale crater plains to the base of Mount Sharp, Curiosity discovered sedimentary rocks consistent with a fluviolacustrine sequence. Curiosity studied ancient lacustrine deposits at Yellowknife Bay on the plains of Gale crater and continues to study ancient lacustrine deposits in the Murray formation, the lowermost unit of Mount Sharp. These investigations include drilling into the mudstone and delivering the sieved less than 150 micrometers fraction to the CheMin XRD/XRF instrument inside the rover. Rietveld refinement of XRD patterns measured by CheMin generates mineral abundances with a detection limit of 1-2 wt.% and refined unit-cell parameters of minerals present in abundances greater than approximately 5 wt.%. FULLPAT analyses of CheMin XRD patterns provide the abundance of X-ray amorphous materials and constrain the identity of these phases (e.g., opal-A vs. opal-CT). At the time of writing, CheMin has analyzed 14 samples, seven of which were drilled from lacustrine deposits. The mineralogy from CheMin, combined with in-situ geochemical measurements and sedimentological observations, suggest an evolution in the lake waters through time, including changes in pH and salinity and transitions between oxic and anoxic conditions. In addition to a geochemically dynamic lake environment, the igneous minerals discovered in the lake sediments indicate changes in source region through time, with input from mafic and silicic igneous sources. The Murray formation is predominantly comprised of lacustrine mudstone and is 150-200 m thick, suggesting long history of lake environments in Gale crater. Curiosity has traversed through the lowermost approximately 30 m of the Murray formation, and each additional sample provides clues about the climate on early Mars. more...

Mars Exploration Rover Opportunity is exploring the rim of 22 km diameter, Noachian-aged Endeavour crater. Marathon Valley cuts through the central region of the western rim providing a window into the local lower rim stratigraphic record. Spectra from the Compact Reconnaissance Imaging Spectrometer for Mars show evidence for the occurrence of Fe-Mg smectite in this valley, indicating areally extensive and distinct lithologic units and/or styles of aqueous alteration. The Alpha Particle X-ray Spectrometer has determined the compositions of 59 outcrop targets on untreated, brushed and abraded surfaces. Rocks in the Marathon Valley region are soft breccias composed of mm- to cm-sized darker clasts set in a lighter-toned, finegrained matrix. They are basaltic in non-volatile-element composition and compositionally similar to breccias investigated elsewhere on the rim. Alteration styles recorded in the rocks include: (1) Enrichments in Si, Al, Ti and Cr in more reddish-colored rock, consistent with leaching of more soluble cations and/or precipitation of Si +/- Al, Ti, Cr from fluids. Coprecipitation of Ge-rich phases with Si occurred in the western area only; high water:rock is indicated. Pancam multispectral observations indicate higher nanophase ferric oxide contents, but the rocks have lower Fe contents. The highly localized nature of the red zones indicate they cannot be the source of the widespread smectite signature observed from orbit. (2) Outcrops separated by approximately 65 m show common compositional changes between brushed and abraded (approximately 1 mm deep) targets: increases in S and Mg; decreases in Al, Cl and Ca. These changes are likely due to relatively recent, surface-related alteration of valley rocks and formation of surface coatings under low water:rock. (3) One target, from the center of a region of strong CRISM smectite signature, shows modest differences in composition (higher Si, K; lower Mn) compared to most Marathon Valley rocks, while another target approximately 40 cm away on the same outcrop does not; a change towards smectite bulk compositions is not observed. The smectite signature likely resulted from alteration under low water:rock such that primary minerals were partially altered to phyllosilicates, but wholesale leaching of cations by fluids did not occur. more...

The Earth formed in a swarm of Moon- to Mars-sized objects that collided together to build our planet. A large body of work has been dedicated to understanding the Earth's composition as being made of single groups or mixtures of chondrites, however, these models cannot account for the isotopic and elemental characteristics of the Earth. Here, we test mixtures of meteorites, including achondrites, analyzed for seven isotope systems (O, Cr, Ni, Ti, Mo, Ca and Sr), to reproduce the isotope compositions of the Earth and Mars. Our Monte Carlo inversion (a numerical method based on generation of random numbers used to invert multiparameter models) yields a new compositional model where Earth and Mars come almost entirely from the same source material. This finding is in striking agreement with recent planetary formation models in which Earth and Mars formed in a common narrow zone of the protoplanetary disk with Mars being ejected to its current position which prevented further accretion. An important outcome of the model is that a significant mass fraction of the Earth could have been made of volatile depleted and refractory enriched planetary bodies such as angrites (among the oldest known achondrites). This conclusion is also in agreement with new Si isotope data in angrites which suggest that a component of angrites would help explain the difference in delta Si-30 between the bulk silicate Earth and its building blocks. Our model matches all isotope compositions for both planets, reproduces the volatile element budget of Mars, and accounts for the enrichment in refractory elements of the Earth and Mars compared to chondrites. (C) 2015 Elsevier B.V. All rights reserved. more...

Phase equilibrium experiments have been performed on a synthetic analog of the Gusev plains basalt composition from the Spirit landing site on Mars. Near-liquidus phase relations were determined over the pressure range of 0.1 to 1.5 GPa and at temperatures from 1125 to 1390 °C in a piston cylinder apparatus and 1 atm gas mixing furnace. The composition is multiply saturated with olivine, orthopyroxene, and spinel near its liquidus at 1320 °C and 1.0 GPa, or 85 km depth on Mars, placing an upper limit constraint on the thickness of the Martian lithosphere at the time of eruption. Our experimental work suggests that the Gusev basalts are anhydrous batch melts of a primitive Martian mantle similar to the composition estimated by Dreibus and Wnke (1984). The temperature of multiple saturation indicates the persistence of high mantle potential temperatures on Mars, similar to those on the modern Earth, until at least the very latest Noachian (3.7 Ga). These high mantle temperatures would be responsible for persistent basaltic volcanism throughout the southern highlands during the first billion years of Mars's history. The source for Gusev basalts differs strongly from the source for shergottite meteorites, reinforcing the idea of the absence of global mantle convection and mixing on Mars. The existence of a relatively primitive mantle reservoir requires that at least part of the mantle underwent little modification during early planetary differentiation. more...

The element iron plays a crucial role in the study of the evolution of matter from an interstellar cloud to the formation and evolution of the planets. In the Solar System iron is the most abundant metallic element. It occurs in at least three different oxidation states: Fe(0) (metallic iron), Fe(II) and Fe(III). Fe(IY) and Fe(VI) compounds are well known on Earth, and there is a possibility for their occurrence on Mars. In January 2004 the USA space agency NASA landed two rovers on the surface of Mars, both carrying the Mainz Mossbauerspectrometer MIMOS II. They performed for the first time in-situ measurements of the mineralogy of the Martian surface, at two different places on Mars, Meridiani Planum and Gusev crater, respectively, the landing sites of the Mars-Exploration-Rovers (MER) Opportunity and Spirit. After about two Earth years or one Martian year of operation the Mossbauer (MB) spectrometers on both rovers have acquired data from more than 150 targets (and more than thousand MB spectra) at each landing site. The scientific measurement objectives of the Mossbauer investigation are to obtain for rock, soil, and dust (1) the mineralogical identification of iron-bearing phases (e.g., oxides, silicates, sulfides, sulfates, and carbonates), (2) the quantitative measurement of the distribution of iron among these ironbearing phases (e.g., the relative proportions of iron in olivine, pyroxenes, ilmenite and magnetite in a basalt), (3) the quantitative measurement of the distribution of iron among its oxidation states (e.g., Fe2+, Fe3+, and Fe6+), and (4) the characterization of the size distribution of magnetic particles. Special geologic targets of the Mossbauer investigation are dust collected by the Athena magnets and interior rock and soil surfaces exposed by the Athena Rock Abrasion Tool and by trenching with rover wheels. The Mossbauer spectrometer on Opportunity at Meridiani Planum, identified eight Fe-bearing phases: jarosite (K,Na,H30)(Fe,AI)(OH)6(S04)2, hematite, olivine, pyroxene, magnetite, nanophase ferric oxides (npOx), an unassigned ferric phase, and a metallic Fe-Ni alloy (kamacite) in a Fe-Nimeteorite. Outcrop rocks consist ofhematite-rich spherules dispersed throughout S-rich rock that has nearly constant proportions of Fe3+ from jarosite, hematite, and npOx (28%, 35%, and 19% of total Fe). Jarosite is mineralogical evidence for aqueous processes under acidsulfate conditions because it has structural hydroxide and sulfate and it forms at low pH. Hematite-rich spherules, eroded from the outcrop, and their fragments are concentrated as hematite-rich soils (lag deposits) on ripple crests (up to 68% of total Fe from hematite). Olivine, pyroxene, and magnetite are primarily associated with basaltic soils and are present as thin and locally discontinuous cover over outcrop rocks, commonly forming aeolian bedforms. Basaltic soils are more reduced (Fe3+/IFetotal ∼0.2–0.4), with the fine-grained and bright aeolian deposits being the most oxidized. Basaltic soil at Meridiani Planum and Gusev crater have similar Fe-mineralogical compositions. At Gusev crater, the Mossbauer spectrometer on the MER Spirit rover has identified 8 Fe-bearing phases. Two are Fe2+ silicates (olivine and pyroxene), one is a Fe2+ oxide (ilmenite), one is a mixed Fe2+ and Fe3+ oxide (magnetite), two are Fe3+ oxides (hematite and goethite), one is a Fe3+ sulfate (mineralogically not constrained), and one is a Fe3+ alteration product (npOx). The surface material in the plains have a olivine basaltic signature (Morris, et al., Science, 305: 833, 2004; Morris, et al., J. Geophys. Res., 111, 2006, Ming, et al., J. Geophys. Res., 111, 2006) suggesting physical rather than chemical weathering processes present in the plains. The Mossbauer signature for the Columbia Hills surface material is very different ranging from nearly unaltered material to highly altered material. Some of the rocks, in particular a rock named Clovis, contain a significant amount of the Fe oxyhydroxide goethite, α-FeOOH, which is mineralogical evidence for aqueous processes because it is formed only under aqueous conditions. more...

The recent identification of large deposits of sulphates by remote sensing and in situ observations has been considered evidence of the past presence of liquid water on Mars. Here we report the unambiguous detection of diverse phyllosilicates, a family of aqueous alteration products, on the basis of observations by the OMEGA imaging spectrometer on board the Mars Express spacecraft. These minerals are mainly associated with Noachian outcrops, which is consistent with an early active hydrological system, sustaining the long-term contact of igneous minerals with liquid water. We infer that the two main families of hydrated alteration products detected-phyllosilicates and sulphates--result from different formation processes. These occurred during two distinct climatic episodes: an early Noachian Mars, resulting in the formation of hydrated silicates, followed by a more acidic environment, in which sulphates formed. more...

In January 2004 the USA space agency NASA landed two rovers on the surface of Mars, both carrying the Mainz Mossbauer spectrometer MIMOS II. The instrument on the Mars-Exploration-Rover (MER) Spirit analyzed soils and rocks on the plains and in the Columbia Hills of Gusev crater landing site on Mars. The surface material in the plains have an olivine basaltic signature [1, 5] suggesting physical rather than chemical weathering processes present in the plains. The Mossbauer signature for the Columbia Hills surface material is very different ranging from nearly unaltered material to highly altered material. Some of the rocks, in particular a rock named Clovis, contain a significant amount of the Fe oxyhydroxide goethite, α-FeOOH, which is mineralogical evidence for aqueous processes because it is formed only under aqueous conditions. In this paper we describe the analysis of these data using hyperfine field distributions (HFD) and discuss the results in comparison to terrestrial analogues. more...

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. more...

The cover shows part of the Larry's Lookout panorama, seen from the Mars Exploration Rover (MER) Spirit during its drive up Husband Hill: the summit is about 200 metres from the rover. Six papers this week report in detail on the MER mission. An Analysis compares predictions used to select a landing site with the conditions actually encountered. This ‘ground truth’ will be invaluable for interpreting future remote-sensing data. Surface chemistry suggests that the upper layer of soil may contain 1% meteoritic material. MER provides a unique glimpse of solar transits of the moons Phobos and Deimos. Rover Opportunity examined wind-related processes, and spectroscopy indicates a dry origin for atmospheric dust. Features from within the Gusev crater give more information on the role of liquid water in Mars's past. An accompanying News and Views puts the MER data in context. Gusev crater was selected as the landing site for the Spirit rover because of the possibility that it once held a lake. Thus one of the rover's tasks was to search for evidence of lake sediments1. However, the plains at the landing site were found to be covered by a regolith composed of olivine-rich basaltic rock and windblown ‘global’ dust2. The analyses of three rock interiors exposed by the rock abrasion tool showed that they are similar to one another, consistent with having originated from a common lava flow3,4,5,6,7,8. Here we report the investigation of soils, rock coatings and rock interiors by the Spirit rover from sol (martian day) 1 to sol 156, from its landing site to the base of the Columbia hills. The physical and chemical characteristics of the materials analysed provide evidence for limited but unequivocal interaction between water and the volcanic rocks of the Gusev plains. This evidence includes the softness of rock interiors that contain anomalously high concentrations of sulphur, chlorine and bromine relative to terrestrial basalts and martian meteorites9; sulphur, chlorine and ferric iron enrichments in multilayer coatings on the light-toned rock Mazatzal; high bromine concentration in filled vugs and veins within the plains basalts; positive correlations between magnesium, sulphur and other salt components in trench soils; and decoupling of sulphur, chlorine and bromine concentrations in trench soils compared to Gusev surface soils, indicating chemical mobility and separation. more...

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. more...

The Mars Exploration Rover Opportunity has investigated the landing site in Eagle crater and the nearby plains within Meridiani Planum. The soils consist of fine-grained basaltic sand and a surface lag of hematite-rich spherules, spherule fragments, and other granules. Wind ripples are common. Underlying the thin soil layer, and exposed within small impact craters and troughs, are flat-lying sedimentary rocks. These rocks are finely laminated, are rich in sulfur, and contain abundant sulfate salts. Small-scale cross-lamination in some locations provides evidence for deposition in flowing liquid water. We interpret the rocks to be a mixture of chemical and siliciclastic sediments formed by episodic inundation by shallow surface water, followed by evaporation, exposure, and desiccation. Hematite-rich spherules are embedded in the rock and eroding from them. We interpret these spherules to be concretions formed by postdepositional diagenesis, again involving liquid water. more...

We interpret recent spectral data of Mars collected by the Mars Exploration Rovers to contain substantial evidence of sulfate minerals and aqueous processes. We present visible/near-infrared (VNIR), mid-IR and Mössbauer spectra of several iron sulfate minerals and two acid mine drainage (AMD) samples collected from the Iron Mountain site and compare these combined data with the recent spectra of Mars. We suggest that the sulfates on Mars are produced via aqueous oxidation of sulfides known to be present on Mars from Martian meteorites. The sulfate-rich rock outcrops observed in Meridiani Planum may have formed in an acidic environment similar to AMD environments on Earth. Because microorganisms are typically involved in the oxidation of sulfides to sulfates in terrestrial AMD sites, sulfate-rich rock outcrops on Mars may be a good location to search for evidence of life on that planet. Whether or not life evolved on Mars, following the trail of sulfate minerals is likely to lead to aqueous processes and chemical weathering. Our results imply that sulfate minerals formed in Martian soils via chemical weathering, perhaps over very long time periods, and that sulfate minerals precipitated following aqueous oxidation of sulfides to form the outcrop rocks at Meridiani Planum. more...

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. more...

The Spirit landing site in Gusev Crater on Mars contains dark, fine-grained, vesicular rocks interpreted as lavas. Pancam and Mini–Thermal Emission Spectrometer (Mini-TES) spectra suggest that all of these rocks are similar but have variable coatings and dust mantles. Magnified images of brushed and abraded rock surfaces show alteration rinds and veins. Rock interiors contain ≤25% megacrysts. Chemical analyses of rocks by the Alpha Particle X-ray Spectrometer are consistent with picritic basalts, containing normative olivine, pyroxenes, plagioclase, and accessory FeTi oxides. Mössbauer, Pancam, and Mini-TES spectra confirm the presence of olivine, magnetite, and probably pyroxene. These basalts extend the known range of rock compositions composing the martian crust. more...

One of the current goals of Martian exploration is to find evidence for extinct (or even extant) life. Carbon (an essential ingredient of life on Earth) is known to occur on Mars as CO2 in the atmosphere and frozen in the polar caps; it is inferred to be present as carbonates in the Martian crust and soils. We are attempting to define and quantify the different carbon reservoirs on Mars, so that we can follow Mars' carbon cycle. This paper discusses a primordial magmatic component that could be the starting point of such a cycle. The nature, distribution and isotopic composition of carbon was measured in a suite of Martian meteorites, comprising Chassigny and 11 shergottites. Other Martian meteorites were not included, as they sample rocks that have been altered by fluids at Mars' surface. Our results, obtained by high-resolution stepped combustion and mass spectrometry, show that the magmatic component has a very variable abundance of 1–100 ppm, with δ13C~−20±4‰. This value is close to magmatic carbon determined for Moon and for Vesta (the parent body of the HED basaltic meteorites), but very different from that of Earth. more...

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. more...