Alteration mineralogy of the Home Plate and Columbia Hills – formation conditions in context to impact, volcanism, and fluvial activity

The Mars Exploration Rover Spirit investigated the igneous and alteration mineralogy and chemistry of Home Plate and its surrounding deposits. Here, we focus on using thermochemical modeling to understand the secondary alteration mineralogy at the Home Plate outcrop and surrounding Columbia Hills region in Gusev crater. At high temperatures (300 °C), magnetite occurs at very high W/R ratios, but the alteration assemblage is dominated by chlorite and serpentine over most of the W/R range. Quartz, epidote, and typical high-T phases such as feldspar, pyroxene, and garnet occur at low W/R. At epithermal temperatures (150 °C), hematite occurs at very high W/R. A range of phyllosilicates, including kaolinite, nontronite, chlorite, and serpentine are precipitated at specific W/R. Amphibole, with garnet, feldspar, and pyroxene occur at low W/R. If the CO2 content of the system is high, the assemblage is dominated by carbonate with increasing amounts of an SiO2-phase, kaolinite, carpholite, and chlorite with lower W/R. At temperatures of hydrous weathering (13 °C), the oxide phase is goethite, silicates are chlorite, nontronite, and talc, plus an SiO2-phase. In the presence of CO2, the mineral assemblage at high W/R remains the same, and only at low W/R, i.e., with increasing salinity, carbonate precipitates. The geochemical gradients observed at Home Plate are attributed to short-lived, initially high (300 °C) temperature, but fast cooling events, which are in agreement with our models and our interpretation of a multistage alteration scenario of Home Plate and Gusev in general. Alteration at various temperatures and during different geological processes within Gusev crater has two effects, both of which increase the habitability of the local environment: precipitation of hydrous sheet silicates, and formation of a brine, which might contain elements essential for life in diluted, easily accessible form

The Microbial Community of a Terrestrial Anoxic Inter-Tidal Zone: A Model for Laboratory-Based Studies of Potentially Habitable Ancient Lacustrine Systems on Mars

Evidence indicates that Gale crater on Mars harboured a fluvio-lacustrine environment that was subjected to physio-chemical variations such as changes in redox conditions and evaporation with salinity changes, over time. Microbial communities from terrestrial environmental analogues sites are important for studying such potential habitability environments on early Mars, especially in laboratory-based simulation experiments. Traditionally, such studies have predominantly focused on microorganisms from extreme terrestrial environments. These are applicable to a range of Martian environments; however, they lack relevance to the lacustrine systems. In this study, we characterise an anoxic inter-tidal zone as a terrestrial analogue for the Gale crater lake system according to its chemical and physical properties, and its microbiological community. The sub-surface inter-tidal environment of the River Dee estuary, United Kingdom (53°21'015.40" N, 3°10'024.95" W) was selected and compared with available data from Early Hesperian-time Gale crater, and temperature, redox, and pH were similar. Compared to subsurface ‘groundwater’-type fluids invoked for the Gale subsurface, salinity was higher at the River Dee site, which are more comparable to increases in salinity that likely occurred as the Gale crater lake evolved. Similarities in clay abundance indicated similar access to, specifically, the bio-essential elements Mg, Fe and K. The River Dee microbial community consisted of taxa that were known to have members that could utilise chemolithoautotrophic and chemoorganoheterotrophic metabolism and such a mixed metabolic capability would potentially have been feasible on Mars. Microorganisms isolated from the site were able to grow under environment conditions that, based on mineralogical data, were similar to that of the Gale crater’s aqueous environment at Yellowknife Bay. Thus, the results from this study suggest that the microbial community from an anoxic inter-tidal zone is a plausible terrestrial analogue for studying habitability of fluvio-lacustrine systems on early Mars, using laboratory-based simulation experiments

Viable metabolisms in a simulated martian environments

Microbes have multiple ways of producing energy. Which of these methods are possible depends on the chemistry of the environment the microbes are in (e.g. not enough of a metal or too much salt), with only specific methods working in certain environments. The same would be true of any waters that might continue to exist on Mars. To narrow down which methods of producing energy would be theoretically possible we simulated martian waters using a collection of minerals that are chemically similar to the chemistry measured by the Mars rover Curiosity in a crater on Mars. We added mud from an estuary to the simulated martian water and identified which microbes were able to grow. We then repeatedly transferred the growing microbes to fresh “martian” water to dilute out the nutrients from the mud. Over time we observed that most of the microbes from the mud have been lost but a few specific microbes were growing well. From this we hope to investigate changes in the chemistry of the water that happen because of these microbes, to try and identify specific chemistries that can be looked for by the future rover missions on Mars seeking evidence of life

Alteration minerals, fluids, and gases on early Mars: Predictions from 1-D flow geochemical modeling of mineral assemblages in meteorite ALH 84001

Clay minerals, although ubiquitous on the ancient terrains of Mars, have not been observed in Martian meteorite Allan Hills (ALH) 84001, which is an orthopyroxenite sample of the early Martian crust with a secondary carbonate assemblage. We used a low-temperature (20 °C) one-dimensional (1-D) transport thermochemical model to investigate the possible aqueous alteration processes that produced the carbonate assemblage of ALH 84001 while avoiding the coprecipitation of clay minerals. We found that the carbonate in ALH 84001 could have been produced in a process, whereby a low-temperature (~20 °C) fluid, initially equilibrated with the early Martian atmosphere, moved through surficial clay mineral and silica-rich layers, percolated through the parent rock of the meteorite, and precipitated carbonates (thereby decreasing the partial pressure of CO2) as it evaporated. This finding requires that before encountering the unweathered orthopyroxenite host of ALH 84001, the fluid permeated rock that became weathered during the process. We were able to predict the composition of the clay minerals formed during weathering, which included the dioctahedral smectite nontronite, kaolinite, and chlorite, all of which have been previously detected on Mars. We also calculated host rock replacement in local equilibrium conditions by the hydrated silicate talc, which is typically considered to be a higher temperature hydrothermal phase on Earth, but may have been a common constituent in the formation of Martian soils through pervasive aqueous alteration. Finally, goethite and magnetite were also found to precipitate in the secondary alteration assemblage, the latter associated with the generation of H2. Apparently, despite the limited water–rock interaction that must have led to the formation of the carbonates ~ 3.9 Ga ago, in the vicinity of the ALH 84001 source rocks, clay formation would have been widespread

Constraints on the formation of carbonates and low‐grade metamorphic phases in the Martian crust as a function of H₂O‐CO₂ fluids

Low‐grade metamorphic hydrous minerals and carbonates occur in various settings on Mars and in Martian meteorites. We present constraints on the stability of prehnite, zeolites, serpentine, and carbonates by modeling the influence of H2O‐CO2 fluids during low‐grade metamorphism in the Martian crust using compositions of a Martian basalt and an ultramafic cumulate. In basaltic compositions with 5 wt% fluid, our models predict prehnite in less oxidized, CO2‐poor conditions (≤0.44 mol kg−1 CO2) on warmer geotherms of 20 °C km−1. At fluid‐saturated conditions, epidote and laumontite are replaced by quartz, calcite, chlorite, and muscovite. In ultramafic compositions with 5 wt% fluid, antigorite (serpentine) is stable at CO2‐poor conditions of ≤0.33 mol kg−1, while talc forms at 0.05–0.56 mol kg−1 CO2. At fluid‐saturated conditions, antigorite is replaced by talc and chlorite, and at higher X(CO2) by magnesite and quartz. Our models therefore suggest that prehnite, zeolites, and serpentine have formed in a CO2‐poor environment on Mars implying that fluids during their formation either did not contain high amounts of CO2 or had degassed CO2. Carbonates and potentially talc would have formed in the presence of a CO2‐bearing fluid and therefore at different alteration stages than for prehnite, zeolites, and serpentine either in the same hydrothermal event during which the fluid composition changed gradually due to cooling and precipitation or by separate and successive alteration events with fluids of different compositions

A review of volatiles in the Martian interior

Multiple observations from missions to Mars have revealed compelling evidence for a volatile-rich Martian crust. A leading theory contends that eruption of basaltic magmas was the ultimate mechanism of transfer of volatiles from the mantle toward the surface after an initial outgassing related to the crystallization of a magma ocean. However, the concentrations of volatile species in ascending magmas and in their mantle source regions are highly uncertain. This work and this special issue of Meteoritics & Planetary Science summarize the key findings of the workshop on Volatiles in the Martian Interior (Nov. 3–4, 2014), the primary open questions related to volatiles in Martian magmas and their source regions, and the suggestions of the community at the workshop to address these open questions

Testing the habitability of distinct simulated martian environments

Habitability of martian waters would have been partially determined by the chemistry arising from interactions with martian lithologies. In this study, the habitability of groundwater chemistries (based on basaltic, iron- and sulfur-enriched lithologies) and the resulting variation in biosignatures was investigated, with microbes from anaerobic estuarine sediment used as an inoculum. The microbial community was monitored by cell counts and 16S rRNA gene profiling. Changes in fluid and precipitate chemistries were measured using ICP-OES and IC, with changes over geological timescales modelled using CHIM-XPT. The fluid chemistries were shown to be habitable, with distinct patterns in cell abundance and growth phases between the chemistries. However, the same genera dominated (Acetobacterium, Desulfovibrio and Desulfosporomusa) regardless of the initial fluid chemistry. In the biotic test group, changes in fluid chemistry were the same in the three chemistries, with an enhanced concentration of aluminium and iron and the removal of sulfate. However, geochemical modelling of the fluids under abiotic conditions over geological timescales revealed similar changes to those in the biotic test groups. Therefore, these samples require further analysis to assess whether we can identify any potentially unambiguous biosignatures that could develop between geologically distinct sites

Colour Peak:An analogue environment for the waters of late Noachian Mars

The surface of Mars cannot sustain liquid water today, but there is evidence water was present during the Noachian era. The transition of the martian climate from the wet Noachian to the dry Hesperian would have resulted in saline and sulfur rich surface waters . Terrestrial analogue environments that possess a chemistry like these proposed waters can be used to develop an understanding of organisms that could have persisted under such conditions. Here we present the chemistry and microbiome of the analogue environment Colour Peak, a sulfidic and saline spring system located within the Canadian High Arctic. In this study, molecular and geochemical techniques were used to investigate the sediment of the Colour Peak springs. Nucleic acids were extracted from the microbes in the sediments and the microbiome was characterised by the amplification and sequencing of 16S rRNA gene amplicons. The elemental composition of the fluids and sediment was determined by ICP-OES and compared with brines determined from the chemistry of the “Rocknest” sand sample at Yellowknife Bay, Gale Crater (Mars) by thermochemical modelling. Gibbs energy values were calculated from this fluid chemistry to identify potentially viable metabolisms. Analysis of the chemistries of the Colour Peak fluids confirmed a chemical composition like the thermochemically modelled fluid, with this justifying the classification of Colour Peak as an appropriate analogue environment to investigate the habitability of former martian aqueous environments. 16S rRNA gene profiling of the Colour Peak microbial community revealed it was dominated by bacteria associated with oxidation of reduced sulfur species and carbon dioxide fixation. Gibbs energy values calculated using the chemistry of the modelled martian fluid demonstrated that the oxidation of reduced sulfur species was also viable in this chemical environment under aerobic and anaerobic conditions. These results demonstrate that microbial sulfide oxidation is thermodynamically viable using both modelled and environmental proxies for former martian aqueous environments. This study highlights that metabolisms utilising the oxidation of reduced sulfur species could have been thermodynamically viable in ancient martian aqueous environments. Further work is needed to assess this proposed viability and the potential for unambiguous biosignature formation

Geochemical bio-signatures in Martian analogue basaltic environments using laboratory experiments and thermochemical modelling

The identification of geochemical bio-signatures is important for assessing whether life existed on early Mars. In this paper, experimental microbiology and thermochemical modelling were combined to identify potential inorganic bio-signatures for life detection on early Mars. An analogue mixed microbial community from an analogue terrestrial fluvio-lacustrine environment similar to an ancient lacustrine system at Gale Crater was used to study microbial dissolution of a basalt regolith simulant and the formation of bio-signatures over a short time frame (1°month) at 14°C, 2 bar. Microbial growth influenced element dissolution (Mg, Fe, Mn, Ca and K) and the formation of morphologies and Fe-Si amorphous layers on mineral surfaces. Thermochemical models were performed at 14°C, 2 bar; the results were compared with experimental data to predict bio-signatures that would occur over geological timescales. The pH was varied to simulate abiotic and biotic experimental conditions. Model results suggest that, at water to rock ratios of 100 to 38, a less complex secondary mineral assemblage forms during biotic dissolution compared to abiotic weathering. Carbonates, quartz, pyrite and hydroxyapatite form under biotic conditions, whereas in the abiotic system magnetite and phyllosilicates would also precipitate. These results could be used to distinguish between abiotic and biotic basalt weathering processes, aiding the interpretation of data from Mars exploration missions

Colour Peak: An analogue environment for late Noachian Mars

The martian surface cannot sustain liquid water today, but there is evidence water was present during the Noachian era. The transition of the martian climate into the Hesperian would have resulted in saline and sulfuric waters. Terrestrial analogue environments that possess a chemistry like these proposed waters can be used to develop an understanding of organisms that could have persisted. Here we present the chemistry and microbiome of Colour Peak, a sulfidic and saline spring system located within the Canadian High Arctic. Nucleic acids were extracted from the microbes in the sediments and the microbiome was characterised by the amplification and sequencing of 16S rRNA gene amplicons. The elemental composition of the fluids and sediment was determined by ICP-OES and compared with brines determined from the chemistry of the “Rocknest” sample at Yellowknife Bay, Gale Crater (Mars) by thermochemical modelling. Gibbs energy values were calculated from this fluid chemistry to identify potentially viable metabolisms. Analysis of the chemistries of the Colour Peak fluids confirmed a composition like the thermochemically modelled fluid, providing justification for the classification of Colour Peak as an appropriate analogue environment to investigate the habitability of former martian waters. Profiling of the Colour Peak microbial community revealed domination by bacteria associated with oxidation of reduced sulfur species and carbon dioxide fixation. Gibbs energy values calculated using the modelled martian fluid chemistry demonstrated that oxidation of reduced sulfur species was also viable in this chemical environment under aerobic and anaerobic conditions. These results demonstrate microbial sulfide oxidation is thermodynamically viable using both modelled and environmental proxies for former martian aqueous environments. This study highlights that metabolisms utilising the oxidation of reduced sulfur species could have been thermodynamically viable in ancient martian aqueous environments. Further work is needed to test this viability and the subsequent potential for biosignature formation