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Tirez was a small and seasonal endorheic athalassohaline lagoon that was located in central Spain. In recent years, the lagoon has totally dried out, offering for the first time the opportunity to analyze its desiccation process as a "time-analog" to similar events occurred in paleolakes with varying salinity during the wet-to-dry transition on early Mars. On the martian cratered highlands, an early period of water ponding within enclosed basins evolved to a complete desiccation of the lakes, leading to deposition of evaporitic sequences during the Noachian and into the Late Hesperian. As Tirez also underwent a process of desiccation, here we describe (i) the microbial ecology of Tirez when the lagoon was still active 20 years ago, with prokaryotes adapted to extreme saline conditions; (ii) the composition of the microbial community in the dried lake sediments today, in many case groups that thrive in sediments of extreme environments; and (iii) the molecular and isotopic analysis of the lipid biomarkers that can be recovered from the sediments today. We discuss the implications of these results to better understanding the ecology of possible Martian microbial communities during the wet-to-dry transition at the end of the Hesperian, and how they may inform about research strategies to search for possible biomarkers in Mars after all the water was lost., (© 2023. The Author(s).)

In the subsurface, the interplay between microbial communities and the surrounding mineral substrate, potentially used as an energy source, results in different mineralized structures. The molecular composition of such structures can record and preserve information about the metabolic pathways that have produced them. To characterize the molecular composition of the subsurface biosphere, we have analyzed some core samples by time-of-flight secondary ion mass spectrometry (ToF-SIMS) that were collected in the borehole BH8 during the operations of the Mars Analog and Technology Experiment (MARTE) project. The molecular analysis at a micron-scale mapped the occurrence of several inorganic complexes bearing PO 3 - , SO x (2 to 4) - , NO x (2,3) - , FeO x (1,2) - , SiO 2 - , and Cl - . Their distribution correlates with organic molecules that were tentatively assigned to saturated and monounsaturated fatty acids, polyunsaturated fatty acids, saccharides, phospholipids, sphingolipids, and potential peptide fragments. SO x - appear to be mineralizing some microstructures larger than 25 microns, which have branched morphologies, and that source SO 3 -bearing adducts. PO 3 -rich compounds occur in two different groups of microstructures which size, morphology, and composition are different. While a group of >40-micron sized circular micronodules lacks organic compounds, an ovoidal microstructure is associated with m/z of other lipids. The NO 2 - /NO 3 - and Cl - ions occur as small microstructure clusters (<20 microns), but their distribution is dissimilar to the mineralized microstructures bearing PO 3 - , and SO 3 - . However, they have a higher density in areas with more significant enrichment in iron oxides that are traced by different Fe-bearing anions like FeO 2 - . The distribution of the organic and inorganic negative ions, which we suggest, resulted from the preservation of at least three microbial consortia (PO 4 - -, and NO 2 - -/NO 3 - -mineralizers PO 4 -lipid bearing microstructures), would have resulted from different metabolic and preservation pathways.

The limits of life of aerobic microorganisms are well understood, but the responses of anaerobic microorganisms to individual and combined extreme stressors are less well known. Motivated by an interest in understanding the survivability of anaerobic microorganisms under Martian conditions, we investigated the responses of a new isolate, Yersinia intermedia MASE-LG-1 to individual and combined stresses associated with the Martian surface. This organism belongs to an adaptable and persistent genus of anaerobic microorganisms found in many environments worldwide. The effects of desiccation, low pressure, ionizing radiation, varying temperature, osmotic pressure, and oxidizing chemical compounds were investigated. The strain showed a high tolerance to desiccation, with a decline of survivability by four orders of magnitude during a storage time of 85 days. Exposure to X-rays resulted in dose-dependent inactivation for exposure up to 600 Gy while applied doses above 750 Gy led to complete inactivation. The effects of the combination of desiccation and irradiation were additive and the survivability was influenced by the order in which they were imposed. Ionizing irradiation and subsequent desiccation was more deleterious than vice versa. By contrast, the presence of perchlorates was not found to significantly affect the survival of the Yersinia strain after ionizing radiation. These data show that the organism has the capacity to survive and grow in physical and chemical stresses, imposed individually or in combination that are associated with Martian environment. Eventually it lost its viability showing that many of the most adaptable anaerobic organisms on Earth would be killed on Mars today.

The recent geomicrobiological characterization of Río Tinto, Iberian Pyrite Belt (IPB), has proven the importance of the iron cycle, not only in generating the extreme conditions of the habitat (low pH, high concentration of toxic heavy metals) but also in maintaining the high level of microbial diversity, both prokaryotic and eukaryotic, detected in the water column and the sediments. The extreme conditions of the Tinto basin are not the product of industrial contamination but the consequence of the presence of an underground bioreactor that obtains its energy from the massive sulfide minerals of the IPB. To test this hypothesis, a drilling project was carried out to intersect ground waters that interact with the mineral ore in order to provide evidence of subsurface microbial activities and the potential resources to support these activities. The oxidants that drive the system appear to come from the rock matrix, contradicting conventional acid mine drainage models. These resources need only groundwater to launch microbial metabolism. There are several similarities between the vast deposits of sulfates and iron oxides on Mars and the main sulfide-containing iron bioleaching products found in the Tinto. Firstly, the short-lived methane detected both in Mars' atmosphere and in the sediments and subsurface of the IPB and secondly, the abundance of iron, common to both. The physicochemical properties of iron make it a source of energy, a shield against radiation and oxidative stress as well as a natural pH controller. These similarities have led to Río Tinto's status as a Mars terrestrial analogue., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Mars has undergone three main climatic stages throughout its geological history, beginning with a water-rich epoch, followed by a cold and semi-arid era, and transitioning into present-day arid and very cold desert conditions. These global climatic eras also represent three different stages of planetary habitability: an early, potentially habitable stage when the basic requisites for life as we know it were present (liquid water and energy); an intermediate extreme stage, when liquid solutions became scarce or very challenging for life; and the most recent stage during which conditions on the surface have been largely uninhabitable, except perhaps in some isolated niches. Our understanding of the evolution of Mars is now sufficient to assign specific terrestrial environments to each of these periods. Through the study of Mars terrestrial analogues, we have assessed and constrained the habitability conditions for each of these stages, the geochemistry of the surface, and the likelihood for the preservation of organic and inorganic biosignatures. The study of these analog environments provides important information to better understand past and current mission results as well as to support the design and selection of instruments and the planning for future exploratory missions to Mars.

Hundreds of impact craters on Mars contain diverse phyllosilicates, interpreted as excavation products of preexisting subsurface deposits following impact and crater formation. This has been used to argue that the conditions conducive to phyllosilicate synthesis, which require the presence of abundant and long-lasting liquid water, were only met early in the history of the planet, during the Noachian period (> 3.6 Gy ago), and that aqueous environments were widespread then. Here we test this hypothesis by examining the excavation process of hydrated minerals by impact events on Mars and analyzing the stability of phyllosilicates against the impact-induced thermal shock. To do so, we first compare the infrared spectra of thermally altered phyllosilicates with those of hydrated minerals known to occur in craters on Mars and then analyze the postshock temperatures reached during impact crater excavation. Our results show that phyllosilicates can resist the postshock temperatures almost everywhere in the crater, except under particular conditions in a central area in and near the point of impact. We conclude that most phyllosilicates detected inside impact craters on Mars are consistent with excavated preexisting sediments, supporting the hypothesis of a primeval and long-lasting global aqueous environment. When our analyses are applied to specific impact craters on Mars, we are able to identify both pre- and postimpact phyllosilicates, therefore extending the time of local phyllosilicate synthesis to post-Noachian times.

Many features of the Martian landscape are thought to have been formed by liquid water flow and water-related mineralogies on the surface of Mars are widespread and abundant. Several lines of evidence, however, suggest that Mars has been cold with mean global temperatures well below the freezing point of pure water. Martian climate modellers considering a combination of greenhouse gases at a range of partial pressures find it challenging to simulate global mean Martian surface temperatures above 273 K, and local thermal sources cannot account for the widespread distribution of hydrated and evaporitic minerals throughout the Martian landscape. Solutes could depress the melting point of water in a frozen Martian environment, providing a plausible solution to the early Mars climate paradox. Here we model the freezing and evaporation processes of Martian fluids with a composition resulting from the weathering of basalts, as reflected in the chemical compositions at Mars landing sites. Our results show that a significant fraction of weathering fluids loaded with Si, Fe, S, Mg, Ca, Cl, Na, K and Al remain in the liquid state at temperatures well below 273 K. We tested our model by analysing the mineralogies yielded by the evolution of the solutions: the resulting mineral assemblages are analogous to those actually identified on the Martian surface. This stability against freezing of Martian fluids can explain saline liquid water activity on the surface of Mars at mean global temperatures well below 273 K.

A search for evidence of cryptic life in the subsurface region of a fractured Paleozoic volcanosedimentary deposit near the source waters of the Río Tinto River (Iberian pyrite belt, southwest Spain) was carried out by Mars Astrobiology Research and Technology Experiment (MARTE) project investigators in 2003 and 2004. This conventional deep-drilling experiment is referred to as the MARTE ground truth drilling project. Boreholes were drilled at three sites, and samples from extracted cores were analyzed with light microscopy, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy. Core leachates were analyzed with ion chromatography, and borehole fluids were analyzed with ion and gas chromatography. Key variables of the groundwater system (e.g., pO(2), pH, and salinity) exhibit huge ranges probably due to surficial oxygenation of overall reducing waters, physical mixing of waters, and biologically mediated water-rock interactions. Mineral distribution is mainly driven by the pH of subsurface solutions, which range from highly acidic to neutral. Borehole fluids contain dissolved gases such as CO(2), CH(4), and H(2). SEM-EDS analyses of core samples revealed evidence of microbes attacking pyrite. The Río Tinto alteration mechanisms may be similar to subsurface weathering of the martian crust and provide insights into the possible (bio)geochemical cycles that may have accompanied underground habitats in extensive early Mars volcanic regions and associated sulfide ores.

The failure of Viking Lander thermal volatilization (TV) (without or with thermal degradation)-gas chromatography (GC)-MS experiments to detect organics suggests chemical rather than biological interpretations for the reactivity of the martian soil. Here, we report that TV-GC-MS may be blind to low levels of organics on Mars. A comparison between TV-GC-MS and total organics has been conducted for a variety of Mars analog soils. In the Antarctic Dry Valleys and the Atacama and Libyan Deserts we find 10-90 mug of refractory or graphitic carbon per gram of soil, which would have been undetectable by the Viking TV-GC-MS. In iron-containing soils (jarosites from Rio Tinto and Panoche Valley) and the Mars simulant (palogonite), oxidation of the organic material to carbon dioxide (CO(2)) by iron oxides and/or their salts drastically attenuates the detection of organics. The release of 50-700 ppm of CO(2) by TV-GC-MS in the Viking analysis may indicate that an oxidation of organic material took place. Therefore, the martian surface could have several orders of magnitude more organics than the stated Viking detection limit. Because of the simplicity of sample handling, TV-GC-MS is still considered the standard method for organic detection on future Mars missions. We suggest that the design of future organic instruments for Mars should include other methods to be able to detect extinct and/or extant life.

Several lines of evidence have recently reinforced the hypothesis that an ocean existed on early Mars. Carbonates are accordingly expected to have formed from oceanic sedimentation of carbon dioxide from the ancient martian atmosphere. But spectral imaging of the martian surface has revealed the presence of only a small amount of carbonate, widely distributed in the martian dust. Here we examine the feasibility of carbonate synthesis in ancient martian oceans using aqueous equilibrium calculations. We show that partial pressures of atmospheric carbon dioxide in the range 0.8-4 bar, in the presence of up to 13.5 mM sulphate and 0.8 mM iron in sea water, result in an acidic oceanic environment with a pH of less than 6.2. This precludes the formation of siderite, usually expected to be the first major carbonate mineral to precipitate. We conclude that extensive interaction between an atmosphere dominated by carbon dioxide and a lasting sulphate- and iron-enriched acidic ocean on early Mars is a plausible explanation for the observed absence of carbonates.

Abstract: The presence of extensive phyllosilicate deposits from the early Noachian of Mars are often interpreted as having formed from neutral to subalkaline solutions. In this paper we examine the Río Tinto fluvial basin, an early Mars analog, that hosts clay production and sedimentation along the entire course of the river. At Río Tinto, phyllosilicate minerals including clays and micas are sourced by volcanosedimentary bedrock of rhyolitic and andesitic composition affected by Carboniferous hydrothermal alteration. Pleistocene to modern acidic weathering of those materials chemically altered the volcanic and sedimentary materials to K/Na–clay–(montmorillonite/smectites)–kaolinite assemblages in paleosoils and fractures while physical weathering degrades phyllosilicates more resistant to acidic attack. During the wet season, phyllosilicates are eroded, transported and deposited from both acidic headwaters and neutral tributaries. During the dry season, sulfates and nanophase oxyhydroxides co-precipitate. Late summer storms that cause fast flooding events mix illite, quartz, feldspars, iron oxides and other minerals in fluvial deposits where these minerals are stabilized and aggrade until the following wet season. As a result, chemical precipitates, primary phyllosilicates and secondary clays form mineral admixtures that explain the compositional diversity of the fluvial deposits. These deposits reveal the persistence of smectites, whose occurrence is explained given that the reaction kinetics under acidic conditions of degradation is lowered by seasonal discharges of the river. The longevity of phyllosilicate minerals within fluvial deposits depends on climatic and geochemical conditions and processes which are in turn are correlated to temperature, persistence of water, hydrological cycling, hydrogeochemistry and composition of the source materials in the basement. These parameters are universal and have to be characterized in order to understand the distribution of mineral composition on any planetary surface, including Mars. [Copyright &y& Elsevier]

One remarkable discovery in Mars geology is the extensive Meridiani iron formation. Although thermal oxidation mechanisms have been proposed for its origin, geomorphology and IR spectroscopy studies support an aqueous scenario as the most feasible environment to host its secondary iron-enriched mineralogy. Terrestrial hydrothermal systems or the iron-rich Precambrian oceans have been proposed as analogs to understand these interesting Martian environments. However, the available TES spectra show hematite as a mineral phase of Mars’ iron rocks, but not the cherty silica that frequently appears in hydrothermal systems or BIFs. The Tinto River Basin, an extreme acidic environment driven by iron biogeochemistry, has water rich in both ferric iron and sulfate. These acidic waters produce ferric iron-enriched sediments dominated by sulfate and oxyhydroxide associations, in which silicates are absent. Moreover, ancient iron formations, pre-Holocene relicts of similar acidic water environments found in the Tinto River Basin, suggest that the primary iron-rich associations changed with time to goethite–hematite associations through dehydration and desulfatation. These features make the Tinto River Basin a reasonable analog for the Mars hematite site if an aqueous origin for these oxides is considered. [Copyright &y& Elsevier]