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Water present on early Mars is often assumed to have been habitable. In this study, experiments were performed to investigate the habitability of well-defined putative martian fluids and to identify the accompanying potential formation of biosignatures. Simulated martian environments were developed by combining martian fluid and regolith simulants based on the chemistry of the Rocknest sand shadow at Gale Crater. The simulated chemical environment was inoculated with terrestrial anoxic sediment from the Pyefleet mudflats (United Kingdom). These enrichments were cultured for 28 days and subsequently subcultured seven times to ensure that the microbial community was solely grown on the defined, simulated chemistry. The impact of the simulated chemistries on the microbial community was assessed by cell counts and sequencing of 16S rRNA gene profiles. Associated changes to the fluid and precipitate chemistries were established by using ICP-OES, IC, FTIR, and NIR. The fluids were confirmed as habitable, with the enriched microbial community showing a reduction in abundance and diversity over multiple subcultures relating to the selection of specific metabolic groups. The final community comprised sulfate-reducing, acetogenic, and other anaerobic and fermentative bacteria. Geochemical characterization and modeling of the simulant and fluid chemistries identified clear differences between the biotic and abiotic experiments. These differences included the elimination of sulfur owing to the presence of sulfate-reducing bacteria and more general changes in pH associated with actively respiring cells that impacted the mineral assemblages formed. This study confirmed that a system simulating the fluid chemistry of Gale Crater could support a microbial community and that variation in chemistries under biotic and abiotic conditions can be used to inform future life-detection missions.

Nitrate-dependent Fe

Modern-day Mars is a dry, harsh environment with no surface water, but billions of years ago it might have been water-rich, like Earth. Scientists believe that, as water was lost on Mars, the remaining water would have become increasingly salty and rich in sulphur. By researching environments on Earth with a similar chemistry, we can develop hypotheses about whether life could have existed on Mars. In our research, we studied Colour Peak, a sulphur-rich, salty spring system in the Canadian High Arctic. This environment contains bacteria that use sulphur to get their energy. By studying water and sediment samples from this region, we confirmed that bacteria that used sulphur as an energy source could potentially have lived on Mars. These results can help scientists who are analysing data from Mars missions to identify evidence of former life on Mars.

Hydrothermal systems that formed as a result of impact events possess all the key requirements for life: liquid water, a supply of bio‐essential elements, and potential energy sources. Therefore, they are prime locations in the search for life on other planets. Here, we apply thermochemical modeling to determine secondary mineral formation within an impact‐generated hydrothermal system, using geochemical data returned for two soils on Mars found in regions that have previously experienced alteration. The computed mineral reaction pathways provide a basis for Gibbs energy calculations that enable both the identification of available geochemical energy, obtained from Fe‐based redox reactions, that could be utilized by potential microbial life within these environments, and an estimate of potential cell numbers. Our results suggest that water–rock interactions occurring within impact‐generated hydrothermal systems could support a range of Fe‐based redox reactions. The geochemical energy produced from these reactions would be substantial and indicates that crater environments have the potential to support microbial cell numbers similar to what has been identified in terrestrial environments.

Authors acknowledge funding from the Science and Technology Facilities Council from the Grant ST/P000657/1. We would also like to acknowledge funding from a Leverhulme Trust Research Project Grant (RPG-2016-153) and thank the Polar Continental Shelf Program (Natural Resources Canada) for logistical field support in Nunavut. The transition of the martian climate from the wet Noachian era to the dry Hesperian (4.1–3.0 Gya) likely resulted in saline surface waters that were rich in sulfur species. Terrestrial analogue environments that possess a similar chemistry to these proposed waters can be used to develop an understanding of the diversity of microorganisms that could have persisted on Mars under such conditions. Here, we report on the chemistry and microbial community of the highly reducing sediment of Colour Peak springs, a sulfidic and saline spring system located within the Canadian High Arctic. DNA and cDNA 16S rRNA gene profiling demonstrated that the microbial community was dominated by sulfur oxidising bacteria, suggesting that primary production in the sediment was driven by chemolithoautotrophic sulfur oxidation. It is possible that the sulfur oxidising bacteria also supported the persistence of the additional taxa. Gibbs energy values calculated for the brines, based on the chemistry of Gale crater, suggested that the oxidation of reduced sulfur species was an energetically viable metabolism for life on early Mars. Publisher PDF

Finding evidence of life elsewhere in the Solar System is dependent on understanding biotic processes that could occur within potentially habitable environments. Here, we describe a suite of high-pressure flow-through reactors that have been developed to investigate biotic and abiotic processes within simulated sub-surface martian and icy moon environments.

The icy satellites of Jupiter and Saturn are perhaps the most promising places in the Solar System regarding habitability. However, the potential habitable environments are hidden underneath km-thick ice shells. The discovery of Enceladus’ plume by the Cassini mission has provided vital clues in our understanding of the processes occurring within the interior of exooceans. To interpret these data and to help configure instruments for future missions, controlled laboratory experiments and simulations are needed. This review aims to bring together studies and experimental designs from various scientific fields currently investigating the icy moons, including planetary sciences, chemistry, (micro-)biology, geology, glaciology, etc. This chapter provides an overview of successful in situ, in silico, and in vitro experiments, which explore different regions of interest on icy moons, i.e. a potential plume, surface, icy shell, water and brines, hydrothermal vents, and the rocky core.

Motivation and Approach Phobos’ proximity to Mars and short orbital period has led to the hypothesis that Phobos could sweep up particles ejected from large impacts on the martian surface [1]; models suggest that Phobos’ regolith could include up to ~250 ppm of martian ejecta material [2,3]. Considering that Mars’ surface has many “Special Regions” that could have been habitable in the past [4], it is not unreasonable to suggest that life could have developed and left behind biomarkers. An impact into one of these areas could eject material containing biologically-significant compounds and deposit it on the surface of Phobos. Therefore, an appealing possibility is that samples collected from the surface of Phobos, by missions such as JAXA’s Martian Moons eXploration (MMX) [5], could contain martian biomarkers. Further investigation into the feasibility of biomarker transfer from Mars to Phobos is hence necessary before returned samples and in situ spacecraft data are analysed. In order to investigate this scenario, light gas gun impact and heating experiments will be used to simulate the conditions a martian rock, containing biologically-relevant material, would undergo throughout the transport process from Mars to Phobos [6,7]. However, prior to carrying out these experiments, the biologically-relevant material must be defined. It is important that this material represents plausible molecular fingerprints of the types of organisms that might have existed in the harsh radiation conditions of Mars’ near-surface for billions of years [8]. Furthermore, they must survive the shock conditions experienced during ejection by a sizeable impact capable of delivering material to reach Phobos’ orbit, and the subsequent deposition onto the surface of Phobos. Biomarkers that could satisfy these criteria include amino acids, polycyclic aromatic hydrocarbons (PAHs), fatty acids and sterols [e.g. 9], some of which may be precursors of chlorinated molecules detected in the Cumberland mudstone at Gale Crater [10]. Previous impact modelling has suggested that a small proportion of near-surface martian material can be ejected at high velocities, capable of reaching Phobos from Mars, and experience shock-pressures low enough to remain solid [11]. However, simulations of martian ejecta impacting Phobos are limited to mass-less [2,3] or “hard rock” projectiles [12] that neglect the reduced shock experienced at the trailing edge of a projectile during the early stages of contact and compression [13], which could aid biomarker survival [14]. Therefore, this study uses the iSALE-2D shock-physics code [15-17] to estimate the pressure and temperature regimes within martian ejecta as it impacts Phobos’ surface. This will enable constraints to be placed on conditions necessary for biomarker survival. Model Parameters Consistency between in situ [e.g. 18, 19] and remote sensing [e.g. 20] observations, and the global meteorite collection [21], indicate that Mars’ global surface composition can be approximated as basaltic. Thus, it can be assumed that basalt is the most likely crustal rock composition to be ejected from Mars. However, this igneous composition is not the most likely rock type in which to find biomarkers. Sedimentary rocks, such as mudstones, indicate favourable past conditions for habitability on Mars and have an increased potential to preserve biomarkers [22]. Organics and bioessential elements have been detected within mudstones at Gale crater [e.g. 23, 24]. However, mudstones are spread sparsely over Mars’ surface, so are less likely to be ejected by impacts. This work, therefore, considers both igneous and sedimentary ejecta, modelling basalt and mudstone projectiles. For the target, the surface composition of Phobos is largely unknown, owing to the lack of direct sampling from the surface; spectral data indicate that the surface resembles D- or T-type asteroids or carbonaceous chondrites, rich in phyllosilicates [25]. Therefore, the closest match to Phobos’ compositional and physical properties will be chosen for the target, with reference to Phobos regolith simulants [e.g. 26]. The simulations will be run over a range of realistic impact parameters, including impactor size (~0.01 - 10 m [27]) and impact velocity (up to 5.3 km s-1 [3,12]). Implications The results from this modelling will provide insight into the survival of martian biomarkers during deposition onto Phobos. Furthermore, the results will inform the subsequent impact and heat experimental simulations. References [1] Murray, J. B., (2011) EPSC-DPS Joint Meeting, 1003. [2] Ramsley, K. R. & Head, J. W., (2013) Planet. & Space Sci., 86, 115-129. [3] Chappaz, L. et al., (2013) Astrobiology, 13(10), 963-980. [4] Rummel, J, D., et al., (2014) Astrobiology, 14(11), 887-968. [5] Usui, T. et al., (2018) 42nd COSPAR Scientific Assembly, B4.2-7-18, 2018. [6] Morland, Z. S. et al., (2020) Proc. 51st Lunar Planet. Sci. Conf., 1096. [7] Morland, Z. S. et al., (2019) Proc. British Planet. Sci. Conf., 88. [8] Kminek, G. et al., (2006) Earth Planet. Sci. Let., 245(1-2), 1-5. [9] Vago, J. L. et al., (2018) in B, Cavalazzi., and F, Westall (Eds.) Springer, 283-300. [10] Szopa, C., et al., (2020) Astrobiology, 20(2), 292-306. [11] Artemieva, N. & Ivanov, B., (2004) Icarus, 171, 84-101. [12] Summers, D., (2019) Life Sci. Space Res., 23, 101-111. [13] Pierazzo, E. & Melosh, H. J., (2000) Meteorit. Planet. Sci., 35, 117-130. [14] Pierazzo, E. & Chyba, C. F., (1999) Meteorit. Planet. Sci., 34, 909-918. [15] Amsden, A. et al., (1980) LANL Report, LA-8095. [16] Collins, G. S. et al., (2004) MAPS, 38, 217-231. [17] Wünnemann, K. et al., (2006) Icarus, 180, 514-527. [18] Filiberto J., (2017) Chemical Geology, 466, 1-14. [19] Cannon, K. M. et al., (2019) Icarus, 317, 470-478. [20] McSween Jr. H. Y. et al., (2003) JGR Planets, 108(E12), 5135. [21] Martian Meteorite Compendium, https://curator.jsc.nasa.gov/antmet/mmc/ [22] Hays, L. E. et al., (2017) Astrobiology, 17(4), 363–400. [23] Stern, J. C. et al., (2015) Proc. Nat. Acad. Sci., 112(14), 4245–4250. [24] Eigenbrode, J. L. et al., (2018) Science, 360(6393), 1096-1101. [25] Fraeman, A. A. et al., (2014) Icarus, 229. 196-205. [26] Morland, Z. S., (2019) Proc. 50th Lunar Planet. Sci. Conf., 2132. [27] Ramsley, K. R. & Head, J. W., (2013) Planet. & Space Sci., 75, 69-95.

Existing martian simulants are predominantly based on the chemistry of the average ‘global’ martian regolith as defined by data on chemical and mineralogical variability detected by orbiting spacecraft, surface rovers and landers. We have therefore developed new martian simulants based on the known composition of regolith from four different martian surface environments: an early basaltic terrain, a sulfur-rich regolith, a haematite-rich regolith and a contemporary Mars regolith. Simulants have been developed so that the Fe2+/Fe3+ ratios can be adjusted, if necessary, leading to the development of four standard simulants and four Fe-modified simulants. Characterisation of the simulants confirm that all but two (both sulfur-rich) are within 5 wt% of the martian chemistries that they were based on and, unlike previous simulants, they have Fe2+/Fe3+ ratios comparable to those found on Mars. Here we outline the design, production and characterisation of these new martian regolith simulants. These are to be used initially in experiments to study the potential habitability of martian environments in which Fe may be a key energy source.

Nisha Ramkissoon, Mark Burchell, Peter Fawdon and Louisa Preston report from an RAS Specialist Discussion Meeting on finding and identifying evidence for life on Mars

Humanity's interest in whether or not we are alone in the universe spans generations, from Giordano Bruno's 16th century musings on other worlds and Giovanni Schiaparelli reporting seeing ‘canali’ in 1877 on the surface of Mars (which were thought to have been created by intelligent life) to alien invasions portrayed in today's movies. However, it is still unclear if other planetary bodies are capable of supporting life. In the search for life there are two broad areas we look into, the requirements of life and actual signs of life. The identification of the key requirements for life enables scientists to focus life detection efforts onto planets and satellites that are considered habitable and more likely to support life. However, our ability to find life or detect signs of life is based on our understanding of life on Earth.

Microorganisms can be used as model systems for studying the biological responses to extraterrestrial conditions. Understanding how cells degrade under such conditions is important for identifying potential biosignatures for future life detection missions. The facilities used to study such responses are extremely challenging and expensive. Therefore, these studies depend on a combination of ground-based simulation and real space experiments. This chapter gives an overview of facilities that are used in space, for example, the EXPOSE facility onboard, the International Space Station, and ground-based simulation facilities. It will also discuss the importance of this research in the context on future life detection missions.

There is evidence that water may exist on Mars as brines in the subsurface. The chemistries of these brines will be greatly influenced by the local lithologies, which would impact on the organisms that could potentially live within them. There are multiple metabolisms that are theoretically viable under martian chemical conditions. In order to better establish which of these are capable of supporting persistent growth under martian conditions, we performed a series of enrichments using four geological simulants for Mars: a global composition, an early and unaltered basaltic composition, a sulfur-rich composition, and a haematite-rich composition. Enrichments were inoculated with sediment from Pyefleet mudflats in the Colne estuary (Essex, UK). Mudflat sediment was added to the simulant materials and a brine based on the chemistry of Rocknest. Enrichments were supplied with a H2/CO2 headspace at 1 bar pressure and incubated for 20 days. The enrichments were repeatedly subcultured into fresh simulant material and brines in order to effectively select for a community actively growing in this chemical environment. The enriched community was characterised through the isolation and identification of microbes, microscopy and the amplicon sequencing of 16S rRNA genes amplified from DNA extracted from each stage of the enrichment. Biotic and abiotic experiments were conducted to identify geochemical changes that occur due to the presence of microbial activity. We will present details on the communities enriched on the different martian simulants, metabolisms identified as present within the actively growing community and geochemical changes that were identified.