This session welcomes all presentations on Mars' interior and surface processes. The aim of this session is to bring together disciplines as various as geology, geomorphology, geophysics, mineralogy, glaciology, and chemistry. We welcome presentations on either present or past Mars processes, either pure Mars science or comparative planetology, either observations or modeling or laboratory experiments (or any combination of those). New results on Mars science obtained from recent in situ and orbital measurements are particularly encouraged, as well as studies related to upcoming missions (ExoMars, Mars 2020, Mars Sample Return).
Ana-Catalina Plesa, Vlada Stamenković, Doris Breuer, Ernst Hauber, Jesse Tarnas, John Mustard, Michael Mischna, and Barbara De Toffoli and the TH2OR and VALKYRIE Teams
While liquid water is not thermodynamically stable at the surface due to the low temperature and pressure conditions, liquid groundwater may still exist in the Martian subsurface [1, 2].
In this study, we use fully dynamical 3D thermal evolution models  and 3D parametrized models  to calculate the depth at which favorable conditions for liquid water are present, assuming that a global subsurface cryosphere exists on Mars today. While fully dynamical 3D models take into account the effect of mantle plumes self-consistently, they are computationally expensive compared to 3D parametrized models that can cover a large range of mantle conditions, although requiring additional parametrizations for thermal anomalies in the interior. In all calculations, we use a 3D crustal model that is compatible with today’s gravity and topography data [5, 6].
Some of the most important parameters that affect the depth of liquid water are the spatial variations of crustal thickness and crustal thermal conductivity, since the crust has a lower thermal conductivity compared to that of the mantle and thickness variations can shift the groundwater table locally closer to the surface (Fig. 1). The amount and distribution of heat sources, and the presence of mantle plumes, can introduce additional perturbations to the depth of groundwater. The surface temperature distribution and the presence of salts and clathrate hydrates considerably affect the depth and locations where subsurface liquid water may be stable. Hydrated magnesium (Mg) and calcium (Ca) perchlorate salts, whose presence has been suggested at various locations on Mars , may significantly reduce the melting point of water ice. In addition to thick regolith layers, clathrate hydrates, if present in the subsurface, would provide an insulating effect reducing the crustal thermal conductivity at least locally [e.g., 8].
The effects of the crustal thermal conductivity and salt abundance on the depth of subsurface liquid water are shown in Fig. 1, where we use the same crustal thickness variations and crustal enrichment in radioactive heat sources in all simulations. The model in Fig. 1a assumes an average crustal conductivity of 3 W/mK, while the model in Fig. 1b has a lower conductivity of only 2 W/mK (see panel 1e for the spatially averaged conductivity profiles that, due to crustal thickness variations, show average values between mantle and crust in the topmost 110 km). Fig. 1d shows the effect of the crustal thermal conductivity on the subsurface temperature profile. For the lower conductivity case the subsurface temperature is warmer, and the groundwater table shifts, on average, 2.5 km closer to the surface. The model shown in Fig. 1c is similar to the one in Fig. 1a but assumes the presence of salts. Instead of using the melting temperature of pure water ice, as was done for the models in Fig. 1a and b, we lower the melting temperature to 199 K  over the entire depth, by assuming that Ca(ClO4)2 is present in eutectic concentration (Fig. 1f). This extreme, and unrealistic, assumption places constraints on the minimum depth at which liquid water may be present in the Martian subsurface today, since kinetic factors such as the flow of groundwater due to gravity may increase the depth of the water table, depending on the total amount of liquid water, porosity and permeability.
In Fig. 1a and b, the depth of the groundwater shows the combined effect of crustal thickness distribution and surface temperature variations. Mantle plumes have only a small effect and may introduce perturbations only if the groundwater is located, on average, at about 5 km depth or deeper. The effect of the crustal thickness is evident in basins, along the dichotomy, and in volcanic provinces, whereas surface temperatures give general water table depth trends with latitude. In Fig. 1c, the depth variations of the groundwater table are mainly caused by the surface temperature distribution, as the groundwater table is located very close to the surface (between 0 – 1 km for latitudes between -57° and 57°). Nevertheless, in all cases (Fig. 1a – c), the water table is significantly shallower in equatorial regions compared to polar regions, mainly governed by lower surface temperatures at the poles.
Our results suggest that the Martian subsurface has had, and still has, the potential to enable deep environments with stable liquid groundwater. Combined with the analysis of geomorphological features at the Martian surface that testify the involvement of water/ice activity and maps of subsurface water ice , such models could provide valuable estimates of the depth of liquid groundwater on past and present-day Mars providing key knowledge on the planet dynamics, evolution and astrobiological potential.
The technology to probe the Martian subsurface at depths of many kilometers is maturing : TH2OR (Transmissive H2O Reconnaissance), a low-mass and average low-power transient electromagnetic sounder capable of detecting the presence of liquid water to depths of many kilometers is currently being developed at JPL . Moreover, mission concepts such as VALKYRIE (Volatiles And Life: KeY Reconnaissance & In-situ Exploration) , which would add to the liquid water sounder a drill capable of accessing depths of 10s-100s of meters or more and employ a (bio)geochemical analysis package on the surface, would provide the measurements necessary to characterize the modern-day subsurface habitability of Mars.
References:  Clifford et al., 2010, JGR, 115(E7);  Stamenković V. et al., 2019, Nat. Astron., 3(2);  Plesa A.-C. et al., 2018, GRL, 45(22);  Breuer D. & Spohn T., 2006, PSS, 54(2);  Plesa A.-C. et al., 2016, JGR, 121(12);  Wieczorek M. & Zuber M., 2004, JGR, 109(E8);  Leshin L. et al., 2013, Science, 341;  Kargel J. et al. 2007, Geology, 35(11);  Marion G. et al., 2010, Icarus, 207(2);  Piqueux S. et al., 2019, GRL, 46.;  Burgin M. et al., 2019, AGU Fall Meeting, P44B-02;  Mischna M. et al., 2019, AGU Fall Meeting, P41C-3466.
How to cite:
Plesa, A.-C., Stamenković, V., Breuer, D., Hauber, E., Tarnas, J., Mustard, J., Mischna, M., and De Toffoli, B. and the TH2OR and VALKYRIE Teams: Mars' Subsurface Environment: Where to Search for Groundwater?, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-698, https://doi.org/10.5194/epsc2020-698, 2020.
Abstract: Calcium sulfate mineral veins cross-cut fluviolacustrine sedimentary rocks at many localities on Mars. Although these veins probably formed under habitable conditions, their potential to retain ancient biosignatures is poorly understood. Here, we report ancient biogenic authigenic pyrite (FeS2) lining a fibrous gypsum (CaSO4.2H2O) vein of probable Cenozoic emplacement age from Permian lacustrine rocks in Northwest England. The observed pyrite distributions and textures suggests that the pyrite formed replacively after gypsum within the veins and was not inherited from the host rock. Spatially resolved ion microprobe (SIMS) measurements reveal that the pyrite sulfur isotope composition (δ34SVCDT) is negatively offset from the host gypsum by ~40‰. We infer that the pyrite was precipitated in the deep subsurface by microorganisms living in porosity at the vein margins, which coupled the reduction of vein-derived sulfate to the oxidation of wall-derived organic matter. This implies that such veins can incorporate biosignatures that remain stable over geological time, which could in principle be detected in samples returned from Mars .
Introduction: Fibrous, antitaxial calcium sulfate veins were encountered by the MER rover Opportunity in Endeavour Crater and are inferred to represent gypsum [2,3]. Similarly, white calcium sulfate veins (anhydrite, bassanite, and perhaps gypsum) cross-cut hundreds of metres of fluviolacustrine and aeolian stratigraphy traversed by the MSL Curiosity rover in Gale Crater, including the Yellowknife Bay and Murray formations [4,5,6,7]. Some of these veins are thought to post-date lithification and to have formed at depths of over 1 km in the subsurface . Veins like these may be encountered in future by the Perseverance and Rosalind Franklin rovers, and have sometimes been discussed as an attractive target for astrobiological investigation, but their potential to preserve biosignatures is poorly understood. Here, we summarise a new study  of ancient biosignatures in ancient (Cenozoic), bedding-parallel, antitaxial veins of white, fibrous gypsum found in Permian lacustrine mudrock. These veins are located in the Eden Shales Formation of the Vale of Eden Basin, Cumbria, NW England, and were sampled underground in situ in the Kirkby Thore gypsum mine.
Figure 1: Pyrite at the margins of a gypsum vein. A: Sketch of hand sample. B: Reflected light photomicrograph showing brassy pyrite with complex microdigitate morphology at gypsum vein margin.
Results: Pyrite was observable to the unaided eye at the margins of the gypsum veins in polished hand samples (Figure 1A); its composition was confirmed with energy-dispersive X-ray spectroscopy. The pyrite displays a complex interfingering boundary with the surrounding gypsum, suggestive of replacive authigenic growth (Figure 1B; we do not consider this morphology itself a biosignature). Gypsum-entombed carbonaceous material of probable organic origin was identified by Raman spectroscopic microscopy in close proximity to the pyrite (from its prominent D- and G-bands); authigenic dolomite is also present. Spatially resolved ion microprobe (SIMS) measurements reveal that the pyrite sulfur isotope composition is consistently very light (δ34SVCDT = –30.7 ‰). Comparison with the sulfate in the vein gypsum (δ34SVCDT = +8.5 ‰; ) indicates a fractionation too large to be explained by non-biological (thermochemical) sulfate reduction (which in any case would be difficult to reconcile with the burial history of the host rock). We infer from these results that the pyrite is likely a product of in situ microbial sulfate reduction coupled to the oxidation of organic matter from the wall rock.
Implications for Mars: Our results imply that the porous margins of calcium sulfate veins in the subsurface can serve as conduits for the flow of sulfate-rich groundwater, and therefore as potential habitats for prokaryotes capable of utilizing this sulfate to oxidize organic carbon. Such habitats may disappear and reappear several times over long spans of geological time as veins reopen under changing stress regimes and re-seal as further sulfate precipitates. Microbial sulfate reduction in our samples was probably stimulated by the low but appreciable organic content of the host rock (TOC ~ 0.5 wt%). Other sources of carbon (e.g., methane) and electrons (e.g., hydrogen) may be available in organic-poor rocks on Mars. In principle, signatures of subsurface life similar to those reported here could be detectable in martian veins, particularly if they were selected to be returned to Earth for SIMS analysis. On Earth, the δ34S biosignature has been identified in rocks as old as 3.47 Ga, although it did not become widespread until after 2.5 Ga [10,11]. Sulfur isotope systematics are not well understood in martian contexts, and the isotopic fingerprints of any indigenous martian organisms may well have differed from those of life on Earth. Nevertheless, sulfide minerals do occur on Mars, and Curiosity Rover has detected isotopically light sulfide (by evolved gas analysis) in Gale Crater sediments, where it could ultimately have originated either from abiotic or from biotic processes . The presence of preserved organic matter in our samples adds to the case that calcium sulfate veins could be an attractive target for analysis on Mars. We note that any biosignatures present in mineral veins cross-cutting sedimentary rocks would have originated in a subsurface habitat markedly different from the depositional environment of the host mudrock. As sampling targets, these rocks may therefore offer insights into two ancient martian habitats — surface and subsurface — for the price of one.
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How to cite:
McMahon, S., John, P., and Philippe, R.: Mars-analogue calcium sulfate veins record evidence of ancient subsurface life, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-1049, https://doi.org/10.5194/epsc2020-1049, 2020.