The Search for Extant Life on Mars as a Focusing Scientific Goal for Future Human Exploration Missions

 

Introduction:  A search for evidence of extant life on Mars provides a focusing goal for human exploration missions that are planned for the 2030s. Finding an example of extant life beyond Earth would be one of the greatest scientific discoveries of all time. Human exploration brings extraordinary improvements over robotic missions in both payload and analytical capability. Environments that may be habitable for modern life on Mars include salts and brines, ground ice, caves, and deep subsurface aquifers [Carrier et al. 2020]. The habitability case for each environment is reviewed as well as how human missions may enable the search. It is important to determine if life exists on Mars prior to human exploration activities to prevent both the risk to Earth of inadvertently bringing potentially harmful organisms from Mars, risks to astronaut health, and the complication of recognizing Mars life once terrestrial contamination has occurred.

Salts and Brines: Salts and evaporites are common at the surface of Mars [Osterloo et al. 2010] with at least 600 regions of chloride salts identified. On Earth, evaporites and associated brines support a wide diversity of microbial communities including phototrophs, lithotrophs, and heterotrophs [Das Sarma and Das Sarma 2017]. Endolithic phototrophs are found associated with gypsum crusts, and halite-entrapped halophilic archaea and bacteria are commonly observed in enclosed brine fluids, having easily detectable carotenoid pigments [DasSarma, et al. 2020]. Halite and gypsum minerals offer radiation protection by attenuating ultraviolet light and provide protection from long-term desiccation by deliquescence [Davila et al. 2010]. Dissolved salts extend the temperature range for maintaining liquid water through freezing point depression and by formation of supercooled liquids, expanding the possibility of life processes at subzero temperatures. Concentrated brines occur in ice vein networks where dissolved salts are excluded during ice formation. Because many salts are hygroscopic, liquid brines might form near the surface in locations that receive periodic frosts. The widespread presence of perchlorate salts on Mars allows brines to form over a large part of the Martian surface due to deliquescence [Rivera-Valentin et al. 2020]. The brines may form at temperatures too low for known terrestrial metabolism but more work is needed to understand their potential habitability. Salt environments are widely accessible to be studied by human missions with standard microbiological sampling methods.

Ice Rich Terrains: Shallow ground ice is widespread on Mars at latitudes above 35^o N /45^o S [Piqueux et al. 2019]. Quasiperiodic climate change results from variations in orbital parameters causing the intensity of incident sunlight at a given latitude to vary over time. As a consequence climate shifts occur and the location and depth of ground ice varies [Mellon and Sizemore, 2022]. Current summer surface temperatures in ice-rich midlatitude regions are sufficient to support life if melting surface ice or frost provides transient liquid water. Sampling ground ice to search for life can be accomplished with 1-2 m drilling systems operated by crews or rovers teleoperated by human crews. Flight prototype life detection instruments fed with a 1m auger drill have successfully identified biosignatures in Atacama Desert samples that were collected and analyzed during Mars mission simulations [Stoker et al. 2022].

Caves: Caves are another high priority environment in the search for extant life on Mars as they protect their interiors from cosmic radiation and energetic solar events, changing surface climatic conditions, and small-scale impact events. Caves can be warmer, wetter, and more protected, therefore more habitable than the surface. More than 1000 candidate caves in volcanic terrain have been identified on Mars from orbital imagery and many occur in the Tharsis volcanic complex area [Cushing et al. 2015]. A cave with natural openings offers direct access to the subsurface, with a relatively stable thermal environment that can persist over geologic time and preserve volatiles while voids with no surface openings are detectable via ground penetrating radar. Microbial life in terrestrial volcanic caves that lives on chemical energy derived from limited organic carbon and minerals is found on and in a wide variety of mineral features, from silica-rich, to carbonate, iron, and other metals distinctive from their basaltic host rock. Such features preserve microbes extremely well in situ. In some cases, obvious moist biofilms are found but in other cases mineral forms trap and preserve microbial structures that are revealed with microscopy (Boston et al. 2001). Cave morphology is complex and human capabilities are essential for exploring them, possibly aided by small helicopters to search for habitable conditions from the surface.

Deep Subsurface: Deep subsurface aquifers might be the longest-lived habitable environment on Mars, possibly existing from the Noachian until now (Onstott et al. 2019) . Physical access to the subsurface will require deep drilling systems, a technology that will likely require human presence to achieve success. Deep drilling would require a low-mass wireline approach as recently demonstrated drilling to 111m in Greenland ice.

 

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