The Importance of Transitional Habitable Zones in the New Mars Underground

Abstract:

While the Curiosity rover continues to provide growing evidence of the salty paleolake origins of Gale Crater, the Perseverance rover will land in Jezero Crater in February of 2021 and begin seeking signs of ancient life within the paleolake deposits. Further, in 2022 the ExoMars Rover will begin its journey to explore the potential paleolake basin of Oxia Planum. All these missions seek to understand the habitable environments within martian paleolakes, and their capability for preserving biosignatures of past life. To accomplish their respective goals, all accessible habitable zones with preservation potential will need to be considered, and this should include transitional subsurface habitable zones within the sedimentary deposits. However, there have only been a few studies of these type of environments on Earth and as such these environments are still poorly understood, especially in the context of the sedimentary and geochemical environment. Across the globe, numerous large paleolakes from the late Pleistocene/early Holocene boundary have gradually transitioned to modern-day saline/hypersaline basins. The sediments of these paleolake basins tend to have complex mineralogies that include a combination of sulfates, phyllosilicates, carbonates, chlorides, other salts, and ancillary minerals. These paleolake basins also tend to maintain groundwater systems that support a diversity of microorganisms though the microbial ecology, specifically the biogeochemical factors that drive community structure, has yet to be fully constrained in any of these environments. A prime example of this environment is the Pilot Valley basin in northwestern Utah, USA and over the past 8 years, our team has studied the Pilot Valley paleolake basin as a Mars analog environment. Our research shows that the organics, brine constituents (e.g. sulfate, nitrate, and chlorine oxyanions), and minerals within the basin sediments provide the necessary components to fuel a diversity of redox reactions. Further, the microbial ecosystem in Pilot Valley is organized into discrete community groups mostly influenced by grain size and other lithological factors. This lithological influence on community structure could impact how, where, and what type of biosignatures get preserved within these transitional subsurface environments.