Extensive geological and mineralogical evidence suggest that ancient Mars once had large volumes of surface liquid water, likely sustained by a thicker atmosphere with greenhouse gases such as H2 and CH4. This environment was potentially more habitable than the cold, baren Mars that we see today. The thick CO2-dominated atmosphere and large volumes of water were originally thought to have been lost to space, an interpretation of an observed enrichment of heavy isotopes. However, in recent work, we substantially modified these hypotheses in two ways. First, we made the first discovery of a long-lasting source of H2 to warm early Mars’ climate over long timescales: crustal hydration (geologic sinks of water). Second, we found that a CO2 atmosphere likely did not always persist, but the Hesperian Period (~3.7 – 3.0 Ga) may have hosted a mildly reducing CO-dominated atmosphere due to geologic sinks limiting OH available to recycle CO to CO2. This significant climate change at Mars over time likely influenced the planet’s habitability, and nitrogen fixation could play a key role in the emergence of life. In two recent investigations, we demonstrated that the formation of the nitrates observed by the Mars Science Laboratory (MSL) can be explained in a warm wet climate (via lightning) and/or a cold icy climate (involving heterogeneous ice chemistry). In more reducing climates, our model also predicts HCN deposition. Recent MSL measurements have discovered several interesting redox pairs in Mars soils (including NOx and HCN; SO3 and H2S; ClO4 and CH3Cl), and these collectively may point to a redox change during Mars’ history. Alternating redox states were previously proposed to warm early climates transiently and may be plausible via stochastic geologic events, and in this work, we discuss future research which aims to identify a subset of early redox climates and timescales that can explain the formation of the recently observed redox dichotomy. Because of its unique and accessible geological record, Mars could serve as the Rosetta Stone for the study of the evolution and habitability of planets in our Solar System (including Earth) and exoplanets.
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