Insights and Challenges for Using the Early Earth to Understand Potentially Habitable Environments on Mars

Abstract:

Many of the challenges for recognizing and reconstructing habitable environments on early Mars are shared by the early Earth. However, a number of key differences obscure the possibility for direct comparison of the evolutionary history of surface environments on the two planets. In particular, while the “faint young Sun” paradox can potentially be solved on the early Earth via high levels of reduced greenhouse gases such as the methane, the early and probably continuous oxidation (and lack of any methane today on Mars) of the Martian surface precludes a similar solution for early Mars. This necessity for a different atmospheric evolutionary history also complicates the interpretation of Martian sedimentary rocks because fundamentally different weathering reactions and different reaction rates would have occurred on early Mars relative to early Earth. In this context, it is possible to evaluate recent claims for Martian paleosols at Gale Crater using both model- and data-based methods of looking at weathering processes there and on the early Earth. The putative paleosol interpretation contrasts strongly with mission scientists, who found, for example, that major element geochemistry of weathering products at Mars’ surface indicate little pedogenesis as compared with similar environments on Earth (McLennan et al. 2014, Science). Mass balance gains and losses of redox-sensitive metals (e.g., Cu and Cr) and other trace elements have been proposed as potential biosignatures for early Earth, do not exhibit similar patterns at Gale Crater. While there are orders of more data for the early Earth, this suggests either that there was little or no biological activity at Gale Crater or that those putative biosignatures were not appropriate for the aqueous environments there because they were derived from non-analogous environments. This disconnect between both paleosols/weathering environments of the early Earth and with predictions of expected weathering trends in comparison to observations from Gale Crater suggest that improvements both to our understanding of Earth analogues for Martian environments and more realistic simulations of the differences in atmospheric evolution for Mars are necessary to advance our understanding of potential or realized habitability on Mars.