Compositional stratigraphy on Mars as evidence of hundreds of millions of years of greenhouse conditions on Mars

Chemical weathering is an important indicator of past climate and redox state [1-3]. On Mars, weathering profiles may have formed in basaltic sediments or volcanic ash that were altered by surface water and were subsequently buried and persevered in the geological record. Orbital remote sensing of the global Martian surface has detected dioctahedral clay minerals within Noachian layered sedimentary rocks, which are consistent with the precipitation-driven pedogenic weathering of mafic sediments [1]. Noachian sedimentary rocks with spectral signatures of subaerial weathering have been detected in thousands of locations across the surface of Mars [1,4].

In this study, orbital imagery, spectroscopy, topographic data and crater chronology are investigated to explore the geologic context, stratigraphy, and relative age of >200 weathering profiles across the Martian southern highlands [4]. The youngest might be Early Hesperian, although virtually all are older than 3.7 Ga. Weathering profiles exist across a wide range of elevations (>11 km), from −5 to 6 km, indicating they developed as a result of top-down, precipitation-driven chemical weathering and this was a global phenomenon. We discovered that almost all exposures show a similar, single stratigraphic relationship of Al/Si material overlying Fe/Mg clays, rather than several, interbedded mineralogy transitions. This points to either a single warming event or, more likely, a chemical resetting scenario in which the most recent event overprints the prior weathering pattern. The time necessary to develop a typical profile is estimated to be several million years, which corresponds to only a portion of the Noachian period. As a result, the broad estimated age span ~700–800 My appears incompatible with a single climate excursion. We consider that the presence of weathering profiles in many geologic units at a wide range of ages over a long period of geologic time and at a wide range of elevations, suggests a top-down, precipitation-driven chemical weathering was global in scope. Fe-mobility was a crucial component of chemically weathering, which happened geologically rapidly under anoxic conditions that might potentially warm the martian surface via reduced greenhouse gas. Collectively, these results indicate that multiple weathering episodes are driven by multiple reduced greenhouse conditions on ancient Mars.

[1] Carter et al. 2015, Icarus, 248, 373-382. [2] Bishop et al., 2018, Nature Astronomy, 2(3), 206-213. [3] Liu et al., 2021, Nature Astronomy,5(5), 503-509.[4] Ye & Michalski, Communication Earth & Environments, 3(1), 1-14.