Topography and mineralogy of clay deposits signify an epoch of warm and humid climate on early Mars

Little is known about the surface of Earth for the first billion years of its history, because subaerial deposits of this age are very poorly preserved in the geologic record. Such deposits could answer important questions about the atmosphere, climate, and emergence of life on primordial terrestrial planets. In contrast, the ancient crust of Mars is far better preserved and has undergone minimal deformation. Outcrops of >3.6 Ga clay sequences where aluminous clay minerals overlie iron/magnesium smectite have been interpreted as evidence for warm and habitable surface environments early in Martian history. Two main hypotheses for the origin of these clay sequences have been proposed: subaerial formation through pedogenic leaching, or subaqueous formation via detrital deposition and/or alteration. The topographic properties of these clay deposits could be used to test between these two hypotheses. However, the few previous topographic analyses were restricted to a handful of sites and usually only measured vertical thickness, which fails to account for strike/dip of geologic layers. Here, we report true thicknesses of clay stratigraphies at 46 outcrops globally, extract vertical profiles of clay mineralogy from orbital reflectance spectra at 14 areas, and investigate the relationship with antecedent topography at Mawrth Vallis. We find strong support for the pedogenic leaching model, especially for the upper aluminous portion. The very high total true thickness of these weathering profiles (global mean 59 m), rivaled only by temperate-tropical analogs on Earth, suggests intense aqueous leaching under ∼0.2−8 m.y. of warm and humid conditions or a longer period of oscillating climates. Our findings add to a growing body of evidence that early Mars experienced epochs of prolonged habitable surface environments conducive to microbial life.