Paleoenvironmental significance of Fe/Mg phyllosilicate and sulfate deposits in Mikumi crater, northern Meridiani Planum, Mars

The region of Meridiani Planum on Mars retains the evidence of multiple surface and near-surface aqueous processes that span from the Noachian to the Hesperian. This makes it an interesting spot for better understanding the characteristics and the evolution of Mars' aqueous environment between these two fundamental epochs. Here we investigate the mineralogy, stratigraphy, and morphology of the Noachian-Hesperian sedimentary materials that fill a 16-km-wide crater named Mikumi, located in the northern part of Meridiani Planum. The analysis was carried out combining observations from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), the High Resolution Imaging Science Experiment (HiRISE), and the Context Imager (CTX). About 20-30 meters, on average, of Fe/Mg phyllosilicate-rich rocks and a thick sequence (75 m) of both mono- and polyhydrated Mg sulfates are detected on the crater floor. Both layers postdate the impact event. Most of the hydrated terrains are extensively polygonally fractured, suggesting the possible action of water-related processes such as desiccation and hydro fracturing on the sediments. Stratigraphic analysis clearly shows that Fe/Mg phyllosilicates were deposited on top of the sulfate sequence, meaning that they are younger than sulfates. This can have interesting implications on paleoclimatic studies of Mars as it shows that a simple timeline going from a clay-rich Noachian to a sulfate-rich Hesperian oversimplifies the evolution of Mars' mineralogy and related aqueous environment at the Noachian-Hesperian boundary.