RECENT PERIODS OF CLAY AND HYDRATED SILICA FORMATION AT MARTIAN CENTRAL PEAKS

Evidence for water is a key part of searching for past habitable environments on Mars and is indicated by the presence of hydrous phases such as clay minerals. Clay detections at central peaks of craters have contributed towards our understanding of Noachian aqueous conditions, but some of these clays may actually represent impact-related in situ clay formation rather than excavation of altered Noachian crust. We will present results from an ongoing global survey of 535 central peaks to assess the distribution and timing of clay and hydrated mineral formation on Mars, including evidence for post-Noachian clay formation. We correlate hydrated mineralogy from CRISM spectral data with stratigraphy and morphology based on HiRISE imagery to determine the pre- or post-impact origin of hydrated minerals. The maximum formation ages of any post-impact minerals are constrained with crater count age dating of the host crater.

Current results from 150 central peaks show that Fe/Mg clays, likely smectite or mixed-layer chlorite/smectite, are the dominant hydrated mineral, followed by hydrated silica and chlorite. Excavated, pre-impact clays are observed in uplifted bedrock and represent half of clay detections. Post-impact clays reside in units emplaced after the impact event and represent a quarter of clay detections. Some of these clays are associated with fluvial features, indicating a possible detrital origin, and cluster from 3.7-4.1 Ga, coincident with the timing of valley network formation. Post-impact clays that are not associated with fluvial features often occur in units characteristic of impact melt and span 2.5-3.8 Ga, suggesting that impact-generated clays have formed throughout Martian history. This also indicates that Fe/Mg clay formation, and therefore water-rock interaction at moderate pH, may have occurred in impact-generated systems as recently as the Hesperian or Amazonian. Hydrated silica occurrences are restricted to 3.4-3.9 Ga and peak from 3.4-3.5 Ga, possibly indicating a shift in global hydrologic conditions that allowed for the increased formation of hydrated silica, either from alteration of impact glass, basaltic precursors, or as diagenetic cements. Current results indicate that habitable conditions likely persisted into the Hesperian and possibly Amazonian, albeit highly localized.