PHYLLOSILICATES IN TYRRHENA TERRA, MARS: A SYSTEMATIC EVALUATION USING CRISM DATA

Recent studies have noted numerous outcrops of phyllosilicate-bearing materials in Tyrrhena Terra, Mars - a southern highlands, Noachian-aged terrain bordered by large impact basins (Hellas and Isidis) and volcanic provinces (Syrtis Major and Hesperia). A few of the phyllosilicate detections proximal to the basins appear in sedimentary settings, but a vast majority are km-scale outcrops associated with craters as rim, central peak, or ejecta materials consistent with subsurface formation followed by impact-induced exhumation.

Determining the exact nature and extent of these phyllosilicate outcrops has the potential to expand our understanding of aqueous alteration in the ancient Martian crust. To that end, we systematically assess the specific mineralogy in Tyrrhena Terra through the analysis of Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) hyperspectral visible and near infrared (~0.4-4 µm) data. Fe/Mg smectites, such as saponite/nontronite, and chlorite/prehnite are the most commonly observed , with less frequent occurrences of illite/muscovite. We also find other types of minerals alongside the phyllosilicates, such as polyhydrated sulfates, primary mafics (olivine, pyroxene), and other hydrated silicate phases. While clear mafic signatures are dominantly in the superposed Hesperian terrains, the relationship(s) of phyllosilicates with other alteration minerals are still under investigation. It is possible that the stratigraphy observed in the greater circum-Isidis region - a distinct sequence of phyllosilicates, sulfates, and mafic minerals - could be more laterally extensive than previously realized. Another likely scenario is that the southern highlands were pervasively altered in a warmer and wetter Noachian era, with subsequent impacts serving to overturn and exhume the altered materials at the same time other hydrated phases (e.g., sulfates) were being formed. We will present the types and distributions of alteration minerals in Tyrrhena Terra and explore their formational scenarios.