AN ANALYSIS OF CRATERS THAT EXHUME BOTH PHYLLOSILICATES AND HYDRATED SULFATES IN TYRRHENA TERRA, MARS

Numerous outcrops of hydrated minerals in the Noachian cratered highland terrain of Tyrrhena Terra have provided evidence of pervasive crustal-water interaction in the earliest portion of Mars’ geologic history. The vast majority of these outcrops are phyllosilicates that have been exposed by impact events, and it is apparent from their location in crater rims, walls, and ejecta that much of the alteration occurred in only the upper few kilometers of the subsurface. A number of outcrops are also observed in central uplift structures, implying exhumation from greater depths. Here we report on the detailed study of three impact craters that not only exhume phyllosilicates but also hydrated sulfates. Analyses of Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) hyperspectral visible and near infrared (~0.4-4 µm) data reveal that the phyllosilicates are Fe/Mg-bearing with diagnostic hydration absorption features near 1.41, 1.92, and some variation in the cation-hydroxyl absorption from 2.29-2.34 µm. Spectra of the sulfates exhibit fairly broad asymmetric bands at 1.45, 1.94, and 2.44 µm that are consistent with Mg- or Fe-polyhydrated sulfates such as copiapite. The three craters are distributed within Noachian dissected cratered terrain (Npld), are relatively fresh, and have diameters of approximately 9.5, 11, and 25km. Analyses of the locations of the hydrated minerals in and around the craters indicate that, in all three cases, the sulfates were stratigraphically beneath the phyllosilicates with pre-impact depths of ~1km and <100 meters, respectively, although phyllosilicates are also observed in the central peak alongside the sulfates in the largest crater. Understanding the conditions that led to the formation and exposure of this unusual combination of hydrated minerals in Tyrrhena Terra will provide important insight into the development of the Noachian crust.