Evidence for Phyllosilicates in the Early Mars Crust at Nili Fossae and Mawrth Valles Using OMEGA and HRSC Data

Hydrated minerals such as Fe-rich smectite clays, have been detected by the spectral imager OMEGA onboard Mars Express (Bibring et al., 2005). These types of phyllosilicates can result from the progressive weathering of volcanic rocks under conditions different from those currently occurring on Mars. Outcrops containing these minerals are located mainly in Noachian-aged regions suggesting that the early Mars conditions before 3.6 Ga were different, with warmer climate permitting weathering by liquid water. Here we examine in detail two regions, Nili Fossae and Mawrth Valles with coordinated OMEGA spectral data and HRSC high resolution images. In Nili Fossae, signatures of smectites are found in fresh outcrops of crust and crater ejecta. Lobate ejecta from a 60 km diameter crater displays abundant smectites that are interpreted to have been excavated during impact from buried reservoirs of smectites. Alluvial fans and sinuous channels are observed in the region but few mineralogic signatures are uniquely correlated to these landforms. Thus while episodes of water flow likely formed these features, the presence of this water did not cause weathering or alteration. In Mawrth valles, Fe-rich, and locally Al-rich smectite clays, are correlated with regions of light toned albedo that were previously interpreted as outcrops of the Noachian crust. The presence of smectites suggests that these rocks are part of the older Martian crust, usually buried under subsequent deposits, but exhumed in Mawrth valles area by erosion. Sediments of the outflow channel are not enriched in hydrated minerals suggesting, like in the Nili area, that the floods were not accompanied by conditions (i.e. sufficient time, or climate) to permit alteration and formation of phyllosilicates.