EXTENSIVE PHYLLOSILICATE LAYER IN NORTHWESTERN NOACHIS TERRA: RELATIONSHIP TO PHYLLOSILICATES IN HOLDEN AND EBERSWALDE?

Evidence for a widespread phyllosilicate-bearing layer has been identified in a distinct region in northwestern Noachis Terra, Mars. This region is bound by Coprates and Eos Chasma to the north, Thaumasia Planum to the west and the Argyre basin to the south. Uzboi Vallis, a major Martian valley system that cuts through Noachis Terra, is the eastern boundary of our region of interest. Two of the potential Mars Science Laboratory (MSL) landing sites, Holden and Eberswalde craters, are sinks in this valley system. Phyllosilicates have been identified in the depositional fans at both craters.

Her Desher Vallis and Nirgal Vallis are two small valley systems that incise the NW Noachis region. Nirgal Vallis, a tributary of Uzboi Vallis, joins the larger system to the south of Holden crater. Her Desher Vallis does not obviously connect to any craters or larger valleys and appears isolated. Mosaicked CRISM mapping observations (resolution 230 m/pixel) indicate that a phyllosilicate-bearing layer stretches along the length of both valley systems and in the interior rims of many craters in the region. Phyllosilicates have been positively identified in CRISM targeted observations (resolution 20-40 m/pixel) of the walls of both Her Desher and Nirgal Valles. A distinct phyllosilicate-bearing layer is observed only a few meters below the surface materials, extending laterally along the walls, consistent with the mapping data. A similar phyllosilicate-bearing layer, located a comparable distance below the surface, is also identified in the interior walls of nearby craters. Spectral characteristics from all observations are consistent with a Fe/Mg smectite, perhaps a mixture of nontronite and saponite.

We will explore the connection between the observed NW Noachis phyllosilicate layer and the phyllosilicates detected in the deltas within Holden and Eberswalde craters. Comparison of spectral properties between the two deposits will provide important insight into whether the phyllosilicates have been eroded from the regional layer and re-deposited, or whether the deltaic phyllosilicates formed by an unrelated mechanism. Understanding these formational scenarios will play a critical role for any in situ investigations conducted by the 2011 MSL rover.