HER DESHER AND NIRGAL VALLES: STRUCTURAL AND MINERALOGICAL EVIDENCE OF SURFACE WATER TRAPPING AND GROUNDWATER RECHARGE IN NW NOACHIS TERRA

Two small valley systems incise NW Noachis Terra: Her Desher and Nirgal Valles. Nirgal is a tributary of Uzboi Vallis, while Her Desher is an isolated valley that does not obviously connect to any outlet. Numerical groundwater models suggest that a Tharsis source region would likely feed groundwater flow through fracture systems in NW Noachis Terra, providing a source of water to form these valleys. Both are geomorphically more similar to valleys formed by groundwater sapping, as opposed to valleys formed by overland flow. There is evidence of strong circum-Tharsis structural control of the networks. The main channel of Nirgal trends sub-parallel to nearby Tharsis-radial graben, with abrupt ~90º bends along the length of the valley; many of Nirgal’s tributaries trend parallel to nearby circum-Tharsis wrinkle ridges. Her Desher also shows sharp 90º bends along its length; portions of the valley align parallel to the regional Tharsis-radial graben. The valley abruptly terminates against these graben, perhaps causing the formation of the pool-like feature at the valley’s southern end. Subtle outflow channels just to the north of Her Desher and Nirgal also terminate at the regional Tharsis-radial graben, suggesting a major water trap and recharge site for a groundwater reservoir.

Nirgal Vallis, Her Desher Vallis and nearby craters all expose the same Fe/Mg-phyllosilicate-bearing layer, with a relatively broadened absorption at ~2.3 µm that suggests a mixture of nontronite and saponite or Mg exchange for Fe in the hydroxyl site of the phyllosilicate phase. New analyses of MTRDR hyperspectral images of Her Desher and Nirgal indicate that hematite and/or polyhydrated sulfate are present in the phyllosilicate layer. In Her Desher, the hematite/sulfate-bearing material comprises approximately 50% (by volume) of the altered layer and is present both above and below the smectite along the entire length of the valley. While the hematite/sulfate can be observed in the alteration layer in Nirgal, its exposure is only ~10% that of the smectite and is inconsistent in extent, present in some areas while lacking in others. Qualitatively, there seem to be more hematite/sulfate exposures in those areas of Nirgal closest to Tharsis and fewer farther away. The evidence suggests that an acidic, sulfur-rich groundwater flowed through an established phyllosilicate-bearing layer and either converted some of the pre-existing smectite into hematite/sulfate or precipitated sulfate salts and Fe-bearing hematite during a later period of alteration.