INSIGHT INTO VALLEY NETWORK ACTIVITY FROM MORPHOLOGY IN THE GREATER MERIDIANI PLANUM REGION, MARS (Invited Presentation)

Valley networks, the most common drainage feature on Mars, are frequently cited as evidence of former climate conditions conducive to sustained overland flow of water. Recent mapping by Davis et al. (2016, Geology) shows an extensive network of inverted channels across Arabia Terra, filling a gap in earlier valley network maps, and illustrating a relatively high concentration of inverted channels in the equatorial greater Meridiani Planum (GMP) region (5°S–5°N, 11°W–6.5°E). The juxtaposition of negative-relief and inverted valley network morphological types in the GMP make it a unique area to assess the nature and relative timing of aqueous processes.

Using Mars Reconnaissance Orbiter (MRO) ConTeXt (CTX; ~6 m/pix) images, we mapped fine-scale valley networks in the lowest exposed stratigraphic unit in the GMP region, ancient cratered highlands (Nhc₁). Fine-scale valley network links are typically ~100 m wide, with a maximum width of 500 m, and <20 m relief. Key regional differences were noted in fine-scale valley network planimetric form, density, and connections to candidate paleolakes. Four principal fine-scale valley network morphologic types are observed: pitted, channel, ridges, and knobs. Transitions between morphologic types along course are common. In the Pampa del Tamarugal region of the Chilean Atacama Desert, there is a similar continuity of landforms associated with groundwater processes, from collapse pits, to pit chains, to curvilinear depressions, to ridges. Drawing upon comparisons with this Earth analog, we propose that the regional differences in observed GMP valley network attributes are related to the mobility of solutes and reflect temporal changes in aqueous processes. An early phase of sustained surface overland flow was followed by a period dominated by groundwater processes, where widespread evaporite deposition transitioned to localized dissolution of solutes that reactivated ancient valley networks. We interpret the newly identified pitted valley networks to reflect late-stage groundwater processes concentrated along the former fluvial conduits, and this localized, reactivation of valley networks occurred during or after exhumation of the Nhc₁ unit.