A Glacial Origin for the Extensive, Linear Inverted Features Within Gale Crater

Abstract:

Previous orbital and in-situ observations at Gale crater have revealed a sedimentary depositional history dominated by lacustrine, alluvial, fluvial, and aeolian processes; however, new insights from this study provide evidence for a late glacial episode based on prominent sedimentary deposits seen from orbit. One particular linear, inverted feature on the eastern floor of the crater has characteristics consistent with a glacial esker origin. When glaciers of sufficient thickness accumulate, water may be transported both on the surface of the ice and at the glacier base. Water flowing at the base carves tunnels in the ice as flow paths for meltwater and sediment carried as bed load from the local watershed and from below the glacier. Sediment may aggrade within the tunnel to form relatively static, inverted features known as eskers. These erosionally resistant structures are observed to extend on the Earth up to hundreds of kilometers and comprise unique morphology distinguishing them from other inverted forms. Terrestrial eskers share a common characteristic of linearity as opposed to their sinuous, fluvial inverted counterparts. The east crater floor esker is highly linear with a straight-line length of 12.94 km, a total length of 14.48 km, and a sinuosity ratio of 1.12 over a total elevation change of ~215 m. Fluvial inverted channels also tend to meander gradually without abrupt changes in direction. The Gale inverted feature displays several sharp directional changes more than 90°, not representative of a fluvial environment. Conventional sedimentary deposition follows the local hydraulic gradient (i.e. no uphill flow/deposition) but several areas along this inverted feature show gradient reversals of up to tens of meters. This phenomenon occurs at the sharp changes in direction, signifying pressurized uphill flow within the ice tunnel followed by subsequent undermining and breakout of the esker flow path to again follow the local hydraulic gradient. This specific sector of the crater also supports the highest probability for extended preservation of glacial ice in the region due to low insolation in this north-south trending depression. The crater rim provides protection against melting during the morning and Aeolis Mons provides even more protection in the afternoon when a majority of melting takes place.