The glacial and periglacial evolution of Coprates Chasma (Valles Marineris, Mars)

Abstract

The search for past and present water on Mars, either as liquid or solid ice, with its important climatological and astrobiological consequences, has been bolstered by novel techniques, and from the release of recent imagery. Although ice is more frequently observed at higher latitudes, evidence of ice and water-related deposits and erosional features has also been identified at equatorial latitudes. This work focuses on the evidence for ancient ice and water-related features in Valles Marineris, Mars' most extensive canyon system, particularly in its deepest southeastern section, Coprates Chasma. We first consider existing information on water, glacial, and periglacial landforms and introduce new findings and imagery based on orbiters and on physical considerations.We examine the peculiar morphologies of two major landslides in Coprates Chasma, which can potentially imply icy conditions at the chasm's base. Analogies to Earth's glacially-induced landslides could hint at similar dynamics and the presence of ice within Valles Marineris. Remarkably intact landforms in Coprates Chasma include pitted cones remnants, commonly deduced to be volcanic cinder cones or mud volcanoes, and here interpreted as landforms more similar to pingos or hydrolaccolites, and then protalus ramparts, ice-affected impact craters, kettle holes from ice melt or sublimation, alases, and rock glaciers. The fluted terrain at the forefront of the Coprates Labes landslide indicate the presence of running water at the bottom of Coprates Chasma about 3 Ga ago. A 20-km long feature, here nicknamed the “Ghost Glacier”, characterized by thin sub-parallel grooves, is interpreted as the remnant of a series of drop moraines abandoned by a local cold-based glacier, possibly one of the Valles Marineris' oldest identifiable features. By examining the contacts between landslide deposits and the terrain of stacked landslide sequences, including craters underneath a landslide body, we deduce that ice in Valles Marineris was present as permafrost or in thin bodies growing locally to at most some meters in thickness. Moreover, the longitudinal profile indicates the lack of a thick glacier within the main valley. We conclude that the past occurrence of glacialism in Valles Marineris was only partly suggestive of glacial conditions found in Earth's Alpine valleys. Additionally, imagery corroborates the existence of a regolith-veneered lacustrine surface, dated to a recent age of 1.1 Ga, over which a massive landslide slid. We propose a tentative hydrological-glaciological history of Coprates Chasma, grounded in published research and our findings.