Ma'adim Vallis Revisited through New Topographic Data: Evidence for an Ancient Intravalley Lake

Abstract

Based on a comprehensive study of the recent topographic map USGS MC-23 SE I-2119 (USGS, 1991,Topographical Map of Aeolis) plus geomorphic and digital elevation, we have constructed a geometric model of Ma'adim Vallis. The sedimentary history of the valley was determined by the damming of the valley by two impact craters, one by the southern crater over Gusev rim and the second located 120 km upstream. The presence of an intravalley lake system is inferred from independent sedimentary analysis and from geomorphic evidence for valley benches. Peripheral and parallel topographic contour intervals of the putative intravalley lake shores suggest that the lake was covered by a thick ice sheet. The longitudinal profile of the valley floor shows a peculiar reverse slope located 300 km upstream from the southern Gusev crater rim. We conclude that this is an accumulation of sedimentary deposits adjacent to the crater rampart which defined the northern boundary of the lake. When the water level reached the lowest point of the west bank, which is 0.4 km below the top of the rim of Gusev crater, it began to floor the surrounding plateau. As the intravalley lake level raised, its ice cover formed a wide lateral bench. Later, this ponded water was funneled into Gusev crater through the breaching of the southern Gusev rampart by an impact crater. The paleolacustrine sediments transported by the lake water that channeled through the rampart debris slope mantled the floor of Gusev crater. The sublacustrine sediments are promising targets for future missions and of high priority in the search for past life.