5 - Lakes in Valles Marineris

Abstract

The Valles Marineris is a large system of troughs extending just south of the Martian equator from about longitude 250° E to 320° E. They trend approximately N 75° W for a distance of about 4000 km, equivalent to nearly the entire width of the United States. The troughs opened about 3.5 Ga ago. Initial collapse along structural planes of weakness formed ancestral troughs, littered with chaotic hills, which filled rapidly with water from ground ice, subterranean aquifers, or nearby valley networks. Ancestral outflow channels spilled out of these troughs and flowed across the adjacent plateaus. Sediments may have filled the troughs gradually, as the basins sank, or were deposited into deep lakes after rapid basin collapse. The major lakes within the Valles Marineris dried up, except for a few late shallow lakes on the trough floors that were locally fed by tributary canyons or lingering valleys. Spur-and-gully erosion, which carved the early walls, apparently also stopped. It is nearly inevitable that lakes or playas with varying levels of surface or ground water existed at some time inside the Valles Marineris. It is difficult to envision the formation of or alteration to hydrated evaporite and iron oxide deposits without some aqueous activity. Ongoing investigations, especially with hyperspectral data, may eventually shed more light on the origin of the ILD and perhaps clarify the still ongoing debate over the extent to which the original ILD were of sedimentary, volcanic, or mixed provenance.