Constraining Paleolake Activity on Mars from Outlet Valley Morphometry

Abstract:

Paleolake basins on Mars have long been studied in an attempt to further understand the hydrology and climate conditions of the ancient martian surface. While the formation of these lacustrine features is evident, the timescale over which they were active and the magnitude of related fluvial activity remains uncertain. The most convincing evidence of paleolake activity comes from hydrologically open basins, or open-basin lakes, that have an outlet valley breaching the basin confining topography, commonly a crater rim. This observation requires that the basin filled completely with standing water before overtopping and incising the outlet valley. The total amount of incision for a given outlet valley is ultimately controlled by a variety of factors, including the susceptibility of the crust to erosion, the topography and hydrology (e.g., gradient, base level) of the terrain exterior to the breach, the magnitude of flooding caused by the initial overtopping of the crater rim, the rate of outflow from the lake once it has reached an equilibrium level following catastrophic breaching, and the duration of time over which the basin contained a hydrologically open lake.

Here we present morphometric analyses of the outlet valleys of 20 open-basin lakes on Mars to help further understand ancient martian paleolake activity. We measured the outlet valley depth of incision as compared to the estimated pre-breach topography, as well as the breach width using CTX stereo-derived digital elevation models. Results from basins with different morphometric parameters (e.g., area, mean depth, volume) are compared to one another to assess any systematic relationship(s) between outlet valley morphometry and paleolake basin morphometry. We also examine these lakes in the context of their broader hydrological setting, and consider how our results compare to terrestrial analogs for the process of crater rim breaching. In particular, we examine the process of catastrophic breaching of natural earthen dams, which we suggest provides the best terrestrial analog to the process of breaching an outlet valley for a crater lake.