CHRONOLOGY OF MULTIPLE CATASTROPHIC FLOODS IN ARES VALLIS, MARS

We investigate the geomorphology and chronology of catastrophic flooding in a major martian outflow channel, Ares Vallis. We use recently acquired stereo and colour images and derived topographic data with grid-spacing of 50 m from the High Resolution Stereo Camera (HRSC) to constrain the detailed flood geomorphology of proximal Ares Vallis. The topography and image data provide morphologic evidence for 6 distinct erosion surfaces that were likely formed by multiple episodes of catastrophic flooding. These surfaces occur at distinct topographic levels, exhibit multiple sets of flood grooves, and contain post-flood crater populations that can be used to provide absolute and relative age constraints on the timing of flooding. Crater count statistics on the flood-eroded surfaces from MRO Context Camera (CTX) (6 m pixel-1) and Thermal Emission Imaging System (THEMIS) visible-light images (18 m pixel-1) indicate that flooding was initiated during the Late Noachian-Early Hesperian at ~3.6 Ga. The morphology data confirm that these initial flooding events occurred on the topographically highest erosion surfaces while subsequent floods abandoned the upper surfaces and continued to modify craters in the topographically lowest, main channel of Ares Vallis into the Late Hesperian-Early Amazonian (~2.5 Ga). We suggest that the erosion surfaces formed as a result of at least 2, but likely 6 distinct flooding episodes. The small downstream chaos terrains of the western branch of Ares Vallis, the multiple collapse terrains in Iani Chaos and Margaritifer Chaos, and the tributary systems of Aram Chaos and Hydapsis Chaos represent multiple sources for water release into Ares Vallis. This analysis supports previous numerical models that require multiple, lower discharge events to explain the time required to recharge the source aquifers and suggests the possibility for multiple, but smaller magnitude flood inundations of the Chryse Planitia basin.