GEOMORPHIC EVOLUTION OF THE MARTIAN HIGHLANDS THROUGH ANCIENT FLUVIAL PROCESSES

Craters in the Martian highlands are preserved in various stages of degradation. As a result of an erosional process active from the Middle Noachian (4.40-3.92 b.y.) through the Hesperian (3.55-1.8 b.y.), ejecta associated with fresh impact craters became etched, hummocky, and dissected by runoff channels. With time, interior gullies became deeply incised and ejecta deposits were entirely removed. Infilling of the craters followed until, in some instances, the craters were completely buried. Only fluvial processes explain these morphologic variations, the size range of affected craters, and the size-frequency distribution curves associated with these crater populations. Based on the number of superposed fresh impact craters, fluvial processes affecting the highlands ceased entirely by the end of the Hesperian. No correlation between cessation of degradation and latitude exists. However, a strong correlation exists between cessation of degradation and elevation. Degradation ended at higher elevations (e.g., 3-4 km; N[5]= approximately 200, Late Noachian) before lower elevations (e.g., 1-2 km; N[5]= approximately 180, Early Hesperian), suggesting that cessation was coupled to desiccation of the volatile reservoir and degassing of a 5-20 bar primordial atmosphere. Volatiles released to the surface by runoff channel formation and seepage may have been part of a complex hydrologic cycle that included periodic, heavy amounts of precipitation. Rainfall was principally responsible for degrading the highlands, eroding impact craters, and redistributing sediments. Rainfall also recharged the highland aquifers, allowing sapping and seepage to continue for hundreds of millions of years. As the primordial atmosphere was lost, cloud condensation, and thus rainfall and aquifer recharge, occurred at progressively lower elevations. Based on estimates on the amount of material removed and duration of degradation, denudation rates averaged 0.0001-0.005 mm/yr. These rates arc equivalent to those in terrestrial periglacial environments.