GEOMORPHIC EVOLUTION OF THE MARTIAN HIGHLANDS THROUGH ANCIENT FLUVIALPROCESSES

Craters in the Martian highlands are preserved in various stages of de gradation. 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.), eject a associated with fresh impact craters became etched, hummocky, and di ssected by runoff channels. With time, interior gullies became deeply incised and ejecta deposits were entirely removed. Infilling of the cr aters followed until, in some instances, the craters were completely b uried. Only fluvial processes explain these morphologic variations, th e size range of affected craters, and the size-frequency distribution curves associated with these crater populations. Based on the number o f superposed fresh impact craters, fluvial processes affecting the hig hlands ceased entirely by the end of the Hesperian. No correlation bet ween cessation of degradation and latitude exists. However, a strong c orrelation exists between cessation of degradation and elevation. Degr adation 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]= appro ximately 180, Early Hesperian), suggesting that cessation was coupled to desiccation of the volatile reservoir and degassing of a 5-20 bar p rimordial atmosphere. Volatiles released to the surface by runoff chan nel formation and seepage may have been part of a complex hydrologic c ycle that included periodic, heavy amounts of precipitation. Rainfall was principally responsible for degrading the highlands, eroding impac t craters, and redistributing sediments. Rainfall also recharged the h ighland aquifers, allowing sapping and seepage to continue for hundred s of millions of years. As the primordial atmosphere was lost, cloud c ondensation, and thus rainfall and aquifer recharge, occurred at progr essively lower elevations. Based on estimates on the amount of materia l removed and duration of degradation, denudation rates averaged 0.000 1-0.005 mm/yr. These rates arc equivalent to those in terrestrial peri glacial environments.