SIMULATING LONG-TERM NOACHIAN LANDFORM EVOLUTION AND PALEOCLIMATE

It is evident from numerous fluvial features (e.g., alluvial fans, deltaic deposits, and valley networks) that Mars was episodically warm enough to support an active hydrological cycle. Valley networks and paleolakes were active around the Noachian/Hesperian (N/H) transition, and most studies of early Martian geomorphology have focused on this relatively short time span. However, the prevailing environmental conditions over the rest of the Noachian Period are not well known. Longer-term conditions are more relevant to biological evolutionary timescales than are shorter-term climatic excursions. Some of the best insights into long-term conditions come from studies of Noachian crater degradation and the interaction of cratering with other processes. Unlike Earth, where landforms are constantly being altered by tectonic, fluvial, mass wasting, and other processes, older geomorphic surfaces are generally well preserved on Mars due to greatly reduced erosion rates since Noachian period. We are using the degree of degradation of craters on the Martian equatorial highlands to reconstruct the sequence of impacts, starting shortly after the last major basin-scale impacts. These degraded craters and relict intercrater surfaces are then used to model the geomorphic processes that shaped the Noachian landscape from the onset of the visible cratering record (~4.0 Ga) up through the time of valley incision (~3.7 Ga). Through landscape evolution modeling of the representative study areas on the Martian highlands, we constrain the long-term Noachian environment. Our initial model runs of a 500 x 500 km area in Noachis Terra (25°S, 28°E) indicate that the rate and pattern of Noachian erosion might have been limited by the weathering rate of the bedrock, if runoff production was high. When the bedrock was set to as readily erodible as regolith, crater rims retreated rapidly, causing the resulting surface topography to be largely obliterated over the model timescale. Also an arid to semiarid climate with a low dependence of runoff upon contributing area best replicates the degree of fluvial dissection and crater degradation in this region.