Long-term precipitation and late-stage valley network formation: Landform simulations of Parana Basin, Mars

We use a computer landform evolution model to show that Noachian-Hesperian-aged, late-stage valley network formation required numerous and repeated moderate flood events rather than one or a few continuous, multiyear, deluge-style flows. We introduce a technique that generates an estimated initial conditions'' digital elevation model (DEM) of the Parana Valles drainage catchment (PDC) prior to valley network incision. We then explored how variations in three classes of environmental parameters related to fluvial processes, and surface material properties evolve the initial conditions DEM. Specifically, we parameterized discharge scaling, evaporation from ponded water, and the effects of an indurated surface crust. Each simulation run produced a model output DEM that was qualitatively and statistically compared to the actual surface DEM. Simulations with an arid to semiarid climate, moderate evaporation rates, and an indurated surface crust provide the best match to the actual surface. Simulated valley network formation requires periods of fluvial activity that last a minimum of 10(3) - 10(4) years under constant deluge-style conditions. However, craters within the PDC in deluge-style simulations overflow and generate exit breaches that cut through all crater walls. Longer simulations (10(5) - 10(6) years) that modeled repeated, episodic flows with interim evaporation avoid universal crater breaching. The paucity of crater rim exit breaches in the PDC and the southern highlands in general implies both that the precipitation was not continuous and that formation conditions were inconsistent with a few short-lived extreme climate excursions such as might be induced by large-scale impacts or other cataclysmic events.