Closed drainage crater basins of the Martian highlands: Constraints on theearly Martian hydrologic cycle

One hundred forty four crater basins with closed drainage have been mapped within the. Martian Noachian highlands that have influent but no outflow ch annels. The influent streambeds to these basins are characterized by steep gradients and elevation head losses on average of 1 km. These closed basins and influent fluvial channels provide a foundation to investigate the proc esses under which water was transferred among the early Martian atmosphere and groundwater reservoirs. Channel gradients reflect basin-controlled wate r table conditions, and the channel head losses suggest sustained drawdowns within the basins due to evaporation and/or sublimation-induced pumping of groundwater. Finite element modeling bf groundwater flow permits estimatio n of minimum rates for evaporation/sublimation for a reasonable set of boun dary conditions and hydraulic conductivities, and suggests a Noachian envir onment with a minimum of 10-20 cm/yr of evaporation and/or sublimation. The flow models are unable ro be reconciled with channel gradients Without int roduction of local precipitation. Acceptable solutions are found with a min imum of 1-2 cm/yr of groundwater recharge.