Formation of Martian flood features by release of water from confined aquifers

Abstract

Many large Martian channels arise full born from discrete areas of chaotic terrain. Estimates of peak discharges based on channel dimensions range from 10 6 to 10 8 m 3/s. It is here proposed that the large channels were eroded by water released rapidly, under great pressure, from deeply buried aquifers. Early in the planet's history the old cratered terrain was probably highly permeable to depths of several kilometers as a result of its volcanic origin and intense brecciation by meteorite impact. Extensive dissection of the old cratered terrain by fine channels suggests that fluvial action was widespread at that time and that warmer climatic conditions prevailed. Much of the water which cut the fine channels was probably removed from surface circulation and entered the groundwater system. Subsequent global cooling trapped the groundwater under a thick permafrost layer and formed a system of confined aquifers. Thickening of the permafrost and warping of the surface created high pore pressures within the aquifers, particularly in low areas. Episodic breakout of water from the aquifers could have been triggered either by impact or by the pore pressure reaching the lithostatic pressure. The rate of outflow would have depended on the aquifer thickness and permeability, its depth of burial, and the diameter of the region over which water had access to the surface. Plausible values give discharges that range from 10 5 to 10 7 m 3/s. Outflow from the aquifer probably caused undermining of the adjacent areas and collapse of the surface to form chaos. Flow ceased when the aquifer was depleted, or when the hydraulic gradient around the chaos, and thus the flow, was so reduced that the flow could freeze. The process could be repeated if the aquifer were recharged.