NETWORKS of small fluvial valleys are extensively developed throughout the ancient heavily cratered terrains of Mars. The existence of the valleys has been cited as compelling evidence for a relatively dense primordial atmosphere capable of maintaining an Earth-like hydrological cycle. Theoretical models of early atmospheric evolution describe the maintenance of a dense CO2 atmosphere and a warm, wet climate until the end of the heavy-bombardment phase of impacting. However, the presence of very young, Earth-like fluvial valleys on the northern flank of Alba Patera conflicts with this scenario. Whereas the widespread ancient martian valleys generally have morphologies indicative of sapping erosion by the slow outflow of subsurface water, the local Alba valleys were probably formed by surface-runoff processes. Because subsurface water flow might be maintained by hydro-thermal energy inputs and because surface-runoff valleys developed late in martian history, when planet-wide climatic conditions were presumably similar to the present, it is not necessary to invoke drastically different planet-wide climatic conditions to explain valley development on Mars. The Alba fluvial valleys can be explained by hydrothermal activity or outflow-channel discharges that locally modified the atmosphere inducing precipitation and local overland flow on low-permeability volcanic ash.
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