Fluvial valleys and martian palaeoclimates

Abstract

NETWORKS of small fluvial valleys are extensively developed throughout the ancient heavily cratered terrains of Mars^1–3. 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^4,5 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^6–8 conflicts with this scenario. Whereas the widespread ancient martian valleys generally have morphologies indicative of sapping erosion by the slow outflow of subsurface water^9,3, the local Alba valleys were probably formed by surface-runoff processes. Because subsurface water flow might be maintained by hydro-thermal energy inputs^10,11 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.