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C.B. Leovy (University of Washington)
Three issues drive our evolving effort to understand the evolution of the atmosphere and surface of Mars: the origin of outflow channels and valley networks, the hemispheric dichotomy, and most recently, evidence from the Mars Global Surveyor pointing toward running water and possibly to standing bodies of water in the recent past.
Outflow channels are widely assumed to have been formed by massive liquid water floods during the Hesperian to early Amazonian periods, while the valley network features are generally believed to have been formed by widespread and repeated surface water flows or groundwater sapping events during the Noachian period. Despite morphologies that strikingly implicate fluvial processes, unresolved difficulties with the flowing water interpretation of these features remain. These include adequate explanations for the fate of the large volume of water required to produce outflow channels and of the mechanisms needed to maintain the warm surface temperatures that would have allowed widespread formation of valley networks by surface erosion or sapping. Thus alternatives to the flowing water hypothesis must still be considered. Recently N. Hoffman (2000, Icarus, p. 126) has proposed that release of carbon dioxide from subsurface clathrate deposits or pressurized liquid carbon dioxide reservoirs could have formed these features by fueling powerful turbidity currents. Even without buried carbon dioxide reservoirs, turbidity currents associated with dust storms are prominent on Mars today, are responsible for widespread erosion and deposition of fine materials, and may have substantially contributed to both outflow channel and valley network features.
Fine particle erosion and transport by wind is highly sensitive to both surface pressure and surface roughness. As a consequence, a deep regolith consisting of a mixture of cobbles, boulders, and fine particles may be unstable with respect to wind erosion. As wind removes fine materials locally, the regolith surface roughens and deepens and becomes susceptible to further erosion. At 100 hPa surface pressure, wind erosion and transport would be two orders of magnitude more effective than today. If pressure during the Noachian was this high or higher, rapid redistribution of fine surface materials and ices could have drastically altered the surface of the planet. This mechanism may have contributed to aspects of the martian hemispheric dichotomy through erosion of the northern plains primarily during the Noachian period. Although there is considerable evidence favoring a depositional origin for these plains, possibly associated with large standing bodies of water, several lines of evidence suggest the contrary, that the northern plains are erosional rather than depositional. A well-instrumented rover mission to a northern plains site with exhumed craters sheltering residual deposits might be able to resolve issues of the origin and evolution of the plains and shed light on the evolution of outflow channels and valley networks.