THE HISTORY OF WATER ON MARS

Even after decades of exploration by spacecraft the history of water on Mars remains a highly contentious issue. Understanding what water was doing on Mars during the Nochian and Hesperian is predicated on investigations that are taking place in three disparate scientific disciplines : geology, astronomy, and meteorology. As our understanding of the geology of Mars has evolved as we have obtained additional data at higher resolution over multiple wavelengths, so has our thinking about water on early Mars. Based on Viking data, the original paradigm was that early Mars had always been cold and dry, highland impact craters had been modified by aeolian or volcanic resurfacing, and the valley networks resulted from groundwater sapping that was sustained by impact melt and/or higher heatflow following accretion. However, topographic data reveal that highland impact craters were eroded by a combination of rainfall and surface runoff that was supported by a global arid climate with higher evaporation rates and/or higher infiltration capacities. Although there is some weak evidence suggesting an earlier stage of valley network formation, a majority of valley networks formed during the last Noachian/early Hesperian boundary. Erosion of the valley networks required substantial amounts of rainfall and surface runoff that was supported by a more regional humid climate and a global hydrologic cycle. Eventually, enough of the primordial atmosphere collapsed into the regolith so that hydrologic condidtions necessary to initiate outflow channel formation were established. From there, the conditions for supporting liquid water on Mars quickly waned. The geologic evidence suggests that conditions can sometimes become favorable for large deposits of ice to accumulate at lower latitudies during the Amazonian.