Mars is the only planet in the Solar System that has landscapes that bear any resemblance to those we see on Earth. The one factor common to both planets is that surfaces have been shaped by flowing water. On Mars, that was a one-off event early in its history, and thereafter shaping the planet has been through continual movement of dust in its thin, but energetic atmosphere, the formation of impact craters and volcanism. Evidence for fluvial processes occurs in the highland regions, which were built mainly by volcanic activity., and stems from careful examination of high-resolution photography from orbiting probes. Whether the various kinds of valleys formed by catastrophic, short-lived floods of melt water released by impacts into deep frozen ground, through steady release of groundwater or actually by precipitation are the ground for speculation and controversy. A means of assessing the possibilities is using accurate data on topographic elevation. Digital elevation models for the Earth, even at coarse resolution (GTOPO30 data at 1 km resolution), map out the intricacy of surface drainage of the continents. A DEM produced by the laser altimeter aboard Mars Orbiter allows not only the various models to be assessed, but enables quantitative work on the amount and rate of water erosion and deposition of sediment when combined with evidence for the age and duration of Mars’ fluvial event (Hynek, B.M. and Phillips, R.J. 2001. Evidence for extensive denudation of the Martian highlands. Geology, v. 29, p. 407-410).
Hynek and Phillips show that the event was long lived, lasting 350 to 500 Ma around 4 billion years ago. Their study was of an area the size of Europe. Scaled up, their findings suggest that of the order of 5 million cubic kilometres of sediment was transported, equivalent to deposition of a 120 metre thick sediment layer in the flat plains of Mars’ northern hemisphere. The average rate of erosion during the event compares closely with that typical of temperate maritime areas of mountains on Earth. It is difficult to see how such prolonged erosion could have taken place without runoff fed by precipitation on the surface, and that implies a much warmer climate and thicker atmosphere than on modern Mars, albeit only for a very early episode in its evolution.