Case for Martian rainfall strengthens

“Everyone knows” about the huge valley systems on Mars, which through their relationships to other aspects of the planet’s features are thought to have formed catastrophically early in its history.  The high-resolution Mars Global Surveyor images and altimetry bring a new perspective to fluvial features (Hynek, B.M. & Phillips, R.J. 2003.  New data reveal mature, integrated drainage systems on Mars indicative of past precipitation.  Geology, v. 31, p. 757-760).  The authors, from Washington University in St Louis USA, show depressions extracted from the altimetry data by simulation of the paths likely to be taken by rain water falling on the surface.  In some areas, the depressions link up in dendritic networks very like those that occur on the Earth’s surface.  Previous data only picked up disconnected valleys.  The newly outlined valleys are V-shaped, unlike the U-shaped systems that developed on Mars probably by sapping as groundwater emerged, either slowly or catastrophically.  Such profiles are good evidence for surface run-off, and that can only indicate precipitation, either of rain, or as a result of melting snow.  Only 11000 kilometres of valley segments can be identified, and are probably relics of a larger ancient system that later events have masked.  Some however, reach to the rims of large craters and seem to post date them.  Probably, the events that carved these systems occurred in Mars’ early history.

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