The evidence comes from the Mars Orbiter Camera (MOC) on NASA's Mars Global Surveyor, which fell silent in early November. This high-resolution imager spotted fresh deposits in images of two crater slopes taken in 2004 and 2005 that did not appear in earlier pictures. "The shapes of these deposits are what you would expect to see if the material were carried by flowing water," says Malin, who is MOC's lead scientist. He argues that liquid water exists underground and collects behind icy dams along crater walls. When those barriers fail, water episodically rushes out, perhaps mixed with salts or other materials, and then flows downhill before evaporating into the thin Martian atmosphere.
I'm 90 percent convinced that the escape of subsurface of liquid water is the best explanation of what they are seeing," says Jim Bell (Cornell University) of the Mars rovers imaging team. He notes that the deposits indicate that the material is thin and flows around obstacles, which argues against landslides. The deposits are very bright, suggesting a fluid consisting of water and dissolved salts rather than one consisting primarily of liquid carbon dioxide. Instruments flying aboard NASA's Mars Odyssey and Mars Reconnaissance Orbiter should be able to pin down the composition of the deposits when they fly over these craters in the next few months. Bell says there is a good chance that the resulting data will enable scientists to confirm or refute the water hypothesis.
The Science paper also presents MOC data on 20 small impact craters created in the past decade. This information will be crucial to scientists who use crater counts to date the ages of surfaces around the planet.