Mars today is a cold, dry desert with an atmosphere so thin that any surface water evaporates or freezes. But spacecraft have found signs of a warmer and wetter past: sulfur-rich salts and other deposits that could only have formed in a water-rich environment, ancient rivers that carved networks of valleys, and evidence of lakes and vast floods. Much of the early water supply remains frozen in polar ice caps and in ice deposits just beneath the surface.
This image from the HiRISE camera on the Mars Reconnaissance Orbiter shows great detail in the highlands of Mars west of the Argyre basin. Light-blue areas are ancient river channels now filled by wind-blown sediments. The widest is about 500 meters (1,640 feet) across. The white dot in the color-coded topographic map (right) indicates the location of this image.