Gullies on Mars

The Gullies of Mars would probably not have been sensational enough
for the title of a vintage Edgar Rice Burroughs story about the
Red Planet. But it would get the attention of planetary scientists today.
First identified in high resolution images of Mars recorded by the
orbiting Mars Global Surveyor spacecraft, the gullies are interpreted
as startling evidence that liquid water flowed across the martian surface
in geologically recent times. Similar channels on Earth are formed by
flowing water, but on Mars the temperature is normally too cold and the
atmosphere too thin to sustain liquid water. Still, it is thought possible
that water did burst out from underground layers and remain liquid long
enough to erode the gullies, while alternative explanations suggest the
erosion was produced by a flowing jumble of solid and gaseous carbon
dioxide. Spanning a few kilometers along the wall of an impact crater
this high resolution image from Mars Global Surveyor shows typical martian
gullies near the top of the crater wall giving way to sand dunes toward the
crater floor. Whitish frost is visible near the top and on the dark sand dunes
below. The muted colors were synthesized from wide angle image data.