Water in Mars

Clearly visible on images produced by Martian probes are runoff and outflow channels,
which are believed to be dry river beds, evidence that water once flowed as a liquid
on Mars. Geological evidence dates the Martian highlands to four billion years
ago, the time in which water was apparently sufficiently plentiful to cause widespread
flooding. Recent theories suggest that at the time, Mars had a thicker atmosphere that
allowed water to exist in a liquid state, even at its low surface temperatures.
The Mars Global Surveyor mission, which has established an orbit around the red
planet and is transmitting early data back to the earth, has found further geological
evidence for the presence of liquid and subsurface water. Such evidence has kept alive
hopes that life may have existed—or may even yet exist, perhaps on a microbial
level—on Mars.