Mars has provoked much speculation on the possibili- (1)
ties of life beyond Earth than any planet in the Solar System. (2) __
The presence of water is a prerequisite for the existing of life. (3) __
Therefore, "follow the water" has been NASA' s chief guide-
line for the exploration of a red planet. Although Mars experi- (4)
ences seasons like on Earth and has polar caps which com- (5) __
posed of carbon dioxide and water ice, today it is bone-dry and
frigidly cold. But evidence is rapidly accumulating Mars was (6) __
once much wetter, with a more clement climate. This evidence
comes from orbiting satellites and from data collected by rov-
ing landers.
      Since 1970s, space probes to Mars have revealed numer- (7) __
ous features apparent carved by flowing water, such as wind-
Lng, branched valleys resembling dried-out streambeds and gi-
ant outflow channels gouged by catastrophic floods. Recent
high-resolution imagery from the Mars Global Surveyor Mars
Orbiter Camera and the Mars Odyssey THEMIS reveal nu- (8) __
merous examples of branched valleys that form tightly-packed,
integrated drainage systems. These channels origins at topo- (9) __
graphic high points; the valleys widen "downstream", some
even displaying inner valleys. The valley networks exhibit
morphometric characteristics, including network densities,
comparative to those of terrestrial drainage basins. These fea- (10) __
tures were most likely produced by rainfall, during wetter,
warmer periods in the past.