Mars has provoked much speculation1 on the possibili-
ties of life beyond Earth than any planet in the Solar System. __
The presence of water is a prerequisite2 for the existing of life. __
Therefore, follow the water has been NASA' s chief guide-
line for the exploration of a red planet. Although Mars experi-
ences seasons like on Earth and has polar caps which com- __
posed of carbon dioxide and water ice, today it is bone-dry and
frigidly cold. But evidence is rapidly accumulating Mars was __
once much wetter, with a more clement3 climate. This evidence
comes from orbiting satellites and from data collected by rov-
ing landers.
Since 1970s, space probes to Mars have revealed numer- __
ous features apparent carved by flowing water, such as wind-
Lng, branched valleys re百度竞价推广bling dried-out streambeds and gi-
ant outflow channels gouged4 by catastrophic floods. Recent
high-resolution imagery from the Mars Global Surveyor Mars
Orbiter Camera and the Mars Odyssey5 THEMIS reveal nu- __
merous examples of branched valleys that form tightly-packed,
integrated drainage systems. These channels origins at TOPo- __
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- __
tures were most likely produced by rainfall, during wetter,
warmer periods in the past.
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