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.