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.