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.