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.