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.