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Mars has provoked much speculation on the possibilities of __1__life on Earth than any other 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 guideline for the exploration of a red planet. Although Mars experiences __4__seasons like on Earth and as polar caps which composed __5__of carbon dioxide and water ice, today it is bone-dry and frigidly cold. But evidence is rapidly accumulating that Mars is once __6__much wetter, with a more clement climate.
This evidence comes from orbiting satellites and from data collected by roving landers.Since the 1970s, space probes of Mars have revealed __7__numerous features apparently craved by flowing water, such as winding, branched valleys resembling driedout streambeds and giant outflow channels gouged by catastrophic floods.Recent high-resolution imagery from the Mars Odyssey Themis reveal numerous examples of branched valleys that from __8__tightly-packed integrated drainage system. These channels origins at topographic high points; the valleys widen “downstream”, __9__some even displaying inner valleys. The valley networks densities,comparative to those of terrestrial drainage basins. __10__ These features were most likely produced by rainfall, during wetter, warmer periods in the past.
