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 apparent 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.
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