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“The arrival
of instrumented spacecraft, beginning with Mariner IV in 1964, showed a
cratered, dry, cold planet that looked almost as inhospitable as our Moon; but
later missions provided
href="http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=2920&mode=thread&order=0&thold=0">evidence
of liquid water on Mars in the recent past. This has led to speculation
about whether simple forms of life, like bacteria, might exist on Mars. The
thin atmosphere of Mars does little to block out damaging radiation from the
sun, and the surface of Mars seems to be sterilized by caustic chemicals like
href="http://www.astrobio.net/news/index.php?name=News&file=article&sid=1431">hydrogen
peroxide, but scientists still hold out hope that
href="http://www.space.com/common/media/video.php?videoRef=b031105_eom_clip4">life
on Mars could survive protected  below the surface.” source...