Mars has water. Not just ice and snow, but liquid, flowing water. It’s salty and scant, but water is one of the essential ingredients for life on Earth—and although the Red Planet certainly wouldn’t be a cushy place to live, scientists think there’s a chance that single-celled organisms could be lurking somewhere on Mars. If we want to find it, we need to look in the places where it’s most likely to be; according to NASA, we need to “follow the water.”
Trouble is, most Mars spacecraft aren’t allowed to “follow the water,” because they’re not clean enough to enter the warm, wet “Special Regions” where Martian life is most likely to have gained a foothold. (Special Regions are defined by the International Council for Science's Committee on Space Research, under their planetary protection policies.) The logic is that if we send a dirty spacecraft into one of these Special Regions, Earth microbes could also thrive there, contaminating Mars and making it so that if we ever do discover microbial life on Mars, we won't know whether it's native to Mars or imported from Earth.
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