(Business Insider) For years, a team of NASA scientists had pondered over the enigmatic dark streaks lining the inner and outer faces of craters on the Martian surface, shown below.
But no longer.
NASA announced the answer to the conundrum Monday: The streaks are evidence of flowing water.
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Most mysterious about these streaks — which can be as long as a football field — was that they appeared to change in size over time, growing longer during Mars' warm summer months and shrinking during colder seasons: