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Dark streaks on sun-warmed slopes hint at unexpected dampness in the Martian tropics. If confirmed, the discovery would mean that water on Mars is more widespread than imagined – possibly adding a wrinkle to efforts to protect Mars from contamination with earthly life forms.

Previous images taken from orbit showed similar streaks that appear to be moving down slopes in the planet's southern mid-latitudes. These dark spots appeared in sunny areas in late spring and summer then faded away, leading scientists to conclude that briny ice under the surface was melting, and that liquid water was seeping up and evaporating.

The latest pictures have doubled the number of southern sites with streaks and added some closer to the equator, most in the deep canyons of Valles Marineris. These dark spots also appear on sunny slopes and then vanish.

"As something that wets and dries again, water is a very attractive explanation," says Alfred McEwen at the University of Arizona in Tucson.

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