This colour-enhanced image shows the apparently damp streaks on the left-hand side (Image: NASA/JPL/University of Arizona)
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.
Water ice wasn't expected to exist in the Martian tropics, where temperatures get high enough that even buried ice should have been vaporised. Small amounts of water might be condensing from the atmosphere rather than seeping up from ice deposits. Either way, McEwen is convinced that liquid water is somehow involved. "No one has come up with a good alternative model," he says.
The fresh evidence for seasonal streaks bolsters the case for briny liquids at work on Mars, says James Head at Brown University in Providence, Rhode Island, who was not involved in the study. Water at the equator would be very surprising, he adds, but it is possible some ice may remain under protective soil cover.
And that may force a rethink of future exploration plans, says McEwen. While spacecraft headed to Mars are carefully sterilised before launch, there is a risk that hardy hitchhiking microbes could make it all the way to the planet's surface. A 2008 report had concluded that tropical Mars is too dry for terrestrial life to survive there, making it a relatively safe landing zone. Evidence of recurring damp spots in the Martian tropics could change that assessment, says McEwen.
Journal reference: Nature Geoscience, DOI: 10.1038/NGEO2014
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