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takyon writes:

Evidence for liquid water on the surface of Mars may have actually been evidence of sand movement:

One of [Alfred] McEwen's most important finds [on Mars] came in 2011, with the discovery of recurring slope lineae (RSL), thousands of temporary streaks along steep slopes, mostly near the equator, that gradually grow and darken as spring turns to summer, as if fed by seeps of water. They soon became cited as the best evidence for liquid water on the surface of Mars today—and also one of the best places to search for microbial life.

But McEwen has now dowsed some of the excitement ignited by his initial finding. In a study published online this month in Nature Geoscience, he and his colleagues analyzed 151 of the streaks, finding that they only occur on slopes steeper than 27° and always peter out when the angle drops below that [DOI: 10.1038/s41561-017-0012-5] [DX]. The researchers interpret this as a sign that the RSL are not formed by water—which would flow down shallower slopes—but rather are dry flows of sand and dust seeking their natural angle of repose.

But there's still hope for life on Mars... or beneath it:

Q: Do you think of Mars as a hospitable place?

A: It's inconceivable to me that there aren't places where there's liquid water today within Mars. If there was ever life on Mars—that originated somewhere—why wouldn't there still be life today in these underground pockets? The surface, on the other hand, is a very harsh environment for life.

Q: Should the search for life be focused on Mars? Or should NASA and other agencies be exploring the ocean worlds that orbit Jupiter and Saturn?

A: I'm heavily involved in one mission—the Europa Clipper. Extant life there is much more likely today than on the surface of Mars. On the other hand, they're further away, and Europa in particular is in a harsh radiation environment. I think we should do both.

Also at The Verge.

Previously: Water on Mars
Is Anything Tough Enough to Survive On Mars?

Original Submission