NASA reports strong evidence that liquid water exists on Mars at the present time.
Spectrographic readings from the Mars Reconnaissance Orbiter indicate the presence of hydrated salts, mostly perchlorates, in a common surface feature called "recurring slope lineae". The best conclusion at this time is that, during the summer months, salty water melts and flows downhill to form the lineae, then freezes during the winter.
This water may, however, be unable to support life. The salt content is incredibly high, which lowers the freezing point and allows it to melt at -23° Celsius. An Antarctic lake with similarly low temperature and high salinity is lifeless, so scientists are conservative in stating the effects this has on the potential for life on Mars.
RSLs have long been considered weak evidence for water on Mars. Current Mars rovers are prohibited from examining them, as they were not sufficiently sanitized before launch, and Earth microbes may have hitched a ride. While the Mars 2020 mission will also not be completely sterilized, I expect probes after that will be, to examine these formations up close.
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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?
