Scientists have revisited a 2015 sighting of water on the surface of Mars and revised the theory to suggest what we saw was sand, and probably not even damp sand at that.
In a new paper titled Granular flows at recurring slope lineae on Mars indicate a limited role for liquid water seven authors from United States Geological Survey (USGS), the University of Arizona, Britain's Durham University and the Planetary Science Institute revisited the study we reported on in 2015 under the headline “WATER SURPRISE: Liquid found on Mars, says NASA”.
That story reported on analysis of “recurring slope lineae” (RSL), described at the time as “dark streaked gullies that appear on some sections of the Martian surface during the summer months.”
The 2015 analysis used information gathered by the Mars Reconnaissance Orbiter (MRO) and led boffins to say that data “unambiguously supports our liquid water-formation hypotheses for RSL.”
The new paper's conclusion is that RSLs “are candidate locations for seeping liquid water on Mars today, but their formation mechanism remains unclear.”
The USGS summary of the new paper stated “The terminal end of the RSL slopes … are identical to the slopes of sand dunes where movement is caused by dry granular flows.”
“Water almost certainly is not responsible for this behavior, which would require the volume of liquid to correspond to the length of slope available, producing more liquid on longer slopes,” the USGS summary stated. “Instead, the 151 RSL examined by the study authors all end on similar slopes despite very different lengths. Additionally … water is unlikely to be produced only near the tops of slopes at these angles and if it were, it should be able to flow onto lower slopes.”
The summary said “Small amounts of water could still be involved in their initiation in some fashion, as hydrated minerals have been detected at some RSL locations,” and concluded “The authors conclude that liquid on present-day Mars may be limited to traces of dissolved moisture from the atmosphere and thin films of water.”
That finding pours cold water on hopes that Mars might be a little more hospitable than it appears and may lead to a rethink about where to land future missions and, for crewed adventures, what we'll need to provide human visitors to the Red Planet. ®