The strange dark streaks on Martian slopes that scientists believed might be running water have turned out to just be flows of sand, according to a new study.
These features of Mars, known as recurring slope lineae or RSL, were discovered in 2011 and provoked controversy ever since, with some suggesting that they were evidence for liquid water flow on a planet that’s supposed to be bone dry. However, a new study seems to crush that theory.
"We've thought of RSL as possible liquid water flows, but the slopes are more like what we expect for dry sand," said Colin Dundas of the US Geological Survey's Astrogeology Science Center in Arizona in a statement.
"This new understanding of RSL supports other evidence that shows that Mars today is very dry."
RSL are dark streaks that show up on Martian slopes in the warm seasons and then fade away in winter, only to reappear the following year. On Earth, the only thing that would cause that behaviour is seeping water. But the study, which Dundas authored, shows that RSL are almost all restricted to slopes steeper than 27 degrees. By observing the streaks with the High Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter (MRO), the study also found that each flow ends on a slope that matches the dynamic “angle of repose” of a sand dune.
"The RSL don't flow onto shallower slopes, and the lengths of these are so closely correlated with the dynamic angle of repose, it can't be a coincidence," said HiRISE Principal Investigator Alfred McEwen at the University of Arizona, Tucson, a co-author of the new report.
Although the report kicks the idea of flowing water to the kerb for now, the scientists weren’t able to figure out what is causing the seasonal flow of the RSL. Why they show up at certain times of year, how they gradually grow and their rapid fading all remain mysteries. As does the presence of hydrated salts in the streaks, which have water molecules bound into their crystal structure.
"RSL probably form by some mechanism that is unique to the environment of Mars," McEwen said, "so they represent an opportunity to learn about how Mars behaves, which is important for future surface exploration."
One possibility is that these salts become hydrated by pulling water vapor from the atmosphere, but that wouldn’t explain why the RSL only show up on some slopes and not others.
"Full understanding of RSL is likely to depend upon on-site investigation of these features," said MRO Project Scientist Rich Zurek of NASA's Jet Propulsion Laboratory, Pasadena, California.
"While the new report suggests that RSL are not wet enough to favor microbial life, it is likely that on-site investigation of these sites will still require special procedures to guard against introducing microbes from Earth, at least until they are definitively characterized. In particular, a full explanation of how these enigmatic features darken and fade still eludes us. Remote sensing at different times of day could provide important clues."
