An article published in the journal “Nature Geoscience” describes a research in which a team of researchers argues that the signs of liquid water flows found on Mars are actually made up of dry sand. The possible existence of what are technically called RSLs (recurring slope lineae), streaks of sand washed by liquid water, was announced in September 2015 by NASA. A new study of data collected by NASA’s MRO (Mars Reconnaissance Orbiter) space probe may, however, show a different situation.

The conclusions of the study of areas such as Hale Crater, which led to the 2015 announcement, seemed very confident despite the fact that the explanations for the existence of liquid water were far from complete. In July 2016, the discovery of new RSLs was announced in some regions of the Valles Marineris’ canyon network. More data were added but doubts about the mechanisms that allowed the water to remain liquid under the conditions existing on Mars remained.

Now other researchers believe they proved that RSLs are actually made of sand that’s dry, not wet. Like their colleagues who conducted the previous studies, they examined the images collected by the MRO space probe’s HiRISE (High Resolution Imaging Science Experiment) camera. In this study they analyzed the Martian slopes stressing the fact that RSLs are only present when they are pretty steep, with a slope higher than 27 degrees, just enough for sand and dust granules to move downhill.

This new analysis was conducted on RSLs of 10 different sites for a total of 151 streaks. In some cases, traces of salts were found that in the previous studies suggested that there was water and could have an antifreeze effect. Actually, the salts could attract the little water vapor present in the Martian atmosphere so at least sometimes the sand could be humid.

Colin Dundas of the U.S. Geological Survey’s Astrogeology Science Center in Flagstaff, Arizona, the article describing this new study’s lead author, stated that his team started from the hypothesis that in the RSLs liquid water flows were possible but the slopes were more similar to what they expected for dry sand. This new explanation supports other evidence that Mars is today very dry.

This new explanation doesn’t explain everything, beginning with the creation and growth of sand streams along the slopes in the course of the seasons. If the salts actually absorb the moisture existing in the atmosphere, they could affect those flows, also changing their color. The mechanism could be strongly linked to Mars’ environment.

In short, the issue of RSLs requires further studies and a new mission with a rover that can go and study it closely might be needed to get final answers. One of the reasons for the excitement about the possibility of the presence of liquid water was related to the hypothesis that there were also life forms but that means that NASA will have to pay extreme attention to the sterilization of any rover to be sent in one of those areas to avoid contamination risks.