In what could turn out to be a set back for water on Mars optimists, researchers have said that the dark features previously proposed as evidence for significant liquid water flowing on Mars have actually been created by sand and dust flows.
According to a study published by the U.S. Geological Survey and colleagues in Nature Geoscience, the streaks created by dust and sand movement i.e. recurring slope lineae, or RSL, may have features resembling seeping liquid water, but they have been formed by moving granular material like sand and dust. This means that the possibility of finding significant volume of liquid water on Mars may be minuscule.
Previous studies have identified RSL features as being growing incrementally, fading when inactive and recurring annually during the warmest time of year on Mars. Observations have indicated that these RSL are mostly found on steep rocky slopes in dark regions of Mars, such as the southern mid-latitudes, Valles Marineris near the equator, and in Acidalia Planitia on the northern plains. The appearance and growth of these features resemble seeping liquid water, but how they form remains unclear, and this research demonstrated that the RSL flows seen by HiRISE are likely moving granular material like sand and dust.
“We’ve thought of RSL as possible liquid water flows, but the slopes are more like what we expect for dry sand,” said USGS scientist and lead author Colin Dundas. “This new understanding of RSL supports other evidence that shows that Mars today is very dry.”
The terminal end of the RSL slopes, said Dundas, 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. Instead, the 151 RSL examined by the study authors all end on similar slopes despite very different lengths. Additionally, said the scientists, 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.
This new research finds that these RSL features are flows of granular material and thus, align with the long-standing hypothesis that the surface of Mars lacks flowing water. Small amounts of water could still be involved in their initiation in some fashion, as hydrated minerals have been detected at some RSL locations. 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.