Few people have inspected the canyons and craters of Mars like Alfred McEwen, a planetary geologist at the University of Arizona in Tucson. He’s the principal investigator for the high-resolution camera on the Mars Reconnaissance Orbiter (MRO), which entered orbit in 2006, revealing features as small as a loaf of bread. With 55,000 observations so far, the craft has exposed about 2% of the planet to spy-camera scrutiny.
One of McEwen’s most important finds 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 doused 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. 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.