Beginning in 2011, scientists using the Mars Reconnaissance Orbiter (MRO)—the best camera looking down on Mars—found hundreds of streaks, about 5 meters wide, that appear seasonally on steep slopes (pictured). They show up during the warm season, grow hundreds of meters long, and then fade as winter approaches. For many years, the team made the obvious interpretation: The streaks meant that, today on Mars, water was flowing, or at least seeping out of the surface. Salts were expected to be present in the water, because they lower the freezing point of water by tens of degrees, and they also make the water less likely to evaporate in Mars’s barely-there atmosphere. But until the researchers directly detected a signature for water in the streaks, or found evidence for salts precipitating out of the water, they were unwilling to declare the case solved. Now, they have found evidence for those salts, using a different instrument on the MRO—one dedicated to the spectroscopic detection of minerals, they report today in Nature Geoscience. They say the salts are mostly likely magnesium perchlorate, magnesium chlorate, and sodium perchlorate. This suggests that the water could get to the surface streaks either from above, via deliquescence—the absorption of water vapor from the atmosphere—or from below, via an underground aquifer.