These dark, narrow, 100-metre streaks called recurring slope lineae flowing downhill on Mars are inferred to have been formed by contemporary flowing water. Recently, planetary scientists detected hydrated salts on these slopes at Hale crater, corroborating their original hypothesis that the streaks are indeed formed by liquid water. The blue colour seen upslope of the dark streaks are thought not to be related to their formation, but instead are from the presence of the mineral pyroxene.

Photograph: Mars Reconnaissance orbiter/University of Arizona/JPL/NASA