Modified clay minerals on Mars Sedimentary rocks exposed in Gale crater on Mars contain extensive clay minerals. Bristow et al. analyzed drill samples collected by the Curiosity rover as it climbed up sedimentary layers in the crater. They found evidence of past reactions with liquid water and sulfate brines, which could have percolated through the clay from an overlying sulfate deposit. Similar sulfate deposits are widespread across the planet and represent some of the last sedimentary rocks to form before the planet lost its surface liquid water, so the results inform our understanding of the geologic processes that occurred as Mars dried out. Science, abg5449, this issue p. 198