New high-resolution color photos of Mars have turned up yet more evidence for the flow of groundwater during the planet's past. NASA's Mars Reconnaissance Orbiter trained its HiRISE camera on Candor Chasma, part of the enormous Martian rift valley, Valles Marineris. The images revealed alternating layers of dark- and light-toned rock interspersed with dark sand dunes [blue]. Cutting across these layers in places were unusual fractures, or joints, up to a kilometer long and bordered by lighter-colored halos. Researchers publishing in this week's Science say that flowing water, liquid carbon dioxide or a mix of the two probably caused the halos by cementing and bleaching the rock around the fractures.