More evidence that Mars was once a very wet planet comes from NASA's Reconnaissance Orbiter: The sweeping, layered terrain shown here in the martian canyon Candor Chasma is riddled with fractures through which liquid seeped and chemically reacted with surrounding rock, perhaps millions of years ago.
Scientists trained the spacecraft's eye on Candor Chasma because other spacecraft had detected hydrated sulfates there. Thanks to the ultrahigh resolving power of the orbiter's instruments, Chris H. Okubo and Alfred S. McEwen at the University of Arizona, Tucson, found light-colored "halos" surrounding the cracks, which are hundreds of meters long (Science 2006, 315, 983). Similar halos that indicate cementing and bleaching with mineral-impregnated water have been found in terrestrial fractures.
The new work "lends support to the idea that a substantive body of groundwater existed on Mars in the past and may exist today," says Stephen Clifford of the Lunar & Planetary Institute in Houston. The layered deposits are shown in light brown, with blue sand dunes.