Bone-dry Mars?

Finding only trace amounts of carbonate minerals on Mars isn't the only strike against the hypothesis that most of the Red Planet was once wet and warm (see p. 298). The presence of large amounts of olivine, a mineral that undergoes rapid transformation into other minerals when exposed to liquid water, also argues against ancient oceans or lakes on Mars.

Using the same spectrometer on the Mars Global Surveyor spacecraft that found the carbonates, Todd M. Hoefen of the U.S. Geological Survey in Denver, Philip R. Christensen of Arizona State University in Tempe, and their colleagues found substantial amounts of olivine in a Martian region called Nili Fossae. The mineral is abundant in a 30,O00-square-kilometer area within Nili Fossae, a complex of depressions and fractures adjacent to an impact basin, the researchers report in the Oct. 24 Science. The absence of water is more than skin deep. A spectrometer aboard the Mars Odyssey spacecraft detected olivine in a layer about 7 km below the rim of the canyon Valles Marineris, Christensen reported earlier this year. The olivine layer suggests the mineral at Valles Marineris "did not encounter subsurface water at any time in its long life, nor has it [encountered] surface water since it was exposed in the wall of the canyon," he notes.--R.C.