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 "Martian Invasion," in this week's issue: Martian Invasion). 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 30,000-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.