A green mineral that wears away when exposed to water is plentiful on Mars, suggesting the planet has been cold and dry throughout its geologic history, U.S. Geological Survey scientists report.

New surface maps of Mars, developed by USGS scientists through a monumental set of 500 trillion calculations, provide amazing clarity and allow for more detailed study of the planet's minerals, including olivine -- the mineral that would have disappeared if the planet were awash in water.

"The large expanses of olivine, about one-million square miles, means chemical weathering on Mars is very low and has been low for most of its geologic history. This information contradicts a popular view of a past warm, wet period in Mars' geologic history," USGS scientist Roger N. Clark said in a prepared statement.

"If the warm period never occurred, other explanations for Mars' large canyons are warranted, and some have been proposed by other researchers."

The finding counters long-standing theories that liquid water once flowed on Mars, not to mention evidence presented in June of recent water on the planet's surface. However, Clark concurs with researchers who believe abundant water probably exists below the surface in a frozen state.

Clark and Todd M. Hoefen at the USGS in Denver, Colo., found the olivine on Mars using data from the Mars Global Surveyor's Thermal Emission Spectrometer. Olivine, a mineral sometimes used in jewelry, is found on Mars in volcanic regions. Exposures include olivine-rimmed craters hundreds of miles across, and with different amounts of iron, giving lighter green and darker green rocks.

Mars Global Surveyor has been in orbit in its surface mapping mission configuration for about 18 months, slowly building images of the surface. At this point, the TES instrument has measured about 3/4 of the surface, measuring a narrow six-mile wide stripe in each two-hour orbit. The result has been surface maps, done at the finest detail of the instrument. For the first time, maps of minerals could be made with clarity never before possible.

The TES instrument senses heat from the surface of Mars; it does so by splitting the heat spectrum, or "color" of the heat into either 143 or 286 wavelengths or colors of thermal infrared light. The variations in the amount of heat emitted by those colors allow scientists to determine what materials are at the surface.

The USGS scientists identified details in the infrared spectrum that indicate what is probably a sulfate in the pervasive dust and many bright soils on Mars. Sulfates have been expected since the Viking Landers found sulfur in the Martian soils in 1976, but the host mineral was never identified. It could be a sulfide (sulfur attached to a metal) or a sulfate -- sulfur attached to three oxygen atoms. The latest study shows that sulfates are abundant, but there could still be small pockets of sulfides.

Putting all this together, about three percent of the surface mapped so far contains abundant olivine, and another three percent contains coarse-grained hematite, consistent with Mars' red color. The origin of the coarse-grained hematite is still unknown. On Earth, it is often associated with water, as in a hot spring or lakebed. But such conditions would also produce other minerals that are not seen on Mars. The presence of water and chemical weathering would also produce clay minerals, which are not seen in the latest data.

"Not seeing minerals that indicate chemical weathering is also consistent with the abundant olivine and that implies the chemical weathering is very low. Thus, a consistent picture is forming that says Mars' surface has remained cold and dry for a long time," said Hoefen.

The findings were presented last week at the annual meeting of the American Astronomical Society Division of Planetary Sciences in Pasadena, Calif.