Mars rover begins scientific work.

A tiny visitor to Mars last week ventured off its landing pad and sank its six wheels into the planet's rust-colored dirt. This week, Spirit, the rover that NASA landed on Mars on Jan. 3 (SN: 1/10/04, p. 22), went for its first stroll. Taking baby steps at its landing site, Connecticut-size Gusev crater, Spirit began its main mission--analyzing rocks and soil for signs that the region may once have contained flowing water and might even have served as a habitat for life.

Spirit drove less than 3 meters from the lander before examining its first target, a pebbly patch of Martian soil. Extending its tool-laden arm, the rover used an image-taking microscope to record the fine-grained texture of the patch. Switching tools, the rover then applied an alpha particle X-ray spectrometer to measure the soil's elemental composition.

The spectrometer revealed that the underlying soil--rich in chlorine, sulfur, silicon, and iron--is similar in composition to soil at the other three sites that U.S. landers have examined. Those similarities may result from Martian winds distributing the soil around the entire planet. If so, says lead scientist Steve Squyres of Cornell University, Spirit's soil findings may not directly represent those of the Gusev crater.

The X-ray spectrometer also found the first traces of nickel and zinc on the planet, says Johannes Brueckner of the Max Planck Institute for Chemistry in Mainz, Germany. He and other researchers presented the findings at a press briefing broadcast Jan. 20 from NASA's Jet Propulsion Laboratory in Pasadena, Calif.

The biggest surprises, says Squyres, came from another instrument, the Mossbauer spectrometer, which detects iron-rich minerals. The soil spectra reveal the presence of the mineral olivine, which typically is found on Earth in volcanic rock. The scientists now plan to look for signs of olivine in nearby rocks, including one dubbed Adirondack. By comparing data from the soil and the rocks, the scientists may be able to determine whether the soil is finely ground rock.

Before the Mossbauer spectrometer took its first measurements, Squyres surmised that the fine-grained particles in the soil are bound only weakly by electrostatic forces. If that were the case, the mechanical force exerted when the spectrometer presses down into the Martian surface would cause soil particles to "collapse and flatten like talcum powder" Squyres says. Yet pictures taken with the microscope after the spectrometer was removed showed little or no change in texture.

Squyres now proposes that sulfate and chloride salts act as a strong chemical glue that holds together the soil particles. The salts may have derived from ancient, gas-belching volcanoes or been transported by a salty ocean that once flowed on the planet, he adds.

Harry Y. McSween of the University of Tennessee at Knoxville says that olivine, such as that detected by the X-ray spectrometer, is rapidly transformed to other compounds in the presence of water. The Martian olivine may argue against the presence of vast amounts of liquid water billions of years ago at the site. Or it could mean that the soil formed after a body of water had evaporated.

Squyres says he's convinced that "lake sediments are in fact buried beneath our [rover's] wheels." It's just not clear how far below, he adds.

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