The rover Opportunity took its first roll on martian soil early on Saturday morning, and scientists offered the most tantalizing evidence so far that the landing site once lay in liquid water.
While ice still exists near the poles, most of Mars, including the equatorial region where Opportunity set down last week, is devoid of water, liquid or frozen. But some scientists believe Mars was warm and wet early in its history and may have possessed an environment where life could have arisen. The $820-million mission of Opportunity and its twin, Spirit, is to look for geological signs of liquid water in Mars' distant past.
At a news conference on Saturday at NASA's Jet Propulsion Laboratory here, Dr. Philip Christensen, a professor of geological sciences at Arizona State University, confirmed that an instrument on Opportunity had detected the iron oxide hematite in the dark pebbles and gravel that dot the landing site.
Furthermore, Christensen, the lead scientist for the instrument, said, "I think the preliminary evidence is consistent with hematite forming at low temperatures in chemical reactions with water."
The instrument, known as the minithermal emission spectrometer, looks at infrared light radiated from the rocks and soils. Certain minerals including hematite emit a distinctive mix of infrared light.
One team member, Dr. Wendy M. Calvin, said Friday morning that hematite had been discovered at the Opportunity landing site, but the leaders of the science mission did not confirm it, saying more checking was required. Christensen said his team finished its analysis on Friday afternoon.
Hematite that forms at low temperatures from sediments at the bottom of a lake or sea has a noticeably different infrared signature than hematite that forms out of hot lava without water. Christensen said that the infrared data argued against the hot lava explanation.
Hematite is what drew scientists to this location, Meridiani Planum, flat plains near the equator. From orbit, the Mars Odyssey spacecraft spotted a deposit of hematite the size of Oklahoma blanketing Meridiani Planum, but it could not tell whether the hematite was in the soil or the rocks or elsewhere. Opportunity was sent to investigate, and researchers hope to learn not only where exactly the hematite is, but also its origin.
Already, the infrared instrument has provided some clues.
While the pebbles and gravel contain hematite, the fine reddish soil below does not. The exposed, light-colored bedrock 20 feet from Opportunity is also hematite free. Christensen said the pebbles must have come from a rock layer, not yet seen, above the light-colored bedrock that had eroded away into pebbles.
"It's got to be there somewhere, and if we're lucky, we'll find it," Christensen said.
Scientists will soon have a better look.
At 4:50 a.m. Saturday, Opportunity made its first drive, methodically rolling 10 feet off the front of its lander, its six wheels then digging into the dirt. It will sit there for the next few days, conducting experiments on the soil beneath it, before moving on to the bedrock.
Right now, the scientists have no idea what the bedrock is made of. "All I can say at the moment is what it's not," Christensen said. The instrument readings showed no hematite, no quartz, no carbonates.
A suite of instruments that Opportunity will press against the rocks should tell more about the minerals there. For instance, if scientists find goethite _ essentially hematite with trapped water _ that would strengthen the argument for past water.
On the Web
For the latest on Spirit and Opportunity, visit NASA's site at http://marsrovers.jpl.nasa.gov.