The mobile robotic geologist has yet to identify any sign of ancient lakebed material that scientists suspect may lie beneath the surface where it has been driving. The rover is, however, approaching a stadium-sized crater that could provide access to older, lower layers.
"We need to use nature's drills, which are craters," said Dr. Ray Arvidson of Washington University in St. Louis, deputy principal investigator for Spirit and its twin rover, Opportunity. Scientists estimate that the meteorite impact that excavated the nearby crater threw rocks out onto the surrounding surface from as deep as 10 to 15 meters (33 to 49 feet), and may have exposed rock layers to that depth inside the bowl.
Spirit can do some digging of its own, too, but not to that kind of depth. It uses its wheels to push the soil aside. The first trench it made reached about 7 centimeters (3 inches) below the surface. Trenching by the rover is helping scientists investigate the possibility of water and ice cycles near the surface on the scale of millions of years or less. By contrast, exploiting the much deeper natural excavation within reach could yield information about whether the area was wet billions of years ago.
NASA flew Spirit to Gusev Crater to find geological clues about past environmental conditions in an area suspected to have held a large lake long ago. Gusev is a Connecticut-size bowl whose walls are too distant to be seen from the Spirit landing site inside the crater. Pictures from Mars-orbiting spacecraft show landscape shapes suggesting that a long valley delivered flowing water into Gusev through a breach in the crater rim.
Spirit's prime mission was planned for three months of operations on Mars' surface. The rover is in such good health, engineers are optimistic it will be able to run considerably longer than that with an extended mission.
Just hours after it landed on Jan. 4 (Universal Time), Spirit looked around and showed its eager science team a broad plain with scattered rocks. A few hills and crater rims punctuated the horizon.
Spirit has surveyed the surface materials in its neighborhood with versatile cameras and a rock-identifying infrared sensor. It has inspected them up close with experiments such as the first microscopic examinations of soil and rock on a foreign planet and composition analysis of the first rock interior ever exposed by cutting into a stone on a foreign planet.
Preliminary assessments of those surface materials are in. "The caprock on the plains is volcanic basalt," Arvidson said. "It has been broken up by impact cratering, and then wind-blown materials have accumulated into a mantle of soil."
One trait of particular interest in Spirit's findings so far is how soil grains stick together, forming a crust in some areas examined. Soil cohesiveness made Spirit's trenching at a site called "Laguna Hollow" take longer for a shallower hole than when Opportunity dug into looser soil for a deeper hole at that rover's landing site halfway around Mars. Sulfur and chlorine identified in the soil may contribute to this trait. "Some process is cementing things together," Arvidson said.
The soil at "Laguna Hollow" also showed hints of an intriguing surface pattern -- roughly polygonal patches about 10 centimeters (4 inches) across, some with pebbles clustered along the borders between the light-colored patches. "This could be complex ripples formed by the wind, or it could be from the soil volume changing," Arvidson said. Potential causes for the swelling and shrinking that could yield such surface patterning include changes in soil moisture content or temperature swings in cohesive soil.
In general, soil is an indicator of more recent environmental conditions than rocks are. Spirit is on the lookout for unusual rocks as it progresses toward the rim of the crater informally named "Bonneville." The crater is roughly 150 meters (about 500 feet) across, and Spirit is more than halfway from its now-empty lander to the crater rim.
"When we get to the rim and look inside, it may very well be a new mission," Arvidson predicted.