"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.