Team members had said Friday that photos showing the ground beneath the lander suggested the vehicle was resting on splotches of ice. Washington University scientist Ray Arvidson said the spacecraft's thrusters may have blown away dirt covering the ice when the robot landed one week ago.
On Saturday, scientists said a more detailed image taken under the lander shows one of the craft's three legs sitting on coarse dirt and a large patch of what appears to be ice - possibly 3 feet in diameter - that apparently had been covered by a thin layer of dirt.
"We were worried that it may be 30-, 40-, 50-centimeters deep, which would be a lot of work. Now we are fairly certain that we can easily get down to the ice table," said Peter Smith, a University of Arizona scientist who is the chief project investigator.
The spacecraft is equipped with a backhoe-like robotic arm that will be used to dig into the ground and retrieve samples for testing in the lander's small laboratories. The lander was sent to a spot on Mars' northern regions in hopes of finding frozen water, but just how deep underground it would be found was unknown.
The robot arm is expected to begin its first digging operations after several more days of testing.
The final proof that the material is ice could take weeks, but close-up color images that were being taken Saturday could improve the researchers' confidence level, said Horst Uwe Keller, the scientist in charge of the camera on the robotic arm. The initial image released Saturday was in black and white.
Once the arm starts digging, dirt and ice it scoops up will be deposited in several small ovens to be heated. Measuring devices will test the resulting gases.
A short-circuit discovered Friday in one of the measuring devices was still being worked on Saturday, Smith said. But engineers were more confident they knew how to work around the problem and get the balky instrument, a gas analyzer, back in operation.
The University of Arizona in Tucson is leading and NASA's Jet Propulsion Laboratory is managing the three-month scientific mission.
Also Saturday, a spokeswoman for the mission said its public Web site was back up after a hacker managed to change the site's lead story overnight.
Sara Hammond said the University of Arizona-hosted site was taken down for several hours Saturday while technicians worked to resolve the problem.
She said the mission update posted Friday afternoon was replaced with a hacker's signature and a link redirecting to the hacker's overseas Web site.