NASA's Spirit rover (search) uncovered more evidence that there was once water on Mars, although not in the quantities its twin Opportunity found traces of halfway around the planet last month, the space agency said Thursday.

Spirit found clues that limited amounts of water altered a volcanic rock, coursing through tiny fissures that crisscross the boulder and cementing together the multiple layers that mask its surface.

The findings were made during a weeklong analysis of the rock in the Gusev Crater (search) region where Spirit landed Jan. 3.

Since then, Spirit has been overshadowed by Opportunity, which found signs that a salty pool of water once sloshed on Mars, ebbing and flowing in an environment that could have supported life eons ago.

Hap McSween, a science team member from the University of Tennessee, Knoxville, said Spirit's find "may be the best we can do at Gusev."

"Evidence of water at Gusev Crater has been hard to come by," McSween said during a news conference at the Jet Propulsion Laboratory (search).

Today, Mars is largely dry and cold. It contains trace amounts of water vapor in its atmosphere and large caps of frozen water at its poles. Spacecraft also have detected significant amounts of ice mixed in the martian soil at high latitudes.

NASA's Jet Propulsion Laboratory launched Opportunity and its twin, Spirit, on a $820 million mission to probe Mars for evidence that water once existed there.