NASA's late-coming Opportunity, which touched down on Jan. 24, has delivered stunning images of and discoveries at its landing site, an area near the equator called Meridiani Planum.

Early last month, the 150-pound (Martian weight) rover examined the rocks of the shallow crater in which it had landed so serendipitously and determined they had been altered by chemical interaction with water. Then, Opportunity trumped its first discovery with images of rocks that displayed the telltale sculpturing of flowing, lapping liquid -- evidence that Mars once held, if not an ocean, then at least a good-sized lake.

Opportunity's headlines and near-flawless performance have not been matched by its twin, however.

Spirit, which landed three weeks earlier within a huge impact basin -- also near the equator -- called Gusev Crater, has been dealing with multiple problems and, so far, has not yet produced a comparable discovery.

First, the rover found itself hemmed in because one of the airbags that cushioned its Jan. 3 landing impact failed to retract, thereby blocking its path. The problem delayed Spirit's roll-off from the landing platform for a week.

Then, the robotic craft scared its mission controllers when it suddenly stopped sending data. The problem turned out to be a glitch in the onboard computer's flash memory. Some frantic and creative reprogramming got the rover back up and running, but last week, another computer problem forced controllers to upload new software again -- and to do the same to Opportunity as a precaution.

These have been hiccups, however. The real disappointment, relatively speaking, has been that Spirit has yet to produce the anticipated smoking gun that water once flowed in Gusev. This has been particularly frustrating to NASA scientists because, of any place on Mars, the basin looks exactly like an ancient lake bed.

Not that the rover has come up completely dry, so to speak. Last month, Spirit's instruments detected hints of a water history in a rock. But those hints were much less conclusive than those Opportunity found of a very wet past on the opposite side of the planet.

Grinding away at the rock -- a dark volcanic specimen called Humphrey by mission scientists -- Spirit's abrasion tool uncovered bright material in the interior crevices and cracks that looked like minerals crystallized out of water.

"If we found this rock on Earth, we would say it is a volcanic rock that had a little fluid moving through it," said Ray Arvidson, of Washington University in St. Louis, the deputy principal investigator for the mission's science instruments. If the interpretation is correct, the fluid -- water with minerals dissolved in it -- may have been carried in the original magma that formed the rock or may have interacted with the rock later, he said.

Whatever the source, however, the water interaction was minimal. The amount suggested by the possible crystals in Humphrey was far less than what Opportunity's similar abrading and close-up examinations revealed in rocks at Meridiani.

On April 4, additional clues from a wind-scalloped volcanic rock suggested repeated possible exposures to water inside Gusev, mission scientists said.

"This is not water that sloshed around on the surface like what appears to have happened at Meridiani," said Hap McSween, a rover science team member from the University of Tennessee in Knoxville. "We're talking about small amounts of water -- perhaps underground."

McSween said the evidence "is in the form of multiple coatings on the rock, as well as fractures that are filled with alteration material and perhaps little patches of alteration material."

The second rock, called Mazatzal, after mountains in Arizona, was partially buried near the rim of the crater named Bonneville, inside the much larger Gusev. Again, after grinding with its diamond-toothed abrasion tool, Spirit brushed two patches on the rock's surface with wire bristles and found a gray, darker layer under the tan topcoat.

The rover even ground further into the rock and found a lighter-gray interior under the darker layer, and a bright stripe that cuts across both.

The stripe seemed to be "a fracture that water has flowed through, potentially with minerals precipitating from that fluid and lining the walls of the crack," said Jeff Johnson, another mission member from the U.S. Geological Survey's Astrogeology Team in Flagstaff, Ariz.

McSween suggested that the light outer coat, dark inner coat and bright veins could have resulted from three different periods of the rock being buried, altered by fluids and unburied.

Despite the two tantalizing hints, Spirit's search has yet to demonstrate that flowing water might have been a plentiful commodity on the Martian surface at one time. The frustrating thing for mission scientists is they do not know for sure why the quick finds at Meridiani have not yet been duplicated at Gusev.

Meanwhile, Spirit has begun to make its way toward an elevated area called Columbia Hills -- named for the shuttle astronauts who perished Feb. 1, 2003. The hills are about 1.3 miles away and represent a weeks-long journey for the solar-powered rover.

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