“The last puzzle piece fell into place a few days ago,” said Steve Squyres of Cornell University, principal investigator for the rover missions. “The rocks here were once soaked in liquid water.”

Opportunity landed on Mars Jan. 25, twenty-one days after its twin, Spirit, landed in Gusev crater on the other side of the planet. Though ice is known to exist in the Martian polar caps, one of the goals of the $820 million rover mission is to look for signs that water existed in liquid form on an earlier, warmer Mars.

Though many scientists have believed this to be true for a long time, the rover provided the first concrete mineralogical evidence. Among other things, Opportunity’s examination of an outcropping of bedrock near its landing site shows the rock contains large quantities of sulfurous salts, something Squyres said indicated the presence of substantial amounts of water.

Just how much these data say about the Martian climate remains unclear. Squyres also said in the press conference those data did not indicate how long ago water was present, how long it lasted, or whether water was present on the planet’s surface.

“This may have simply been groundwater percolating through stuff [rock] that was laid down in a different manner,” he said.

The new data do little to clear up the debate over what the climate on ancient Mars was like, or whether it was ever warm and wet enough, for a long enough period of time, to allow life to develop, scientists said.

One other thing Squyres is sure of is that whenever water was present, it was a long time ago.

“There is nothing like this going on on Mars today. This was a different Mars,” he said.

Opportunity found several specific signs the rocks it studied had been shaped by water, scientists said.

Spherical particles found in the rocks appear to have formed from material precipitating out of solution, Squyres said. If the particles were material ejected from volcanos or meteor impacts, as some suggested, they would have deformed the rocks where they landed in ways that were not seen by Opportunity.

Scientists also believe that flat, regularly-shaped cavities in the rocks are molds left by the growth of crystals, which later disappeared, possibly dissolved by flowing water.

The large amount of salts, detected by a device Opportunity carries called an alpha-particle x-ray spectrometer, which is used to determine the chemical composition of materials, were also important indicators.

“The only way you can form such a high concentration of salt on Earth, normally, is to dissolve it in water and have it evaporate out,” Squyres said.

Finally, another part of Opportunity’s toolkit called a Mossbauer spectrometer detected the presence of jarosite, a mineral Squyres said will form only in the presence of water.

The findings could affect the nature of future missions to the red planet.

“It’s clear that we’re going to have to do a sample return,” said NASA’s associate administrator Ed Weiler, after Squyres proposed that the best way to answer some of the remaining questions would be to bring rocks from Mars back to Earth for laboratory examination.

Scientific goals in future Mars missions will be combined with research into things like soil toxicity, aimed at preparing for a manned mission, Weiler said.

Meanwhile, the rover team plans to keep up its investigation as long as the rovers remain operational.

Squyres said he hopes examining other areas of the bedrock around Opportunity will confirm whether the rock was formed under water, or by some other process.

Plans for both Opportunity and its twin, Spirit, involve scouting out nearby craters, where researchers hope to find older material and a greater variety of features than they have seen so far.

“We forsee that we could keep [the rovers] going for several months,” said project scientist Joy Crisp.