Big quote:

Steven Squyres, the Cornell astronomer and principal investigator for the Mars Exploration Rovers, and a team of 17 other mission scientists summarized Opportunity's most recent discoveries in their report in Science.

Just north of a crater named Erebus, Squyres and his colleagues say, the instruments and camera aboard Opportunity examined layers of a sandstone outcrop named Olympia. The outcrop is crisscrossed by small troughs only inches deep, which the team's geologists determined could not have been formed by wind erosion or volcanism. The only possibility, they concluded, was that they were created by flowing water in the far distance past, 2 billion or more years ago, when Mars might -- just might -- have been warm and wet.

"Water flow was more extensive across the ancient Meridiani surface," the scientists said in their report.

In another finding, the scientists used Opportunity's rock abrasion tool, known as the RAT, at outcrops near the rim of Erebus Crater to grind 2-inch circles of rock surface for analysis by the X-ray spectrometer aboard. Various layers of the sandstone bore increased levels of sulfur and magnesium -- evidence that soluble salts had once evaporated from salt water there, the scientists said.

Near the same crater, Opportunity steered toward a feature on an outcrop the scientists named Lemon Rind because it appeared to be partially covered by a thick skin. There the RAT exposed what are most probably salt crystals, remaining after the water evaporated long ago. And just below the surface of a nearby rock dubbed Kalavrita, the RAT uncovered tiny round spheres of hematite, an iron mineral abundant on the Martian terrain that most often forms in water the way rust does.

Along its route, Opportunity also crossed a flat area much like a playa, or dry lake bed, on Earth. Squyres and his colleagues concluded that a "pan" of sulfate particles must have formed there when ancient volcanic rock surfaces were leached by acid waters that have long since evaporated.

When the rover reaches the walls of the huge Victoria Crater, it will find the thickest stack of layered rocks it has yet encountered, and to Ray Arvidson of Washington University in St. Louis, who is Squyres' deputy, the prospect of analyzing those rocks is truly exciting. Acids in ancient Martian water might not mean an easy environment for life to exist, but at the bottom of Victoria, the scientists hope the signs of water might be different.

Full article here:

http://sfgate.com/cgi-bin/article.cgi?file=/c/a/2006/09/08/MNG5TL0AOJ32.DTL