NASA's Mars rover Opportunity has found "bulletproof" evidence that liquid water—and potentially habitable conditions—once existed on the red planet's surface, scientists say.

The find is a vein of gypsum—a mineral that forms in the presence of liquid water—that's been nicknamed Homestake.

The mineral vein is "about as wide as your thumb and a few tens of centimeters long," Cornell University geologist Steve Squyres said yesterday at a meeting of the American Geophysical Union in San Francisco.

Opportunity has found water-related minerals on Mars before, but those discoveries were in sandstones. Before consolidating into rock, the stones' grains might have blown many miles from their sources, obscuring the history of the minerals.

By contrast, the Homestake vein lies in bedrock, which means "this stuff formed right here," said Squyres, who is the principle investigator for the rover mission.

In Mars's ancient past "there was a fracture in the rock. Water flowed through it. Gypsum precipitated. There is no ambiguity about this," he explained.

"This is the single most powerful piece of evidence for liquid water on Mars that has been found by the Opportunity rover," Squyres added.

"Here, both the mineralogy and the [form of the outcrop] scream water."

Crater "Like a New Landing Site"

The gypsum vein was found on a rise, informally known as Cape York, on the rim of the 14-mile-wide (23-kilometer-wide) Endeavour Crater, which Opportunity reached a few months ago.

The team had been driving toward the crater for years, knowing that the rocks there would be older and hopefully different from what Opportunity had been previously exploring during its eight-year sojourn.

"These rocks are very ancient," Squyres said, adding that "as soon as we got to Cape York, it felt like the mission started over again. It was like a new landing site."

Gypsum bands like the one in Homestake aren't the only signs of water along Endeavor's rim.

Orbiting instruments have seen the chemical signatures of smectite, a form of clay, which is another mineral associated with water, said rover team member Ray Arvidson of Washington University in St. Louis.

What's more, the water-related minerals previously found by the rover had suggested a harsh, high-acid environment.

But gypsum can form under much more neutral conditions that could have been friendly to life, Cornell's Squyres said. And if clays exist, they also tend to form under much more neutral conditions.

Mars Rover Nearing Winter Base

Further exploration, however, will have to wait until the next Martian spring.

The solar-powered Opportunity is currently preparing to hunker down in a winter base where it can receive enough sunlight to maintain observations through the oncoming Martian winter.

A suitable site has already been picked, sloped at the ideal angle to keep the rover's solar panels pointed at the sun.

In addition to reduced sunlight, "this is the dustiest part of the year for the solar panels," said team member Bruce Banerdt of NASA's Jet Propulsion Laboratory in Pasadena, California.

In the past, windstorms have cleared dust from the panels, but it's been a while since the last such "cleaning event," he said.

The combination of dust and the lowering angle of the sun have cut the rover's electrical power to 40 percent of maximum. But that's not enough to threaten the rover in the same way as a winter power drain ended operations for Opportunity's twin rover Spirit.

"This is a very different situation than Spirit was experiencing [before it died]," he said. "We're not talking about surviving. We're talking about getting enough energy on the panels so we can stay active.

"As of this morning [December 7], she's in excellent health," Banerdt said of Opportunity. "Really, she hasn't changed much in the last year."