NASA's two Mars rovers, well past their 90-day prime missions, have entered a dramatic new phase of exploration, researchers said today. On one side of the planet, the Opportunity rover is working its way down a steep slope into Endurance Crater, slowly creeping back in time as it discovers older and clearly different type rocks. Evidence is mounting that shallow seas once pooled in this region, periodically drying out and reforming.

A beautiful view from the Opportunity rover shows the layered rocks inside Endurance Crater. Credit: NASA/JPL Download a larger version here

"Everything we see is completely compatible with very shallow water, wetting and drying, you have enough water to wade around in or something and it evaporates away and maybe that happens over and over and over again," said project scientist Steve Squyres.

On the other side of Mars, Spirit has found one of the strangest rocks discovered to date, one that defies easy explanation. But it contains high levels of hematite, a compound that can form in the presence of water.

"It's felt to us in the last couple of weeks that the mission has started over again," Squyres said. "We're into totally new stuff in both places and it's been fairly wild."

Spirit currently is focused on a rock dubbed "Pot of Gold" that was discovered at the base of the Columbia Hills. Named after the fallen shuttle astronauts, the rolling hills are located some two miles from Spirit's landing site in the vast Gusev Crater.

Pot of Gold features strange-looking rock globules on the ends of fragile stalks. Squyres said they are not concretions, like the so-called "blueberries" found in abundance by Opportunity during its exploration of Meridiani Planum. Scientists don't know what to make of Pot of Gold or what clues the rock might hold about Mars' past.

"I don't know how these things formed and its driving me nuts, to be perfectly honest," Squyres said.

A microscopic image from Spirit shows the unexplained formations. Credit: NASA/JPL Download a larger version here

But Spirit will use all the tools at its disposal to get to the bottom of the mystery.

"We went to Gusev Crater looking for a water story," Squyres said. "We touched down we looked around, we saw a bunch of lava. We didn't see a water story. What we saw off in the distance at two-and-a half kilometers away, looking almost unreachable, were the Columbia hills. And we began to hope, after a few weeks of looking around at Gusev, that maybe somehow we could get there and find something different.

"And then ... we completed this very long trek across the plains, hoping we'd find something truly new and different in the hills. Well, we have. We may have a water story here."

But hematite can form with or without water.

"The key is that you can tell the difference between a watery origin for hematite and a non-watery origin if, if you can measure the other stuff that goes along with it," Squyres said. "If you can figure out what other minerals are present, if you can look for patterns in the elemental chemistry. If this rock has seen water, there ae certain key clues we would expect to see in the chemistry that might tell us which it is."

As a result, Spirit's research team plans to take its time. "We have been roaring across the plains as fast as we could go for many weeks and we are now done with that," Squyres said. "We are going to go very, very methodically, through the chemistry here, through the mineralogy. ... We're going to try to take one of these rocks apart. We've got some very, very exciting times ahead of us with Spirit."

Chris Voorhees, a mechanical systems engineer at the Jet Propulsion Laboratory in Pasadena, Calif., said both rovers remain in good health, although engineers continue to study data indicating the drive train powering one of Spirit's six wheels is suffering from a lubrication problem.

Spirit's view of the "Pot of Gold" rock. Credit: NASA/JPL Download a larger version here

Once researchers complete their studies of Pot of Gold and other nearby targets, Spirit will be parked in an orientation that will let sunlight fall on the suspect wheel. Heaters also will be turned on in a bid to loosen up and distribute lubrication that might have migrated away from the gear train.

Just in case that doesn't work, engineers are practicing techniques for driving Spirit with just five of its six wheels.

"It's not even close to being the end of the world, but it does present new challenges," Voorhees said.

On the other side of Mars, Opportunity is slowly making its way into Endurance Crater in hopes of gaining access to older, deeper rock formations that might provide more clues about Mars' watery past. Right now, Opportunity is parked on a 23-degree slope. Just in front of it is an even steeper slope, slanting downward at 35 to 40 degrees.

Engineers operating a mockup at the Jet Propulsion Laboratory said extensive testing shows Opportunity can safely continue down the slope and climb back out again later. No final decisions have been made, but "the end conclusion was that the test results were very positive to being able to take these additional steps," Voorhees said.

This graphic shows Opportunity's drives into Endurance Crater. Credit: NASA/JPL Download a larger version here

Researchers are eager to drive Opportunity deeper into Endurance because they see clear evidence that rock formations just ahead of the rover have a different history from those found at higher levels.

"I think there are really two take-away messages from our explorations down in Endurance Crater so far," Squyres said. "One is, there's an awful lot of salt down there, considerably more than many of us initially suspected.

"That means there was a lot more water involved in doing this than we originally thought. What we hope to do here is work our way down the stack and try to really understand just how much sulfate there really is in this place, how many meters of this stuff is here."

Once that number is in hand, scientists can calculate how much water must have been involved.

"So we're starting to think, and now I'm moving from scientific findings to scientific hypothesis, we're starting to think the materials that we're seeing deeper down in the crater may have been stirred, mixed, blown around by the wind. We think at some point they may have been actually ground up into fine particles and been mixed around by the wind."

The evidence so far indicates "there having been more water than we originally thought, but periods of wetting and drying, wetting and drying where it gets wet for a while, it dries up and the wind blows things around, it gets wet again, and so forth," Squyres said.

"I don't think ... that we've got compelling evidence for what I would call an ocean in the sense of it being a very, very, deep body of water," he said. "But the more sulfates we find, the more water episodes, I guess, would have had to have taken place, the more total water would have had to flow through the system to account for all that sulfate."

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