NASA's Opportunity rover, studying exposed bedrock in the crater where it landed by chance in January, has found clear evidence that Mars once supported a wet, habitable environment, one that would have been suitable for life, scientists announced today.

The rocky outcrop traversed by the Mars Exploration Rover Opportunity is visible in this three-dimensional model of the rover's landing site. Credit: NASA/JPL/Ames/Cornell Download a larger version of this image here.

Putting multiple lines of evidence together, "it's hard to avoid the conclusion that this stuff was deposited in liquid water," said chief scientist Steve Squyres. "Now, what's the significance of this? The purpose of this mission was to go to Mars and see whether or not it once had habitable environments.

"We believe, at this place on Mars, for some period of time, it was a habitable environment. This was a ground water environment, this was the kind of place that would have been suitable for life. Now that doesn't mean life was there. We don't know that. But this was a habitable place on Mars at one point in time."

Ed Weiler, NASA's associate administrator for space science, said the agency's "ultimate quest at Mars is to answer the age-old question: Was there life, is there life on Mars? Today's results are a giant leap toward achieving that long-term goal."

Opportunity and its twin, Spirit, were launched to Mars last year to search out signs of water in the geology of two very different landing sites. Spirit landed inside Gusev Crater, which scientists believe once held a vast lake. So far, Spirit has encountered primarily volcanic rocks and soil. But scientists are hopeful lakebed deposits may be found in a nearby crater or elsewhere.

Opportunity landed on the other side of the red planet, in an Oklahoma-size region known as Meridiani Planum where hematite, a mineral that forms in the presence of water, had been detected from orbit. But today's announcement did not involve hematite. By pure chance, the rover came to rest in a small crater that featured exposed bedrock.

"Ever since Opportunity touched down on Meridiani Planum the night of Jan. 24 and we first opened our eyes and took a look around and saw this marvelous outcrop of layered bedrock literally right in front of us, we've been trying to puzzle out what this outcrop has been trying to tell us," Squyres said.

"For the last two weeks, we've been attacking this outcrop literally with everything we have, every single piece of our payload has been brought to bear on this. Over the last couple of weeks, the puzzle pieces have been falling into place and the last puzzle piece fell into place a few days ago. We have concluded the rocks here were once soaked in liquid water."

But that is just one part of the puzzle.

"One question is were these wonderful layered rocks actually laid down in liquid water? We don't have an answer to that one yet," Squyres said. "We're working on it, we're making some headway, we've got some tantalizing clues in that direction, we may have something for you in another week to two weeks.

"But the second question is were these rocks acted upon, were they altered by liquid water? And the answer to that, we believe definitively, is yes."

He presented four lines of evidence to support that view.

First, he cited the presence of tiny, spherical formations embedded in the rock that have been exposed due to weathering. Detailed examination indicates they most likely are "concretions," small, rounded bodies that form when minerals precipitate from water and build up around a nucleus in sedimentary rock of different composition.

A microscopic image of the rock outcrop taken by Opportunity on sol 15. Credit: NASA/JPL/Cornell/USGS Download a larger image here

The bedrock studied by Opportunity also features lenticular voids that most likely are the result of crystals that formed through precipitation and then disappeared due to erosion or chemical changes.

Opportunity's Alpha Particle X-ray Spectrometer found high concentrations of sulfur in the rocks and the rover's Mossbauer spectrometer found jarosite, an iron sulfate hydrate.

"This is a pretty unusual mineral," Squyres said. "It's fairly rare. ... There's a lot of jarosite in this rock. Because it's a sulfate hydrate, this is a mineral that you've got to have water around to make it.

"So a combination of probable concretions, these things that we think are crystal molds, the sulfur, the sulfates and particularly the jarosite, you put that story together and it's hard to avoid the conclusion that this stuff was deposited in liquid water."

And that could be the key to future exploration in the search for past life on Mars.

"If you ask yourself on Earth what kinds of rocks really well preserve evidence for very ancient life, for biochemistry, one of the best kinds of rocks for doing this is rocks that contain minerals precipitated from water," Squyres said. "Because what happens is, not only does the water provide the medium in which that biochemistry takes place, but as minerals precipitate they can trap the evidence of that and preserve it very well for very long periods of time."

But Squyres said the rover team does not yet know how long ago the water might have existed or how long it was present. The team doesn't even know if the water existed on the surface or below it.

"I want to again differentiate between a standing body of liquid water in which stuff settled out and having the sediments, the rocks, already there and water percolating through it," he said. "We cannot yet tell you with certainty that these rocks were laid down in a lake, in a pool, in a sea of water. We do not know that yet. So I can't talk about depth because I don't know if there was any. This may have simply been ground water percolating through rocks that were put down in a different fashion."

This image, taken by the panoramic camera on the Mars Exploration Rover Opportunity, shows a close up of the rock dubbed "El Capitan," located in the rock outcrop at Meridiani Planum, Mars. This image shows fine, parallel lamination in the upper area of the rock, which also contains scattered sphere-shaped objects ranging from 1 to 2 millimeters (.04 to .08 inches) in size. There are also more abundant, scattered vugs, or small cavities, that are shaped like discs. These are about 1 centimeter (0.4 inches) long. Credit: NASA/JPL/Cornell Download a larger version of this image here.

But he offered two intriguing scenarios that could explain what Opportunity has discovered.

"One possibility is that we had an eruption of volcanic ash," he said. "Ash settles out, maybe there were a number of eruptions and you build up layers of a finely layered, fine-grained rock fundamentally basaltic in character, but you've got lots and lots of pore space in it. And then subsequent to that, water percolates through that rock and it deposits sulfates, it changes the chemistry. So it's fundamentally a process of alteration of this ash as water percolates through it."

In the second scenario, "you had a salty sea at this location and you had water in that sea, you might have had currents, you might have even had waves, I don't know. We see things that might be the result of water sloshing around, so we'll go and take a look at that.

"Then, as the water evaporates away, crystals of salt, sulfate salts are deposited from that, they settle out, more salts are deposited, the water evaporates away, maybe that happens multiple times and you build up layers and layers of these salts which then subsequently can have water percolate through them and cause recrystallization and so forth.

"Two very different scenarios," Squyres said. "Both of those are fundamentally possible based on what we see right now."

But make no mistake, he added. "There's nothing like this going on on Mars today. This is a window into the past of Mars, it's totally different from anything we see happening at the surface today."

Video coverage for subscribers only:
   VIDEO: WATCH TODAY'S ANNOUNCEMENT NEWS CONFERENCE QT
   VIDEO: 3-D TOPOGRAPHIC MODEL OF OPPORTUNITY CRATER QT
   VIDEO: PANORAMA OF ROCK OUTCROP WITH NARRATION QT

   MORE: PREVIOUS MISSION COVERAGE!
   SUBSCRIBE NOW