Opportunity currently is exploring Mars along with its twin, Spirit.
"Liquid water once flowed through these rocks -- it changed their texture and it changed their chemistry," said Steve Squyres of Cornell University, the principal investigator for science instruments on both rovers. "We've been able to read the tell-tale clues the water left behind, giving us confidence in that conclusion."
On Jan. 24, Opportunity touched down in a relatively smooth, flat area called Meridiani Planum, about halfway around the Martian equator from Spirit's landing site, called Gusev Crater. Meridiani was chosen because data collected by the orbiting Mars Global Surveyor had determined it was rich in a mineral called gray hematite, an iron-derived compound that forms almost exclusively in the presence of water.
Opportunity's landing turned out to be extremely fortuitous. The 150-pound (Martian weight) rover ended up in the middle of a small and navigable impact crater that contained an exposed, "marvelous outcrop of layered bedrock," Squyres said.
"Every single piece of our payload" was brought to bear examining that outcrop, he said. Clues from the rocks' composition, such as the presence of sulfates, and the rocks' physical appearance -- including niches where crystals grew -- helped make the case for a watery history.
"NASA launched the Mars Exploration Rover mission specifically to check whether at least one part of Mars ever had a persistently wet environment that could possibly have been hospitable to life," James Garvin, lead scientist for Mars and lunar exploration at the National Aeronautics and Space Administration headquarters, told reporters. "Today we have strong evidence for an exciting answer: 'Yes.'"
That strong evidence arrived in four parts, Squyres explained.
First, the rock structure contained tiny spherical objects -- nicknamed "blueberries" by the mission team -- that appear to have formed the same way as concrete hardens when water is added to its ingredients.
Second, the rock is "shot through with tabular holes," he said, suggesting it once held crystalline structures that subsequently have either been washed away or evaporated.
Third, the rock contains a high level of salt, a strong sign of the chemical action of water.
Fourth, the rock contains jarosite, an iron sulfate-hydrate that requires water for its formation. Jarosite suggests the rock may have underlain an acidic lake or an acidic hot springs environment.
Put these components together, Squyres explained, and there is only one plausible conclusion: "For a certain amount of time, Mars was habitable."
Conclusive evidence notwithstanding, many questions remain. Foremost is whether the minerals were deposited as sediments, accumulating over a long period of time at the bottom of a lake or river, or whether they were formed by the chemical action of groundwater.
Another question is how pervasive the mineralogy of the Opportunity crater is on the Martian surface. Over the next month or so -- depending on the operational lifetime of the rover -- Opportunity will roll across Meridiani to see if other rocks will yield similar compositions. The Spirit rover will attempt a similar exploration of Gusev.
There is a bigger crater about 800 yards away from Opportunity's current site, rover team member Joy Crisp told reporters. Orbital images of the site show the crater is rimmed by a bright rock outcropping that looks virtually identical to the outcropping that produced the water evidence, she said.
On a longer timescale, NASA scientists must begin planning a new mission to collect some of the mineral-bearing rocks at the Meridiani site and return them to Earth for more detailed study. To date, none of the next generation of Mars probes -- the Mars Reconnaissance Orbiter, scheduled to liftoff in 2005, the Phoenix Mars Scout in 2007 and the Mars Science Laboratory in 2009 -- will carry the capability to retrieve Martian soil.
Such a mission has been compelled by Opportunity's discovery.
"It's the kind of thing that is absolutely the best-case scenario," said David Grinspoon, principal scientist at the Southwest Research Institute's space studies department in Boulder, Colo.
"We've had suggestive evidence (for water) from orbital missions for some time -- this is a smoking gun," Grinspoon told United Press International.
Multiple instruments aboard Opportunity all turned up data associated with the presence and actions of water, he noted.
"Opportunity's situation represents probably one chance in a thousand," said Grinspoon, author of "Lonely Planets: The Natural Philosophy of Alien Life" (Harper Collins, 2003). Not only did the rover land in a crater, which contained the exposed outcropping of subsurface rock, it also came to rest looking directly at the outcropping, he said.
"If this hadn't happened, they might have had to drive around for weeks to find something as promising," he explained.
Grinspoon said the discovery now requires an attempt to retrieve Martian soil samples, perhaps from the very area Opportunity is exploring.
"They can't age (the rocks) because there are no instruments aboard for that purpose," he said, so NASA needs to bring samples back to Earth for analysis.
"When they calculate how old the rocks are, they'll know how long ago water flowed on the surface," he added.
Phil Berardelli is UPI's Science & Technology Editor. E-mail email@example.com