Soon after its chance hole-in-one landing in a Martian crater last January, NASA's robotic geologist Opportunity sidled up to an expanse of bedrock poking from the rusty ground.
For the next six weeks, the rover snuffled around the site like a mechanical anteater. Using its cameras, chemical probes and drilling arm, Opportunity collected information about the weathered, impassive stone, from its chemical composition to its curious layering.
Two hundred million miles away, the robot's masters cloistered themselves around the clock in rooms with blacked-out windows and pored over the screenfuls of incoming data.
Finally, they made a dramatic announcement: The evidence showed that an ancient sea had gently lapped against the rock outcrop on the now parched, desolate plain.
It has been almost a year since that revelation. No other discovery has topped the excitement of confirming that a large body of water once rippled on Mars.
But scientists say the 96 gigabits of geologic and atmospheric information beamed back by Opportunity and its twin rover, Spirit, as they roamed the equatorial lowlands has greatly enhanced -- and in some ways profoundly altered -- their understanding of the Red Planet.
For a period in its early history, Mars apparently was much warmer and wetter than the frigid wasteland it is today. Shallow, briny pools covered at least part of the planet's surface more than 3 billion years ago, ebbing and flowing with the Martian seasons, judging by patterns etched in the rocks.
"It's a very different Mars than the one we thought we knew," the rover science mission's principal investigator, Cornell University astronomer Steve Squyres marveled in an interview.
"Every day we find new surprises," agreed Ray Arvidson, a Washington University planetary geologist and Squyres' deputy.
Water apparently played a key role in the origin and early evolution of Mars.
Evidence suggests the interstellar rubble that rapidly coalesced to form the planet included sizeable amounts of ice, said Case Western Reserve University planetary geologist Steve Hauck, part of a team that reviewed eight years' worth of Mars satellite data for a report in last Thursday's journal Science.
Martian volcanoes -- the largest in the solar system -- could have spewed subsurface water and steam skyward, altering the climate.
The same subsurface water also may have indirectly doomed Mars' atmosphere, Hauck said. Something -- perhaps water's impact on the heat-transferring capacity of Mars' interior rock layers -- caused the planet's metal core to cool. Like a dying engine, the core could no longer generate powerful, protective magnetic fields. Without that planetary shielding, relentless blasts of charged particles from the sun stripped away Mars' atmosphere. Surface water vaporized.
On Earth, water is the elixir of life. Whether that was ever true on Mars is impossible to say for now. Without more evidence, the most that rover science team members are willing to say is that water made Mars capable of supporting life for a time.
If micro-organisms did somehow manage to blossom in the Martian shallows, they would have had to have been a hardy lot. The rovers' chemical analysis of rocks that were once soaked by the ancient sea shows the water was a salty, corrosive brew, choked with sulfuric acid, possibly a consequence of the sulfurous ash that rained down from Mars' volcanoes. The watery environment would have been comparable to polluted mine runoff in certain spots on Earth.
"It's not the place you'd expect most organisms would like to live," said geologist Matt Golombek of NASA's Jet Propulsion Lab. Yet in similarly extreme terrestrial surroundings, like the acid-tainted waters of Spain's Rio Tinto River, iron- and sulfur-loving microbes thrive.
Modern Mars is just as inhospitable, with raging dust storms and landing-site temperatures that swing from a balmy 32 degrees in daytime to minus 148 at night. That the rovers are still exploring and beaming back data is a small miracle.
Their predecessor, a tiny six-wheeled NASA cart called Sojourner, scooted around a rocky Martian plain for nearly three months in 1997, taking pictures and examining rocks until its communications gear failed.
NASA engineers figured the more robust but complex Spirit and Opportunity might last six months. Spirit passed its 411th Martian day, or "sol," (39 minutes longer than on Earth) today; Opportunity, which landed later and on the opposite side of the planet, is at 390 and counting.
The rovers have survived software hiccups, arthritic mechanical parts, and a film of power-sapping dust on the solar panels that recharge their batteries.
"Nobody dreamed we'd make it through the [Martian] winter," despite tricks such as driving the rovers up hillsides to expose their solar panels more directly to the low-hanging winter sun, said bio-geochemist David Des Marais of NASA's Ames Research Center.
Scientists had expected the rovers' energy supply to dwindle as the ever-present dust accumulated, but the electrical levels stabilized at about 70 percent.
"What we're learning is that a lot of dust stays in the atmosphere," said physicist Geoffrey Landis of Cleveland's NASA Glenn Research Center, who supervises the dust-tracking experiments. "That turns out to be a wonderful, unexpected gift."
Some of the brick-red dust does settle on Spirit and Opportunity, but not enough to block all sunlight. Wind tunnel tests showed it would take a 100-mph gust to fully clean the solar arrays. Typical Martian breezes probably are not that strong, so Landis said it's likely that a combination of air movement and wind-induced vibration of the panels limits dust buildup.
The Martian winds are a consequence of temperature shifts as the thin envelope of carbon dioxide encircling the planet heats up and cools down. Opportunity tracked those trends -- the first time that has been done in Mars' lowest atmospheric layer -- with an instrument that takes temperature readings from ground level to more than a mile high.
"If you look at the data we're getting, it's like a hot summer day in a cornfield on Earth," Landis said.
Morning sunlight heats the bare ground, causing warming air to rise and mix with colder layers aloft, creating turbulence. By 4 or 5 in the Martian afternoon, a warm air mass sits atop the cooling lower layer -- a classic temperature inversion familiar to smog-bound city dwellers. The inversion lasts through the night, then disappears at dawn.
A better understanding of Mars' atmosphere should help NASA planners narrow their aim when planning where to set down future landers. Lacking precise information about air density, temperature and winds -- all of which can affect an incoming craft's trajectory -- the scientists needed a big landing area to account for drift.
Spirit and Opportunity's landing "footprints" had to be 50 miles long by 6 miles wide. Identifying level, obstacle-free sites on Mars with that much room, and that also held promising geologic clues about past water, took more than two years.
NASA hopes to shrink future Mars footprints by as much as 80 percent. A smaller landing zone "means you can go a lot more places," said Golombek, who led the rovers' site selection effort.
The next-generation exploration vehicle -- a car-size, nuclear-powered craft that Des Marais calls "a rover on steroids" -- should also be more mobile. With extra power, "a lot more of the surface of Mars should be laid open for inspection," he said. Launch is set for 2009.
For now, NASA scientists are wringing all possible information out of the precious robotic explorers.
"They could drop dead tomorrow," Squyres said. "We've always got that in our heads. We plan for months in the future, but drive like there's no tomorrow. When these rovers die, there'll always be some tantalizing thing just out of reach."