In an announcement that could mark a historic moment in the search for life on other planets, scientists say they have discovered convincing evidence that liquid water once existed on Mars and created the conditions necessary for life.
During the past two weeks, NASA's rover, Opportunity, has been poking, grinding, and probing a thin necklace of bedrock peeking from a nearby crater wall. Now, researchers say, these images and measurements overwhelmingly point to one conclusion: that groundwater coursed through the rock at some point in the past, molding and reshaping it.
Though no direct evidence of life has been found, the data suggest an environment of acidic water capable of supporting primitive microorganisms. Moreover, with continued study during coming weeks, scientists hope to learn whether ponds or even lakes might have covered Opportunity's landing site, further raising the potential for past life on Mars.
For NASA, the discovery means that the rover mission has already achieved what it was created to do - find evidence of liquid water. But more broadly, it gives momentum - and additional direction - to the search for extraterrestrial life, and it at last offers a measure of certainty to the century-old search for water on the the Red Planet.
"It's gratifying to find what you are looking for," says Bruce Betts of the Planetary Society in Pasadena, Calif. "Finding evidence of the action of past liquid water is key to not only understanding the geology, but also opens the opportunity of past life on Mars."
The findings might fall far short of what turn-of-the-century astronomers imagined when they pointed their telescopes toward Mars and saw what seemed to be a vast system of canals. But for modern-day scientists, it offers much-needed confirmation that liquid water did, in fact, play a role in Mars' past.
The evidence has come from each of the Opportunity rover's suite of instruments. The rover's grinding wheel has drilled holes in the bedrock outcrop less than an inch deep - but deep enough to analyze the rock's older insides. There, scientists have found significant quantities of sulfur, which is "very hard to explain without the presence of water," says Steve Squyres, the mission's chief scientist. Even more persuasive is the presence of jarosite, a rock that forms in large amounts of water.
In addition, pictures show small slots in the rock the size of pennies, suggesting that crystals precipitated out of water to form the notches, then dissolved later as the water disappeared. And tiny spherical stones the size of BBs litter the site. These "blueberries," as they have been called, could have formed in several different ways, says Dr. Squyres, but the "most straightforward" explanation is that they are materials that solidified in groundwater and then were shaped and smoothed by it.
Taken together, the evidence paints a clear picture, Squyres says: "It adds up to a story of water flowing through rock and changing the rock."
Ironically, the findings did not involve hematite. Scientists originally sent Opportunity to its current site because the area showed signs of hematite, a substance that usually forms in water. They have found it at the site, but not in the bedrock, and the bedrock has offered its own evidence of the past presence of water.
To be sure, the discovery is significant in itself. Yet Squyres and others say its true importance is what it says about the potential for past life on Mars. Immediately, this dimple of a crater on the vast equatorial plains of Meridiani has become a tantalizing target for future study.
"If life evolved, there may be evidence," says Dr. Betts. "But these rovers are not designed to look for that."
Opportunity, and its twin currently studying the other side of Mars, are not robot paleontologists. They are not designed to dig for fossils or search for traces of organic material. They are geologists, and both still have work to do. The other rover, Spirit, is heading towards a crater in hopes of finding evidence of water there.
But Opportunity still has more to do in its small crater before it heads out to look for signs of past water elsewhere on Meridiani. An area of the outcrop shows indications of cross-bedding, a process that occurs in lake beds. The discovery of bromine also suggests that the area could have been submerged in standing water that gradually disappeared.
"These rocks could have some interesting stories to tell," says Squyres.
They are the stories that scientists would most like to read. During recent years, the circumstantial evidence that huge amounts liquid water once flowed over Mars has become almost overwhelming. Telltale swirls in the dust at the Spirit site and the hematite on Meridiani seemed to suggest that rivers and lakes were a part of the Martian past. But scientists lacked proof.
Monday's announcement certainly gives them that - at least in a limited way. While Mars currently has water on the surface at the poles, it is locked up in ice. But researchers want to know if there were times at lower latitudes when water was not locked up at the poles and was flowing over the surface.
Corroborating evidence from "down and dirty exploration" has been lacking until now, says Brown University planetary geologist James Head III. "This is really exciting, because it gives us multiple lines of evidence" for concluding that early in Mars' history "it had environments favorable to the formation and evolution of life."
Indeed, Mars' past instantly becomes more interesting if the rovers find evidence that water stood on the surface for long periods of time, forming ponds and lakes. For that discovery, however, scientists will have to wait - at least a little while.
"Significant quantities of water on Mars raise the chance that there was life on Mars," says Ken Croswell, author of "Magnificent Mars." "And the most important discovery with regard to Mars would be that there was once - or now is - life on Mars."
• Staff writer Peter N. Spotts contributed to this report from Boston.