Three weeks after reporting that the Opportunity rover's landing site on Mars was once wet, scientists went even further on Tuesday, declaring that the now-barren rocks were formed at the bottom of an ancient body of saltwater.
The findings, announced at a NASA news briefing in Washington, represent an important link in a chain of evidence hinting that the Red Planet was wet enough and warm enough for a long enough time to support the development of life.
Moreover, if organisms ever did arise, their fossils should still exist within Martian rock, the scientists said.
"If you have an interest in searching for fossils on Mars, this is the first place you want to go," Ed Weiler, NASA's associate administrator for space science, told journalists. He and other space agency officials said the findings could well affect future missions to Mars.
"Opportunity's latest science returns from Mars have profound implications for future exploration," NASA Administrator Sean O'Keefe said.
One step at a time
Members of the rover science team based their conclusions on a microscopic analysis of the bedrock exposed just a few yards (meters) from the spot where Opportunity settled after its landing almost two months ago, at the bottom of a shallow crater in Meridiani Planum.
Three weeks ago, the researchers announced that water once "drenched" the site, based on the presence of sulfate salts and the way crystals within the rock dissolved. But Cornell University's Steven Squyres, the mission's principal scientific investigator, shied away from saying whether the water merely percolated through the subsurface or pooled as bodies of standing water.
As recently as last week, scientists kept mum on the question of groundwater vs. standing water. But after outside experts on sediments reviewed the microscopic imagery, Squyres and his colleagues decided to take the plunge on Tuesday.
"We think Opportunity is now parked on what was once the shoreline of a salty sea on Mars," Squyres said.
The scientists can't yet say how long ago liquid water covered the area, or for how long, or exactly how deep the water was. NASA said more light could be shed on those questions if Opportunity completes its planned odyssey to the wall of another crater with a thicker exposure of bedrock. On Monday, the rover emerged from the crater where it landed and is preparing to drive about a half-mile (700 meters) to the new crater, named Endurance.
Tracing the evidence of ancient water is the primary goal of NASA's $820 million twin-rover mission, which launched Opportunity and Spirit to the Red Planet last summer. Spirit landed on the other side of Mars, in a 90-mile-wide (140-kilometer-wide) crater that scientists suspected was once a lakebed. So far, Spirit has found evidence that a small amount of water made its way to the surface inside molten rock — but nothing on the scale detected by Opportunity.
Squyres took pains to distinguish between the earlier round of "water on Mars" findings and the new results. "It's like the difference between water you can draw from a well and water you can swim in," he told journalists.
Bruce Jakosky, a planetary scientist at the University of Colorado who was not involved in the latest research, said the distinction is subtle but important. "The difference is that this tells us about the climate at the time that the deposits were formed," he told MSNBC.com.
Subsurface water could have been present on a localized basis even if conditions at the surface were barren and dry — but the evidence for water at the surface itself tells scientists that "conditions must have been conducive to support water in a very widespread way," he said.
Cross-bedding and festooning
The key to the findings reported Tuesday lies in the fine layers within the bedrock. During just one day, Opportunity's microscopic imager snapped 152 pictures of a rock nicknamed "Last Chance" to gather data for analysis. Other pictures were taken of an outcrop called "Upper Dells." All those images were combined to form a wide, up-close mosaic.
In the microscopic view, experts saw unmistakable signs of cross-bedding, in which some of the rock layers lie at angles to the main layers; and festooning, which are smile-shaped curves produced by the shifting of the loose sediments' rippled shapes under a current of water.
"Ripples that formed in wind look different than ripples that formed in water," said science team member John Grotzinger, a sedimentologist at the Massachusetts Institute of Technology. Some of the patterns in the Martian rock could have been created by wind, but others provided reliable evidence of water's role, he said.
Grotzinger determined that the grains of sediment around Opportunity's landing site were shaped into ripples by water at least 2 inches (5 centimeters) deep, and perhaps much deeper. The water would have been flowing at a speed of 4 to 20 inches (10 to 50 centimeters) per second, he said.
The water in which the rock formed need not have been an ocean or even a year-round lake, Grotzinger said. Instead, it could have been a salt flat, or playa, sometimes covered by shallow water and sometimes dry, on the edge of an ocean or in a desert basin. "We're clearly dealing with some kind of transiently wet, transiently dry environment," he said.
To provide an extra level of confidence in what they were seeing, science team members shared their results with six outside experts before going public. "The results were very positive," NASA's Weiler said.
One of the experts, David Rubin of the U.S. Geological Survey, said he was "astonished" when he saw the data. "There on Mars are sedimentary structures just like we see on Earth. ... Even in the best counterexample I could come up with, there probably would be water on the surface, if not above the surface," he told journalists.
Signs of ancient life?
The newly announced findings raise the tantalizing prospect that conclusive traces of fossil organisms could be found within Martian rock.
Looking for fossil-like features in Martian rock has been a popular pursuit ever since researchers saw "nanofossil" structures in a meteorite from Mars known as ALH84001 in the mid-1990s. Some have even pointed to threadlike or macaroni-shaped features in the Spirit and Opportunity imagery.
The earlier controversy over nanofossils has made most mainstream scientists wary of using astrobiology's "F-word." Squyres emphasized that the likeliest fossil discoveries would relate to microbial life — organisms that would not be visible to Opportunity's microscopic imager, but could be detected by future probes.
If nothing else, Opportunity's findings have whetted NASA's appetite for more. For example, Weiler noted that a more advanced rover, the Mars Science Laboratory, was due for launch in 2009.
"Meridiani has now become the prime landing site for that rover," he said. "It will carry astrobiology instruments for the first time."
James Garvin, NASA Headquarters' lead scientist for the moon and Mars, said earlier missions — the 2005 Mars Reconnaissance Orbiter and the 2007 Mars Phoenix lander — also would follow up on the twin rovers' discoveries.
"The ultimate goal of our program in the near term, by early in the next decade, is to go to places like Meridiani, like this site we've been talking about here today, and bring the materials from there back home to Earth," Garvin said.
Once NASA's Mars exploration program gets to that stage, it's only a matter of time before the deepest questions about the Red Planet are resolved, Weiler said.
How much time?
"If there is life on Mars, or was life on Mars, I think in this century we're going to know the answer to that question," he said with a smile, "but not in the next three weeks."