The landing site of the Mars rover Opportunity was once drenched with water, providing an environment that could have supported life, NASA scientists announced at a press conference Tuesday.
Water once covered or infused the small crater in which the rover sits, then it gradually evaporated away, leaving high concentrations of salt behind. A lake or ocean at the site might have once been the size of one of North America's Great Lakes. Or the area might have been loaded with groundwater that rarely if ever reached the surface.
And while no signs of biological activity are likely to be uncovered by the current mission, scientists are ecstatic that they now know exactly where to look for past life on Mars.
"We think we've found enough evidence at the Opportunity site that liquid water was in abundance for some period of time," Benton Clark III, a Mars Exploration Rover team member, said in a telephone interview prior to the press conference.
Researchers still don't know how long the liquid water lasted and whether any remains today.
"That's the $64,000 question," Clark, who is also chief scientist of space exploration at Lockheed Martin, told Space.com.
Stephen Squyres, principal scientist for the rover mission, said NASA researchers do believe Meridiani Planum, the broader area where Opportunity landed, had a habitable climate for some period of time.
"It doesn't mean life was there," Squyres said. "But this was a habitable place."
Ultimately, the Opportunity site is a place where scientists would love to grab samples of soil and rock, to return to Earth for laboratory study. Meanwhile, more work is needed to pin down whether the water was just in the ground or covered the surface to some depth. Further investigation across a broader area might also reveal how long ago the water was present.
"I don't think we'll be able to tell much about age or duration until we get out of this hole and look at other outcrops," Squyres said. "Eventually, we'll have to bring some of this stuff back."
He said to expect more strong science in the coming weeks.
Opportunity has been studying an outcropping of apparently ancient bedrock that's about the height of a curb.
The geologic signs of Martian water include Opportunity's discovery that the outcrop was full of sulfate salt deposits arranged in layers, with embedded spherical particles that can be formed by liquid water.
Initially, scientists thought the puzzling BB-sized spherules were bits of volcanic material that may have formed from droplets of molten volcanic glass or meteor impact ejecta. But further study, including up-close looks at cross sections of these "blueberries," as scientists called them, led researchers to believe they are concretions of material, once dissolved in water, that gradually settled in porous rock.
"It grows around in a spherical shape until it fills in the pores of a rock," Squyres said.
One key to how the spheres formed is the fact that they do not deform the layers in which they are embedded, as might have happened if the spheres were created in volcanic eruptions and then fell to the surface.
"Had they dropped in, they would have pushed the layers down," said John Grotzinger, a rover science team geologist from the Massachusetts Institute of Technology. And had the spheres dropped in during catastrophic events, they would not be spread so randomly through all the rock layers, he said.
Opportunity also found odd indentations about the size and shape of a penny in the rock outcrop. Squyres said geologists see similar formations in Earth rock where crystals that once sat in a rock face had eroded away over time or dissolved due to a change in water chemistry.
More convincing were closer observations with two instruments on the rover arm, the Mössbauer and alpha particle X-ray spectrometers, as well as the mast-mounted miniature thermal emission spectrometer. Those devices found a pattern of salt deposition found in the slow evaporation of water.
"This is a window into the past on Mars unlike anything that is happening today," Squyres said.
Water is essential to life as we know it. Clark said the discovery, especially the sulfate deposits, could make the past existence of life on Mars very possible.
"There are organisms that use sulfate as an energy source," he said.
Researchers have long known that Mars was warmer and wet in ancient times. Few disagree that running water carved many of the huge canyons and other gullies and river-bed features seen in countless photographs of the Red Planet. Hints of shorelines have suggested lakes or oceans. Significant amounts of water ice also sit frozen at both of Mars' poles.
Bob Craddock, science adviser to the under secretary for science at the Smithsonian Institution, said the new findings gave him goose bumps. "This is why we put landers on the surface," he said.
Craddock is an expert on Mars geology and is not involved in the rover mission. He said the NASA scientists were being "very scientific" in their cautious approach to interpreting the findings.
Craddock told Space.com he's leaning toward standing water as the explanation for the many lines of evidence presented Tuesday. The presence of one mineral in particular, called kieserite, is typically left behind only when a sea evaporates, he said.
Don't expect any evidence of life to be detected by the current rovers, however. "Unless we find a dinosaur bed, we're not going to be able to [find signs of past life] in situ very easily," Craddock said. Instead, samples would need to be returned to an Earth lab for more study, he said.
Opportunity and its identical robot twin, Spirit, are part of an $820 million mission designed to search for water on Mars. Both rovers operate near the equator on opposite sides of the planet.
The rovers landed in January, each designed for 90 days of operation. Mission managers say the rovers could last into summer, but only if their batteries and other equipment hold out. Changing seasons and dust accumulations have already begun to affect the amount of power the rovers' solar arrays can generate.
Opportunity has gleaned other images and measurements hinting at the existence of Martian water. Its landing site is rife with the mineral hematite, which typically forms in the presence of water. The hematite is more abundant outside the shallow depression where the rover is now.
Over the next week or two, the robot will try to characterize the hematite to add to the understanding of the planet's water history.
Scientists then want to look into how extensive the water was. There are no clear shoreline features suggesting any boundary to any possible ancient lake or ocean in Meridiani Planum.
Opportunity will trek nearly a half-mile (740 meters) eastward to a crater called Endurance, which has a bright rim that might be a larger version of the outcrop just investigated. Endurance is about 525 feet (160 meters) wide and 100 feet (30 meters) deep.
Then, if Opportunity lasts, the robot would head south about 1.2 miles (2 kilometers) toward mottled light-and-dark terrain, where bright areas seen from Martian orbit might be more outcroppings of bedrock.
Next steps for NASA
Jim Garvin, NASA's lead scientist for Mars and the moon, said Meridiani Planum is now an early front-runner for a Mars sample return mission. That's primarily because it still has the field pretty much to itself.
Garvin said that one of the problems confronting NASA's earlier plans to launch a Mars sample return mission is that scientists could only conjecture as to where a promising touchdown site would be. Now with Opportunity's findings, NASA has at least one solid idea as to where to look.
"Now we have a candidate," Garvin said.
Whether Meridiani Planum remains the front-runner, however, depends on what NASA spacecraft find between now and the end of the decade, when planning for a Mars sample return mission is expected to be in full swing.
Garvin said the findings announced Tuesday will be used to fine-tune the Mars Reconnaissance Orbiter to locate and identify outcroppings similar to those at Meridiani Planum. The orbiter is due for launch next year.
Mars is presently 156 million miles (251 million kilometers) from Earth, or about 1.7 times the Earth-Sun distance. It is visible above brilliant Venus in the evening sky.
Space News staff writer Brian Berger contributed to this report from Washington. Space.com Senior Science Writer Robert Roy Britt also contributed to this report.