NASA's Opportunity rover has found evidence that its landing site in Mars' Meridiani Planum region was once covered with salty water — but could that ancient environment have given rise to life? Probably not, the rover team reported Tuesday, but if life arose elsewhere on the planet, it might have been able to survive there.
The latest conclusions are based on a thoroughgoing analysis of Martian rock layers at Endurance Crater, as well as an earthly simulation that produced the same sorts of minerals seen on the Red Planet. Researchers determined that in ancient times, the soil at the Meridiani Planum site was acidic, oxidizing and sometimes wet — conditions that would probably pose stiff challenges to life as we know it.
"Life that had evolved in other places or earlier times on Mars — if any did — might adapt to Meridiani conditions, but the kind of chemical reactions we think were important to giving rise to life on Earth simply could not have happened at Meridiani," said Harvard University's Andrew Knoll, a member of the rover science team and a co-author of research appearing in Wednesday's issue of the journal Earth and Planetary Science Letters.
In the issue, nine papers by 60 researchers discuss what Meridiani Planum was like eons ago, and provide comparisons to harsh habitats on Earth. The rover team's principal investigator, Steve Squyres, told MSNBC.com that the papers represented the best effort to date to put everything Opportunity has learned into perspective.
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"Until this set of papers, all that we had written about the rocks at Meridiani was contained in one four-page paper that came out a year and a half ago," he said. "What we wanted to do was, in a peer-reviewed journal where we had no page limit, sit down and tell the story really well."
Squyres said the researchers looked at extreme environments on Earth — such as salt flats and acidic rivers — to size up the chances for life on ancient Mars under similar conditions.
"You can go to environments on Earth that are arid, oxidizing and acidic, and are full of life," he explained. "The question is the chemistry. ... Getting the kind of biochemistry going to initiate life at that high acidity level could be tough."
The newly published research builds on findings from the first months of Opportunity's mission. The rover landed on Mars in January 2004 — three weeks after its twin, Spirit, touched down on the opposite side of the planet. The $820 million twin-rover mission was originally slated to last only 90 Martian days, or sols, but Spirit is still going strong after a full Martian year , and Opportunity is due to pass that milestone on Dec. 12.
"Each day we cross our fingers and pray that they're going to keep going, because they could drop dead at any time," Squyres said. "But they're pretty tough machines."
The heart of the latest research is the analysis of a 23-foot-thick (7-meter-thick) stack of rock layers exposed inside Endurance Crater, which is about a half-mile (750 meters) away from Opportunity's landing site. The rover spent six months inside the crater's rim, from June to December 2004.
"This whole story only becomes clear if your rover lasts for hundreds of sols, and you're able to find a great big hole in the ground, and you have engineers who are smart enough to get you down that slope and enable you to sample all that stratigraphy," Squyres said.
Scientists found three divisions within the Endurance rock stack:
- The lowest, oldest portion had the signature of dry sand dunes.
- The middle portion had windblown sheets of sand. Particles in those two layers were produced in part by previous evaporation of liquid water.
- The upper portion, with some layers deposited by flowing water, corresponded to the layers of rock that Opportunity had found in the shallower crater where it first came to rest.
Materials in all three divisions were wet both before and after the layers were deposited by either wind or water, the researchers said. They described chemical evidence that the sand grains deposited in the layers had been altered by water before the layers formed. Scientists analyzed how acidic water moving through the layers after they were in place caused changes such as the formation within the rocks of hematite-rich spherules informally known as blueberries.
Researchers tried to duplicate the geological processes they believe took place on the Red Planet.
"We made some simulated Mars rocks in our laboratory, and infused acidic fluids through them," Nicholas Tosca from the State University of New York, Stony Brook, said in a NASA news release. "Our theoretical model shows the minerals predicted to form when those fluids evaporate bear a remarkable similarity to the minerals identified in the Meridiani outcrop."
NASA said the stack of layers in Endurance Crater resulted from a changeable environment perhaps 3.5 billion to 4 billion years ago. In physical appearance, the area may have looked like salt flats occasionally holding water, surrounded by dunes — similar to the White Sands region in New Mexico.
In Endurance Crater, as in White Sands, "you see evidence for multiple episodes of the water table going up and down," Squyres said. One mark on the Endurance Crater stack looks like a "bathtub ring," showing the point to which Martian water once rose, he said.
In chemical and mineralogical makeup, an acidic river basin named Rio Tinto, in Spain, provides a useful model for the ancient Martian environment, according to a paper by David Fernandez-Remolar of Spain's Centro de Astrobiologia and four colleagues.
Many types of microbes live in the Rio Tinto environment — one of the reasons for concluding that ancient Meridiani could have been habitable. However, the organisms at Rio Tinto are descended from populations that originated in less acidic and stressful habitats. If Meridiani had any life, it might have had to come from a different habitat, the researchers concluded.
Still mad about Meridiani
Does this mean Squyres and his colleagues are losing interest in Meridiani as a site where traces of ancient life might be found? Not in the least.
Squyres said that Meridiani is "actually a very good place" to look for microfossils of Martian life, since the chemistry is conducive for preserving the structures of tiny organisms. He and Harvard's Knoll both said the Martian blueberries would be of particular interest.
Knoll told MSNBC.com that in the Rio Tinto area, paper-thin slices of iron deposits were found to be "full of microfossils." Therefore, the blueberries, which are basically deposits of a type of iron, could have a "reasonably high likelihood of preserving any biosignatures of life that might have been present."
"If I could see a blueberry in my lab, I'd be a happy man," Knoll said.
Knoll is interested in other areas of Mars as well.
"Meridiani excites us because we know it, but Meridiani is a tiny, tiny portion of the Martian surface," he said. "I would frankly like to see us take two or three carefully sited robotic missions over the next decade or so to places that are different from Meridiani, older than Meridiani."
Armed with the data from all those sites, scientists could intelligently choose where to send a robotic probe to retrieve a Martian sample and bring it back to Earth for up-close study. "That's going to be extraordinary when it happens," Knoll said. "So you want to do that pretty carefully."
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