Opportunity Finds Beachfront Property on Mars
The rocks in the outcrop that NASA’s Opportunity rover has been exploring for the past several weeks "were not just altered and modified by water; they were actually formed in water, perhaps [in] a shallow salty sea," NASA Associate Administrator Ed Weiler said Tuesday.
|Cross-beds point to flowing water origins. Credit: NASA/JPL/Cornell|
Three weeks ago, NASA scientists announced that they had uncovered mineral evidence that water had percolated underground through Opportunity Ledge, altering its chemical composition. Opportunity Ledge is the bedrock outcrop in Eagle Crater, where the rover landed. Last week, the mission science team reported that the tiny gray spherules embedded in the rock were hematite-bearing concretions, and that they, too, had been formed through a process involving water. The new finding extends the water story still further, confirming that water was present not only underground but on the surface as well.
"It’s a fundamental distinction," said Steve Squyres, principal investigator for the Mars Exploration Rover (MER) mission. "It’s like the difference between water you can draw from a well and water you can swim in. Opportunity is now parked on the shoreline of what was once a salty sea on Mars."
|Variable grain sizes.
Conclusive evidence for the new discovery comes from crescent-shaped crossbedding visible in a region of the outcrop known as Last Chance. MIT geologist John Grotzinger, a member of the mission science team, said the patterns looked like "smiles" frozen into the rock. Cross beds are layers within a sedimentary rock that form at intersecting angles. They are typical of sediments deposited by moving wind or water. Windblown sediments, however, do not form the smile-shaped patterns seen in Last Chance.
"Through the experiments that have been done here on Earth and the natural environments that we can observe here on Earth, we feel very confident that this adds up to a story about ripples moving in water rather than in wind, at velocities of maybe 10 to 50 centimeters a second, maybe a mile an hour," said Grotzinger.
The crescent shapes are small: they are only a couple of centimeters (about an inch) across. They form when gently flowing water pushes tiny ripples of sediment ahead of it along the shoreline of a shallow body of water, or along the edge of a slow-moving stream.
|Tibetan plains imaged from space are rich in lakes.
Dave Rubin, of the US Geological Survey, said present-day conditions in the Qiangtang Basin on the Tibetan Plateau might be similar to conditions at Meridiani Planum at the time the outcrop rocks formed. Although the Qiangtang Basin is in an arid region – it gets only about an inch of rain each year – "if you dig down under the surface in places, there’s soft, wet, sandy salty sediment," he said. The Qiangtang Basin today represents what Rubin called "a transitional environment," perhaps similar to conditions in Meridiani Planum as its water dried up and deposited the outcrop. Rubin was a member of a six-person scientific committee assembled by NASA to review the findings of the mission science team.
Previously detected chemical evidence – different concentrations of bromine seen in different layers of the outcrop – suggested to scientists that the rock was composed of salt deposits left behind by evaporating water. But the science team considered that evidence inconclusive. So they instructed the rover to take a series of some 150 microscopic images of Last Chance, a section of the outcrop where Pancam images revealed possible cross beds. They then pieced together the images into a set of mosaics. "Those mosaics, with their very high resolution," said Squyres, "were able to capture the essential features that we needed to go after this problem."
Although the new finding is a clear indication that at Eagle Crater water was present above ground at some point in the past, Squyres cautioned, it does not necessarily mean that Mars was ever warm enough for liquid water to be stable at the martian surface. It’s possible, he said, that the sediments that make up Opportunity Ledge formed in an ice-covered sea or lake.
|Mars Global Surveyor image of landing site, showing Endurance Crater. Click image for larger view.
Credit: Malin Space Systems/NASA/JPL
"One way that you could make these evaporitic salts is by evaporating the water away and concentrating the brine until the salts fall out. But another way you could do it, I think, would be if you had a cover of ice. Ice that freezes out tends to be much more pure" than the brine that it freezes out of, he said. Any salts in the water thus become increasingly concentrated beneath the ice. "I think you can make rocks like this simply by freezing an ice cover. And you certainly could have currents of water flowing beneath the ice."
It is not possible yet to determine the size of the body of water that formed the outcrop, or for how long it existed. Answering that question depends, in part, on figuring out whether the outcrop is an isolated formation or is representative of the larger Meridiani Planum region. Squyres is hopeful that the science team will get a better understanding of the regional context of the Eagle Crater outcrop in a few weeks, when Opportunity arrives at Endurance Crater, about 700 meters (just under half a mile) distant from the rover’s present position.
Pancam images suggest that Endurance Crater may contain a wall several meters deep, composed of the same material that makes up Opportunity Ledge. "That enables you to look farther and farther into the martian past, at a greater slice of martian history," Squyres said. To construct a more complete geologic history of Meridiani Planum and its relationship to other regions on Mars, however, scientists will have to wait until future orbital and lander missions arrive at Mars.
One of the goals of that future exploration will be to search for evidence that life once existed, or perhaps still exists, on Mars. Tuesday’s announcement that Opportunity Ledge formed in water bolsters the notion that the area was once habitable. "If we’re correct in our interpretation," Squyres said, "this was a habitable environment on Mars. This was a shallow sea. These are the kinds of environments that are very suitable for life." The environment might also have done a good job of preserving evidence of any life present there, he added.
Opportunity’s discoveries make Meridiani Planum a likely spot to conduct such future astrobiological investigations in situ. It also makes the region a good place to hunt for rocks to bring back to Earth for further analysis. NASA plans to fly a mission to Mars in 2014 that will return samples to Earth in 2016.