March 2, 2004: Some rocks at Opportunity's landing site in Meridiani Planum on Mars were once soaked in liquid water. Members of the Mars Exploration Rovers' international science team presented the evidence today to news reporters at NASA Headquarters in Washington, DC.
"Liquid water once flowed through these rocks. It changed their texture, and it changed their chemistry," said Cornell University's Steve Squyres, the principal investigator for the science instruments on Opportunity and its twin, Spirit. "We've been able to read the tell-tale clues the water left behind, giving us confidence in that conclusion."
Here are some of the clues that water formerly pervaded an outcropping of rocks where Opportunity has been working:
(1) The rover's alpha particle X-ray spectrometer found lots of sulfur in the outcrop. Related clues from that instrument and the miniature thermal emission spectrometer suggest the sulfur is in the form of sulfate salts (similar to Epsom salts). On Earth, rocks containing so much salt either formed in water or, after formation, were soaked in water a long time.
[see caption]Above: These spectra show that a rock dubbed "McKittrick" near the Mars Exploration Rover Opportunity's landing site at Meridiani Planum, Mars, possesses the highest concentration of sulfur yet observed on Mars. [More]
(2) The rover's Moessbauer spectrometer detected jarosite, a hydrated iron sulfate mineral that could result from the target rock spending time in an acidic lake or acidic hot springs environment.
(3) Pictures from Opportunity's panoramic camera and microscopic imager show many thin, flat holes--"about the size of pennies," says Squyres--in an outcrop rock selected for close-up examination. These holes, or "vugs," match the distinctive appearance of Earth-rock vugs that form where crystals of salt minerals grow inside rocks that sit in briny water then disappear by eroding or dissolving.
The cameras have revealed spheres the size of BBs embedded in outcrop rocks. Researchers call them "blueberries"-- although they're not blue, they're gray. The spherules are not concentrated at particular layers within the rock, as they would be if they originated outside the rock and were deposited onto accumulating layers while the rock was forming. Instead, the spherules are scattered. This means they are probably what geologists call "concretions" that formed from accumulation of minerals coming out of solution inside a porous, water-soaked rock.
(5) Some of the spherules in pictures from the microscope appear to have stripes that correspond to layering of the matrix rock around them. This would be consistent with the interpretation that the spherules are concretions that formed inside a wet rock.
There is still much to learn: When was the area wet? And how long did the wet conditions last? How was the water collected--e.g., in a salty lake or sea? How deep was the water? Scientists and engineers plan to keep Opportunity busy in the days ahead looking for more clues that might answer some of these questions.
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