The Mars rover Spirit has found new evidence that the red planet was once quite wet.
A malfunctioning wheel on the rover accidentally unearthed whitish mineral deposits when it scraped through the top layer of soil.
When the rover scientists saw the white patch in the subsoil, they scanned it with an instrument called an x-ray spectrometer and discovered that it was 90 percent pure silica, said Albert Yen, a geophysicist at NASA's Jet Propulsion Laboratory.
There are two ways such silica-rich deposits might have been formed, Yen said, but both require water.
One is that the minerals formed when volcanic gases dissolved in water percolated up through the soil, leaching away other minerals and leaving silica behind.
"That's a process that's been fairly well documented in Hawaii in some of the deposits on Mauna Kea," Yen said.
Alternatively, silica-rich water might have bubbled out of hot springs and then evaporated, leaving the deposits found by Spirit.
Either way, Yen said, "it is hard to concentrate this much material without the action of water."
At the time it came across the deposits, Spirit had been exploring a Connecticut-size basin named Gusev Crater (see a map of Mars).
It had previously found other indications of water in the area, but this was the best evidence yet, Yen said. He announced the find yesterday at a meeting of the American Geophysical Union in Acapulco, Mexico.
"This is a remarkable discovery," added Steven Squyres, the lead scientist of the rover team, in a statement.
"The fact that we found something this new and different after nearly 1,200 days on Mars makes it even more remarkable. It makes you wonder what else is still out there."
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