Oct. 13, 2004 — If water ever flowed over Mars' surface, it was a one-night stand, say geochemists studying the properties of an unusual mineral allegedly found by the Opportunity rover at Meridiani Planum.

The possible detection of a normally short-lived mineral called jarosite seems to indicate that there was liquid water on at least Meridiani Planum. But the moisture didn't last long and hasn't been back since.

"Jarosite is basically a salt," said geochemist Megan Elwood Madden of Virginia Polytechnic Institute.

Madden and her colleagues have used what is known about jarosite on Earth to model the conditions that would allow it to be created and last on Mars. Their results are presented in this week's issue of the journal Nature.

On Earth, jarosite is found in very acidic and dry places, like in the tailings of abandoned mines of Colorado and other arid regions. It's also created in volcanic rocks that are being altered by acidic, sulfur-rich waters near volcanic vents.

In short, jarosite is what remains after acidic iron and sulfur-laden waters dry up. It too would dissolve and wash away if water returned, like dried sea salt in the rain.

On Mars, however, the original minerals that created the iron and sulfur-rich water are still there — sitting right next to the alleged jarosite — as if the water up and left in the middle of dinner.

"For both to exist together means the weathering process couldn't have gone to completion," said Madden.

Madden and her colleagues simulated what they thought would be a reasonable history for Meridiani Planum jarosite and found that the best explanation for the mineral is a geologically short period of liquid water on Meridiani Planum.

That period could have been as brief as a meteorite impact melting subsurface ice into water, lasting days or weeks, or as long as an orbital situation that made Mars slightly warmer for centuries. But researchers don't really know.

"I tend to agree," said jarosite investigator Alexandra Navrotsky of the University of California at Davis. "You needed fluid for a short time."

Both researchers cautioned that all the work on jarosite was more or less speculative because NASA has yet to release the hard data behind their claims of finding the mineral.

"It would be good to see the data to confirm it's jarosite" and not some other sulfur-containing mineral, said Navrotsky.

But even with confirmation, she said, it's important to keep in mind that the data only really tells us about the water history of Meridiani Planum, not the whole of Mars.

"We only have a limited amount of information and several implications," Madden agreed. "The story will get more interesting as NASA data is released."