Image: Spirit rover
A wide-angle picture from the Spirit rover shows its own robotic arm poised over bright churned-up material in Gusev Crater — material that could represent a "Martian salt lick."
By Alan Boyle Science editor
updated 2/18/2005 7:16:10 PM ET 2005-02-19T00:16:10

When the Spirit and Opportunity rovers arrived on Mars more than a year ago, it seemed as if their landing sites were worlds apart. But now that Spirit has taken the high ground, the mission's top scientist says both rovers are coming across similarly salty chemistry — and that hints at a tale of ancient, most likely acidic water on both sides of the Red Planet.

The latest discovery is a patch of bright soil serendipitously churned up by Spirit's spinning wheels in the Columbia Hills — a spot so thick with magnesium sulfate salt that Cornell University's Steve Squyres, the principal investigator for the rover missions, described it as a "Martian salt lick."

The concentrations of salt lend further support to the view that Spirit's current surroundings in Gusev Crater, like those of Opportunity, were formed through interaction with water.

"The two places are looking more similar in terms of their chemistry," Squyres told journalists here Friday at the annual meeting of the American Association for the Advancement of Science. "We're starting to see these high salt concentrations that we initially saw right away at [the Opportunity landing site at] Meridiani."

Does that mean that the Spirit site was once covered by a salty sea, as scientists believe was the case at the Opportunity site? Not necessarily, Squyres said. But the salt concentrations — found in the soil patch as well as in nearby bedrock — indicates at least that ground water was at work.

Squyres and his colleagues theorize that when Mars was young, water with a high concentration of sulfuric acid seeped through a type of basaltic rock containing magnesium, such as olivine. The chemical interaction would produce a "broth" of salty water, Squyres said.

"There was a lot of water flowing around, and it went away and left the salt behind," he said.

What's next?
The next step is to analyze layered rock outcroppings in the Columbia Hills for clues as to how they were formed. Particular patterns of cross-bedding could point to the lapping of an ancient sea, just as they did in Opportunity's case.

Squyres marveled that Spirit was just coming into the prime of its science mission, 400 Martian days after it landed in Gusev Crater. The rover's landing site was in the midst of some relatively unremarkable basalt deposits, and it had to travel 2.5 miles (4 kilometers) to get to the "good stuff," he said.

Meanwhile, Opportunity is plugging along, traveling southward toward some intriguing etched terrain and ultimately Victoria Crater, where a 130-foot-deep (40-meter-deep) geologic record awaits.

The rovers have spectacularly exceeded their original mission life of 90 days. "It shows you how important longevity and mobility are on a mission," Squyres said. The mission, which originally carried an $820 million price tag, is being extended in six-month chunks, with the next renewal due in April.

'Sense of urgency'
Even Squyres knows the rovers won't last forever — particularly Spirit, which is having more of a problem with dusty solar arrays and must be positioned to make optimal use of limited sunlight. He said mission planners must soon decide whether to send Spirit farther up Husband Hill, or downhill toward a feature that scientists have dubbed Tennessee Valley.

For the rovers, what was once a slow and steady exploration is turning into a race against time and chance.

"There's a sense of urgency," Squyres admitted. "We don't know how long they're going to be around."

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