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WASHINGTON, May 23 (Reuters) A little NASA Mars rover has come across soil that scientists said suggests hot springs may have percolated long ago on the Martian surface, providing an environment conducive to life.

The six-wheeled rolling robot Spirit, exploring the expansive Gusev Crater just south of the Martian equator, detected a patch of light-colored soil that was 90 per cent pure silica, an indicator of the past presence of water, they said yesterday.

''It was astonishing,'' Steve Squyres of Cornell University in Ithaca, New York, a senior scientist in the rover project, said in a telephone interview. ''We had never seen anything like this on Mars before.'' Scientists using the rover to glean information about Mars -- the fourth planet from the sun, after Earth -- said the discovery is the latest strong piece of evidence that dusty and desolate Mars was once a wet and wild.

Wondering whether life on Earth is unique in the cosmos, scientists are eager to determine whether Mars was ever habitable, perhaps by microbial life forms. Water is considered a vital ingredient for life.

Silica is found widely on Earth in several forms including quartz and opal. It is a common sand component. And it often is deposited in places like hot springs where scalding water interacts with rock.

Scientists are offering a couple of explanations for the Martian silica. One is that it may have resulted from the interaction of soil with acid vapors produced eons ago by volcanic activity in the presence of water.

Another more intriguing theory, they said, is that the silica was spawned in hot springs like those seen at Yellowstone National Park, straddling Wyoming, Montana and Idaho in the United States.

'Potential habitat for life' The fact that the soil was 90 percent silica got the scientists' attention. ''Typically on Earth, the only way you can achieve that level of enrichment is by pumping water through rocks,'' Steve Ruff of Arizona State University in Tempe, another scientist in the project, said by telephone.

Scientists have amassed a lot of evidence of large deposits of water ice at the Martian poles, signs of limited surface water elsewhere and abundant evidence that large bodies of liquid water flowed freely on the surface in the past.

''If indeed we are seeing a hydrothermal hot spring, these places on Earth are great hosts for life,'' Ruff said. ''We know that cyanobacteria and various other bugs and life forms really thrive in these kind of hot spring environments. If that's what we've found on Mars, we have certainly found a potential habitat for life.'' Spirit has been exploring Mars for three years, far longer than the three-month mission for which it was intended.

Because one of its wheels no longer rotates, the rover scuffs up the ground as it rolls through the mix of hard rock and loose soil on the surface. Venturing through a low range of hills inside the crater, it scraped up and exposed some bright soil that turned out to be the silica.