It has been two years since two U.S. robotic rovers landed on Mars to begin a search for water. The plan was for them to roam for about three months and then die because of the red planet's extremely rugged conditions. But to the surprise of everyone involved, the robots continue operating and adding to scientists' understanding of the red planet. 

Spirit reached the surface of Mars by parachute on January 3, 2004 and Opportunity touched down the same way on the opposite side of the planet three weeks later. Since then, the six-wheeled rovers have traveled about 11 kilometers, analyzing soil and rock for signs of water, evidence that Mars might have been hospitable to life.

No one is more astonished at their longevity than Cornell University geologist Steven Squyres, the mission's principal investigator.

"Well, believe it or not, it's the second anniversary of Spirit's landing on Mars," he said.  "These rovers were expected to last for 90 days and it has been two years now."

The U.S. space agency NASA thought that the planet's extremely frigid temperatures and dusty atmosphere would keep their lives short. The dust was expected to build up on their solar panels, limiting their ability to recharge their batteries from the sun's energy.  But luckily, whirlwinds blew the dust away several times, extending their lives.

The mission's major finding came early, within the first 90 days. If the rovers had worked only that long, NASA's key objective would have been satisfied. The Opportunity rover found chemical evidence in bedrock that a shallow saltwater sea existed at some time in Mars' past. Spirit drilled into volcanic rocks and found minerals suggesting they had been carried by water and collected in the lava's tiny pores as it hardened.

Opportunity's finding has recently been challenged by researchers who say that the chemical signatures it found in the bedrock could be explained by volcanic ash as well as water.

While that debate continues, the robots keep finding new variations of bedrock in the different areas they are exploring.  Steven Squyres says the geological information they have collected has increased evidence about ancient Martian environments, including wet conditions possibly suitable for simple life forms.

"During the first 90 days of Spirit's mission, we were on flat lava plains and everything was the same," he noted.  "But as a result of lasting so long, we have been able to explore a place called Columbia Hills, and for the last 400 and some odd days, we have been exploring in those hills. Every time we turn a corner, every time we go over a ridge, it seems like there is something new. All that diversity, all those discoveries were really enabled by the extraordinarily long life of the rover."

The Cornell University geologist says that the composition and texture of six types of rock the rovers have inspected suggest that Mars was once a hot, violent place with volcanic explosions. He says water was around, perhaps in hot springs in some cases and trace amounts in others.

The rovers have had some minor problems while making their discoveries. One of Spirit's front wheels would not turn in 2004 until engineers began driving it in reverse. Opportunity's mechanical arm failed to extend last November because of a stalled motor, but technicians were able to fix it.

Despite these hitches, Mr. Squyres says the twin robots endure.

"I have no idea how long they are going to keep working. They could die tomorrow or they could last another year," he explained.  "We have no way of knowing, so we just take it a day at a time."