Nearly two years after NASA's twin rovers parachuted to Mars, a Jekyll-and-Hyde picture is emerging about the planet's past and whether it could have supported life.

Both Spirit and Opportunity uncovered geologic evidence of a wet past, a sign that ancient Mars may have been hospitable to life. But new findings reveal the Red Planet was also once such a hostile place that the environment may have prevented life from developing.

"For much of its history, it was a very forbidding place," said mission principal investigator Steven Squyres of Cornell University.

Scientists stressed that the rovers were investigating a snapshot in geologic time and that it's possible that other regions of Mars that have yet to be explored could have had a different environment.

The new analyses were presented Monday at an American Geophysical Union meeting in San Francisco.

Since landing on opposite sides of Mars in January 2004, the six-wheel rovers found conclusive evidence that the planet once had water based on examination of water-altered bedrock at their respective sites.

But the sedimentary rocks in the Martian plains where Opportunity landed also painted a picture of a past environment some 3 billion to 4 billion years ago that fluctuated between being very acidic and arid — conditions that were probably unfavorable to life.

The Gusev Crater region where Spirit touched down revealed an even more violent history. Three outcrops examined by the rover displayed deposits of water-altered debris from explosive events. Hot ash rained from the sky and space objects bombarded the surface about 4 billion years ago. During that time, water was present, but not a large amount.

Scientists acknowledged that such harsh environments probably would have posed challenges for life to start, but they did not rule out the possibility that limited life forms could have thrived under these extreme conditions.

They pointed to similar environments found on Earth in which primitive creatures adapt and live in radical habitats including ocean hot springs, volcanic craters and polar glaciers.

At the very least, scientists said, the sites where the rovers landed may have been suitable for life when water was present. These areas deserve further scrutiny, including a possible sample return of Martian rock or soil to Earth to determine whether fossilized life exists, they said.

Also Monday, the European Space Agency said its Mars Express spacecraft found evidence that Mars underwent a major global climate change. Data collected by the spacecraft showed that Mars once was moist and had surface water that disappeared about 3½ billion years ago, leaving the planet dry and cold.

Present-day Mars is dusty, dry and cold with no apparent sign of life on its barren, rust-colored surface.

Since 2004, the twin rovers have dazzled scientists with discoveries that point to a more lively ancient environment.

The solar-powered, golf cart-sized robotic explorers outlasted their primary, three-month mission long ago and are operating on overtime. They are managed by NASA's Jet Propulsion Laboratory in Pasadena.

In August, Spirit climbed to the top of a Martian hill as tall as the Statue of Liberty. After a month at the summit, the rover is making its way down to explore a basin that holds geologic promise. Spirit recently did some nighttime viewing of the sky and photographed a lunar eclipse of Mars' moon Phobos.

Meanwhile, Opportunity's robotic arm became stalled while trying to examine a layered outcrop and engineers are working to fix it. The rover is facing rough driving on its way across plains to Victoria Crater, about eight times bigger than Endurance Crater, a 430-foot-diameter crater previously studied by Opportunity.