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updated 10/28/2010 6:31:34 PM ET 2010-10-28T22:31:34

NASA's stuck Mars rover Spirit has found more evidence that water trickled beneath the Red Planet's surface in the past perhaps within the last few hundred thousand years.

The sandy spot where Spirit got bogged down last year harbors stratified layers of dirt with different compositions close to the surface, a new study reveals. Researchers suspect these layers were caused by seepage of thin films of water on Mars, perhaps from melting frost or snow.

This seepage could have occurred during cyclical climate changes when Mars was tilted more on its axis, researchers said. The water may have moved down into the sand, carrying soluble minerals deeper than less-soluble ones, they added. [ Most Amazing Mars Rover Discoveries ]

The axis tilt of Mars varies over time scales of hundreds of thousands of years. The fact that Spirit found these layers in the dirt rather than locked away in rock further suggests the water was seeping relatively recently, rather than billions of years ago, researchers said.

"Once you freeze that evidence in a rock, it can stay there for a long time," said Bruce Banerdt, a project scientist for the Mars Exploration Rovers project at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "But you don't expect to maintain evidence in loose dirt for long periods of time."

Buried water in Martian history
The relatively insoluble minerals near the surface include what is thought to be hematite, silica and gypsum, according to researchers. Iron-rich ferric sulfates, which are more soluble, appear to have been dissolved and carried down deeper by water, they added.

None of these minerals is exposed at the surface, which is covered by wind-blown sand and dust.

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"The lack of exposures at the surface indicates the preferential dissolution of ferric sulfates must be a relatively recent and ongoing process since wind has been systematically stripping soil and altering landscapes in the region Spirit has been examining," rover deputy principal investigator Ray Arvidson of Washington University in St. Louis, explained in a statement.

The new study, which appears in the Journal of Geophysical Research, is based on observations made by Spirit before it stopped communicating with Earth in March of this year. The findings contribute to an accumulating set of evidence that Mars may harbor small amounts of liquid water at some periods during ongoing climate cycles.

Spirit, its rover twin Opportunity and other NASA Mars missions have found evidence of wet Martian environments billions of years ago that may have been favorable for life. Observations by the Phoenix Mars Lander in 2008 and various orbiters since 2002 have identified buried layers of water ice at high and middle latitudes and frozen water in polar ice caps.

Spirit still sleeping
The twin Mars rovers finished their three-month prime missions in April 2004, then kept exploring in bonus missions. One of Spirit's six wheels quit working in 2006.

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In April 2009, Spirit's left wheels broke through a crust at a site called "Troy" and churned into soft sand. A second wheel stopped working seven months later. Spirit could not maneuver into a position slanting its solar panels toward the sun for the winter, as it had done for previous winters.

Engineers anticipated it would enter a low-power, silent hibernation mode, and the rover stopped communicating March 22 of this year. Spring begins next month at Spirit's site, and NASA is listening to see if the rover reawakens, officials said.

"Most of us have high hopes," Banerdt told SPACE.com. "Our models say she could start communicating any day now. But we also recognize that this is an extremely risky situation for Spirit, and there are so many unknowns that we just can't be sure."

Among those unknowns, according to Banerdt: how much dust blankets Spirit's solar panels, how cold the rover's interior got and the current surface conditions where it bogged down.

Researchers took advantage of Spirit's months at Troy last year to examine in great detail soil layers the wheels had exposed, along with neighboring surfaces. Spirit made 13 inches of progress in its last 10 backward drives before energy levels fell too low for further driving in February.

Those drives exposed a new area of soil for possible examination if Spirit does awaken and its robotic arm is still usable, researchers said.

"With insufficient solar energy during the winter, Spirit goes into a deep-sleep hibernation mode where all rover systems are turned off, including the radio and survival heaters," said rover project manager John Callas of JPL. "All available solar array energy goes into charging the batteries and keeping the mission clock running."

The rover is expected to have experienced temperatures colder than it ever has before, and it may not survive. If Spirit does get back to work, the top priority is a multi-month study that can be done without driving the rover, researchers said.

The study would measure the rotation of Mars through the Doppler signature of the stationary rover's radio signal with enough precision to gain new information about the planet's core. The rover Opportunity has been making steady progress toward a large crater, Endeavour, which is now approximately 5 miles away.

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