The soft Martian soil that ensnared the rover Spirit holds evidence that liquid water still trickles below the planet's surface. But this serendipitous discovery may be Spirit's last.

Spirit's wheels broke through the crust of a sand pit called "Troy" in April 2009, after five years spent mostly exploring a region called Home Plate. NASA officially gave up on trying to extract the rover in January.

But the soil exposed by Spirit's spinning wheels carries clues that Mars may still be wet. The newly exposed surface layers include minerals thought to be hematite, silica and gypsum, which don't dissolve easily in water. But layers of iron sulfate minerals, which do dissolve easily, lie centimeters below the crust.

These layers suggest water, maybe in the form of frost or snow, seeped into the ground relatively recently and carried the soluble minerals deeper into the soil. The seepage could have happened during cycles in Mars' history when the planet tilted further on its axis.

None of these soluble minerals are exposed at the Martian surface, which indicates the soil interacted with water recently, and probably continuously. Because the Martian surface is constantly being sculpted by wind, these layers would have eroded away if they were laid down long ago.

Water could also have come from fumaroles or hot vents beneath the surface, the rover team reports in the Journal of Geophysical Research.

"On Earth, fumaroles and hydrothermal systems provide the environmental conditions, water, nutrients and energy sources needed to sustain robust microbial communities," the team wrote. A prime example is the acidic, iron-rich hot springs in Yellowstone National Park.

"It seems likely that the region in and around Home Plate may have likewise supported a habitable environment," the team wrote. Although there may not have been enough water to sustain life for long.

In 10 attempts to break free, Spirit only managed to drive 13 inches backwards through the soft soil before NASA gave up in January. Spirit shut down completely on March 22 to hibernate through the Martian winter. If the rover wakes up again when Martian spring arrives next month, there will be fresh soil to explore. The researchers also plan to take advantage of Spirit's immobilization to investigate whether Mars's core is liquid or solid by precisely tracking the rover's radio signal.

But Spirit has sat through colder temperatures this winter than it ever experienced, and its solar panels were stuck facing away from the sun. It may not wake up. Scientists are crossing their fingers, but not holding their breath.

Image: NASA/JPL-Caltech/Cornell University

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