Stuck in the sand and with time to spare, Spirit hits the jackpot.
Analysis of soil samples from where the Mars rover Spirit is stuck shows evidence of subsurface water.
The sand is stacked with less soluble compounds at the surface and increasingly more soluble minerals farther down.
The finding is a fluke. Scientists added the research after the rover got stuck and engineers were trying to figure out how it could extricate itself.
Stuck in the sands of Mars, the grounded Spirit rover unearthed evidence of subsurface water in the planet's recent past.
"It's total serendipity," Washington University planetary scientist Ray Arvidson told Discovery News. "We're driving backwards, the right front wheel doesn't work, so wherever we went we had to drag it along. It's like pushing a shopping cart with a bad front wheel. You don't push it, you pull it, but the wheel has torque."
The rover ended up getting stuck, breaking through the crust and -- surprisingly -- discovering telltale byproducts of water passing through the exceptionally silica-rich patch of soil.
"This sand wasn't normal looking," Arvidson said.
So, with nowhere to go and time to spare, scientists started a layer-by-layer look at what likely will become Spirit's final resting spot.
The soil studies were conducted between April 2009 and January 2010 as engineers tried to figure out a way to extricate Spirit from its sand pit. By March, the Martian winter silenced Spirit, which was no longer able to get enough sunlight to charge its batteries. The rover remains in hibernation, out of radio contact with its control team at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
"We're still waiting patiently and optimistically," rover lead scientist Steve Squyres, with Cornell University, wrote in an email to Discovery News.
Meanwhile, analysis showed the crust where Spirit sits is comprised of relatively insoluble minerals like silica, hematite and gypsum, but each layer down contains increasingly more soluble compounds which typically result when ferric sulfates are dissolved by water.
Arvidson's best guess is that in the relatively recent past -- a hundred thousand years or so -- when the planet's tilt was more askew, snow fell in the area where Spirit now sits, then melted, moistening the sand to the point where water seeped down into the ground.
"It wouldn't be puddles, but films of water that was pulled by gravity downward," Arvidson said. "You couldn't take a drink of it. For one thing, it'd be salty and for another there's not much of it."
"This is telling us something about the modern water cycle on Mars," he added.
Arvidson and colleagues report their findings in the Journal of Geophysical Research.