Spirit's right-front wheel, visible in this October 2009 image, has not worked since 2006. It is the least-stuck of the rover's six wheels at the current location, called "Troy."NASA/JPL-Caltech
— Mired in sand, NASA's Mars rover Spirit has a new life awaiting as a stationary lander.
— Soil that trapped rover is filled with water-soluble materials that the rover will test.
— Spirit's new science roster also includes experiments to assess Mars' core and its atmosphere.
No one would begrudge NASA's Mars rover Spirit — six years into a mission pegged for 90 days, stuck in sand and lacking power to phone home — retirement.
Instead, in the spirit of making lemonade out of lemons, scientists are preparing a new round of studies uniquely suited for a stuck Mars lander.
"I don't think anyone should have the expectation that this rover is going to go sprinting across the countryside again," lead rover scientist Steve Squyres with Cornell University, said at a Mars exploration strategy session in California last week.
"But there is science that is specially enabled from staying in one place for a long time and it turns out these rovers are pretty good landers," he said.
Silenced since March by low power — the rovers have solar arrays to harvest energy from the sun — Spirit is expected to spring back to life within a month as the Martian spring advances across Gustev Crater, where Spirit landed on Jan. 3, 2004.
An identical twin, Opportunity, touched down three weeks later and remains on the move on the other side of the planet. Both rovers were designed to search for signs of past water, believed to be a key ingredient in the recipe for life.
Spirit's final resting place may turn out to be a gold mine. The rover sits in an area called Troy on the lip of a small crater located west of Home Plate in Gusev Crater. Crippled by two broken wheels — it started off with six — the rover has been unable to extricate itself from the slippery sand, which is laced with water-soluble minerals.
"It's not a coincidence that we got stuck where we did. This soil here is bizarre. It's weird, it's different and it is fascinating in its chemistry and mineralogy," Squyres said. "We have the opportunity to really hunker down on a spot of soil that's really interesting and understand it in detail."
Scientists believe they are seeing the effects of trickling water, with layers of deposits.
"There is a wealth of materials here," Squyres said. "And we have a testable hypothesis here of what might be going on."
Another set of experiments, overseen by rover project scientist Bruce Banerdt with NASA's Jet Propulsion Laboratory in California, will attempt to zero in on what's inside the planet.
Scientists don't know much about Mars' core, whether it is solid, liquid or a combination of the two. With Spirit stationary, its communications signal can be used as beacon to precisely track the planet's spin. Over a period of months, that information can be used to deduce the size of Mars' core.
Banerdt said the radio science experiment would work even if Spirit could travel several meters, a scenario Squyres deemed highly unlikely. "We were having a tough time driving with five wheels, and with four wheels going long distances is pretty much out of the question," he said.
Rounding out Spirit's new science agenda are studies of how the atmosphere interacts with ground.
"If you're moving all the time, you find some interesting feature and then you leave it and you never see it again," Squyres said.
Or, the rover gets it in view again, but the angle is different or the lighting has changed, so you can't differentiate between visual effects and actual changes in the atmosphere, he added.
Spirit has instruments that can look at dust storms and other atmospheric phenomena from both microscopic and broad-view perspectives.
Of course, Spirit's new campaign depends on the rover reviving from its prolonged winter hibernation. The crippled craft was unable to position itself to pick up much of the winter's sun, so scientists don't know how long it will take for the rover to recharge.
Based on the temperatures the rover was expected to experience, it should have been able to recharge itself by September or October.
It's also possible that the rover's master clock was shut down by the cold, in which case Spirit's recovery could take months, Squyres said.
"My money is still on that vehicle coming back to life and my guess is that it's probably going to happen in the next month," he said.