image from Spirit rover

As of Sol 1939 (June 17, 2009), Spirit's total odometry remains at 7,729.93 meters (4.80 miles).

More than six (Earth) years after it parachuted onto Mars' Gusev crater, the long-lived rover Spirit is transitioning to a new role as a stationary craft.

Members of NASA's Mars Exploration Rover mission announced the end to their efforts at extricating the rover, which has been stuck in a patch of soft sand on the west side of the Home Plate plateau since last April, in a teleconference Jan. 26.

The six-wheeled vehicle has been operating with only five functioning wheels since its left front wheel seized up in 2006. Its left rear wheel stopped working in November, making escape even more difficult; and while recent drive attempts have shown progress, the Martian winter is also approaching fast.

The team is now focusing on tilting Spirit's solar arrays toward the sun to maximize the vehicle's chance of surviving the winter -- and of going back to work in the spring (September or October on Earth).

"We have hope that Spirit will survive this cold dark winter that we have ahead of us and be ready to do more science come springtime," said Steve Squyres, principal investigator of the mission and Cornell's Goldwin Smith Professor of Astronomy. "There's an enormous amount of new science that we can do with a stationary vehicle, and we're very excited about getting going on that science."

One example: By tracking Spirit's radio signal over several months, researchers could observe how the planet's axis of rotation changes, or wobbles, as a result of gravitational interactions with its moons and the sun. The nature of that wobble could reveal whether the planet has a solid or liquid core.

"If Mars has a solid core of iron it will wobble in a certain way; but if it has a liquid core it will wobble in a slightly different way. And with six months of tracking, we think we can tell the difference," Squyres said.

A stationary Spirit could also closely observe how the Martian atmosphere and surface interact; and characterize the soil around the rover.

"The soil here is not like normal Martian soil," Squyres said. "It's extraordinarily rich in sulfate salts -- and it makes it horrible stuff to drive in, and that's a big part of why Spirit got stuck where it did -- but scientifically, this stuff is tremendously important."

In the composition and layering of the soil, researchers hope to find new clues about the geological history of the region.

Getting Spirit through winter will be a challenge. The lack of sunlight could result in Spirit going into fault mode, shutting down all electronics except its clock and some heaters for all but a few minutes each day. If that happens, the rover could be silent for months. And the frigid temperatures, expected to dip below -40C (-40F) could endanger the craft's electronics.

But the team is optimistic.

"We're not giving up on Spirit," said Squyres. "Once springtime comes, if the vehicle is … talking to us, we feel there is a lot of really exciting science ahead, including some stuff that I think is truly groundbreaking."

Meanwhile, on the other side of the planet, Spirit's tireless twin rover Opportunity marks two milestones this month. On Jan. 14, the rover's 2,124th sol, or Martian day, on the planet, its odometer rolled over to 19 kilometers (11.8 miles). And it celebrated its sixth Earth-year anniversary on the planet Jan. 24.

Opportunity is making steady progress toward the 22-km-wide Endeavor Crater, stopping to analyze rocks along the way. One intriguing finding, a basketball-sized piece of basalt called Marquette Island, was "unlike any Martian rock anyone has ever seen anywhere," Squyres said.

The rock's coarse grains indicate it was ejected from deep below the planet's surface and possibly from far away.

Marquette could yield clues about the nature of the original rock in Meridiani Planum (the wide plain Opportunity is traversing) long before the formation of sulfate-rich sediments ubiquitous to the region now, Squyres said.

Scientists think the Meridiani sediments formed when acidic groundwater interacted with a kind of precursor rock called protolith, producing sulfate salts. Since Marquette originated in the planet's crust, it could be a representative sample of that protolith.

It's too soon to say for sure, but stay tuned, Squyres said. "We're working through that story now."