Having logged more than 2,000 sols (Martian days) since landing on the Red Planet in early 2004, the rovers Spirit and Opportunity are the Methuselahs of Mars. Next month marks the sixth anniversary of their respective thump-and-roll arrivals at Gusev crater and Meridiani Planum, and they've endured some 23 times longer than than the 90-day missions NASA handlers planned for them.
But while Opportunity continues its long-haul trek to reach a big crater named Endeavour, its twin has been stuck in one spot for six months. Back in May, Spirit was being maneuvered around a low, 300-foot-wide plateau dubbed Home Plate when it broke through a thin crusty layer on the rim of a small crater and sank to its hubcaps in soft dust. NASA engineers have been trying to extricate their six-wheeled explorer ever since.
Spirit's general condition is good, but its prospects for a clean getaway are not. The mission team has found it difficult to mimic how the fine dust behaves in the low gravity and thin atmosphere of Mars. When the rover sank it also bottomed out on a rock that's touching its underbelly.
The craft's infirmities haven't helped. Its right-front wheel hasn't worked since 2006, failing weeks after reaching Home Plate and forcing the rover to roll backward to get around. Bouts of electronic amnesia come and go, episodes when the computer's flash memory simply fails to record what's going on.
After assessing the situation for many weeks, engineers at the Jet Propulsion Laboratory began a series of carefully designed maneuvers in late November that they hoped would free Spirit from its dusty dilemma. Slowly turning the wheels inched the craft backward a bit, but a new problem arose when Spirit's right-rear wheel abruptly stalled. During a subsequent test a few days later, it stalled again.
The cause isn't yet known, but the most likely reasons are motor failure, a gearbox jam, or a small rock wedged in the wheel — none of which would be good news for the sandtrapped craft. More tests are planned today and tomorrow. "Mars does not owe us a solution to this problem," laments rover driver Scott Maxwell, "and there might not be one."
Meanwhile, all that churning has unearthed a scientific consolation prize. The disturbed soil is enriched in sulfates, in far greater amounts than have been found elsewhere on the planet's surface (which is generally rich in sulfur anyway). "Sulfates are minerals just beneath the surface that shout to us that they were formed in steam vents, since steam has sulfur in it," explains Ray Arvidson (Washington University), a Martian-surface specialist. "Steam is associated with hydrothermal activity — evidence of water-charged explosive volcanism."
In fact, all of Home Plate appears to be an ancient volcanic hotspot, a place where erupting lava came in contact with ice-rich ground and exploded, lofting countless shards of basaltic rock that plopped all over the landscape. In 2007 the rover trudged through a deposit of nearly pure silica, further evidence of volcanic activity.
So while everyone would rather have Spirit on the move again, NASA scientists still have plenty to do as the rescue unfolds. "This is some of the best evidence from either rover for past water activity on Mars," Maxwell notes. "From a scientific point of view, it's like having your car break down in front of Disneyland."