Those Mars rovers just keep on trucking. It’s been nearly four years since Spirit and Opportunity touched down separately on opposite sides of the red planet, but they’re both still going strong. Steve Squyres of Cornell and John Callas of NASA provided an update here on how they’re doing – and how they’re still making surprising discoveries.

First to Spirit, which has been something of the underdog of the rovers. But finally, scientifically, “it’s caught up to Opportunity,” said Squyres. And it’s all because of a problem mission managers have been cursing for some time: the fact that Spirit’s right front wheel is stuck, and whenever they have to move the six-wheeled vehicle they have to drag the stuck wheel behind it like a recalcitrant shopping cart.

But this awkward movement means that there’s a deep trench behind the rover wherever it moves. And that recently revealed a glistening white trail in the wheel trench. Chemical analyses show that the stuff is nearly entirely made of silica — the stuff that makes up glass — with a touch of titanium. And that, says Squyres, suggests two possible environments: either hot springs or volcanic fumaroles, both of which mean heat plus water — and potentially life. It’s the first time Spirit has found potentially habitable environments, the kind of warm watery places where microorganisms like to thrive.

But Spirit is also running out of time. The Martian winter is approaching, and mission managers have about two weeks to get it into a position where it can catch as much sunlight as possible during the long winter. “It’s a scramble” to get it into place in time, Callas told reporters. The rover is running low on battery power — dust has covered its solar arrays — and it can move only every other day, taking a rest period in between to recharge its batteries. Rover operators expect the solar arrays to go down to just 30 percent power over the winter – and that may or may not be enough for it to survive. Its first winter, the batteries stayed around 70 percent; the second winter, about 55 percent. The team has many backup plans, such as turning off the survival heater for a miniature chemical analyzer on board, to keep the juice flowing as long as possible.

Meanwhile, on the other side of the planet, Opportunity continues to shine. It is exploring the Victoria crater, grinding holes into the layers of rock formations that encircle the crater like a ring in a bathtub. Opportunity doesn’t have such a problem with survival, because the wind patterns there mean that gusts regularly clear off its solar panels.

It’s anyone’s guess as to how long these rovers might eventually last. But no one can say they’ve been slackers.