Mars rovers Spirit and Opportunity both turn 10 this month, the latter stunning scientists as it continues its slow and steady surveillance of the Red Planet long after its expected expiration date.

Spirit landed on Mars on Jan. 4, and was followed 21 days later by its nearly identical twin Opportunity. Despite landing on nearly the exact opposite sides of the planet, the rovers shared the same goal of identifying sites where water would have existed anciently.

Spirit initially made its way to a crater researchers believed might have once held a lake. Once it arrived, however, it was greeted by a thick layer of volcanic deposits that acted as a barrier between the rover and any possible lakebed sediments.

On the other side of the planet, Opportunity was treated with much better luck, landing - as it turned out - only a short distance from a rock that quickly revealed evidence of a previously water-rich environment.

As both rovers set out toward more distant destinations, the expectation among those back on Earth was that neither probe would live to celebrate its first birthday.

Spirit drove to a set of hills, making a portion of the journey with a dead wheel. It eventually became stuck in a sand trap in 2009 where it remained until, a year later, the rover apparently froze death - though not before achieving what it set out to do.

In the hills where it explored, researchers found "compelling evidence of an ancient Mars that was a hot, wet, violent place, with volcanic explosions, hydrothermal activity, steam vents - nothing like Mars today," the rovers' principal investigator, Steve Squyres of Cornell University, said.

Opportunity, meanwhile, has been hard at work investigating the outcrops on the rim of what's known as Endeavour Crater, which is 14 miles wide.

"Opportunity is still in excellent health for a vehicle of its age," said John Callas, a project manager for NASA's Mars Exploration Rover Project who has worked on the Spirit and Opportunity missions for more than 13 years. "The biggest science may still be ahead of us, even after 10 years of exploration."