Pasadena, Calif. ? NASA’s twin rovers reached out their robotic arms to touch the surface of Mars on Monday, marking the first day of the joint $820 million mission that both spacecraft were in full swing.

Opportunity and Spirit, 6,600 miles apart on opposite sides of the planet, began the work week gearing up for in-depth analyses of the soil and rocks beneath their wheels.

Opportunity rolled onto the Martian ground on Saturday, a week after it landed. Spirit arrived Jan. 3 but broke off its science work nearly two weeks ago after software problems crippled the vehicle.

On Monday, mission manager Jennifer Trosper said Spirit had joined Opportunity back at work, even as engineers worked out the last kinks in its software.

“We have two operational rovers on Mars,” Trosper said at NASA’s Jet Propulsion Laboratory.

Opportunity stretched out its robotic arm and photographed in detail each of the four instruments it carries. The arm — formally known as an “instrument deployment device” — is the most complex mechanism on each rover and was reported to be working well.

Spirit resumed use of its own arm, picking up where it left off. NASA planned for it to brush the dust off a volcanic rock dubbed Adirondack, allowing the rover’s to photograph it in extreme close-up.

NASA launched the pair of vehicles to try to find geological evidence of past water activity on Mars. That could show the planet was hospitable to life, perhaps billions of years ago.

It appears Opportunity has not had to venture far to find such evidence: It has already discovered an iron-rich mineral called gray hematite, and preliminary measurements suggest it is of a variety that forms in liquid water.

Spirit, in contrast, may have to drive hundreds of yards, to a nearby crater called Bonneville, to uncover similar geologic proof.

“Spirit is the driving mission. We are already theorizing how to drive far and fast,” Trosper said.