The Mars rover Opportunity is making its way back out to the Martian plains nearly a year after descending into a large Martian crater, Victoria, to examine exposed ancient rock layers.
“We’ve done everything we entered Victoria Crater to do and more,” said Bruce Banerdt, of NASA’s Jet Propulsion Laboratory in Pasadena, California who is project scientist for Opportunity and its rover twin, Spirit.
Opportunity is now preparing to look at loose cobble rock on the plains which are approximately fist-size and larger, which were thrown long distances when objects hitting Mars blew deep craters into the planet; even deeper than the recently explored Victoria crater.
“Our experience tells us there’s lots of diversity among the cobbles,” said Scott McLennan of the State University of New York, Stony Brook. McLennan is a long-term planning leader for the rover science team. “We want to get a better characterization of them. A statistical sampling from examining more of them will be important for understanding the geology of the area.”
Opportunity entered Victoria Crater on Sept. 11, 2007, after a year of exploring from the rim. Once a drivable inner slope was identified, Opportunity used contact instruments on its robotic arm to inspect the composition and textures of accessible layers, then drove close to the base of a cliff called “Cape Verde,” part of the crater rim, to capture detailed images of a stack of layers 20 feet tall. The information Opportunity has returned about the layers in Victoria suggest the sediments were deposited by wind and affected by groundwater.
“The patterns broadly resemble what we saw at the smaller craters Opportunity explored earlier,” McLennan said. “By looking deeper into the layering, we are looking farther back in time.” At approximately a half mile in diameter, it is the deepest seen by Opportunity.
Engineers are programming Opportunity to climb out of the crater at where it entered. A spike in electric current drawn by the rover’s left front wheel last month settled debate about whether to keep trying to get closer to the base of Cape Verde on the steep slope. The spike resembled one seen on Spirit when that rover lost the use of its right front wheel in 2006. Opportunity’s six wheels are all still working after 10 times more use than they were designed to perform, but the team took the spike in current as a reminder of the pitfalls involved. Yet, unbelievably, the rover is still working well.
“If Opportunity were driving with only five wheels, like Spirit, it probably would never get out of Victoria Crater,” said JPL’s Bill Nelson, a rover mission manager. “We also know from experience with Spirit that if Opportunity were to lose the use of a wheel after it is out on the level ground, mobility should not be a problem.”
Opportunity drives with its robotic arm out of the stowed position because a shoulder motor has degraded over the years to the point where the rover team didn’t want to risk having it stop working while the arm is stowed on a hook. If the motor were to stop working with the arm unstowed, the arm would remain usable; not a good thing.
“Both rovers show signs of aging, but they are both still capable of exciting exploration and scientific discovery,” said JPL’s John Callas, project manager for Spirit and Opportunity.