It is important to remember though that MER began as, and continues to be a discovery driven mission. "If we find something great along the way, we'll slam on the brakes and stop,” assured Squyres. "But it's time now to move on."

The MER team’s extended mission proposal focused on three primary science objectives: finding more Matijevic Formation, the most ancient Martian strata the mission discovered on Cape York at Matjevic Hill in 2012–2013; studying the gully top to bottom; and analyzing the bedrock inside Endeavour Crater to see how it differs from the same kind of bedrock found on the plains.

Opportunity drove into the extended mission in September when she departed Marathon Valley and headed for Spirit Mound to search for the ancient Matijevic Formation bedrock that has been elusive for the last four years. “This mound, we thought, was our best pre-gully chance to find more Matjevic Formation,” said Squyres. “That's why we went all the way down there. Having answered the question – it turns out it's not there – then the thing to do is boogie as fast as we can to the top of the gully."

As the rover begins the long and winding exit across slopes forming this section of Endeavour's western rim, the rover began deviating from the original extended mission route. "We will be deviating from it more," said Squyres.

In detailing the map that accompanied their extended mission proposal, the MER officials last year "drew some dots" on a traverse "as reference points to describe the landmarks that exist along the way," Squyres explained. "But do not make the mistake of thinking that we intend to stop and do science at each science waypoint. In fact, some of the waypoints don't have any particular significance and we're not going to visit at all," he added. "The only science waypoints that really matter are 1, Spirit Mound, and 13, the top of the gully.

Nearly 13 years into a mission that was originally slated for 90-days, Opportunity continues to amaze. Aside from a stuck "on" heater that forces her to shut down every night, a broken shoulder joint, a "frozen" actuator in her right front steering wheel, and the loss of her long-term or Flash memory, the rover has been performing "much the same as when we took her out of the box," said Bill Nelson, chief of MER engineering, at the Jet Propulsion Laboratory (JPL), home to all of NASA's Mars rovers.

Fortunately, the Martian spring has not darkened Endeavour with a dust storm, not yet anyway. Even so, Opportunity is again being called upon to demonstrate her MER mettle in ways and over a span of time that would present a challenge to any Mars rover. In coming months, the robot will have to confirm her ability to climb and rove nearly sideways sol after sol on steep slopes like she did in Endurance and later, after a planet encircling dust storm, Victoria Crater. Perhaps too, she will rely on her robot DNA that somehow enabled her twin, Spirit, to scale record-setting heights on Husband Hill.

"The MER rovers are pretty good mountain goats when it comes down to it,” said Lead Flight Director Mike Seibert, also a rover planner (RP) at JPL. "We've pushed the rover pretty hard in the past few weeks. And we're going to keep pushing pretty hard as we continue to climb up the rim.”

The real challenge for this rover apparently lies with the terrain. "The high tilts and finding a pathway that is within the capability of the rover is somewhat of a worry," said Nelson. "But Opportunity has done pretty much everything we've asked. As long as we don't ask more than what she's capable of, we believe the rover will continue performing admirably."

Like always, Opportunity is doing the work, making the effort sol after sol after sol, chugging along in a very unforgiving environment. The science ahead, it seems, is screaming, and the parallels to old world scenarios playing out on Earth are uncanny.

The MER mission's research at the ancient gully will not only be the first study of such a Martian geological feature so rich with history, but Opportunity is the only rover or land-based 'bot that has access to the remnants of Mars' Noachian Period, the time when water was likely flowing in various ways and in numerous places on the Red Planet. If the rover and the scientists do find evidence that the gully was carved by water it would certainly bolster that long-held theory.

"If we show that this gully was carved by water, then that will tell us something about water on Mars a long time ago," said Squyres. "But we're not going to debate what formed this thing, we're just going to go and find out.”

The road ahead will be brutal. “It's rocky. It's steep. It's challenging. And we have a hugely important science objective out in front of us," Squyres summed up. "We're playing for pretty high stakes right now.”