Guided by mineral mapping from orbit, the rover is exploring outcrops on the northwestern slopes of Solander Point, making its way up the hill much as a field geologist would do.
The outcrops are exposed from about 2 meters to about 6 meters above the surrounding plains, on slopes as steep as 15 to 20 degrees. The rover may later drive south and ascend farther up the hill, which peaks at about 40 metres above the plains.
"This is our first real Martian mountaineering with Opportunity," said Steve Squyres of Cornell University, the principal investigator for the rover.
"We expect we will reach some of the oldest rocks we have seen with this rover - a glimpse back into the ancient past of Mars," said Squyres.
The hill rises southward as a ridge from Solander Point, forming an elevated portion of the western rim of Endeavour Crater. The crater spans 22 kilometres in diameter.
Key targets on the ridge include clay-bearing rocks identified from observations by the Compact Reconnaissance Imaging Spectrometer for Mars.
This segment of the crater's rim stands much higher than "Cape York," a segment to the north that Opportunity investigated for 20 months beginning in mid-2011.
Opportunity reached Solander Point in August after months of driving from Cape York. Researchers then used the rover to investigate a transition zone around the base of the ridge.
The area reveals contact between a sulfate-rich geological formation and an older formation.
The sulfate-rich rocks record an ancient environment that was wet, but very acidic. The contact with older rocks may tell researchers about a time when environmental conditions changed.
Opportunity first explored the eastern side of Solander Point, then drove back north and around the point to explore the western side.
"We took the time to find the best place to start the ascent," said Opportunity's project manager, John Callas of NASA's Jet Propulsion Laboratory, Pasadena, California.
"Now we've begun that climb," he said.
The rover began the climb on October 8 and has advanced farther uphill with three subsequent drives.
"We're in the right place at the right time, on a north-facing slope," Callas said.