Opportunity is within 'Marathon Valley' on the west rim of Endeavour Crater exploring for phyllosilicate clay minerals.

The rover is operating in persistent RAM mode (not using Flash for data storage). On Sol 4120 (Aug. 26, 2015), Opportunity used the Rock Abrasion Tool (RAT) to brush the freshly ground surface target, 'Pvt. Robert Frazer' to prepare it for in-situ (contact) measurements. After the surface was cleaned, the Microscopic Imager collected an image mosaic.

This was followed by the placement of the Alpha Particle X-ray Spectrometer (APXS). The APXS integration was performed on the evening of the next sol. Both Panoramic Camera (Pancam) and Navigation Camera (Navcam) panoramas were sequenced over several sols along with additional readouts of Flash Bank 7 memory.

On Sol 4125 (Sept. 1, 2015), the rover drove to the west about 46 feet (14 meters) in order to approach a new geological contact that may exhibit characteristics of alternation.

As of Sol 4125 (Sept. 1, 2015), the solar array energy production was 384 watt-hours with an atmospheric opacity (Tau) of 0.668 and a solar array dust factor of 0.582.

Total odometry is 26.42 (42.52 kilometers), more than a marathon.