Opportunity is in '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). The rover is in the midst of a grind campaign on the surface target, 'Pvt. Robert Frazer.'

On Sol 4114 (Aug. 20, 2015), Opportunity ground about 2.5 millimeters into the surface to expose fresh outcrop using the Rock Abrasion Tool (RAT) on the end of the robotic arm.

This was to be followed on the next sol by a brushing to remove the grind tailings from the surface site. But, a sequencing error prevented the RAT from initiating the brush activity.

The project diagnosed the problem on the Surface System Testbed (SSTB) rover at JPL and confirmed the corrective action.

Meanwhile, the rover continued the remote sensing of the valley with several Panoramic Camera (Pancam) panoramas collected of the North Wall of Marathon Valley plus some 13-filter Pancam images of selected surface targets.

An Alpha Particle X-ray Spectrometer APXS measurement of atmospheric argon was collected on the evening of Sol 4116 (Aug. 22, 2015).

The plan ahead is to complete the brushing of the freshly ground surface target for detailed in-situ (contact) measurements.

As of Sol 4119 (Aug. 25, 2015), the solar array energy production was 404 watt-hours with an atmospheric opacity (Tau) of 0.652 and a solar array dust factor of 0.590.

Total odometry is 26.41 miles (42.51 kilometers), more than a marathon.