This image taken by the front hazard-avoidance camera on NASA's Mars Exploration Rover Opportunity shows the rover's arm extended to examine the composition of a rock using the alpha particle X-ray spectrometer.

Opportunity took this image during the 1,826th Martian day, or sol, of the rover's Mars-surface mission (March 13, 2009).

The spectrometer is at a target called "Penrhyn," on a rock called "Cook Islands." As Opportunity makes its way on a long journey from Victoria Crater toward Endeavour Crater, the team is stopping the drive occasionally on the route to check whether the rover finds a trend in the composition of rock exposures.