These two images, taken five Martian days (sols) apart by the front hazard-avoidance camera on NASA's Mars Exploration Rover Opportunity, document the Martian sky above the rover's Endeavour Crater location becoming dustier.
The image on the left was taken during Sol 3296 of Opportunity's mission on Mars (May 2, 2013). The one on the right was taken during Sol 3301 (May 7, 2013) with the rover at the same location but its arm in a different position. In between those dates, local-scale dust storm activity west of Opportunity's location had reduced clarity of the sky over Opportunity. Effects can be seen in the Sol 3301 image. Note the softened shadow of the rover and the obscured horizon on the right.
The location of the rover when these images were taken was on the "Cape York" segment of the western rim of Endeavour Crater. During this period Opportunity was investigating the rock target "Esperance," which is on the pale patch of rock just to the left of the shadow of the rover's arm. After that investigation, the last planned for Cape York, the rover team plans to drive Opportunity toward a segment of the rim to the south, called "Solander Point."