From press release:

Dusty Solar Panels on Spirit

The deck of NASA's Mars Exploration Rover Spirit is so dusty that the rover almost blends into the dusty background in this image assembled from frames taken by the panoramic camera (Pancam) during the period from Spirit's Sol 1,355 through Sol 1,358 (Oct. 26-29, 2007).

Dust on the solar panels reduces the amount of electrical power the rover can generate from sunlight each sol. Earlier self-portraits by Spirit, such as one taken on Sol 586, offer a comparison view of cleaner solar panels.

The vertical projection used here produces the best view of the rover deck, though it distorts the ground and antennas somewhat. The eight-pointed star shape near the front of the rover (bottom of the image) marks the location of the camera mast, which is out of view of the Pancam atop the mast.