NASA Mars Exploration Rover Spirit took this image using the left eye of its navigation camera during the 1,813th Martian day, or sol, of Spirit's mission on Mars (February 7, 2009). Contrast has been stretched to make dust on the solar panel more easily visible.
Engineering data from Spirit's power subsystem indicated that some dust blew off the rover's solar array on the preceding day, Sol 1812 (February 6, 2009).
This image and one from Sol 1811 (February 5, 2009) (PIA11798) provide a before-and-after comparison of the dust visible on Spirit's rear solar array. The reduction in the dust coating is difficult to discern in the images. The cleaning event was detected by a change in electrical output from the solar panels.
Spirit's daily energy supply from the solar array increased by about 30 watt-hours, to about 240 watt-hours from 210 watt-hours. (Thirty watt-hours is enough to illuminate a 30 watt bulb for one hour.) Before the Sol 1812 cleaning, only 25 percent of sunlight hitting the array was getting past the dust to be used by the photovoltaic cells. That factor improved to 28 percent after the cleaning event.
Spirit took this image at about 2:07 p.m. local solar time at Spirit's location inside Gusev Crater. The rover's position was next to the northern edge of the low plateau called "Home Plate," visible beyond the solar panel.