The Imager for Mars Pathfinder (IMP) collected the photons you see here. I dithered the image to only 256 shades of gray, down from the 4096 the Imager is capable of, so I could convert to gif.
Also, the image has NOT been corrected for the electronic shutter effect of the CCD. That's why the pixels on the right side of the image are higher in intensity than those on the left side.