This is pretty hard to make out, but that faint milky smudge in this swath of green appears to be Mars' moon Phobos. Commentor fredk posted the image on the unmannedspaceflight forums.
The image was taken from the surface of Mars by the Curiosity rover on Sept 21. The strange potato-shaped moon rises in the West and sets in the East, and circles the Red Planet more than twice a day.
(Via The Planetary Society blog)