Io's Sodium Cloud.

Explanation : Io has its own cloud. Jupiter 's most active moon is visible on the left of the above false-color photograph , with its left side illuminated by sunlight. But what is happening on Io 's right side? Here a plume of gas from the active volcano Prometheus also reflects sunlight. A second active volcano, Pele , is also visible: marked by the red spot just below Io 's center. Surrounding the moon is a yellow haze originating from gas ejected by Io's volcanos . Sodium atoms in the gas cloud are particulary efficient at reflecting yellow light . Several points of light are background stars.