Io is the closest large moon of Jupiter and was named for a priestess of Hera who became one of the lovers of Zeus. Io has the most volcanic activity of any moon in the solar system, and its entire surface is covered with lava every few thousand years. NASA notes that this photo is based on real infrared, green and ultraviolet-light images and has only been adjusted to show the contrast. Io has an irregular elliptical orbit and is slightly larger than our own moon. It was discovered in 1610 by Galileo.