Astronomy Picture of the Day . ... In the early frame above, Ganymede , the largest moon in the solar system, is off the lower right limb of the planet, while intriguing Europa is visible against Jupiter's cloud tops, also near the lower right. ... In later frames, as planet and moons rotate (right to left), red spot junior moves behind Jupiter's left edge while the Great Red Spot itself comes into view from the right. Also finally erupting into view at the right, is Jupiter's volcanic moon, Io . ...