SETI Institute#PPOD: The shadow is not that of Europa but a second moon (Io), which is not in this frame. Europa is slightly smaller than Earth's Moon and is tidally locked. Jupiter's Great Red Spot (GRS) is a vast cyclonic storm system about three times the size of Earth when Voyager flew by. Since 1979, the GRS has continuously shrunk, slowly changing its shape from an oval to a circle. It is now a little over the size of the Earth only. Credit: NASA / JPL-Caltech / Kevin M. Gill
