Jupiter and Its Moons Photograph courtesy NASA This family portrait, a composite of the Jovian system, includes the edge of Jupiter (with the Great Red Spot visible) and Jupiter's four largest moons, known as the Galilean satellites. From top to bottom are Io, Europa, Ganymede, and Callisto. The smallest of these four moons, Europa is about the size of Earth's moon.
Io is the first Galilean moon of Jupiter, it is slightly larger than Earth’s moon. Io experiences intense tidal heating due to its elliptical orbit and orbital resonance with Europa and Ganymede. This makes Io the most geologically active moon in our solar system.
JUPITER’S MOONS — CLOSE-UP These four views show the great surface variety present on Jupiter’s large satellites. We see volcanic deposits on Io, ice fractures on Europa, grooved terrain on Ganymede, and impact basins on Callisto. The relative ages of these surfaces range from Io, the youngest, through Europa and Ganymede, to Callisto, the oldest. Prepared for NASA by Stephen Paul Meszaros
Gibbous Europa-Although the phase of this moon might appear familiar, the moon itself might not. In fact, this gibbous phase shows part of Jupiter's moon Europa. ESA and NASA have together started preliminary development of the Europa Jupiter System Mission , a spacecraft proposed to better study Europa.