The Galilean satellites

The Galilean satellites

Fig. 9.9 . A composite of the four largest moons of Jupiter, which are known as the Galilean satellites. From left to right and increasing distance from Jupiter, the moons shown are Io, Europa, Ganymede, and Callisto, . The images of Ganymede and Io are from the Galileo spacecraft, while those of Callisto and Europa are from the Voyager 1 or 2 spacecraft. In order of increasing distance from Jupiter, Io is closest, followed by Europa, Ganymede, and Callisto. Io is subject to the strongest tidal stresses from the massive planet. These stresses generate internal heating, which is released at the surface and makes Io the most volcanically active body in our solar system. Europa appears to be strongly differentiated inside with a rock/iron core, a surface layer of bright water ice, and possibly local or global zones of liquid water between these layers. Tectonic resurfacing brightens terrain on the less active and partially differentiated moon Ganymede. Callisto, furthest from Jupiter, appears heavily cratered and shows no evidence of internal activity. (Courtesy of NASA/JPL/DLP.)

Copyright 2010, Professor Kenneth R. Lang, Tufts University