Jupiter's Galilean Satellites
Jupiter and Three Galilean Satellites - PIA00358.
Jupiter with Io Crossing - PIA00455.
Jupiter with Satellites Io and Europa - PIA00144.
Jupiter's Galilean Satellites

Jupiter's Galilean Satellites

Courtesy of NASA/JPL/DLR

http://photojournal.jpl.nasa.gov/catalog/PIA01299


This composite includes the four largest moons of Jupiter which are known as the Galilean satellites. The Galilean satellites were first seen by the Italian astronomer Galileo Galilei in 1610. Shown from left to right in order of increasing distance from Jupiter, Io is closest, followed by Europa, Ganymede, and Callisto.
The order of these satellites from the planet Jupiter helps to explain some of the visible differences among the moons. 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 have a rock/iron core, an ice layer at its surface, and local or global zones of water between these layers. Tectonic resurfacing brightens terrain on the less active moon Ganymede. Callisto, furthest from Jupiter, appears heavily cratered at low resolutions and shows no evidence of internal activity. This composite picture shows the satellites scaled to a common resolution of 10 km (6 mi) per picture element.
NASA's Galileo spacecraft acquired the Io and Ganymede images in June 1996, the Europa images in September 1996, and the Callisto images in November 1997.