Galilean satellites(gal-ă-lee -ăn) The satellites Io, Europa, Ganymede, and Callisto of Jupiter, discovered in 1610 by Galileo Galilei and, independently, by the German astronomer Simon Marius. They are bright enough to be seen with the aid of binoculars and have been studied in detail by Pioneer and Voyager spacecraft and the Galileo spaceprobe. The Galilean satellites are Jupiter's largest satellites by far, comparable in size with the small planets. Ganymede, the largest satellite in the Solar System, is slighter bigger than Mercury in diameter, and all of them except Europa are larger than the Moon. Each is in synchronous rotation, keeping one face permanently turned toward Jupiter. Their maximum surface temperatures vary between the 120 K of Io to 155 K for Callisto, probably as a result of differences in albedo; the albedo of Io is 0.61, of Europa 0.64, of Ganymede 0.42, and of Callisto as low as 0.20. They show a progressive decrease in bulk density from Io, the closest (3.57 times that of water) through Europa (2.97) and Ganymede (1.94) to Callisto, the farthest (1.86), indicating that the proportion of rocky material to ice is greater for the denser satellites. Ganymede and Callisto, unlike Io and Europa, are both heavily cratered bodies. The three Galilean satellites closest to Jupiter in order of distance – Io, Europa, and Ganymede – follow paths that are locked in to each other by orbital resonance. Thus for every single orbit completed by Ganymede, Europa completes two and Io four. One consequence of this orbital resonance takes the form of gravitational effects. Europa's gravitational influence on Io has perturbed it into a more eccentric orbit around Jupiter, taking it both closer to and farther away from the giant planet and exposing it to the tidal stresses that produce the active volcanoes observed by the Voyager and Galileo spacecraft. Europa in its turn is acted upon by Ganymede and as a result its geology may have been subject to the effects of tidal friction and heating. It has been suggested that as Callisto's distance from Jupiter increases owing to the tidal exchange of angular momentum between it and the giant planet, it too will eventually reach a 2:1 resonance with Ganymede. See also Jupiter's satellites; Table 2, backmatter.
Galilean satellites[‚gal·ə¦lē·ən ′sad·əl‚īts]
The four largest and brightest satellites of Jupiter (Io, Europa, Ganymede, and Callisto).