From left to right: Ganymede, Callisto, Io, and Europa.
The Galilean satellites are the four largest moons of Jupiter. In order
of increasing distance from the primary, they are Io, Europa, Ganymede,
and Callisto. They were discovered independently
by Galileo Galilei and Simon Marius in 1610. Galileo proposed that they be named the Medicean stars, in honor
of his patron Cosimo II de Medici; however, the present names are due to
All four Galilean moons are easily visible in a small telescope or binoculars. Io, Europa, and Ganymede periodically line up with the result that their gravitational interactions during the alignments force the three moons into non-circular orbits; the moons' varying distances from the planet lead to tidal distortions, and subsequent tidal heating, caused by Jupiter's gravity. The heating, in turn, fuels extreme volcanism on the innermost of the Galilean moons Io and, evidence suggests, helps maintain a liquid sea beneath the frozen crust of the second Galilean moon, Europa.