The Moons of Jupiter
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The moons of Jupiter are natural satellites orbiting Jupiter, the largest planet in the solar system. Jupiter possesses a large number of moons, ranging from massive bodies comparable to terrestrial planets to small irregular rocks. This large count results from Jupiter's powerful gravity, which can capture small objects passing nearby.
Four of the largest moons—Io, Europa, Ganymede, and Callisto—are known as the Galilean moons, having been observed by Galileo Galilei through a telescope in 1610. This discovery demonstrated that other worlds not orbiting Earth exist, and it had bearing on the debate concerning the center of the solar system.
The Galilean Moons
Io is the most volcanically active body known in the solar system; its volcanism is driven by tidal heating from Jupiter and other moons. Europa is surrounded by a shell of ice, with an ocean of liquid water thought to exist beneath, making it one of the most significant locations in the search for habitable conditions. Ganymede is the largest moon in the solar system and larger than the planet Mercury, and is the only one known to possess its own magnetic field. Callisto is a heavily cratered world, indicating that its surface has remained largely unchanged over a long period.
Groups of Small Moons
Beyond the four large moons, there exist inner moons orbiting very close to the planet, some of which are ring moons associated with Jupiter's ring system. Further out lie many small moons, numerous among them orbiting along elongated or retrograde paths, meaning they orbit opposite to the planet's rotation. This characteristic suggests that most of them were captured bodies rather than moons formed in place.
Observation
Jupiter's moons have been observed with ground-based telescopes and space-based observatories such as Hubble, and detailed images have been obtained by spacecraft that have passed by or orbited the planet, including Voyager and Galileo, and subsequently Juno. Current research emphasizes Europa and Ganymede, particularly in measuring ice shells and confirming the presence of subsurface water.