Io is one of Jupiter's 50 named moons. (Jupiter, the largest planet in our solar system, also has 12 "provisional moons" that are identified with numbers.) Io, brightly colored by sulphur eruptions, is the most volcanically active body in the solar system.

Photograph courtesy NASA

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  • On March 8, 1979, Voyager 1 discovered volcanoes on Io, one of Jupiter’s moons. These were the first active volcanoes discovered outside Earth.

    Io’s volcanism is a result of tidal heating. Io has a very eccentric (or oval-shaped) orbit around Jupiter. Io’s orbital eccentricity interacts with the gravitational pull of Jupiter in such a way that the interior of Io heats up. The heat creates so much pressure that hot gases and other materials are ejected at Io’s surface: volcanoes.

    Active volcanoes have been discovered on two other moons: Enceladus, a moon of Saturn, and Triton, a moon of Neptune. Io, however, is still the most volcanically active place in the solar system. (And yes, that includes Earth!)

  • Term Part of Speech Definition Encyclopedic Entry
    eject Verb

    to get rid of or throw out.

    gas Noun

    state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.

    gravitational pull Noun

    physical attraction between two massive objects.

    interact Verb

    to work with or meet.

    interior Noun

    internal or inland.

    Jupiter Noun

    largest planet in the solar system, the fifth planet from the Sun.

    moon Noun

    natural satellite of a planet.

    Encyclopedic Entry: moon
    pressure Noun

    force pressed on an object by another object or condition, such as gravity.

    volcano Noun

    an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

    Encyclopedic Entry: volcano