• (written as Io)
    One of the satellites of Jupiter discovered in 1610 by Galileo. 3660km across and an average of 422 000km from Jupiter’s centre, Io has an orbit taking 1.8 days to complete. Space probes have shown Io as a world whose yellow-red surface is made up of vast sulphur deposits, and Io’s surface is marked by huge sulphur volcanoes. This vulcanism, which may also involve silicate material like those found in terrestrial rocks, is still active, and is driven by energy transferred from Jupiter by tidal effects. Io also interacts with Jupiter’s magnetic field and affects its radio emissions.

Information & Library Science

  • abbreviation in Internet addresses, the top-level domain for British Indian Ocean Territory


  • acronym forinput/output
    (written as IO)


  • acronym forintelligence officer
    (written as IO)
  • noun an officer responsible for intelligence.