Io

Io (Jupiter I) is one of the original four moons found centuries ago (the Galilean Moons), the nearest to Jupiter of these four, and is the fifth outward of Jupiter's currently known moons. Its radius is 1821 km (a bit over a quarter or roughly 2/7 of Earth's), its mass is 8.9×10²²kg (about 1/66 of Earth's), with a mean density about three and a half times that of water (a bit over half or about 7/11 Earth's). Io's orbital period is 1.75 days and it is tidally locked. Its orbit is synchronized with that of Ganymede (with 1/4 Ganymede's orbital period) and Europa (1/2), an example of orbital resonance. Of interest are the "real" volcanos on the surface, i.e., with temperatures above 1000 K rather than cryovolcanos like those found on other solar system moons. Flyby observations of Io have been made by the Pioneer, Voyager, Galileo, and Juno missions.

The Io torus (or Io plasma torus) is a torus-shaped cloud surrounding Jupiter, consisting of plasma from IO's volcanic eruptions, shaped by Jupiter's magnetic field. It is an example of a gas torus, a term for such a torus of gas around a planet, typically associated with one of its moons.