(well-known Jupiter moon)
Io (Jupiter I) is the first of Jupiter's first four
discovered moons, of them and the fourth outward from the planet
according to current knowledge.
Its radius is 1821km (a bit over a quarter or 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 7/11 Earth's).
Io's orbital period is 1.75 days and it is tidally locked.
Its orbit is in sync with Ganymede (1/4) and Europa (1/2).
Of interest are the "real" volcanos on the surface,
i.e., with temperatures above 1000K
rather than cryovolcanos like those found on other solar system moons.
Flyby observations of Io have been made by
the Pioneer, Voyager, and Galileo missions.
The Io torus (or Io plasma torus) is a gas torus,
a torus-shaped cloud surrounding a planet typically
associated with a moon.
In Io's case, it is plasma from its volcanic eruptions
shaped by Jupiter's magnetic field.
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