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In Greek mythology, Io (pronounced "EYE oh," though "EE oh" is also acceptable) was the daughter of Inachus, a river god. One day Zeus noticed her and she quickly became one of his many lovers. Their relationship continued until Hera almost caught them -- a fate avoided by Zeus turning Io into a heifer. However, Hera was not completely fooled and demanded Zeus give her the heifer as a present.

Once Io was given to Hera, she placed him in the charge of Argus to keep her separated from Zeus. Zeus then commanded Hermes to kill Argus, which he did. Io escaped across the Ionian Sea to Egypt, where she was transformed back into human form by Zeus. In Egypt Io gave birth to Epaphus.

The term Io fly is derived from the gadfly Hera sent to torment Io into fleeing to Egypt after Argus was slain.

Io is also the name of one of the moons of Jupiter. It is the innermost of the four large, or Galilean, moons of Jupiter.

Io is most noteworthy for its volcanic nature, and is the most volcanically active body in the Solar System. Unlike volcanoes on Earth, Ionian volcanoes emit sulfur or possibly sulfur dioxide.

The energy for all this activity probably derives from tidal interactions between Io, Europa, Ganymede and Jupiter. These three moons are locked into resonant orbits such that Io orbits twice for each orbit of Europa which in turn orbits twice for each orbit of Ganymede. Though Io always faces the same side toward its planet, the effects of Europa and Ganymede cause it to wobble a bit. This wobbling stretches and bends Io by as much as 100 meters and generates heat through internal friction.

Io also cuts across Jupiter's magnetic field lines, generating an electric current. Though not a large source of energy compared to the tidal heating, this current may carry more than 1 trillion watts with a potential of 400,000 volts. It also strips ionized atoms away from Io at the rate of a thousand kilograms per second which form a torus of intense radiation around Jupiter that glows brightly in the ultraviolet. Particles escaping from this torus are partially responsible for Jupiter's unusually large magnetosphere, their outward pressure inflating it from within. Recent data from the Galileo probe indicate that Io may have its own magnetic field.

Some of Io's volcanic plumes have been measured rising over 300 km above the surface before falling back, the material being ejected from the surface at approximately one kilometer per second. The volcanic eruptions change rapidly; in just four months between the arrivals of Voyager 1 and Voyager 2 some of them stopped and others started up. The deposits surrounding the vents also changed visibly.

Unlike most of the moons in the outer solar system Io may be somewhat similar in bulk composition to the terrestrial planets, primarily composed of molten silicate rock. Recent data from the Galileo probe indicates that Io has a core of iron (perhaps mixed with iron sulfide) with a radius of at least 900 km.

Io's surface is almost completely lacking in craters, meaning it must be very young. In addition to volcanoes, the surface includes non-volcanic mountains, numerous lakes of molten sulfur, calderas up to several kilometers deep, and extensive flows hundreds of kilometers long of low viscosity fluid (possibly some form of molten sulfur or silicate). Sulfur and its compounds take on a wide range of colors which are responsible for Io's variegated appearance.

Analysis of the Voyager images led scientists to believe that the lava flows on Io's surface were composed mostly of various compounds of molten sulfur. However, subsequent ground-based infrared studies indicate that they are too hot for liquid sulfur; some of the hottest spots on Io may reach temperatures as high as 2000 K (though the average is much lower, about 130 K). One current idea is that Io's lavas are molten silicate rock. Recent Hubble Space Telescope observations indicate that the material may be rich in sodium. There may potentially be a variety of different materials in different locations.

Io has a thin atmosphere composed of sulfur dioxide and perhaps some other gases.

Unlike the other Galilean satellites, Io has little or no water. This is probably because Jupiter was hot enough early in the evolution of the solar system to drive off the volatile elements in the vicinity of Io but not so hot to do so farther out.

Data for Io:

  • Diameter: 3632 km
  • Surface area: 41 million km2
  • Orbital radius: 421,600 km
  • Orbital period: 1.76 days
  • Rotational period: 1.76 days
  • Mass: 8.92*1022 kg
  • Density: 3.55 g/cm3
  • Orbital inclination: 0.04°

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