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Io is the innermost of the four Galilean moons of Jupiter. It is named after the Greek mythological figure Io, one of the many lovers of Zeus (who is also known as Jupiter in the Roman mythology).

Although the name "Io" was suggested by Simon Marius soon after its discovery in 1610, this name and the names of the other Galilean satellites fell into disfavor for a considerable time, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, Io is simply referred to by its Roman numeral designation as "Jupiter I", or simply as "the first satellite of Jupiter".

Volcanism

Galileo probe images revealed active volcanoes. The eruption in the top inset reaches 140km high, the other is 75km high.

Io is most noteworthy for its volcanic nature; it is the most volcanically active body in the Solar System. Similarly to volcanoes on Earth, Ionian volcanoes emit sulfur and sulfur dioxide. Originally it was thought that many lava flows consisted of sulfurous substances. However, nowadays it is thought that many of them are molten silicate rock like on the Earth.

The energy for this activity probably derives from tidal interactions among Io, Jupiter, and two other moons of Jupiter, Europa and Ganymede. The three moons are locked into Laplace-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.

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

Another source of energy is that Io 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 1,000 gigawatts with a potential of 400 kilovolts. It also strips ionized atoms from Io at the rate of a thousand kilograms per second. Due to the rapid rotation of Jupiter's magnetic field, these particles are swept along the orbit in front of Io where they 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 might have its own magnetic field.

The location of Io with respect to the Earth and Jupiter has a strong influence on the Jovian radio emissions as seen from the earth: When Io is visible, radio signals from Jupiter increase considerably.

Physical characteristics

Io exhibits extraordinary variations in color and brightness as shown in this color-enhanced image.

Unlike most 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), the core's radius being at least 900 km.

When Voyager I first returned images of Io in 1979, scientists expected to see numerous craters, the density of which across Io's surface would give clues to the moon's age. However, they were surprised to discover that Io's surface is almost completely lacking in craters, due to the tremendous amount of volcanic activity constantly reshaping the landscape. Since the surface features visible today were formed relatively recently, Io's surface is described as "young", as is the Earth's. In contrast, celestial bodies with heavily cratered features, such as Earth's Moon, are considered to have "old" surfaces, since they have remained in their current state for billions of years.

In addition to volcanoes, Io's surface includes nonvolcanic mountains, numerous lakes of molten sulfur, calderas up to several kilometres deep, and extensive flows hundreds of kilometres long of low-viscosity fluid (possibly some form of molten sulfur or silicate). Sulfur and its compounds take on a wide range of colors and 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 Kelvin (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 be a variety of different materials in different locations.

Io has a thin atmosphere composed of sulfur dioxide and perhaps 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 hot enough to do so farther out.

Io in fiction

• Io plays an important role in both the book and the film of Arthur C. Clarke's 2010: Odyssey Two (1984). The spacecraft Discovery is found tumbling end over end in orbit around Io, coated with sulfur from the erupting volcanoes beneath it.
• Peter Hyams, the director of 2010, had previously made a film called Outland (1981), set in a mining colony on Io, although the moon itself has little importance to the plot.
• In the television cartoon saga Exosquad, Io is the location of an Exofleet base and the scene of several critical battles between Terran and Neosapien forces.
• In the science fiction TV series Babylon 5, Io is home to an Babylon colony, second in size only to the colony on Mars.

See also

• List of geological features on Io
• List of mountains on Io
• List of paterae on Io

External links

• Bill Arnett's Io webpage
• The Calendars of Jupiter
  ... | Thebe | Io | Europa | ...

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