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Io (moon)

Discovered byS. Marius
G. Galilei
Discovered in1610
Orbital characteristics
Mean radius421,600 km
Revolution period1d 18h 27.6m
Is a satellite ofJupiter
Physical characteristics
Equatorial diameter3632 km
Surface area41,000,000 km2
Mass8.94×1022 kg
Mean density3.55 g/cm3
Surface gravity1.81 m/s2
Rotation period1d 18h 27.6m
Axial tilt°
Surface temp
K130 K2000 K
Atmospheric characteristics
Atmospheric pressuretrace kPa
Sulfur dioxide%

Io is innermost of the four Galilean moons of Jupiter.

Table of contents
1 Volcanism
2 Physical characteristics
3 See also
4 External links


Io is most noteworthy for its volcanic nature - it 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. 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, 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.

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.

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

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.

When Io was first imaged by Voyager I 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. Io's surface is sometimes described as "young" as is the Earth's, even though it is estimated that most bodies in the solar system were created around the same epoch, roughly some 4.3-4.6 billion years ago. In contrast, celestial bodies with heavily cratered features, such as Earth's Moon, are considered to have "old" surfaces.

In addition to volcanoes, Io's 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.

Satellite picture of Io taken from the Galileo probe. The picture shows two volcanic eruptions (magnified in the insets). The one on the horizon is 140km high, the other one is 75km high. ()

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 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 hot enough to do so farther out.

See also

External links