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| Click image for description | |||||||
| Discovery | |||||||
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| Discovered by | S. Marius G. Galilei | ||||||
| Discovered in | 1610 | ||||||
| Orbital characteristics | |||||||
| Mean radius | 421,600 km | ||||||
| Eccentricity | 0.041 | ||||||
| Revolution period | 1 d 18 h 27.6 min | ||||||
| Inclination | 0.040° | ||||||
| Is a satellite of | Jupiter | ||||||
| Physical characteristics | |||||||
| Mean diameter | 3643.2 km | ||||||
| Surface area | 41,000,000 km2 | ||||||
| Mass | 8.94×1022 kg | ||||||
| Mean density | 3.55 g/cm3 | ||||||
| Surface gravity | 1.81 m/s2 | ||||||
| Surface Gravity (Earth = 1) | 0.1847 | ||||||
| Rotation period | 1d 18h 27.6m | ||||||
| Axial tilt | ° | ||||||
| Albedo | 0.61 | ||||||
| Surface temp. |
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| Atmospheric characteristics | |||||||
| Atmospheric pressure | trace kPa | ||||||
| Sulfur dioxide | % | ||||||
Io is the innermost of the four Galilean moons of Jupiter. It is named after Io, one of Zeus's many love interests in Greek mythology.
Although the name "Io" was suggested by Simon Marius soon after its discovery, this name and the names of the other Galilean satellites curiously fell into disfavor for a considerable time, and was not revived in common use until the mid 20th century. In much of the earlier astronomical literature, it is simply referred to by its Roman numeral designation as "Jupiter I" or as the "first satellite of Jupiter".
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 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 kilometer 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; these atoms 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.
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. Io's surface is described as "young", as is the Earth's, since the surface features visible today were formed relatively recently. 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 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 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 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 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.
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