Io (moon)
Galileo spacecraft true-color image of Io. The dark spot just left of the center is the erupting volcano Prometheus. The whitish plains on either side of it are coated with volcanically deposited sulfur dioxide frost, whereas the yellower regions contain a higher proportion of sulfur.
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| Discovery | |||||||||
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| Discovered by | Galileo Galilei | ||||||||
| Discovery date | 8 January 1610 | ||||||||
| Designations | |||||||||
| Jupiter I | |||||||||
| Adjectives | Ionian | ||||||||
| Orbital characteristics | |||||||||
| Periapsis | 420000 km (0.002807 AU) | ||||||||
| Apoapsis | 423400 km (0.002830 AU) | ||||||||
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Mean orbit radius
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421700 km (0.002819 AU) | ||||||||
| Eccentricity | 0.0041 | ||||||||
| 1.769137786 d (152853.5047 s, 42.45930686 h) | |||||||||
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Average orbital speed
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17.334 km/s | ||||||||
| Inclination | 0.05° (to Jupiter's equator) 2.213° (to the ecliptic) |
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| Satellite of | Jupiter | ||||||||
| Physical characteristics | |||||||||
| Dimensions | 3,660.0 × 3,637.4 × 3,630.6 km | ||||||||
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Mean radius
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1821.6±0.5 km (0.286 Earths) | ||||||||
| 41910000 km2 (0.082 Earths) | |||||||||
| Volume | 2.53×1010 km3 (0.023 Earths) | ||||||||
| Mass | (8.931938±0.000018)×1022 kg (0.015 Earths) | ||||||||
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Mean density
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3.528±0.006 g/cm3 | ||||||||
| 1.796 m/s2 (0.183 g) | |||||||||
| 0.3755±0.0045 (estimate) | |||||||||
| 2.558 km/s | |||||||||
| synchronous | |||||||||
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Equatorial rotation velocity
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271 km/h | ||||||||
| Albedo | 0.63±0.02 | ||||||||
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| 5.02 (opposition) | |||||||||
| Atmosphere | |||||||||
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Surface pressure
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trace | ||||||||
| Composition by volume | 90% sulfur dioxide | ||||||||
Io /ˈaɪ.oʊ/ (Jupiter I) is the innermost of the four Galilean moons of the planet Jupiter. It is the fourth-largest moon, has the highest density of all the moons, and has the least amount of relative water of any known object in the Solar System. It was discovered in 1610 and was named after the mythological character Io, a priestess of Hera who became one of Zeus's lovers.
With over 400 active volcanoes, Io is the most geologically active object in the Solar System. This extreme geologic activity is the result of tidal heating from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean satellites—Europa, Ganymede and Callisto. Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km (300 mi) above the surface. Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of Io's silicate crust. Some of these peaks are taller than Mount Everest. Unlike most satellites in the outer Solar System, which are mostly composed of water ice, Io is primarily composed of silicate rock surrounding a molten iron or iron-sulfide core. Most of Io's surface is composed of extensive plains coated with sulfur and sulfur-dioxide frost.
Io's volcanism is responsible for many of its unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various subtle shades of yellow, red, white, black, and green, largely due to allotropes and compounds of sulfur. Numerous extensive lava flows, several more than 500 km (300 mi) in length, also mark the surface. The materials produced by this volcanism make up Io's thin, patchy atmosphere and Jupiter's extensive magnetosphere. Io's volcanic ejecta also produce a large plasma torus around Jupiter.
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