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 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. | |||||||||
| Discovery | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Discovered by | Galileo Galilei | ||||||||
| Discovery date | 8 January 1610 | ||||||||
| Designations | |||||||||
| Pronunciation | /ˈaɪ.oʊ/ | ||||||||
| Jupiter I | |||||||||
| Adjectives | Ionian | ||||||||
| Orbital characteristics | |||||||||
| Periapsis | 420000 km (0.002807 AU) | ||||||||
| Apoapsis | 423400 km (0.002830 AU) | ||||||||
Mean orbit radius | 421700 km (0.002819 AU) | ||||||||
| Eccentricity | 0.0041 | ||||||||
| 1.769137786 d (152853.5047 s, 42.45930686 h) | |||||||||
Average orbital speed | 17.334 km/s | ||||||||
| Inclination | 0.05° (to Jupiter's equator) 2.213° (to the ecliptic) | ||||||||
| Satellite of | Jupiter | ||||||||
| Physical characteristics | |||||||||
| Dimensions | 3,660.0 × 3,637.4 × 3,630.6 km | ||||||||
Mean radius | 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) | ||||||||
Mean density | 3.528±0.006 g/cm3 | ||||||||
| 1.796 m/s2 (0.183 g) | |||||||||
| 0.3755±0.0045 (estimate) | |||||||||
| 2.558 km/s | |||||||||
| synchronous | |||||||||
Equatorial rotation velocity | 271 km/h | ||||||||
| Albedo | 0.63±0.02 | ||||||||
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| 5.02 (opposition) | |||||||||
| Atmosphere | |||||||||
Surface pressure | 5 to 40 nbar | ||||||||
| Composition by volume | 90% sulfur dioxide | ||||||||
Io is a moon of the planet Jupiter. It is Jupiter's third biggest moon with a diameter of 3642 km, slightly bigger than Earth's moon.
Io has about 400 active volcanos. It is the most volcanically active body in the Solar System. Volcanoes erupt massive volumes of silicate lava, sulphur and sulphur dioxide. This constantly changes Io's appearance.
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