Io (moon)

“IO” redirects here. For I/O, see input/output.

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

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/[1]
Alternative names	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
Orbital period	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[2]
Mean radius	1821.6±0.5 km (0.286 Earths)[3]
Surface area	41910000 km2 (0.082 Earths)
Volume	2.53×1010 km3 (0.023 Earths)
Mass	(8.931938±0.000018)×1022 kg (0.015 Earths)[3]
Mean density	3.528±0.006 g/cm3[3]
Surface gravity	1.796 m/s2 (0.183 g)
Moment of inertia factor	0.3755±0.0045[4] (estimate)
Escape velocity	2.558 km/s
Rotation period	synchronous
Equatorial rotation velocity	271 km/h
Albedo	0.63±0.02[3]
Surface temp. min mean max Surface 90 K 110 K 130 K[5]
Apparent magnitude	5.02 (opposition)[6]
Atmosphere
Surface pressure	5 to 40 nbar
Composition by volume	90% sulfur dioxide

Io has about 400 active volcanos. It is the most volcanically active body in the Solar System. This is due to tidal heating because of friction between Jupiter and the other moons, Callisto, Ganymede, and Europa. ^[7]Volcanoes erupt massive volumes of silicate lava, sulphur and sulphur dioxide. This constantly changes Io's appearance.

The picture of Jupiter's moon Io was produced by combining the best images from both the Voyager 1 and Galileo Missions. Superbly detailed Voyager 1 images cover longitudes from 240 W to 40 W and the nearby southern latitudes.

Io has highest density of any moon, the strongest surface gravity of any moon, and the lowest amount of water of any known astronomical object in the Solar System.

Due to the same tidal locking that makes the Moon have the same side always facing Earth, Io always has the same side facing Jupiter. Io is a prolate spheroid, pulled out of round by tidal stress from Jupiter’s gravity.