|Discovered by||Galileo Galilei|
|Discovery date||January 7, 1610|
|Alternative names||Jupiter I|
|Periapsis||420,000 km (0.002 807 AU)|
|Apoapsis||423,400 km (0.002 830 AU)|
|Mean orbit radius||421,700 km (0.002 819 AU)|
|Orbital period||1.769 137 786 d (152 853.504 7 s, 42 h)|
|Average orbital speed||17.334 km/s|
|Inclination||2.21° (to the ecliptic)
0.05° (to Jupiter's equator)
|Dimensions||3,660.0 × 3,637.4 × 3,630.6 km|
|Mean radius||1,821.3 km (0.286 Earths)|
|Surface area||41,910,000 km2 (0.082 Earths)|
|Volume||2.53×1010 km3 (0.023 Earths)|
|Mass||8.9319×1022 kg (0.015 Earths)|
|Mean density||3.528 g/cm3|
|Equatorial surface gravity||1.796 m/s2 (0.183 g)|
|Escape velocity||2.558 km/s|
|Equatorial rotation velocity||271 km/h|
Io is the most volcanically active body in the Solar System. Volcanoes erupt massive volumes of silicate lava, sulphur and sulphur dioxide, constantly changing Io's appearance. This new basemap of Jupiter's moon Io was produced by combining the best images from both the Voyager 1 and Galileo Missions. Although the subjovian hemisphere of Io was poorly seen by Galileo, superbly detailed Voyager 1 images cover longitudes from 240 W to 40 W and the nearby southern latitudes.
In the same way that the Moon always has the same side facing Earth, Io always has the same side facing Jupiter. The movie shows two speeded-up rotations of Io (a single rotation really takes 1.77 days), and begins with a view of the Jupiter-facing hemisphere. With rotation in an easterly direction, after two seconds the volcano Prometheus (on the equator) comes into view. The massive red deposit around Pele (seconds 5-10) is the most distinctive expression of volcanic activity on Io, and just to the north-west is the horse shoe-shaped Loki Patera, the most powerful volcano on Io.
- Thomas, P. C.; et al. (1998). "The Shape of Io from Galileo Limb Measurements". Icarus 135 (1): 175–180. doi:10.1006/icar.1998.5987.
- Yeomans, Donald K. (July 13, 2006). "Planetary Satellite Physical Parameters". JPL Solar System Dynamics. http://ssd.jpl.nasa.gov/?sat_phys_par. Retrieved 2007-11-05.
- "Classic Satellites of the Solar System". Observatorio ARVAL. http://www.oarval.org/ClasSaten.htm. Retrieved 2007-09-28.