250px-Io highest resolution true color
Io in true color.
Discovered by Galileo Galiei
Discovery date January 8, 1610
Other names Jupiter I
Pronounced ai'ou
Orbital Characteristics
Periapsis 420,000 km (0.002 807 AU)
Apoapsis 423,400 km (0.002 830 AU)
Semimajor axis Unknown
Orbital Period 1.769 days (152,853 seconds, 42 hours)
Average orbital speed 17.334 km/s (38,775 mph, 62,402 kph)
Eccentricity 0.0041
Satellite of Jupiter
Physical Characteristics
Volume 2.53×1010 km3 (0.023 Earths)
Mass 8.9319×1022 kg (0.015 Earths)
Axial tilt Unknown
Mean density 3.528 g/cm3
Mean radius 421,700 km (0.002 819 AU)
Rotation period synchronous
Albedo 0.63 ± 0.02
Surface temperature (min) 90 K (-297 °F, -183 °C)
Surface temperature (avg) 110 K (-261 °F, -163 °C)
Surface temperature (mean) 110 K (-261 °F, -163 °C)
Surface temperature (max) 130 K (-225 °F, -143 °C)
Apparent magnitude 5.02 (opposition)

Io (or Jupiter I) is a moon of the planet Jupiter. Discovered by famous astronomer Galileo Galilei in 1610, it is among 4 others discovered by him.

Io is the fourth largest moon in the Solar System, and one of the best known. It was named after the mystical charecter of the same name from Greek mythology, a priestess of Hera who fell in love with Zeus.

Io is unique in that it contains over 400 active volcanos on its surface, making it the most volcanically active object in the Solar System. Some of the mountains on its surface reach higher than the peak of Mount Everest. Partly due to the lava flows on Io's surface and the various compounds of sulphur it contains, it is coloured in an odd mixture of yellow, red, green and black. It has either a molten iron or iron sulfide core.

Io has been viewed by numerous spacecraft, including the two Discovery crafts and the Galileo craft.

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