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Io Facts

Jupiter’s moon Io is the most active volcanic natural satellite in our Solar System. The lava which erupts from its active volcanoes can travel up to 10 miles high (or kilometres). The moon is named after Io in Greek mythology, a woman who turned into a cow during a dispute between Zeus and Hera.

When was Io discovered?

Io was discovered with three other moons (Europa, Ganymede and Callisto) in 1610 by Galileo Galilei. The discovery of the Jovian moons led to the understanding that the planets in our Solar System orbit the Sun, rather than the Universe orbiting the Earth.

When Galileo first discovered the moons of Jupiter he called them the Medicean planets, after a powerful Italian Medici family. He also referred to the moons numerically as I, II, III, and IV. This system was used until the mid-1800s when the satellites were given their mythological names. It was decided this would be best as the numerical system would become very confusing as new moons were being discovered.

How did Io get its name?

The innermost moon of Jupiter got its name from a mortal woman in Greek mythology who got turned into a cow when Zeus and Hera, King and Queen of the gods, had a marital spat.

Before it was named Io, Galileo named the moon Jupiter I.

Orbit Facts

  • Io is Jupiter’s innermost of Galileon moons and is the fifth natural satellite out from Jupiter.
  • Io orbits 421,700 km from Jupiter’s centre.
  • Io orbits Jupiter in 42.5 hours.
  • Io orbits Jupiter twice for every one orbit of Europa and four orbits to every one orbit Ganymede completes.
  • Both Europa and Ganymede exert periodic gravitational influence on Io, this is known as orbital resonance.
  • The orbital resonance and the tidal forces from Jupiter are the primary heat source of Io’s volcanic activity.
  • Io rotates around Jupiter synchronously, keeping one hemisphere pointing toward Jupiter (leading hemisphere) and the other pointing away (trailing hemisphere).

Related:Which planet has the most moons?

Geological Facts

  • Io has over 400 active volcanoes on the surface of the small moon. This makes the satellite the most actively volcanic object in our Solar System.
  • Io’s volcanoes produce plumes of sulphur and sulphur oxide which can reach 500 km above the surface of the moon.
  • The volcanic activity of Io has shaped a lot of its geological features.
  • The lava flows and volcanic plums have created massive surface changes.
  • Io’s surface is covered in lava flows which can be as long as 500 km (300 miles).
  • Io also has over 100 mountains, and some of them are larger than Mount Everest.
  • Io is made from silicate rock which surrounds an iron-sulfide core.
  • Io’s surface is primarily yellow because it is covered in sulphur and sulphur-dioxide frost. The frost is created due to atmospheric volcanic gases freezing and falling to the surface.

Expeditions to Io Facts

  • In 1973 Pioneer 10, and its sister mission Pioneer 11 (1974), both take close up measurements of Io.
  • Voyager 1 takes data of Io on a flyby of the moon in March 1979.
  • Voyager 2 takes further snapshots of Io in July 1979 at a greater distance upon its flyby.
  • From 1992 to 2002 the Galileo spacecraft takes the closest images and data captures of Io of any space mission.
  • In 20o0 the Cassini spacecraft takes snapshots of Io on its way to Saturn.
  • In 2007 the New Horizon mission took further snapshots of Io as it fly by and its way to to the outer edges of our Solar System.

Related:How long would it take to get to Jupiter?
Related:How many Earths can fit in Jupiter?


Picture of Io

Image of Io (moon)

Io Quick Facts

Mass:89,319,379,731,108.9 billion kg (0.015 x Earth)
Equatorial Diameter:3,643.2 km
Equatorial Circumference:11,445.50 km
Orbit Distance from Jupiter:421,800 km
Orbit Period:1.769 Earth days
Mean Surface Temperature:-163.15°C
First Record:January 8, 1610
Recorded By:Galileo