A Jovian moon and one of Galilean Moons

Quick Facts

  • Diameter: ~ 3,640 KM

  • Adjective used: Ionian

  • Pronunciation: EYE Oh

  • Alternate names: Jupiter I

  • Ionian Day:

  • Ionian Year:

  • Type: Volcanically-active natural satellite

  • Distance from Jupiter: ~ 422,000 KM

Io, also known as Jupiter I, is the innermost Galilean moon of the planet Jupiter and it’s the most volcanically active cosmic body in the solar system. It is fourth-largest natural satellite in the solar system and has the highest density out of all them.

Io is the fifth moon in line from planet Jupiter. It has average orbital distance of 422,000 KM from Jupiter. It takes about 1.7 Earth-days to orbit Jupiter. Io is tidally locked, so the same side always faces Jupiter.

Io was discovered on 8 January 1610 by astronomer Galileo Galilei.

Io is slightly larger than Earth's Moon. It has a mean diameter of 3,640 KM, in comparison to 3,470 KM diameter of Moon. It is somewhat ellipsoid in shape. Among the Galilean satellites, in both mass and volume, Io is the third largest, ahead only of Europa.

Io was the first natural satellite to be discovered after Earth’s Moon. Astronomer Galileo Galilei discovered Io on January 8, 1610, along with the discovery of three other moons of Jupiter – Europa, Callisto, Ganymede, were the first moons to be discovered that were orbiting a cosmic body other than Earth.

Named after

Io is named after the Greek mythological character Io who was one of mortal lovers of Zeus, the Greek god of sky and counterpart of Roman Jupiter. Astronomer Simon Marius named this natural satellite of Jupiter as Io in 1614.

With more than 400+ active volcanoes, Io is also the most geologically active object in the solar system. This is because of tidal heating which results from friction generated within Io’s interior as it is being pulled by Jupiter and remaining Galilean moons in opposite directions.

Io’s surface also have numerous numbers of mountains which are uplifted due to extreme compression of the Io’s crust which is made of silicate. Io is mainly composed of silicate rock, molten iron or iron sulfide core.