Io (Moon of Jupiter)

Io Facts

Io is one of Jupiter’s moons and is the fifth moon in distance from Jupiter. Here we take a look at some amazing facts about Io.

Io moon

Who Discovered Io?

Io was discovered in 1610 by Galileo Galilei along with Europa, Ganymede and Callisto.  Galileo first called the moon Jupiter I. In the mid-1800s, the moon was renamed Io.

The name Io has a bit of a tasteless story, as Io is named for the daughter of Inachus, whom Zeus raped. He turned Io into a cow to hide the incident from his wife.

Io Overview

  • The diameter of Io is an amazing 2,263 miles (3,642km).
  • It is 261,597 miles (421,000km) from Jupiter.
  • It completes its orbit of Jupiter in 1.8 days.
  • The average temperature on Io is -327°F (-164°C).

How old is Io?

  • Io is about 4.5 billion years old, which is around the same age as Jupiter.
  • Io is similar the size of the moon and the third largest of Jupiter’s moons.

Volcanoes on Io

  • It is an extremely active volcanic moon and is the most active in the solar system. It has more than 400 active volcanoes on the surface. That’s quite something. They are constantly erupting.
  • This volcanic activity is due to tidal heating. This is because Io is stretched and squeezed by Jupiter’s strong gravitational pull as well as by the gravitational pull of other satellites, which aren’t as powerful as Jupiter itself. Io’s rock surface bulges up and down by as much as 100m during this process. This impacts Io’s volcanic activity in a similar way to which Earth’s oceans react to the moon. This is what creates the tidal activity we mentioned.


  • The volcanic plumes rise about 190 miles (300km) above the surface of the moon. You can see that these certainly are large volcanoes. This showers the surface with sulphur, sulphur dioxide particles and rocky ash.

Magnetic Fields on Io

  • Io has an iron core, just like on Earth. This means that it could have its very own magnetic field.
  • When Io orbits across Jupiter’s powerful magnetic field, it creates 400,000 volts of electricity across its surface. That is a lot of electricity.
  • Now this is quite something. Jupiter’s magnetic field literally strips off 1 ton of material from Io every single second!

Io Just Like Pizza

  • Io is often compared to a pizza. That’s pretty strange, but this is because of its odd surface which is made up of sulphur dioxide, which comes from the volcanoes. It has a very thin atmosphere which is also made up of sulphur dioxide.
  • Io has quite a few mountains, some of which are taller than Mount Everest. The average heights of Io’s peaks are about 4 miles (6km).

Io mountain peaks

  • Io is mostly made up of silicate rocks, and its surface is covered in sulphur particles from those volcanoes it’s so famous for, as well as frost that is created as the atmosphere gases freeze and fall to the ground. It is also covered with lava lakes and floodplains of liquid rock.
  • The surface of Io is constantly changing colours from shades of red, yellow, white, black and green!

Photos of Io

  • Several spacecraft have flown by Jupiter and its moons. Pioneer 10 arrived first, in 1973, followed by Pioneer 11 in 1974. Voyager 1 and Voyager 2 returned with amazing photos during their flybys. The Galileo spacecraft passed as low as 162 miles (261km) over the surfaces of the Galilean moons with some very detailed images.


So there are some amazing facts about Io. Do you know any other facts about this moon of Jupiter?

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