Io is a moon of Jupiter. It was discovered in 1610 by Italian astronomer Galileo Galilei and is one of the four Galilean satellites.

Diameter: 3,636 km
Mass: 8.93 x 1022 kg
Composition: Iron and rock
Orbit: 422,000 km from Jupiter

Io is the third-largest moon of Jupiter and the fourth largest in the Solar System. It is slightly larger than Earth’s moon. As the fifth moon from Jupiter, it was discovered at the same time as Ganymede, Europa and Callisto by Galileo. When first observed, Io and Europa were seen as just one entity. On a second viewing, they were able to be distinguished as separate satellites. Since the discovery of the Galilean Satellites, a further 63 moons of Jupiter have been identified.

The four Galilean moons were originally named the Medicean planets by Galileo, after the Medici family. Only when more satellites were discovered were unique names attributed to them. All of Jupiter’s moons are named after mythical characters. In keeping with this, Io is named after the daughter of Inachus, who went on to be one of Zeus’s many lovers.

Io consists of an iron core surrounded by silicate rock. The surface is covered in sulphur. The moon has over 400 active volcanoes. It has a colourful surface characterised by volcanic activity. Below the shell, heat caused by tidal pressure means that liquid lies below the crust. Tidal activity on Io causes evident peaks and craters in its surface, some of which are over 100 metres both in height and depth. The volcanoes on Io are active and the regular escape of liquid rock means that the surface of Io is constantly renewed. This volcanic activity is responsible for the changing patterns and colours on the surface. Volcanic plumes from the erupting volcanoes are evident up to 500 km above the surface. Unlike the colder satellites of Jupiter, Io has no water.

Io completes an orbit in 42.5 hours. It orbits Jupiter between Thebe and Europa, with one side always facing the planet. The side that faces Jupiter is known as the subjovian hemisphere, with the other side being referred to as the antijovian hemisphere.

In 1972 and 1973, Pioneer 10 and 11 were the first spacecraft to fly by Io. Data and photographs collected at the time provided good estimates of the size and mass of Io and also determined that its composition was mostly rock rather than water. Later in 1976, two separate Voyager missions found Io to be volcanically active and took pictures which clearly showed a multi-coloured surface. More recently, during the 1990s and 2000s, the Galileo spacecraft flew near Io and collected data about the composition of the core and surface. It was during this mission that it was determined that the core was composed of iron. Io has since been closely observed from Earth through telescopes and also from the Earth’s orbit by the Hubble telescope.

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