The moons of Jupiter: an introduction

The planet Jupiter is known to have at least 69 moons. Of these, eight are reckoned to be “natural” satellites in that they were formed from material left in orbit after the formation of the host planet, but the rest (the “irregular” moons) have been captured by Jupiter’s gravity during the billions of years since that happened.

The eight natural satellites comprise two groups. There are four small inner moons – Metis, Adrastea, Amalthea and Thebe – and four outer moons that are each the size of planets in the inner solar system. These moons are Io, Europa, Ganymede and Callisto. These are also known as the “Galilean” moons because they were discovered by Galileo in 1610.

(The Galilean moons, to the same scale. A composite image created by Stephen Paul Meszaros on behalf of NASA)

Most of the irregular moons belong to one of four groups, each of which was originally a single minor planet that was torn apart when it got too close to Jupiter. Each group contains a single moderate-sized satellite, these being Himalia, Carme, Ananke and Pasiphae.

The names of the moons derive from Greek mythology. Originally the names were those of women (including nymphs) who were seduced by Zeus (Jupiter in Roman mythology). However, even Zeus’s amorous activities were insufficient for the naming on this basis of all the moons that have been discovered in more recent years, mainly thanks to the Voyager space probes. Since 2004 newly discovered moons have been given the names of offspring of Zeus and his conquests.

Most people know about Saturn’s ring system, but may not be aware that Jupiter also has rings. These are not visible from Earth but have been detected by probes flying past the planet. The rings were probably created by meteor impacts on the inner planets that ejected dust into Jupiter’s orbit.