The Galilean Moons of Jupiter

Curio Vol 2 | Issue 7

Left to right: Io, Europa, Ganymede and Callisto

Jupiter, the largest planet in the Milky Way, is 1,321 times larger than Earth. What you might not know is that its large size is not outdone by its incredible gravitational pull. Jupiter’s gravity has in-fact ‘trapped’ space masses that have been hurtling through the universe, and turned these meteors into moons that orbit Jupiter.

There are 67 moons that orbit the galaxy’s largest planet, (63 of them born out of the pull described above) with only four being the ‘natural’ satellites of the giant gas planet.

These four natural satellites are known as the Galilean Moons.


As the name might suggest, the Galilean moons were discovered by the Italian astronomer, physicist, philosopher and mathematician, Galileo Galilei. It was in January 1610 that Galileo had made improvements to his telescope, thus allowing him to hone his observational astronomy in even greater detail.

The four moons were the first objects found to orbit another planet other than Earth. This discovery not only contradicted the view of Ptolemy (that everything orbited our planet) but also demonstrated that some objects in space could not be seen by the naked eye alone.

The four moons which were originally known as Jupiter I, II, III and IV, were renamed as Io, Europe, Ganymede and Callisto respectively. All of these names derive from the lovers of the Greek God Zeus.


Io, slightly larger than the Earth’s moon, is the closest of the Galilean moons to Jupiter. It is also the most geologically active object, and driest known object in the solar system. It completes one orbit of Jupiter in just 42.5 hours.

Io has over 400 active volcanoes, a result of tidal heating that is generated from interior friction of its core. This happens because gravity from Jupiter and gravitational fields from the other three Galilean moons creates a volatile, radioactive environment.

Io produces plumes of sulfur dioxide that rise as high as 500km from their volcanic eruptions; its lava flows painting the surface in shifting tones of yellow, red, green, white and black. Io’s unusual thermodynamics is also responsible for doubling the power of Jupiter’s magnetic field.


Europa is the smallest of the Galilean satellites and is primarily made of silicate rock with a water-ice crust. From a distance, it is scratched with cracks and streaks (called lineae), and has the smoothest known surface of any known object in the Solar system, with barely a crater or mountain peppering its crust. Europa completes one orbit of Jupiter in about 84 hours.

Uniquely, Europa has an atmosphere that is mostly oxygen, and scientists have hypothesised that a water ocean exists beneath the thick ice shell of the planet. Whilst this ice layer is likely to be a few kilometres thick, the presence of a liquid ocean beneath would suggest that this moon could conceivably be a source of habitable life.


Ganymede is the largest of the moons and is bigger than the planet Mercury. It is the biggest moon in the entire solar system, and is composed of silicate rock and water-ice, similar in make up to its smaller sister, Europa. Ganymede completes one orbit of Jupiter in one week and three hours.

A saltwater ocean lies beneath the surface of Ganymede (much like Europa), although, it has wedged between Hexagonal Ice and Tetragonal Ice. Like Europa, Ganymede’s ocean has posed questions of its potential habitability. Ganymede is also the only moon in the Solar System to have a magnetosphere, likely created by its liquid iron core and its convection currents.


Callisto is the last and outermost of the Galilean moons, and completes one orbit of Jupiter in 2 weeks and 3 days. Callisto has the oldest and most heavily cratered surface in the solar system; seen from afar, it is almost looks like a bubble where a galaxy of stars resides within.

Callisto’s ancient surface is only made up of craters, light plains, dark and smooth plains and multi-ring structures. The density of these craters are so saturated that any new craters made by impact erase adjacent ones. Multi-ring basins such as Valhalla, a giant impact structure 3,800km in diameter are mysterious concentric rings reminiscent to the cross-section of a tree.

These ring structures were possibly created by very oblique impacts but also a giant impact that punctured the crust and entered an underlying layer of warm ice below. These multi-ring structures are thought to be between 2–4 billion years old.


The quartet of the Galilean moons that orbit Jupiter are intriguing in their apparent physical differences. Io is a bright, volcanic and sulfurous mass. Europa is a cold, silicate sphere with an icy crust. Ganymede emits its magnetosphere via is tumbling liquid iron core; and Callisto sits with its pocked marks, the result of millions and millions of crater impacts.

What the moons share in common is the planetary mass they orbit around. Moving at different speeds, at different times, covering different distances as they dance around Jupiter; the Galilean moons circle the gas giant, orbiting at their own pace and gravity.

Individuals in their own right — a family in others.

Curio is released weekly on Mondays. Words and Illustrations by Rob Lee.