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Jupiter's Galilean Moons

by Ron Kurtus (updated 18 January 2022)

The four largest moons orbiting the planet Jupiter are called the Galilean moons or satellites, named after Italian astronomer Galileo Galilei who first observed them in 1610.

(German astronomer Simon Marius also claimed to have seen the moons around the same time but did not publish his observations, so Galileo is given the credit for their discovery.)

The names of the moons are Io, Europa, Ganymede, and Callisto. They are all bigger than our Moon and are also further from the surface of Jupiter and our Moon is from Earth.

Questions you may have include:

  • How do the moons compare with Earth's Moon?
  • What are details about each Galilean moon?
  • What are interactions between the moons?

This lesson will answer those questions.

Comparison with Earth's Moon

The following chart compares Io, Europa, Ganymede, and Callisto with the Earth's Moon:

  Distance from planet Radius Mass
Earth's Moon 384,400 km 1737 km 7.342*1022 kg
Io 422,000 km 1815 km 8.94*1022 kg
Europa 671,000 km 1569 km 4.80*1022 kg
Ganymede 1,070,000 km 2631 km 1.48*1023 kg
Callisto 1,883,000 km 2400 km 1.08*1023 kg

Details about moons

There are important facts about the Galilean moons.


Io is volcanically active and has a surface covered by sulfur in different colorful forms. Io's volcanoes are driven by hot silicate magma.

Its interior has a layered structure similar to Earth. Io has a core, and a mantle of at least partially molten rock, topped by a crust of solid rock coated with sulfur compounds.

Io goes around Jupiter in a slightly elliptical orbit. Jupiter's gravity causes Io's surface to rise 100 m (300 feet) high. This generates enough heat for volcanic activity and to drive off any water.


Astronomers believe that Europa has twice as much water as the Earth. Its surface is mostly water ice. Astrobiologists also feel that Europa has the potential for sustaining life forms.

Europa has a core, a rock envelope around the core, a thick, soft ice layer, and a thin crust of impure water ice. Global subsurface water layer probably lies just below the icy crust.

The heat needed to melt the ice in a place so far from the sun is thought to come from inside Europa, resulting primarily from the same type of tidal forces that drive Io's volcanoes.


Ganymede is the largest moon in the solar system. It is larger than the planet Mercury. It also is the only moon known to have its own internally generated magnetic field.

Ganymede also has a core, rock envelope around the core, thick soft ice layer, and a thin crust of impure water ice.


Callisto's surface is heavily cratered, providing a visible record of events from the early history of the solar system. However, the very few small craters on Callisto indicate a small degree of current surface activity. Layering at Callisto is not well defined and appears to be mainly a mixture of ice and rock.


Three of the Galilean moons influence each other.

Io affects the orbital periods Ganymede and Europa. This results in Europa's orbital period being twice Io's period and Ganymede's period being twice that of Europa. In other words, every time Ganymede goes around Jupiter once, Europa makes two orbits and Io makes four orbits.

The moons all keep the same face towards Jupiter as they orbit, meaning that each moon turns once on its axis for every orbit around Jupiter.


The Galilean moons or satellites are the four largest moons orbiting the planet Jupiter. They are called that after Italian astronomer Galileo Galilei who first observed them.

The names of the moons are Io, Europa, Ganymede, and Callisto. They are all bigger than our Moon and are also further from the surface of Jupiter and our Moon is from Earth.

Three of the four moons influence the motion of each other.

Live and interesting life

Resources and references

Ron Kurtus' Credentials


Jupiter: Moons - NASA

Moons of Jupiter - Wikipedia

Astronomy Resources


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