jupiter badge: name and stats
Chart of relative sizes of Galilean satellites.
Io Europa Ganymede Callisto
Io specs - Radius: 1,821.3 kilometers; Mass: 8.9319x10^22 kilograms; Gravity: 1.796 m/s^2
Europa specs - Radius: 1,569 kilometers; Mass: 4.8x10^22 kilograms; Gravity: 1.315 m/s^2
Ganymede specs - Radius: 2,634 kilometers; Mass: 1.4819x10^23 kilograms; Gravity: 1.428 m/s^2
Callisto specs - Radius: 2,410.3 kilometers; Mass: 1.0757x10^23 kilograms; Gravity: 1.236 m/s^2
Inner core
Metallic hydrogen
Great Red Spot
Comet Shoemaker-Levy 9 impact sites

The planet Jupiter and its satellites form a miniature version of a planetary system. Currently Jupiter has 63 satellites and new ones might still be discovered, however only 16 are considerably large. Probably the bigger satellites were formed together with Jupiter, while the smaller ones were later captured by its strong gravitational field. Voyager 1 first registered a faint ring system around the planet. The rings can be observed by telescope with infrared light. However, the most remarkable of all satellites are the four biggest moons, discovered by Italian astronomer Galileo Galilei in 1610.

Io, the innermost Galilean satellite is an active fiery world. Io's diameter is a bit smaller than our Moon's. Io is rocky with a large iron core. It is the most volcanically active and colorful place in the Solar System. Scientists have seen Io's volcanic eruptions spewing material up to heights of 190 miles. Most of that material falls back to the surface changing the surface color. It also makes rings around the active volcanoes that can be white, yellow or red, due to sulfur dioxide.
Europa is an intriguing icy world. It surface resembles a frozen lake containing numerous cracks and ridges. Some of them are hundreds and even thousands kilometers long. While the bluish formations on the surface indicate old icy plains, the reddish regions along the ridges are young and contain non-ice components. Scientists think that the reason for this strange appearance is tidal forces, which stretch the surface producing cracks. Such tidal forces could cause material from the interior to be squished out forming ridges. The tidal heating also is the main reason for the existence of the subsurface ocean under the icy cap. This ocean makes Europa one of the most viable places for life in the Solar System.
Ganymede, the third Galilean satellite, is completely different. Most of its dark surface is covered with craters, which are flatter than those on most rocky planets and moons. The reason is that over time ice on Ganymede flows like a glacier. Beside craters, there are also smooth regions with ridges and grooves, that are probably the result of tectonic activity. These areas are younger than the cratered regions, but older than the formations on Europa. Ganymede might also has a subsurface ocean, however, if it does it lies deep under the surface.
Callisto, the most distant of Galilean moons, is almost the same size as Mercury. Although it is comparable in structure to Ganymede, it has a completely different appearance. Its crust is a mixture of ice and rocks. The old dark surface is covered with impact craters and a thick dusty layer. The craters differ in age and size. The larger craters are surrounded with multiple rings. There are no signs of tectonic activity or of a subsurface ocean.
The strong gravitational force of Jupiter combined with the tidal resonance between Io, Europa and Callisto causes geological activity. As a result the moons are alternatively stretched and squeezed. Such stretching and squeezing generates so much heat that the interior melts. This drives Io's volcanoes and subsurface oceans on Europa and Ganymede.
Tidal resonance of three Jovian Moons

Io, Europa and Ganymede are in tidal resonance. That means that every time Ganymede completes one rotation around the Jupiter, Europa makes two and Io makes four.

europa icon

All the ice on Europa contains more water than all of Earth's oceans put together. If this ice ever melts (when the Sun became a red giant for example) water will cover the entire surface in one giant ocean.

magnetic field of Ganymede

Ganymede is the largest satellite in the Solar System. It's even bigger than the planet Mercury. It is also the first moon discovered to have its own magnetic field.

Tidal resonance of Callisto

Scientists expect that in the next 600 years Callisto will became tidally locked with the rest of the Galilean satellites. Then one rotation around the Jupiter will be equal to the eight of Io's rotations.


Galileo Galilei

Italian astronomer, mathematician and physicist best known for his defense of heliocentrism and for the discovery of Jupiter's four largest moons.

Illustration: Gabriel Alvarado-Marin (SETI Institute)



A straight line passing through the center of a figure, usually a circle, and whose endpoints lie on the periphery of the figure.

tidal forces

The flexing and pulling that occurs when the gravitational pull of a planet tugs on one of its satellites, causing the satellite's crust to flex up and down.


Tectonic activity

Tidal resonance

Eruption on Io
Eruption on Io
Mission New Horizons
Description Eruption from the Tvashtar region of Jupiter's moon, Io, as captured by New Horizons in 2007. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute