Galilean Moons Fun-facts

The four largest satellites of Jupiter, Io, Europa, Ganymede, and Callisto are known as the Galilean Moons. They were discovered in January 1610 by Galileo Galilei, and they were the first objects found to orbit another object. But, apart from this, is there anything else that makes the Galilean moons extraordinary? The answer to this is yes. So let’s see what may classify as Galilean moons fun-facts, by checking them one by one.

A draft of the letter that Galileo sent to Leonardo Donato, Doge of Venice, reporting the discovery of the four moons around Jupiter. Courtesy of University of Michigan Special Collections Library


Galilean moons in numbers

Io in numbers

Io is the third largest amongst the Galilean satellites. It has a radius of 1,821 km, which makes Io the fourth largest satellite in the Solar system. It is named after Io, a priestess of Hera, who became a lover of Zeus. It has a mass of 8.93×1022 kg and a mean density of 3.528 g/cm3. The surface gravity on Io is 0.183 g(~ 1.8 m/s2).

Io is located at a mean distance of 421,700 km from Jupiter, and it needs 1.8 days to orbit Jupiter. Just like our Moon, its rotational period is synchronous to its orbital period, thus from Jupiter, one can see only one side of Io.

The mean temperature of Io is -163°C (-263°F), but the temperature ranges between -183°C (-297.5°F) and -143°C (-225.6 °F).

True-color image of Io taken from Galileo probe. The dark spot near center (left) is the erupting volcano Prometheus. The light colored plains are due to volcanically deposited sulfur dioxide frost, while the yellower regions contain a higher proportion of sulfur. Courtesy of NASA.

Europa in numbers

Europa is the smallest of the Galilean satellites. The satellite is named after Europa, the Phoenician mother of King Minos and lover of Zeus. It has a radius of 1,580 km (slightly smaller than the Moon), making it the sixth-largest satellite in the Solar system. It has a mass of 4.8×1022 kg and a mean density of 3.014 g/cm3. The surface gravity on Europa is 0.134g (~ 1.3 m/s2).

Europa is located at a mean distance of 671,100 km from Jupiter, and the satellite needs 3,5 days to orbit Jupiter. Just like Io, it has a synchronous rotational period.

The mean temperature of Europa is -171°C (-276°F), but the temperature ranges between -223°C (-370°F) and -148°C (-235°F).

Image of Europa, taken from the Galileo probe, in approximately true color. On the lower right side of the image a prominent crater (Pwyll) can be clearly seen. The darker regions around Europa’s surface are areas where the ice has higher mineral abudance. Courtesy of NASA.

Ganymede in numbers

Ganymede is the largest amongst the Galilean satellites and also the largest in the Solar system. It is named after Ganymede, a young, handsome man from the Greek mythology, that Zeus kidnapped to make him the cupbearer of the Gods. It has a radius of 2,634 km and a mass of 1.48×1023 kg. The mean density of Ganymede is 1.942 g/cm3, while the surface gravity on the largest satellite of Zeus is 0.146g (~ 1.43 m/s2).

The mean distance between Ganymede and Jupiter is 1,070,400 km, and it needs 7.15 days to orbit Jupiter. Ganymede has also a synchronous orbital period.

The mean temperature of Ganymede is -163°C (-262°F), but the temperature varies between -203°C (-333°F) and -121°C (-186°F).

Ganymede as seen by the Galileo probe. The regions that have a lighter color, regions of recent impacts, and the whitish polar cap at upper right, are regions rich in water ice. Courtesy of NASA.

Callisto in numbers

Callisto is the second-largest satellite of Jupiter and the third-largest in the Solar system. It is named after the nymph Callisto, who was associated with the goddess of hunting Artemis, and she was also a lover of Zeus. The satellite has a radius of 2,410 km (almost equal to the size of Mercury), and it has a mass of 1.08×1023 kg. The mean density of Callisto is 1.834 g/cm3, while the surface gravity of Callisto is 0.126g (~ 1.26 m/s2).

The satellite orbits Jupiter at a mean distance of 1,882,700 km, and it needs 16.7 days to complete an orbit around Jupiter, and it has a synchronous orbital period.

The mean temperature in Callisto is -139°C (-218°F), but the temperature may vary between -193°C (-316°F) and -108°C (-163°F).

Callisto image obtained from the Galileo probe. The most obvious feature is the heavily cratered surface. Courtsey of NASA.


Structure of the Galilean moons


Unlike the Moon’s heavily cratered surface, Io practically lacks craters. The surface of Io is dominated by lava flows, smooth plains, and tall mountains. Such features point towards a young geological surface, where craters are buried by volcanic material. The latter is also responsible for the color of Io, which resembles a pizza.

Io has over 400 active volcanoes which makes it the most geologically active object in the Solar system. The extreme geological activity is the result of tidal heating from friction that is generated within Io’s interior as it is pulled between Jupiter and the other satellites.

Io’s composition is mostly silicate rock and iron. This makes Io, the satellite that is the closest to the terrestrial planets than any other satellite. These volcanoes produce plumes of sulfur and sulfur dioxide that can reach as high as 500 km. Additionally, Io has more than 100 mountains that have formed by compression at the base of Io’s crust.

Based on our current knowledge, we believe that Io has a silicate rich crust and mantle and a core that contains roughly 20% of its mass. If we consider the case of an iron-rich core, the core has a radius of 350 – 650 km. But if we consider an iron-sulfide-rich core, then the core has a radius between 550 – 900 km. The mantle of Io is at least 75% of magnesium-rich material, and it has a similar composition with meteorites.


Starting from its center, observations made by the Galileo spacecraft suggest that Europa has an iron-nickel core and a rocky mantle. But our interest on Europa it’s neither on its core, or mantle, but between the latter and its surface.

Europa’s surface lacks mountains and craters. It is also the object with the smoothest surface in the Solar system. There are just a few craters on its surface, which is dominated by cracks and streaks. Based on its appearance, it is commonly accepted that a water ocean exists beneath the surface of Europa, which could hold extraterrestrial life. Observations with the Hubble Space Telescope, have shown the presence of water vapor plumes, which are thought to be caused by erupting cryo-geysers.

Composite image of Europa displaying the suspected plumes of water vapor. The eruption takes place at 7 o’clock position. The water is believed to come from the proposed ocean below Europa’s surface. The image of the plumes is from Hubble’s Imaging Spectrograph. The image of Europa, superimposed on the Hubble image, comes from data from the Galileo and Voyager missions. Courtesy of NASA & ESA.


Just like Earth, Ganymede is fully differentiated, consisting of a liquid iron-nickel-sulfur rich core, a silicate mantle, and an outer layer of liquid water and water ice. The size of each layer is not accurately known, and they depend on the composition of the mantle and the abundance of sulfur in the core.

The presence of a liquid metal-core, and the convection within it, can explain the presence of the magnetic field on Ganymede. The core can extend out to 500 km and based on theoretical assumptions, the temperature in the core of Ganymede is around 1225°C – 1425°C (2240°F – 2600°F).

Even at the early stages of space exploration, scientists were suggesting that Ganymede has a thick layer of water between its icy surface and another layer of ice atop the mantle. We confirmed this assumption in the 1990s when Galileo flew by Ganymede. A couple of years ago, by considering realistic thermodynamics for water and salt effect, scientists suggested that Ganymede may have a stack of several ocean layers separated by different phases of ice, with the lowest liquid layer being adjacent to the mantle. Such water layer structuring, together with the right temperature, makes Ganymede also a good candidate to detect extraterrestrial life.


Callisto has one of the oldest surfaces amongst all objects in the Solar system. As an effect of this, it is also the most heavily cratered. The surface layer lies upon an icy structure that has a thickness between 80 and 150 km thick. Below the surface, it is assumed that a salty ocean with a depth of 150-200 km is located. Support for the ocean existence comes from the fact that Callisto reacts to Jupiter’s magnetic field. The presence of water in liquid phase is more likely if an antifreeze exists such as ammonia. Thus, if such conditions are satisfied, the ice/water layer can be 250-300 km thick. If there is no water in liquid form, then the ice layer extends for around 300 km.

Observations from the Galileo probe suggest that Callisto is not a fully differentiated object. Thus, Callisto’s interior structure is characterized by a density increase with depth due to the presence of rocky material. If Callisto has a core, the latter is small (around 600 km), and with a density that is not consistent with an iron-rich core (3.1 and 3.6 g/cm3). If this scenario is true, then Callisto is different from the nearby Ganymede, which appears to be fully differentiated.


Galilean moons interesting facts

  • Galileo named the Moons after the Medici family. The four satellites are also known as Jupiter I, II, III, and IV.
  • The four Galilean moons contain 99.99% of the mass of the 79 satellites that orbit Jupiter.
  • Europa, Ganymede, and Callisto, may host life.
  • Io is the most volcanically active world in the Solar system, with more than 400 active volcanoes.
A sequence of images of Io taken from the New Horizons probe, showing Io’s volcano Tvashtar spewing material 330 km above its surface. Courtsey of NASA.
  • Io has 100 to 150 mountains. Their average height is around 6km, but the South Boösaule Montes has a height of 17.5 ± 1.5 km.
  • Io has an extremely thin atmosphere. It consists mainly of sulfur dioxide (SO2), with traces of sulfur monoxide (SO), sodium chloride (NaCl, salt), and atomic sulfur and oxygen. As a fun fact of a fun fact, sulfur dioxide is used to preserve dried food.
  • If the current scientific models are correct, then Europa has twice the amount of water the Earth does.
  • The volcanic ejecta of Io create a large plasma torus around Jupiter.
  • Europa’s thin atmosphere is composed primarily of oxygen.
  • Ganymede is larger than Mercury in size, but due to its low density has half the mass.
  • If Ganymede was in orbit around the Sun, it would classify as a planet.
  • Ganymede is the only moon in the Solar system which has a magnetic field.
  • If our assumptions on Ganymede are correct, then the largest satellite of the Solar system hosts the largest ocean.
  • Ganymede’s atmosphere is made out of oxygen in molecular form and perhaps also ozone.
  • Callisto has more craters than any other object in the Solar system.
  • Callisto is most likely the largest object in the Solar system that is not fully differentiated.
  • The Valhalla crater in Callisto is 360 km and has concentric rings around it that extend out to 4,000 km.
Image of the Valhalla crater on Callisto, taken from Voyage 1. Courtesy of NASA.
  • Callisto has a very thin atmosphere composed of carbon dioxide and oxygen.


2 thoughts on “Galilean Moons Fun-facts”

  1. Wow! Thank you so much for sharing this information with us. I hadn’t heard of the Galilean moons before reading this article, but they all seem to have a really rich history (and are all easy on the eyes). I love learning about outer space, and this just shows how vast our solar system truly is. I have saved your site and will share it with my friends and family. God bless you!

    1. Anestis Tziamtzis

      Dear C.N.

      I would like to thank you for time, and your comments. The Galilean moons are really our last hope on detecting life outside our planet and within our Solar system. I really wonder how people will react if we find life forms within the ocean of Europa or Ganymede.



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