Ganymede Facts

Ganymede is the biggest moon in the Solar System. Ganymede would have been considered as a planet if it was directly revolving around the sun. Even though it’s that big, it is hardly visible from the naked eye as the Jupiter’s brightness hides this moon.

Ganymede Profile

Interesting Ganymede Facts

  • Ganymede is one of the 79 moons of Jupiter.
  • It is the largest known moon in the entire solar system.
  • Ganymede is larger than the planet Mercury.
  • It was discovered by Galileo Galilei in 1610.
  • The moon has a radius of 2631.2 km.
  • It takes roughly 6 to 7 days to complete one orbit around Jupiter.
  • Ganymede was probably formed only of few million years after the planet Jupiter itself.
  • It is believed that it took around 10,000 years for the formation of Ganymede.
  • Ganymede has been named after a figure from Greek mythology. It was the name of Zeus’s cupbearer.
  • The moon is 1.07 million kilometers away from Jupiter.
  • It is the only moon and the entire solar system to have its own magnetosphere.
  • In 1973 Pioneer 10 by NASA became the first spacecraft ever to fly by Ganymede. After this several other spacecrafts flew past the Jovian moon.
  • Ganymede is one of the few celestial bodies where the possibility of finding life is the highest.
  • Ganymede would have been considered as a planet if it was directly revolving around the sun.
  • Ganymede has been mentioned in various works of science fiction such as movies, TV shows, books, etc. It is also one of the most popular settings for video games when it comes to celestial bodies.


Ganymede is one of the 79 moons of the planet Jupiter. It is the largest and the most massive out of all of the planet’s moons. It is the ninth-largest object in the solar system. It lacks a well-defined atmosphere. It is larger than the planet Mercury and only slightly smaller than the planet Mars.

Ganymede can hardly ever be seen from the earth with the naked eye as the light of Jupiter obscures most of its moons. Jupiter is one of the brightest objects in the night sky.

Ganymede was probably formed by the accumulation of particles in the sub-nebula of the planet Jupiter. The sub nebula of Jupiter was a huge disc-like structure surrounding it after its formation.

It is believed that Ganymede took about 10,000 years for its formation which is much shorter than Callisto which is believed to have taken 100,000 years.   


The first person to observe Ganymede through a telescope was Galileo Galilei. In 1610, he observed three star-like objects near Jupiter which turned out to be Ganymede, Callisto, and a combination of light from Io and Europa. A few days later, he saw all the four main moons of Jupiter separately.   

Simon Marius made independent discoveries about the moons of Jupiter at about the same time as Galileo Galilei. He may have discovered them a few days earlier as well. Still, Galileo Galilei is credited for its discovery as he was the first one to publish information about this discovery.

Ganymede was named after a boy who was abducted by Zeus (Jupiter) and acted as a cupbearer for all the gods by succeeding Hebe. The name was suggested by Johannes Kepler. It is the only moon of Jupiter that has been named after a male figure.

Ganymede is sometimes also called Jupiter III. If Ganymede was orbiting the sun directly instead of orbiting the planet Jupiter it could have been considered a planet because of its size and other characteristics.

Ganymede was the first moon discovered revolving around a planet other than the Earth. This gave strong proof for the theory that suggests that the sun was at the center and the planets revolved around it contrary to the ancient belief of the earth being in the center.

Chinese sources say that in some obscure unreliable ancient Chinese astronomical records, an astronomer named “Gan De” may have detected this moon of Jupiter with the naked eye.


Ganymede has a diameter of 5268 km and a radius of 2631.2 km which is 0.41 times than that of the Earth.

Ganymede is larger than the planet Mercury but not as dense. Ganymede only has around half the mass of Mercury.

It is also larger than Pluto and only slightly smaller than Mars. It is nearly 4.5 billion years old and is nearly the age of Jupiter itself.

Ganymede is the most massive and the largest moon in the entire solar system. It is one of Jupiter’s four main moons which are also known as Galilean moons. An object needs to fulfill three criteria to be considered a planet that is directly revolving around the sun, having sufficient mass, and clearing the space around it.

As Ganymede does not fulfill the criteria of directly revolving around the sun, it is not considered as a planet.

Surface and Composition

The surface of Ganymede can have very old and highly created regions as well as relatively young lighter regions full of grooves and ridges.

The grooves have most probably been formed as a result of tectonic activities or due to the release of water from under its surface.

These can be as tall as 2000 feet and even stretch for as far as thousands of kilometers. The dark patches may be made up of clay or organic materials.

Some of the substances present on the surface of the moon Jupiter are carbon dioxide, hydrogen sulphate, sulphur dioxide, magnesium sulphate, sodium sulphate, cyanogen, etc.

Ganymede has a density of 1.936 grams per centimeter cube. The mass fraction of ices of

Ganymede is smaller than that of Callisto, which suggests its rocky nature. It is mainly composed of rocky material and water-ices. Traces of some volatile ices of ammonia may be present as well.

The composition of the rocky parts of Ganymede is not exactly clear but it is believed to be mostly consisting of a large amount of iron oxide and a small amount of metallic iron. Ganymede has a well distinguished internal surface. It has a core that is made up of iron and ironing sulphides.

Then, it has a mantle that is made up of silicates and finally a crust consisting of water in the form of solids and liquids. The crust mostly made up of ice is very thick at about 800 km.

Its daytime temperatures can range from – 182 °C to -112 °C. At night the temperatures can drop as low as -193 °C. Ganymede may have a saltwater ocean below its surface.

Magnetic field

Ganymede is the only natural satellite of any planet that is known to have its own magnetosphere. Ganymede has a permanent magnetic moment which is independent of Jupiter’s magnetic field.

The value of the magnetic field of Ganymede is close to 1.3 × 1013 T·m3. This value of magnetic moment is more than thrice the value of the magnetic moment of the planet Mercury. Ganymede’s magnetic moment is tilted to the opposite direction of the magnetic moment of Jupiter that is against the Jovian magnetic moment at an angle of 176 °.

The way the magnetospheres of Ganymede and Jupiter interact is quite similar to the way that the Earth’s magnetic field and Solar winds do.

Ganymede is also known to have an induced dipole magnetic field in addition to the dipole moment. The induced magnetic field is believed to be directed from Jupiter. Or it could also be directed towards Jupiter, but it is not clear.

Scientists believe that its magnetic properties of Ganymede are generated as a result of the presence of an iron-rich metallic core at its center. This is similar to the way the magnetic field generated on the earth.

Even though Ganymede has a well-differentiated iron core and a known magnetosphere, the magnetosphere of Ganymede remains mysterious to mankind as this magnetosphere has some features that are not found in many such similar celestial bodies.

Orbit & Rotation

Ganymede takes about 7-8 Earth days to revolve around Jupiter. Ganymede orbits The Giant planet at a distance of close to 1,070,400 km.

As it is at a distance of about 1.07 million kilometers from Jupiter, it is the third Galilean satellite after Io and Europa.

Distance from Jupiter

Ganymede is about 1,070,400 km away from Jupiter.


Scientists recently discovered the presence of a thin atmosphere of oxygen above the surface of Ganymede. This has been confirmed by the Hubble Space Telescope.

However, this atmosphere is too thin and weak, and it is doubtful that it could ever harbor life. The surface of Ganymede is highly irregular. Being the largest moon in the solar system as well as a potential world for the sustaining of life, Ganymede is one of the most speculated celestial bodies in the solar system.

Hence, various spacecrafts have visited Ganymede and several more projects are waiting to be approved and confirmed for the future as well. These will help us understand and study the nature of Jupiter’s moon better and in more detail.

Exploration of Ganymede

 The moon Ganymede was discovered by Galileo Galilei on 7th January 1610. It was discovered along with three more moons of Jupiter.

The discovery of Ganymede and the other moons of Jupiter is very significant as it proved that the Earth is not the center of the universe and that all the planets in the solar system do not revolve around it.

Ganymede has been visited by several spacecrafts. The first spacecraft to fly by Ganymede was Pioneer 10 in 1973. The following year that is in 1974 Pioneer 11 also flew by Ganymede. Both of these spacecrafts were launched by NASA. 

The space probes provided a lot of information such as physical characteristics, atmosphere, magnetosphere, etc. in great detail.

The next to space probes flew past Ganymede in 1979. They were called Voyager 1 and Voyager 2. In 1995 the spacecraft called Galileo flew by Ganymede six times. These flybys were called G1, G2, G7, G8, G28, and G29. G2 flew by just over 264 km above the surface of Ganymede while G1 was the spacecraft that discovered the magnetic field of the Jovian moon.

The most recent flyby of Jupiter’s moon Ganymede was done by New Horizons by NASA in 2007.

New Horizon was on its way to Pluto and it also stopped by Europa. Several other space missions have been proposed to be sent to Ganymede.

Some of them are in the process while others have not yet been approved. All the spacecrafts that have flown by Ganymede so far have been launched by NASA.

Potential for Life

No human being has ever visited Ganymede and no evidence of life has ever been found on it. Still Ganymede is one of the celestial bodies where the possibility of finding life is the highest.

As scientists have discovered a huge saltwater ocean under the surface of the Jovian moon it is quite possible that the moon could sustain life.

However, these oceans are much saltier than those on the earth. Also, the moon has a very thin atmosphere that is not fit for human beings.

Hence, if life does exist on Ganymede it will be a different kind of life that is, an alien life, and not the kind of life that we find on the earth.

Ganymede in Pop Culture

Jupiter has a famous ring system, particularly the Galilean moons which include Ganymede, Io, Europa, and Callisto are famous when it comes to being an important setting in the works of science fiction.

It has had significant mention in several books, films etc. Some such books are Seete Ship by Jack Williamson, Farmer in the Sky by Robert A. Heinlein, Jupiter Five by Arthur C. Clarke, The Runaway Robot by Lester Del Rey, The Moons of Jupiter by Alice Munro, The Deceivers by Alfred Bester, Larklight by Philip Reeve and many more.

Some works of films and  television on Galilean moons are Cowboy Bebop, 2010, Starhunter, Red Dwarf, Cloud Atlas, Ready Jet Go!, The Expanse, Power Rangers, Space Battleship Yamato, Geneshaft, Return to Jupiter etc.

Some of the video games that feature Ganymede are Planet of Death, The Lost Episodes of Doom, Jupiter’s Masterdrive, Star Cruiser, Carnage Heart, Shadowgrounds, etc. 


  • 365 BC: First possible observations of Ganymede
  • 1610: Galileo Galilei discovers Ganymede along with 3 other moons of Jupiter
  • 1973-1974: Pioneer 10 and Pioneer 11 flyby Ganymede and provide information
  • 1979: Voyager 1 and Voyager 2 provide more details about the Jovian moons
  • 1995: NASA’s Galileo spacecraft makes six close flybys of the moon
  • 2019: NASA’s Juno flies by Ganymede to provide polar images of the moon


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