Ganymede is considered to be the largest moon within our solar system and the ninth largest object in the solar system. The third of the 79 moons of Jupiter, it also holds the distinction of being the only moon with its own magnetic field. Ganymede’s magnetic field causes electrified glowing gas auroras to appear in both the south and north poles.

Ganymede diagram Introduction

It has bright, large areas of grooves and ridges that cut across the darker, older regions. These grooved areas are a possible hint that Ganymede experienced some dramatic changes in its surface in the far past. Scientists also believe that there is a good chance that Ganymede may have an underground ocean.

Ganymede Statistics:

  • Orbits: Jupiter
  • Discovered By: Galileo Galilei
  • Discovery Date: January 7, 1610
  • Diameter: 5,262.4 km
  • Orbital Period: 7.16 days
  • Mass: 1.48 x 10^23 kg (2.0 Moons)
  • Orbit Distance: 1,070,400 km
  • Surface Temperature: – 163 degrees C


Ganymede was one of a total of four moons discovered by Galileo Galilei in 1610. Galileo had some difficulty distinguishing whether two of the moons were actually one. He later discovered all four of what are called “Jovian moons.”

It’s important to remember that up until this point, astronomers assumed that everything in the celestial sky orbited around our Earth, including the sun. They hadn’t considered that there were planets or galaxies that were independent. Galileo’s discovery changed everything as they understood it, recognizing that planets orbited around the sun and that moons orbited around the planets.

It’s thought that Simon Marius may have made the same discovery about the moons around the same time that Galileo made his discovery, and may have actually found the moons up to a month before Galileo; however, Galileo gets the credit because he was the first to publish finding the moons.

When Galileo first found the Jovian moons he wanted to call them the “Medicean planets,” named after the Medici family. He listed the four moons in Roman numerical order as: I, II, III, and IV. These names would remain for a few centuries until the mid-1800s when astronomers realized that more moons were being discovered and a numbering system would be confusing. At that time, the four moons got their names of Io, Europa, Ganymede, and Callisto.

Ganymede was named after a mythological character. He was a young, beautiful boy who was taken to Olympus by the Greek god Zeus, disguised as an eagle. Ganymede took on the job of being the cupbearer for the gods of Olympus.  While Zeus is the name of the Greek god, the Romans called him Jupiter.

Formation, Structure and Surface:

Ganymede is made up of three layers: a sphere of metallic iron at the core, which is responsible for generating the magnetic field; a mantle, which is a spherical shell of rock that surrounds the core, and a third spherical shell of almost entirely ice that surrounds the rock shell and the core.

The shell of ice is incredibly thick and is thought to be nearing 497 mi/800 km thick. It’s what is showing on the very surface of Ganymede, and it’s believed that there may be some rock mixed in with the ice.

Spacecraft that have done flybys of Ganymede have sent back images to indicate that the moon has had a rather rough geological history. It appears that the surface has two types of terrain: around 40% of the surface is covered by a lot of dark cratered areas, and the other 60% is covered by lightly grooved areas that form patterns across the surface.

The grooves are called “sulcus” which means burrows or grooves, and it’s believed that these were formed due to faults created when the water below the surface was released. The grooves have ridges that are as tall as 2,000 ft/700 m, and can extend for thousands of kilometers over the surface. The grooves also don’t have very many craters in them.

Ganymede Aurora Magnetosphere

It’s thought that the darker and more cratered areas of the surface are believed to be the moon’s original crust, and the lighter and smoother areas are younger. The largest area on Ganymede has been given the name of the Galileo Region.

Images of Ganymede’s large craters how that they are really quite flat. They don’t have the usual depressions in the center that most craters have on other moons and planets. Scientists think that this might be due to a gradual and slow movement of the icy surface. These unusual craters are called “palimpsests” which are also considered to be phantom craters.

The palimpsests are from 50-500 km in diameter and have both dark and brighter rays of the ejected material around the crater edges. The “rays” have a tendency to be brighter in those that are in the grooved terrain and darker in the dark terrain craters. Images have shown that Ganymede also has a lot of “bumps” on the surface which could be rock formations.

Scientists think that there is a saltwater ocean beneath the surface of Ganymede. The Hubble Space Telescope took a detailed look at the moon in 2015 and studied the auroras. The focused on how the auroras change between the magnetic fields of both Jupiter and Ganymede. The process called “rocking” that was seen in the auroras gave scientists the evidence that they wanted to make the assumption that the ocean underneath is salty, and actually saltier than our Earth oceans.

Atmosphere and Magnetosphere:

Ganymede is considered to be a low density moon, even though it is larger in size than the planet Mercury. In 1996, the Hubble Space Telescope let astronomers find that Ganymede had a thin atmosphere of oxygen. The atmosphere is too thin to be considered a good candidate for life to survive on Ganymede.

Ganymede is the only satellite within our solar system to have its own magnetosphere. This type of magnetosphere is usually only found with planets. Both Earth and Jupiter have their own magnetosphere, which is region that is shaped like a comet where charged particles are deflected or trapped. The magnetosphere of Ganymede is actually embedded in Jupiter’s magnetosphere.

Could Life Exist?

Scientists don’t hold any hope that Ganymede could have life on it. Ganymede’s internal structure is one that has high pressure at the base of the ocean and any water that would exist there would be changed into ice. Without hot-water vents to bring nutrients to the ocean it is unlikely that life could occur.

Interesting Information:

If Ganymede were orbiting a sun instead of Jupiter it would be a planet. The moon is larger than the planet Mercury and has the highest mass of any of the satellites and over twice the mass of our moon.

In Greek mythology, Ganymede was a Trojan prince that became the cupbearer for the gods of Olympus. The name was a suggestion by Johannes Kepler, the German astronomer and mathematician.

Ganymede is orbiting Jupiter at 39,165 km/h and it takes the moon 7.1 Earth days to complete an orbit around Jupiter.

Ganymede is like most moons in our solar system in that it is tidally locked to its parent planet, showing only one face all of the time.

For every complete orbit that Ganymede has around Jupiter, Europa orbits Jupiter twice, and Io completes four orbits.

Ganymede is made up of almost equal amounts of water ice and rock.

A 2014 published study indicated that there is a good chance that there might be several layers of saltwater stacked beneath the surface of Ganymede, and may extend down as far as 124 miles.

It’s thought that the lowest layer of liquid on Ganymede may exist next to the rocky mantle.

Ganymede has 40% of its surface that is covered with a lot of dark cratered areas that are thought to be from comet or asteroid impacts.

The cratered dark regions on Ganymede are nearing 4 billion years old and the lighter area are thought to be younger.

Scientists think the younger, lighter regions of Ganymede’s surface are due to geological disruption from tectonic activity that caused tidal heating that created friction, orbital, and rotational energy that spread out as heat over the crust.


Since 1973 there have been six spacecraft that have visited Ganymede: Pioneer 10 and 11 did flybys in 1973 and 1974, sending information as well as blurry images of the moon. In 1979, Voyager 1 and 2 sent better quality images of Ganymede that displayed the terrain that was grooved as well as the discovery that Ganymede was bigger than originally thought. It was thanks to the data from Voyager 1 and 2 that Ganymede replaced Saturn’s moon, Titan, as the largest satellite in our solar system.

In 1995 the Galileo spacecraft passed Ganymede at a really low altitude of 162 mi/261 km to view the surfaces of the Galilean moons. It sent back a huge amount of new information including the discovery of Ganymede’s magnetic field and the potential for an ocean below its surface.

On its way to Pluto and the Kuiper Belt, the 2007 New Horizons mission sent higher resolution images of Ganymede to create topography and composition maps of the entire satellite.

NASA and the United States Geological Survey released the first detailed map of Ganymede in 2014. They included incredibly detailed images as well as an animated video that made use of all of the observations sent back from Voyager 1 and 2 spacecraft and the Galileo spacecraft.

Ganymede Terrain Exploration

Facts about Ganymede Moon for Kids:

  • Scientists have discovered that there is something inside of Ganymede that is assisting in generating a strong magnetic field. It’s believed that the core at the center of Ganymede is allowing the moon the only magnetic field of any satellite in our solar system.
  • Ganymede may have an ocean below its surface that is over a liquid iron and nickel core.
  • If Jupiter had grown larger it would have turned into a second sun in our solar system and Ganymede would have been one of its planets.
  • Scientists think that Ganymede was formed when Jupiter was just beginning to form in the early solar system and that several smaller objects came together to create the moon.
  • The Hubble Space Telescope confirmed that Ganymede has a very thin atmosphere that appears to contain oxygen. It’s thought that the oxygen is released as surface ice has the hydrogen and oxygen molecules broken apart due to solar radiation.
  • There have been quite a few missions that were talked about to continue exploration of Ganymede, but most were either cancelled or are still in consideration.

Pop Culture:

  • Many of the moons of Jupiter have been the topic of film, novels, and art.
  • In Isaac Asimov’s 1940 short story, Christmas on Ganymede, the aliens on Ganymede are introduced to the Earth holiday.
  • In other Isaac Asimov’s short stories is the 1941, Not Final! and the 1942 Victory Unintentional, the humans living on Ganymede are in conflict with those that live on Jupiter.
  • Arthur C. Clarke’s novels 2061: Odyssey Three and 3001: The Final Odyssey, Ganymede is a warm planet in orbit around the new sun Lucifer and has a large equatorial lake.
  • Robert Heinlein had Ganymede as a backdrop or main focus of many of his stories including:
  • Space Cadet, The Rolling Stones, Farmer in the Sky, Double Star, Time for the Stars, and I Will Fear No Evil.
  • The 1954 novella by Paol Anderson, The Snows of Ganymede, a group of terraformers visit a settlement on Ganymede called “X.”
  • The Lester del Ray 1963 novel, Outpost of Jupiter, had a plague strike the human settlement Ganymede outpost. In another del Ray novel, Space Jockey, Ganymede is filled with distrustful terrestrials as it was once a penal colony.
  • Almost all of Phillip K. Dick’s 1950s and 1960s novels had mention of Ganymede.
  • Piers Anthony’s novel series from 1983-1986 called Bio of a Space Cadet had changed the name of Ganymede to Cuba.
  • The Power Rangers television series had Ganymede as a hiding place that was chosen by Zordon.
  • The 1993-1999 television series, Babylon 5, had a ship buried under the surface of Ganymede.
  • In the series, Return to Jupiter, Ganymede is the mining colony location.
  • The 2017 television show, The Expanse, features Ganymede as the planet where a majority of the food production for the solar system occurs.