Callisto is the second largest moon orbiting Jupiter and is the third largest moon in our entire solar system. Callisto is covered in craters which means that it has had been hit by a lot of asteroids and comets throughout its history. Callisto is considered to have the most heavily cratered object in our solar system.

Scientists believe that even though the surface of Callisto is rough looking, it may have a liquid ocean under its surface. The surface is also known for having bright areas that are thought to be ice and darker patches where the ice may have eroded.

Callisto diagram Introduction

Callisto is one of the four Galilean moons that also include Io, Europa, and Ganymede. Callisto is the outermost of these moons, orbiting just outside of the main radiation belt of Jupiter.

Scientists believe that if Jupiter had continue to grow it would have developed the nuclear fusion required to turn it into the second sun within our galaxy. If this had happened, all 79 of the moons orbiting Jupiter, including Callisto, would have been planets. This is one of the reasons that astronomers often refer to Jupiter and its moons as a “mini solar system.”

Callisto Statistics:

  • Orbits: Jupiter
  • Discovered By: Galileo Galilei
  • Discovery Date: January 7, 1610
  • Diameter: 4,860.6 km
  • Mass: 1.08 x 10^23 kg (1.5 Moons)
  • Orbital Period: 16.69 days
  • Orbit Distance: 1,882,700  km
  • Surface Temperature: – 139 degrees C


The largest of Jupiter’s moons are Io, Europa, Ganymede, and Callisto. They are called the Galilean moons, named after Galileo Galilei for his 1610 discovery of all four. Each moon is larger than the dwarf planet Pluto, and Ganymede is bigger than the planet Mercury and the largest moon in our solar system.

On January 7, 1610, Galileo turned his telescope toward Jupiter and was surprised to realize that there were moons circling it. He had a bit of difficulty telling the difference in two of the moons, because he wasn’t sure if they were two or a single moon.

Up until this point, astronomers thought that Earth was the center of the universe and everything, including the sun, orbited around Earth. They also thought that Earth was the only planet and ono other moons existed. Galileo’s discovery not only changed the entire concept of science, but made the scientists realize that Earth rotated around the sun.

Galileo named the four Jovian moons after a powerful and political Italian family, the Medicis. He called each of the four moons by Roman numerals: I, II, III, and IV. Around the mid-1800s, the moons were given the names that they have today, and the former “IV” is now Callisto.  As time passed and telescopes improved, additional moons were discovered around Jupiter.

Callisto is the name of a mythological Greek character. She was a nymph that Zeus, the king of the gods, fell in love with. Zeus’ wife, Hera, changed her into a bear and then Zeus turned her into a star and put her high up in the sky. She is known as the Great Bear, or Plough, part of the constellation Ursa Major.  Zeus and Callisto had a son and he is noted in the Little Bear Constellation.

Formation, Structure and Surface:

Up until the 1960s, little was known about Callisto. Astronomers used their telescopes but even though Callisto is a large moon, it was too far away to get any details. All that astronomers could tell was that the surface of Callisto didn’t seem to be as noticeable as the surface of Io and Ganymede.

Callisto also has a low reflection called an “albedo” and scientists figured that it also had a low density. No one saw any evidence of the emission of water, so they assumed that its surface was just rocky.

In the 1970s, the Pioneer 10 and 11 spacecraft flew by Jupiter and its moons, but little information seemed to be found out beyond what the telescopes had seen. In the late 1970s, the Voyager mission took more in-depth images of Callisto and revealed data that was not known before.

The images showed a more refined status of Callisto’s temperature and density as well as the fact that Callisto had a lot more craters than most of the other moons.

These new images immediately changed Callisto from being one of the most boring objects in the solar system to one of great interest. The craters were assumed to be the result of comet and asteroid impacts.

The Galileo spacecraft did a total of 12 flybys and sent back higher resolution images and a lot more data about Callisto than scientists had previously had. The flybys assisted in mapping the surface of Callisto and in finding out that the moon had a thin atmosphere of carbon dioxide.

Callisto Base Formation

They also found that there was evidence that Callisto may have a subsurface ocean. The evidence of a potential ocean was first depicted in the regular fluctuations of the magnetic field of Callisto as it orbited Jupiter.

This implies that there are electrical currents inside the Callisto that the parent planet stimulates. The current itself must be created from something that is conductive and this led to the assumption that there was salty liquid water in the form of an ocean under the rocky surface.

More recently, scientists believe that the ocean on Callisto may be deeper than was previously assumed, as well as the possibility that the ocean didn’t exist at all. The presence of an ocean might put Callisto as a potential candidate for a location that could support life.

Callisto is believed to be around the same 4.5 billion year age as Jupiter, which means that it was probably created when the planet was very young. Callisto doesn’t show any evidence of any surface geological activity, so other than the impacts of asteroids and comets, the surface may not have changed from what it looked like when it first formed.

Callisto is the farthest out of the four Galilean moons and it takes about 7 Earth days for it to complete its orbit around Jupiter. Because of the distance beyond the main radiation belt of Jupiter, Callisto also has a reduced tidal influence than the other three moons. Callisto is tidally-locked, like a majority of moons in our solar system. This means that only one side of Callisto faces Jupiter as it orbits

Atmosphere and Magnetosphere:

One of the big discoveries about Callisto happened during an examination of some of the 2007 Hubble Space Telescope images. Scientists realized that Callisto had an effect on the Jupiter auroral bursts in its atmosphere.

While Jupiter has auroras that it generates on its own, there seemed to be additional phenomena as it interacted with the four largest moons: Europa, Io, Ganymede, and Callisto.

While the auroral effect had been previously noted for three of the four moons, this was the first time that they had seen evidence that the same thing was happening with Callisto.

Using a computer model that was released in 2018, scientists believe that the four largest moons may have actually been formed with a little help from Saturn. It’s thought that as the core of Saturn increased it also increased it gravitational influence on baby planets known as planetesimals that were in the inner solar system. This influence may have been enough to assist in the formation of the Galilean moons.

Callisto’s magnetosphere seems to be directly influenced by Jupiter. It experiences increases and decreases, depending upon the proximity of the moon from its parent planet. More recent computer models have been used to see how the magnetic fields of both Callisto and Jupiter interact.

The results of the research has led scientists to believe that Callisto may have a subsurface ocean as well as locating oxygen in its atmosphere.

Could Life Exist?

Callisto may have a salty ocean under its surface and an atmosphere, but it lacks tectonic activity. Scientists believe that there is an alternative to the requirement to tectonic activity in generating energy, but the odds are low that Callisto could harbor life.

Callisto Earth Exploration

Interesting Information:

  • Callisto is a big moon! It’s almost as large in diameter as the planet Mercury, however, it is low density, so it only has one-third of Mercury’s mass.
  • The reason that Jupiter’s four largest moons weren’t discovered before 1610 is that they can’t be seen with the naked eye. Galileo was the first person to design and use a telescope that could look into the celestial skies to research what was out there.
  • If Jupiter had continued to grow and become the second sun in our solar system, Callisto and all of Jupiter’s moons would be planets. Instead, Callisto is a moon that orbits a planet.
  • Callisto is home to more craters than any other known object in our solar system. The sheer volume of craters has made scientists believe that it is one of the solar system’s oldest landscapes.
  • The craters and landscape on Callisto are never changed due to the lack of tectonic activity or volcanism that could alter the surface.
  • Callisto is made up of around 60% iron and rock and 40% ice. It’s believed that some areas in and near the ice could contain liquid water.
  • If you look at Callisto through a telescope it appears brighter than the Earth’s moon. The reason for the brightness is that Callisto is covered in a thick layer of ice which reflects the sun better than our own moon’s surface.
  • The other three Galilean moons, Io, Europa, and Ganymede are affected by the gravitational pull of Jupiter. Callisto is farther away from Jupiter and isn’t affected by the same gravitational pull of its parent planet and this means that it also doesn’t have as much tidal heating which contributes to the energy needed to melt the surface of the moon.
  • Callisto is really far away from Jupiter!  It’s over one million miles from its parent planet. To give an idea of comparison, our moon orbits around Earth at a distance of 238,855 miles.
  • It takes Callisto 16.7 days to complete an orbit around Jupiter and when it finishes the orbit it will have traveled over 7 million miles.
  • The speed that Callisto is traveling is 18,400 mph/29,530 kph.
  • Callisto sets the record for having the most craters of any known object in our solar system. Its biggest crater is named Valhalla, for the hall of the Norse god Odin where their fallen warriors are taken after they die.
  • The large crater Valhalla is nearing 3,800 km in diameter.
  • Callisto’s thin atmosphere is made up of carbon dioxide and possibly molecular oxygen.
  • The formation of Callisto is thought to have been from gas and dust that encircled Jupiter when it was first forming 4.5 billion years ago.


Calisto and some of the other moons orbiting Jupiter has been the focus of a number of spacecraft that have done multiple flybys, including the Galileo mission in the 1990s and 2000s.

NASA’s current JUNO mission is at Jupiter and will be looking at the environment and atmosphere of Jupiter as well as taking images at a distance of Callisto.

Facts about Callisto Moon for Kids:

  • Callisto is half icy and half rocky, with a surface that has silicate dust, rock particles, carbon dioxide ice, water ice, and hydrocarbon particles.
  • There doesn’t seem to be any evidence that Callisto was tidally heated.
  • Callisto’s surface features have probably been created due to impacts by a massive number of asteroids and comets.
  • There are so many craters on Callisto that if any new ones happen they will probably hit older ones and destroy them, making a new impact crater.
  • The Galileo spacecraft detected a thin atmosphere on Callist that is mostly made up of carbon dioxide.
  • At one time, NASA was studying Callisto as a potential location to set up a basecamp for human expeditions to the Jovian system.

Pop Culture:

  • Callisto, like many of the moons of Jupiter, has been the subject for science fiction writers for many years.
  • The 1940s Isaac Asimov novel, The Callistan Menace, shows Callisto with an atmosphere of oxygen and carbon dioxide and vegetation and lakes.
  • Robert Heinlein’s 1950s book, Farmer in the Sky, mentions creating an atmosphere on Callisto for colonists.
  • Philip K. Dick, author of Blade Runner, and The Minority Report, wrote a short story called “The Mold of Yancy” about colonists living on Callisto in a totalitarian society.
  • Callisto is featured by authors in a number of other books including:
  • Anne McCaffrey, The Rowan
  • Piers Anthony, The Bio of a Space Tyrant.
  • Rob Lopez, Callisto: Dystopian Space
  • Ian Stewart and Jack Cohen, Wheelers
  • Kim Stanley Robinson, Blue Mars and Galileo’s Dream