Callisto Facts

Callisto the second-largest moon of Jupiter after Ganymede. It is also the third-largest moon in the entire solar system after Jupiter’s moon Ganymede and Saturn’s moon Titan. It is the outermost orbiting Galilean moons of Jupiter. 

Callisto Profile

Interesting Callisto Facts

  • Callisto is the second-largest moon of Jupiter.
  • It is the third-largest moon in the entire solar system.
  • It is the outermost Galilean satellite of Jupiter.
  • It was discovered by Galileo Galilei in 1610.
  • It has a radius of 2410 km.
  • Callisto is 99% the size of the planet Mercury.
  • It has been named after one of the lovers of the god Zeus in Greek mythology.
  • It takes nearly 17 Earth days to revolve around Jupiter.
  • The first spacecraft to visit Callisto were Pioneer 10 and Pioneer 11 by NASA in the 1970s.
  • Callisto is nearly 4.5 billion years old.
  • The discovery of the Galilean moons is one of the most significant ones in the history of astronomy as this discovery provided the strong basis to the heliocentric theory that stated that the sun was at the center and other planets revolved around it. This is contradictory to the ancient belief of the earth being the center of the universe and all the moons and stars including the sun revolving around it.
  • The potential for life on Callisto has led to the moon becoming a popular and important setting for works of art in science fiction. Callisto has been mentioned in several books, comic books, TV shows, films, and most importantly video games.


The Jovian moon was discovered by Galileo Galilei in 1610 along with the other major moons of Jupiter namely Ganymede, Io, and Europa. Collectively four satellites of Jupiter called the Galilean satellites.

Callisto has been named after one of the lovers of Zeus from Greek mythology. Callisto was the daughter of Lycaon. She was a nymph and was often associated with Artemis, the goddess of hunt. This name was suggested by Simon Marius.

However, the names of the Galilean satellites came into common use long after their discovery and it was suggested in the mid-twentieth century. Callisto was also sometimes referred to as Jupiter IV meaning the fourth satellite of the planet Jupiter.

Scientists believe that these Galilean moons are formed by the materials which were remaining in Jupiter’s disc after the formation of the planet itself. This makes the moon at least 4 billion years old.


Callisto does not have a uniform color or it does not have the same appearance throughout its surface. For most of its part appears to be a silverish yellow in color.

As it is very far away from the earth and is often outshined by Jupiter it cannot be seen by the naked eye. However, advanced telescopes can be used to observe Callisto.


It has a radius of 2,410 km and is only slightly smaller than the planet Mercury. Hence had it been orbiting the sun directly it would have been considered as a dwarf planet or just as a planet.

This is more than 29 times the radius of the planet itself.  It is also nearly 4.5 times farther away from Jupiter than Io which is the closest moon to the planet. It is approximately 778 million kilometers away from the Sun.

Surface and Composition

Callisto’s surface is covered by bright scars or patches which are actually craters. The surface of the Jovian moon is heavily crated and hence it was initially thought to be a dead moon.

Callisto is mostly composed of water ice and other rocky material with some traces of ammonia and other volatile ices as well.

The surface of Callisto is icy and heavily crated. The craters on this moon are of different shapes and sizes. Craters may be created in the form of multiple rings or they may also be bowl shaped.

The composition of the surface of Callisto is thought to be very much similar to the composition of the entire moon itself. It is actually a little different than the other Galilean moons.

Callisto has two hemispheres namely the leading hemisphere and the trailing hemisphere.

  • The leading hemisphere is much darker than the trailing hemisphere whereas in the case of the other Galilean satellites the reverse is true. The leading hemisphere is rich in sulphur dioxide.
  • The trailing hemisphere of the Moon contains carbon dioxide in abundance. Some relatively young impact craters on the surface are also rich in carbon dioxide.

Some of the dark areas on the surface of the Moon have properties similar to those of D-Type asteroids.

The crater density of Callisto’s surface is so high that it can also be considered near saturation. This means that if any new crater is formed on its surface an old one would be erased.

In earlier times Callisto was considered as a dead moon. But today, its surface does not contain any mountains, volcanoes, or other tectonic features and is hence relatively simpler than that of the other Galilean satellites.

It mostly contains planes that are further categorized as cratered plains, light plains, and bright and dark smooth plains.

The impact craters on the surface of Callisto are seen as bright spots on the surface and they are shiny too. It is speculated that the craters shine because their tips are covered with water ice. This has led to the speculations that there might be an ocean of liquid water under the surface of Callisto and the Jovian moon could hence be a potential site for sustaining life.

This is one of the most important discoveries as far as the Jovian moon system is concerned.

Callisto has a differentiated interior structure. Its surface which mostly consists of plains and impact craters forms its lithosphere. It is about 80 to 150 km thick.

There might be a salty ocean under the surface that could go on from being 150 to 200 km deep. Under this, there might be a layer of an extremely conductive liquid which should be at least 10 km thick.

Magnetic field

The magnetic field of Callisto is extremely weak. This is because of its great distance from the planet Jupiter. The magnetic field of Callisto is approximately 300 times weaker than that of Europa.

Distance from Jupiter

Callisto is at a distance of 1,880,000 km from the planet Jupiter.


An extremely thin layer called the atmosphere was detected above the surface of Callisto in 1999. Most of the exosphere and atmosphere consists of carbon dioxide and traces of hydrogen and oxygen.

Exploration of Callisto

Callisto has been observed by many spacecraft as part of a mission of exploring the Jovian moon system. However, it has never won an individual or exclusive destination for a space mission.

The first spacecrafts which were sent to observe Callisto were Pioneer 10 and Pioneer 11 by NASA in the 1970s. They were sent to explore Jupiter and Jovian moon system.

They provided a little amount of information about the moon of Jupiter. A major step in the exploration of the Jovian Moon system is with NASA’s Voyager 1 and Voyager 2 in 1979.

Voyager 1
Voyager 1

They conducted a flyby to Calisto and provided nearly half of the information of its surface in great detail. This also helped in accurately measuring the mass, size and temperatures on the Jovian moon.  

Another space mission called Galileo conducted several flybys exploration of the planet Jupiter and its moons from 1994 to 2003. It came extremely close to Callisto eight times. In 2001 it went as close as 138 km above the surface of Callisto.

Galileo Mission
Galileo Mission

The Cassini spacecraft en route to Saturn also flew by Callisto in the year 2000. In 2007 The New Horizons spacecraft en route to Pluto provided more spectral images of the Jovian Moon.

The European Space Agency or the ESA is expected to launch another mission to explore Jupiter and its moons in the year 2022. It is called the Jupiter Icy Moon Explorer or JUICE.

The Europa Jupiter System Mission or the EJSM was formerly proposed as a joint machine by NASA and the ESA which was supposed to be launched in 2020.

Galilean Satellites as Important Scientific Discoveries

In ancient times people believed that the Earth was the center of the universe and all the other planets in the solar system the moon and the sun revolved around the Earth. This was called the geocentric astronomical model.

 In the 16th century, Copernicus introduced another model which was called the heliocentric model.

This module was the total contradiction to the geocentric model because it stated that the sun was at the center and the planets revolved around it. This became a heated topic of debate.

Scientists divided amongst themselves into the two parts. This lasted for several years. In 1610 when Galileo Galilei discovered the four Galilean satellites it was an important development in the history of astrology.

As the moons of Jupiter were the first satellites to be discovered that orbited a planet other than the earth. This created a strong argument in favor of the heliocentric model. The heliocentric model was accepted quite later.

Potential for Life

No evidence of life has yet been discovered on the moon of Jupiter. But it seems to be one of those bodies of the solar system where life could possibly exist. Callisto is one of the few bodies in the solar system where the possibility of finding life is the highest.

Other bodies of the Solar system include Europa, a moon of Jupiter, Titan, a moon of Saturn, and many other moons of both of these planets.

The possible presence of a sub-surface ocean and the traces of oxygen and hydrogen found in the atmosphere give support to this speculation. No human being has ever visited Callisto.

A conceptual study called the Human Outer Planets Exploration (HOPE) was conducted by NASA in 2003. This regarded the exploration of the outer parts of the solar system by human beings in the future.

Callisto was considered as an important target in this mission. It is also stated that Calisto could possibly become a site or a station that would help us explore other celestial bodies like Europa or some of the moons of Saturn.

NASA has also predicted that human space mission to Callisto could be possible by the decade of the 2040s.

Callisto in Pop Culture

As one of the potential sites for the discovery of life and being a base to discover outer part of the solar system, Callisto has become a fascinating object for science fiction writers. Callisto is popular in the earth’s culture more as a part of the planet Jupiter’s moon system than individually.

Nevertheless, it has been mentioned in several movies, books, TV shows, and video games both individually and as a part of the Jovian Moon system.

Some of the books include Beyond the Wall of Sleep (1919) by H. P. Lovecraft, Galileo’s Dream (2009) by Kim Stanley Robinson, etc. Some such films or TV shows include Jupiter Moon (1990), Cowboy Bebop (1998), etc. Some such video games are Zone of the Enders: The 2nd Runner (2003), G-Police, etc.


  • 1610: Callisto is discovered by Galileo Galilei
  • 1973-1974: Pioneer 10 and 11 become the first spacecrafts to flyby Callisto
  • 1979: Voyager 1 and Voyager 2 provide more details about Callisto
  • 1994-2003: NASA’s Galileo has eight close encounters with Callisto
  • 2000: NASA’s Cassini obtains high-quality infra-red spectra of the Galilean satellites
  • 2007: NASA’s New Horizons provides more information about Callisto


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