Jupiter's Moon, Callisto: Facts & Overview

Instructor: John Williams
Callisto is the second largest moon of Jupiter. It has many characteristics that make it important to scientists and astronomers. This lesson discusses several facts about Callisto, one of the Galilean Moons.


Callisto: The Second Largest Galilean Moon

The term moon refers to any of the naturally occurring bodies in space that orbit planets within the universe. These objects are also known as natural satellites, which means that they have a regular pattern of orbit around the host planet.

Earth, for example, has one moon, and typically, we can see it during the evenings as it circles the planet. This is due to the gravitational (attractive) pull that the Earth has on the moon to keep it in orbit.

Other planets are orbited by moons as well. While Earth has only one moon, many planets have more than one and, therefore, scientists will often name them to keep them organized. Jupiter, the largest planet in our solar system, has 67 (yes, 67!) identified moons. One of these moons is Callisto, and we will discuss this particular member of Jupiter's moon collection.

Callisto as a Galilean Moon

Galileo Galilei was a 16th-17th century astronomer from Italy. In 1610, he discovered the four largest moons of Jupiter, and they are collectively known as the Galilean Moons. These four bodies are some of the most massive objects in the solar system aside from the planets. The second largest of these moons is Callisto, and it is the third largest moon in the entire solar system.

Surface Characteristics

Callisto is primarily composed of rocks and ice. However, scientists believe that this landscape is not actively changing, and that it lacks geological activity. Therefore, Callisto has been nicknamed the 'Dead Moon.' Callisto's surface is covered with craters, or deep depressions, and these structures were detected in the 1970s by the Pioneer and Voyager spacecrafts.

Callisto is also interesting in that scientists believe it may be able to support life. The presence of water, carbon dioxide, and other key molecules associated with living organisms suggests that this could be the case. For these reasons, scientists are extremely interested in the possibility of finding living organisms on this moon.


Callisto has an atmosphere that is composed primarily of carbon dioxide. The atmosphere also contains oxygen similar to that found in Earth's atmosphere. It is believed that this atmosphere was formed from gases found on Jupiter. Due to Callisto's gravity, it is able to attract gases from its host planet.

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