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Ganymede is the largest moon of Jupiter and the solar system. In addition, it is the only one that has a magnetic field.

Discovery of Ganymede

On January 7, 1610, Galileo Galilei, already equipped with a rudimentary telescope, observed that three fixed stars appeared near Jupiter.

Because of their small size, these three stars were totally invisible to the naked eye.

As he noted in his diary, on the night of January 13, 1610, he saw four stars, and realized that the first three had moved.

Jupiter, the Galilean moons. Credit: web “”

He concluded that the stars were actually objects revolving around Jupiter.

We now know that these objects that Galileo discovered are four moons of the planet Jupiter. They are universally known as “Galilean moons.”

The definitive names given to these four moons are: Europa, Ganymede, Io and Callisto.

• Europe has a diameter of 785 km.
• Io has a diameter of 3,643 km.
• Callisto has a diameter of 4,900 km, it is located about 1,800,000 km from Jupiter.
• Ganymede has a diameter of 5,622 km.

In order of distances to Jupiter, Ganymede is the seventh closest. It completes its orbit in approximately seven days.

4 moons
Sizes of Io, Europa, Ganymede and Callisto. Credit: web “”
Jupiter’s moons crossing the giant planet. Credit: web “”.

Ganymede in fiction

Due to Ganymede’s interest as the largest natural satellite in the solar system, it occupies a preferential place for the plot of various works of science fiction.

Significant examples are:
• “Christmas on Ganymede” (1940), short story by Isaac Asimov.
• “The Star Farmer” (1950), by Robert A. Heinlein.
• “The Snows of Ganymede” (1954), novel by Paul Anderson.
• “I visited Ganymede” (1972), by José Rosiano.
• “My preparation for Ganymede” (1975), by José Rosciano.
• “Buddy Holly is alive and well in Ganymede” (1992), by Bradley Denton.

In addition to the central theme, Ganymede is mentioned in many other works, both in books, as well as in film and television.

Ganymede surface

Ganymede’s surface is a mixture of two types of terrain where silicates predominate.

This surface shows two different types of terrain: dark and light. The dark regions, saturated with impact craters and formed four billion years ago, cover about a third of the satellite.

Ganymede has a crust of ice that floats on top of a muddy mantle that can contain a layer of liquid water.

The crust appears to be divided into some tectonic plates, like those on Earth.

Although there is no evidence of recent tectonic activity, already solidified lava flows have been observed.

On all terrains there are many impact craters, 3,000 to 3,500 million years old, similar to those on the Moon.

A chain of craters on Ganymede. Credit: NASA, Wikipedia.

There are also slightly more recent craters, which have ejection rays.

Ganymede’s craters are fairly flat, without mountains, and lacking the central depressions common to craters on the Moon and Mercury.

The most striking feature on Ganymede is a dark plain called Galileo Regio, as well as a series of concentric rings that are remnants of an ancient impact crater.

Ganymede interior

The most recent observations suggest that Ganymede has a differentiated structure in three layers:

• a small core of cast iron or iron and sulfur, in the center,
• a rocky silica mantle, surrounding the iron core
• a crust of ice and liquid water on the outer mantle.

The metallic center suggests a greater degree of warming of Ganymede, than had previously been proposed.

Diagram of the internal structure of Ganymede. Credit: Kelvinsong. Wikipedia.

Ganymede’s inner ocean may hold more water than all the oceans on Earth combined.

Ganymede has atmosphere

The Hubble Space Telescope has found evidence of a thin oxygen atmosphere, which includes ozone and atomic and molecular oxygen.

Oxygen is produced when radiation reaching Ganymede’s surface ice breaks it down into hydrogen and oxygen. Hydrogen, which is very light, is lost to space.

Ganymede also has its own magnetosphere, which is generated by the movement of conductive material inside.

Ganymede’s magnetic field is probably the product of convection movements within the cast iron core.

Space probes that have photographed Ganymede

Since Galileo Galilei discovered Ganymede on January 7, 1610, all astronomers have been interested in getting to know this moon better.

When it was possible to get closer to Jupiter with space probes, all of them have been providing new information of interest.

The Pioneer 10 was the first to get images from a short distance.

Voyager probes refined the images and provided better values ​​of the physical characteristics of Ganymede

The Galileo probe discovered its underground ocean and its magnetic field.

In addition to the four Galilean moons, observations from Earth and space probes have estimated the total number of Jupiter moons at 67.

The European Space Agency is planning a mission to the Jupiter system for the year 2022. There will be several flybys of the Galilean moons and the launch of the probe around Ganymede.