- Ganymede is one of the many satellites, or moons, that orbit round the great planet, Jupiter.
- None of these moons can be seen with the naked eye but the four largest ones, Io, Europa, Ganymede and Callisto, can be seen with a small telescope.
- They were first seen and described by Galileo in 1610, and so they are known as the “Galilean moons”
Figure 1. The Galilean moons of Jupiter seen through a small telescope. Ganymede is the moon third in distance from the planet
- They are sometimes also called the moons of the “Jovian System”. “Jovian” is the adjective from Jupiter, so Jovian System means the system of Jupiter.
- Astronomers refer to the “Jovian System” because Jupiter with its moons in orbit is very similar to the Solar System with the planets in orbit.
- Ganymede is the third moon from Jupiter.
- Ganymede is the largest moon in the entire Solar System. It is larger than the planet Mercury.
Figure 2. Ganymede, the largest moon in the Solar system. An image from the NASA Galileo spacecraft
- In spite of its size Ganymede is called a moon and not a planet, because it orbits Jupiter and not the Sun.
- Ganymede has a diameter of 3,267 miles (5,262 kilometres).
- Ganymede’s distance from Jupiter is 664,718 miles (1,070,400 kilometres).
- The moon travels 4,176,546 miles (6,725,518 kilometres) to complete one orbit
- It takes only 7.1 Earth days to complete this long orbit, travelling at a speed of 24,321 miles (39,165 kilometres) an hour.
- Like the other Galilean moons and like the Earth, Ganymede has a number of layers.
- At the centre is a core of iron, which exerts a strong magnetic force.
- Covering the iron core is a mantle of rock and, covering the rock is a thick layer of ice.
- The layer of ice is thought to be as much as 500 miles (800 kilometres) thick.
- The darker areas which can be seen in Figure 2 are thought to be much older than the lighter areas.
- The darker areas have deep craters, showing previous volcanic activity, and a very rough surface.
- The lighter areas have grooves and ridges but quite a smooth surface.
- Scientists believe that the lighter area was formed on top of the darker, rougher crust.
- The surface of Ganymede shows millions of years of geological activity.
Figure 3. The surface of Ganymede, showing the deep grooves present in the lighter, and younger, areas.
- Ganymede has an atmosphere containing oxygen, but not enough to support any form of life.
- Ganymede gets his name, like the other moons, from characters in Greek mythology who were close to the Greek sky god, Zeus, known to the ancient Romans as Jupiter.
- Ganymede was a beautiful young boy whom Zeus (Jupiter) captured and carried to the top of Mount Olympus, where the Gods lived.
- Zeus captured Ganymede by turning himself into an eagle and swooping down on the boy as he played. He then grabbed him in his talons and flew off with him.
- Ganymede became the page and cupbearer of the god Zeus (Jupiter).