As can be expected from the largest planet in our solar system, Jupiter also features the largest of all moons.
The diameters of the four Galilean moons, named after Galileo Galilei, who discovered them, are nearly those of planets. These four moons have been known since 1609 and were named Io, Europa, Ganymede and Callisto. Io, which appears in an ochre color, is the moon with the highest volcanic activity in the solar system.
The Voyager space probe was even able to capture some of the eruptions taking place on Io. For millions of years, the moon has been covered in a thick layer of lava. Its diameter is approximately 3630 kilometers and it orbits Jupiter at a distance of 422,000 kilometers.
Only marginally smaller than Io is Europa with a diameter of 3140 kilometers. The Galileo orbiter revealed to the surprised scientific community that there is water on this moon. Continental ice plates "float" on a layer of water and mud. At a mean distance of approximately 670,000 kilometers from Jupiter, Europa completes an orbit within 3.5 days.
The largest moon in the solar system is Ganymede. It has a diameter of 5260 kilometers and is thus larger than the planet Mercury. Ganymede as well as Callisto have a lower density than the two other Galilean moons. Both moons are at greater distances from Jupiter (Ganymede at 1.07 million kilometers, and Callisto at 1.88 million kilometers) and consist of rock and ice. They are both covered in craters. The largest crater on Callisto is called Valhalla.
With a diameter of 4800 kilometers, Callisto is nearly the size of Mercury. The four inner moons of Jupiter are called Metis, Adrastea, Amalthea and Thebe. They are considerably smaller than the Galilean moons and, with the exception of Amalthea, were only discovered in 1979 by the Voyager space probe. Amalthea was discovered in 1892. It is the largest of the four inner moons of Jupiter and has a diameter of 210 kilometers. The distances of the inner moons from the planet are between 128,000 (Metis) and 220,000 (Thebe) kilometers.
Beyond the Galilean moons, Jupiter has another eight satellites that were only discovered in 1980 thanks to the Voyager space probe. Leda, Himalia, Lysithea and Elara orbit Jupiter in the same direction as the inner moons. However, there are another four satellites orbiting Jupiter at even greater distances from the planet and in reverse direction to the other moons. They are believed to be planetoids originating from the asteroid_belt that were at some stage captured by the planet. These satellites are called Ananke, Carme, Pasiphae and Sinope and they revolve about the planet in retrograde orbits and thus in the opposite direction to the other moons.
The outermost of the Jupiter moons is Sinope at a distance of 23,700,000 kilometers from the planet. It was discovered by the Voyager probe, which also revealed the existence of a weak ring system around Jupiter. Inside the orbits of the moons and at a distance of fifty to sixty thousand kilometers from the planet, we find the main ring of Jupiter. It consists of small dust particles and extends over 30 kilometers, emitting only minimal light. This is the main reason why it was only discovered by the space_probe, which flew past the planet at a comparatively small distance.
