Above: Image © A composite of Jupiter's four largest moons, which are known as the Galilean satellites. Shown from left to right in order of increasing distance from Jupiter: Io, Europa, Ganymede, and Callisto, Public Domain
Did you know? Mercury and Venus are the only two planets in the solar system that have no moons.
Earth is not the only planet with a moon. From Mars to Neptune, most planets have several moons, and many of these moons have fascinating characteristics. Our own moon is grey and barren, but others have atmospheres, volcanoes, and even environments that may be capable of sustaining life!
Mars has two moons, called Phobos and Deimos. These small, potato-shaped objects are thought to be asteroids that travelled too close to Mars and were captured by the planet's gravity. Such moons are referred to as “captured asteroids.”
Selected moons of solar system and Earth, to scale
Jupiter has 67 known moons, more than any other planet. Many of these moons are captured asteroids, due to Jupiter’s huge mass and the planet's proximity to the asteroid belt. Four of Jupiter's moons are of particular interest to astronomers: Io, Europa, Ganymede, and Callisto. These are called the Galilean moons, after their discoverer, Galileo Galilei, who first observed them through his telescope in the early 1600s.
Io is the most geologically active object in the solar system. Active volcanoes are everywhere on this moon. While lo's surface is quite cold, its proximity to Jupiter means that planet’s gravity is constantly stretching the moon's interior. This stretching creates an enormous amount of friction, which keeps Io’s mantle hot and molten.
Europa has a surface of ice formed by water. Just as Jupiter's gravity encourages volcanic activity on Io, the planet's pull is thought to create a liquid ocean below the surface of Europa. The presence of liquid water is considered essential for life, which makes Europa an object of great interest to scientists hoping to discover life beyond Earth.
Did you know? Jupiter’s moon Ganymede is the largest moon in the solar system. But Earth’s moon is the largest moon relative to the size of its parent planet.
Ganymede is the largest moon in the solar system. It is actually bigger than the planet Mercury! The fourth Galiliean moon, Callisto, has one of the oldest and most heavily cratered surfaces in the solar system. Some of Callisto's blemishes date back almost as far as the formation of the solar system itself.
Saturn has almost as many moons as Jupiter—62 have been discovered so far. One of Saturn's most interesting moons is Titan, which is the only moon in the solar system with a substantial atmosphere. This atmosphere allows for wind and rain, and there are even lakes on the moon's surface! Unfortunately, it is unlikely that life will be found on Titan, since its rains and lakes are composed of liquid methane, not water. Also, the surface temperature is a chilly -180°C. Titan is still an object of intense study, since it is one of the few rocky objects in the solar system with a real climate, not to mention the farthest one from the sun.
An illustration of a retrograde orbit, like the one followed by the moon Triton around the planet Neptune
Uranus has 27 moons, although most of them were not discovered until after 1986, when the Voyager 2 probe flew by. The moons of Uranus are the only ones in the solar system whose names are unrelated to the mythology of their parent planet. Although Uranus was named after the Greek god of the sky, its moons are named after characters from English literature, including plays by Shakespeare and poetry by Alexander Pope.
The most distant planet, Neptune, has 13 known moons. The largest of these, Triton, follows a “retrograde” (or backwards) orbit around Neptune. If you were looking down on Neptune’s north pole, the planet would rotate counter-clockwise, while Triton would orbit clockwise. As a result, Neptune pulls back on the orbit of Triton, causing it to fall closer to the planet. Astronomers believe that, sometime in the future, Triton will come so close to Neptune that the planet’s gravity will tear the moon apart. Such an event would turn Triton into a massive ring system around Neptune, larger than the one surrounding Saturn!
So while the moon may be the Earth's travelling companion in space, our planet's natural satellite is far from unique in the solar system. Not only do several other planets have multiple moons, these moons boast some fascinating characteristics, ranging from backwards orbits to potential sources of extraterrestrial life.