The moons in the Solar Program vary enormously in size. The biggest are bigger than the planet Pluto; the smallest are kilometre-sized lumps of rock. Moons more than about 300km across are spherical, while smaller moons tend to become irregular and potato-shaped. The larger moons were discovered from Earth. The smaller ones were identified by space probes visiting the major planets. Jupiter’s four largest moons (To, Europa, Ganymede and Callisto) are known collectively as the Galilean moons, after the Italian astronomer Galileo Galilei. He discovered them by looking through a telescope in 1610. Nevertheless, they had been not revealed in all their glory until the Voyager 1 space probe flew past Jupiter in 1979. Ganymede is the largest of all of the Solar System moons. It is a spherical body bigger than the planet Mercury.

Surface features

Moons’ surface functions are also immensely varied. Many moons show the scars of past bombardment by room rocks, but their present surface material may be quite various. Earth’s Moon, for example, has a crater-marked, dry, rocky surface. Much more distant moons like Uranus’s four largest (Oberon, Titania, Umbriel and Ariel) have cratered ice and rock surfaces. Saturn’s Mimas has one large crater known as Herschel, which covers a third of the tiny moon’s face. The cratered plains of Uranus’s Miranda are interspersed with cliffs and deep canyons. These suggest that it was shattered by a huge collision within the past, only to reassemble itself once more.

Some moons are studded with volcanoes. Astronomers had been amazed when Voyager 1 revealed active volcanoes on Jupiter’s 10, the first to be discovered outside Earth. In 1989, active volcanoes were discovered on a second moon, Neptune’s Triton. This is surprising as Triton is the coldest place within the Solar Program, with an average temperature of -235°C. Plumes of black dust from the volcanoes have marked the moon’s surface area. Most moons have no atmosphere. Saturn’s Titan, the second largest of all of the moons, is an important exception. It is a ball of rock and ice, but its surface is hidden by a thick orange smog, about 90% of which is nitrogen, the rest probably methane.