II.

Solar System Moons

The best-known natural satellite is Earth’s moon. The Moon is unusually large relative to the size of its primary (Earth) and has a diameter about one-fourth the diameter of the planet. The Moon’s surface, like the surfaces of most of the natural satellites in the solar system, is heavily cratered and geologically inactive.

Neither Mercury nor Venus has any natural satellites, but Mars has two small moons: Phobos and Deimos. Jupiter has more than 60 natural satellites, four of which are quite large: Io, Ganymede, Callisto, and Europa. These large moons were discovered by Galileo with an early telescope in 1610—the first moons detected around another planet. Active volcanoes cover Io, and scientists believe that oceans of water may hide beneath the icy crusts of Ganymede, Callisto, and Europa. All four of these moons are larger than the dwarf planet Pluto, and Ganymede is larger than the planet Mercury, as well. Saturn also has more than 60 natural satellites, the largest of which is Titan. Titan is bigger than Mercury, and is the only moon with a thick atmosphere. Enceladus, one of Saturn’s smaller moons, has active volcanism in the form of geysers that send out vast plumes of liquid water from the south polar region. Uranus has at least 27 moons, none of which is as large as Earth’s moon. Miranda, one of Uranus’s smaller moons, shows signs of terrific upheavals on its surface. Neptune’s largest natural satellite, Triton, is slightly larger than Pluto. Its surface appears to be continually reshaped by the freezing and thawing of nitrogen.

The dwarf planet Pluto has three moons. Its largest moon, Charon, is half as large as Pluto itself and was discovered in 1978. Some astronomers consider the pair a double dwarf planet. Eris, a dwarf planet larger than Pluto, has a small moon named Dysnomia. The odd football-shaped dwarf planet 2003 EL61 has two small moons.

Astronomers have detected satellites around other types of solar system bodies. At least 100 asteroids are thought to possibly have satellites. Confirmed asteroid moons are given official names and catalog designations by the IAU. The first asteroid moon was discovered in 1993 when the Galileo space probe photographed the asteroid Ida and its moon Dactyl. In the outer solar system a number of centaurs and Kuiper Belt Objects are known to have satellites. To date, satellites have not been confirmed around any comets.