Moons are natural satellites that orbit planets, and they vary greatly in size, composition, and number. For example, Jupiter has over 79 known moons, including the largest moon in the solar system, Ganymede. In contrast, Mercury and Venus have no moons at all. Moons can be rocky, icy, or gaseous, and some even have atmospheres. Saturn's moon Titan is notable for its thick atmosphere and lakes of liquid methane. The study of moons helps scientists understand the formation and evolution of planetary systems, as well as the potential for life beyond Earth.