moons

Moons, or natural satellites, are bodies that orbit planets or other larger bodies. They range in size from tiny rocks to the Earth's Moon, which is unusually large relative to its planet. In the Solar System, most planets have one or more moons; Mercury and Venus have none, while Mars has two small moons, Phobos and Deimos. Some dwarf planets also have moons, such as Pluto's system.

Moons form and evolve through several processes. They can form in a circumplanetary disk around a young planet (co-formation), be captured by a planet's gravity, or be formed by impact ejecta that coalesces into a satellite (as with Earth's Moon). Once formed, a moon's orbit is governed by gravity and tidal forces with its planet; many are tidally locked, showing the same face to their planet. Orbits can be near-equatorial, irregular, or retrograde; resonances between moons can shape orbital evolution.

Sizes and compositions vary widely. They range from rocky inner satellites to icy outer ones. Some have atmospheres, such as Saturn's Titan, which has a thick nitrogen-rich atmosphere. Several moons show active geology or geologic processes, e.g., Io's volcanism, Enceladus's icy plumes, and Europa's suspected subsurface ocean. Subsurface oceans are also suspected on Ganymede and Callisto. Surface features include craters, troughs, and tectonics; some cryovolcanism and plumes indicate ongoing activity.

Moons are important for studies of planetary formation, evolution, and potential habitability. They provide laboratories for understanding gravity, geology, and interior structure. The Moon has been the focus of human exploration; beyond Earth, many missions have studied outer planet moons, including Galileo to Jupiter and Cassini to Saturn. Future missions such as Europa Clipper and JUICE aim to explore icy moons for clues about oceans and habitability.