Photo: A moon.

Photograph courtesy NASA/JPL

With volcanoes, frozen oceans, and methane seas, moons are often much more than balls of rock circling a body other than the sun. In fact, the only definitive thing that separates many moons from planets like the Earth and Mars is what they revolve around. Planets circle the sun; moons circle the things that circle the sun—planets, dwarf planets, and other so-called small solar system bodies. Otherwise, moons are diverse and fascinating worlds unto their own.

Take Europa, for example. One of four planet-size moons in orbit around the planet Jupiter, it has an almost glassy surface of ice that scientists say may cover an ocean that is 31 miles (50 kilometers) deep. Tidal forces between Jupiter and Europa are believed to generate enough heat to keep the ocean liquid. Observations with the Hubble Space Telescope also indicate Europa has a tenuous atmosphere of oxygen. The combination of water, a heat source, and an atmosphere raises the possibility that Europa harbors life.

The best known moon is the one in orbit around planet Earth. It is Earth's only natural satellite and the only extraterrestrial body that humans have visited. Gravity on the moon is one-sixth of Earth's gravity, allowing astronauts to take giant leaps on its surface. The moon lacks an atmosphere, but spacecraft have found water ice at both poles, deposited from impacting comets. According to a leading theory, a Mars-size body smacked Earth about 4.5 billion years ago and the debris from the collision accumulated to form the moon.

Unlike Earth's moon, many moons formed from the same material that glommed together and gave rise to the body they orbit. Other moons are asteroids captured into orbit by a larger body's gravity. Only the dwarf planet Pluto's moon Charon is thought to have formed from a collision like the one that gave rise to Earth's moon.

No matter how they form, the moons are many. Only Mercury and Venus are moonless. Earth has one, Mars two. Neptune has 13 and Uranus 27. The gas giant Jupiter has 63 known moons. Saturn has at least 60—and 42 of those have been discovered since 1997. The dwarf planet Pluto has three moons. Eris, another dwarf planet, has one moon. Dozens more moons orbit small solar system bodies.

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