<P> The evolution of moon systems is driven by tidal forces . A moon will raise a tidal bulge in the object it orbits (the primary) due to the differential gravitational force across diameter of the primary . If a moon is revolving in the same direction as the planet's rotation and the planet is rotating faster than the orbital period of the moon, the bulge will constantly be pulled ahead of the moon . In this situation, angular momentum is transferred from the rotation of the primary to the revolution of the satellite . The moon gains energy and gradually spirals outward, while the primary rotates more slowly over time . </P> <P> The Earth and its Moon are one example of this configuration . Today, the Moon is tidally locked to the Earth; one of its revolutions around the Earth (currently about 29 days) is equal to one of its rotations about its axis, so it always shows one face to the Earth . The Moon will continue to recede from Earth, and Earth's spin will continue to slow gradually . In about 50 billion years, if they survive the Sun's expansion, the Earth and Moon will become tidally locked to each other; each will be caught up in what is called a "spin--orbit resonance" in which the Moon will circle the Earth in about 47 days and both Moon and Earth will rotate around their axes in the same time, each only visible from one hemisphere of the other . Other examples are the Galilean moons of Jupiter (as well as many of Jupiter's smaller moons) and most of the larger moons of Saturn . </P> <P> A different scenario occurs when the moon is either revolving around the primary faster than the primary rotates, or is revolving in the direction opposite the planet's rotation . In these cases, the tidal bulge lags behind the moon in its orbit . In the former case, the direction of angular momentum transfer is reversed, so the rotation of the primary speeds up while the satellite's orbit shrinks . In the latter case, the angular momentum of the rotation and revolution have opposite signs, so transfer leads to decreases in the magnitude of each (that cancel each other out). In both cases, tidal deceleration causes the moon to spiral in towards the primary until it either is torn apart by tidal stresses, potentially creating a planetary ring system, or crashes into the planet's surface or atmosphere . Such a fate awaits the moons Phobos of Mars (within 30 to 50 million years), Triton of Neptune (in 3.6 billion years), Metis and Adrastea of Jupiter, and at least 16 small satellites of Uranus and Neptune . Uranus's Desdemona may even collide with one of its neighboring moons . </P> <P> A third possibility is where the primary and moon are tidally locked to each other . In that case, the tidal bulge stays directly under the moon, there is no transfer of angular momentum, and the orbital period will not change . Pluto and Charon are an example of this type of configuration . </P>

Events that led to the formation of solar system