The origin of the moon is a mystery as old as the tides. No other planet boasts such a singular and striking companion: Mercury and Venus have no moons at all, Mars has two tiny spud-shaped satellites, and the many moonlets of the gas giants are tiny in comparison with their mother planets. Pluto has, in Charon, a mate that rivals its relative size, but two or three Charons—or Pluto itself, for that matter—could fit inside the moon. Ours is bigger. Even if the Apollo samples hadn't dashed all hopes, certain inconvenient laws of physics make the old lunar-origin theories suspect. The main problem has to do with angular momentum, a measure of rotation in a system. In the Earth-moon system, Earth spins on its axis in the same direction that the moon travels in its orbit. Physical laws require that the combined momentum of these two intimately linked rotations stay the same over time.
Now consider another aspect of this pas de deux: The moon is moving away from Earth by more than an inch a year. Scientists in the 1930s calculated that rate from ancient astronomical records, and laser reflectors planted on the lunar surface by Apollo astronauts confirmed it. The moon was much closer to its parent when it formed more than 4 billion years ago—probably 15 times closer. Today it's about 240,000 miles away. Originally the distance might have been only 16,000 miles—just four Earth-radii away. It would have loomed 15 times larger in the sky, had anyone been around to see it.
The laws governing angular momentum insist that, if the moon was once closer to Earth, then Earth must have been rotating faster. The same principle makes a figure skater twirl more quickly when he tucks in his arms and legs. The days back then probably lasted just five hours, Canup says. Even so, dynamic models reveal that the ancient Earth was spinning too slowly to toss off a chunk of its own heft, as in the fission model. It was spinning too quickly, on the other hand, to make the capture of a moon in a close orbit likely. And co-accretion wouldn't have put enough spin on the system. When these failings became evident, shortly after Apollo, lunar scientists' disillusionment was complete. Planetary scientist William Hartmann went back to the drawing board.
