Flip-Flop: Did the Moon Do a Turnabout?

Posted on January 30, 2009 

Flip-Flop: Did the Moon Do a Turnabout?

For thousands of years only one side of the moon was visible to humankind as a result of synchronous rotation, a sort of orbital lockstep that keeps the moon rotating once for every lap it takes around Earth. Astronomers had to settle for this near-side view until 1959, when a Soviet craft took the first photographs of the moon’s far side. But could the view from Earth have been different early in lunar geologic history?

In a paper in press for the journal Icarus, geophysicists Mark Wieczorek and Mathieu Le Feuvre of France’s National Center for Scientific Research’s Institute of Earth Physics in Paris postulate that our natural satellite was once rotated 180 degrees, with the current far side of the moon facing Earth. A large impact roughly four billion years ago could have temporarily disrupted the moon’s rotation, the researchers say, allowing it to eventually settle back into so-called spin-orbit synchrony either in its original orientation or rotated 180 degrees. (Wieczorek says that the tidal bulges on the lunar surface induced by Earth’s gravity, which deform the moon into an elongated shape that helps stabilize its position, would prevent the moon from easing into synchrony at any intermediate orientation.)

Wieczorek and Le Feuvre first examined the size and velocity necessary for a sufficiently spin-disrupting asteroidal or cometary strike, turning up a few possible candidates based on cratering records on the lunar surface.

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