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.
"Just based on the physics, it's very, very, very probable that at least one and perhaps more of these impacts did this to the moon," Wieczorek says. "The second question, and this is the harder part, is finding if there's any evidence of this or not."
The researchers sought that evidence by examining the placement and age of craters across the lunar surface. If the moon's orientation had remained constant throughout its history, there should be more impact cratering on its western hemisphere, which is the leading hemisphere in the moon's orbit in its current orientation. (Wieczorek likens this to driving a car in a storm—more rain hits the front windshield than the rear.)
The analysis revealed that whereas the younger impact basins follow this pattern, the older ones tend to be found on the trailing side of the moon, indicating that the moon has swiveled 180 degrees about its axis since those ancient craters formed. According to their calculations, in fact, the arrangement of older impact basins near the now-trailing eastern hemisphere has less than a 0.3 percent probability of happening by chance.
A few caveats that Wieczorek and Le Feuvre are careful to note: Ages for most of the 46 impact basins studied are not well constrained, and some older basins might be obscured by ejecta from younger craters nearby, which would mean the current data set is incomplete. Better topographic maps now being pieced together by lunar orbiters such as India's Chandrayaan 1 and Japan's Kaguya could help clarify the historical record of asteroidal and cometary impacts.
H. Jay Melosh, a planetary scientist at the University of Arizona Lunar and Planetary Laboratory who has studied the effects of impacts on the moon's orientation, finds the new proposal quite plausible. Although Wieczorek and Le Feuvre's in-depth analysis of the cratering data may spark some arguments over the details, Melosh says, "the overall picture is both reasonable and well documented."