I've been teaching a class of high-school seniors about how the Moon's spin rate and orbital period became locked together, in what astronomers call synchronous rotation. Thanks to a combination of slight irregularities in the lunar interior and Earth's ocean tides, one hemisphere of the Moon must continually face toward Earth and the other must always be turned away from us. Sure, the Moon nods a bit this way or that, due to libration, but basically we're stuck with looking at its near side permanently.
However, a new study, about to be published in the journal Icarus, probes whether there once was no "Man in the Moon" to gaze upon.Mark Wieczorek and Mathieu Le Feuvre, researchers at the Institute of Earth Physics of Paris, find that the Moon's eastern hemisphere has a statistical excess of big and very old impact basins. Yet the Moon's current orientation argues for just the opposite: its western side, which faces forward as the lunar sphere zips around Earth, should have the lion's share of basins. If the Moon has been synchronously locked this way for its entire history, Wieczorek and Le Feuvre calculate there's only a 0.3% likelihood that this mismatch could have occurred by chance.
So they explore how big a push it would have taken to knock the Moon out of lock and flip it front to back. A big one, it turns out: an object at least 30 miles (50 km) across would have to strike a glancing blow fast enough to gouge out a basin a couple hundred miles (350 to 500 km) in diameter. Of the known lunar basins, only six could have done the trick, and the best candidate seems to be Smythii. It's large enough, 350 miles (570 km) across, and it's on the lunar equator.
The putative strike must have taken place at least 3.8 billion years ago, when the Moon was roughly only half as far away as it is now. The strike wouldn't have changed the Moon's spin axis much, but it would instantly alter the spin rate. Primordial Earthlings would have seen the lunar face swing side to side like a pendulum. If this forced libration exceeded 90°, the Moon would have briefly rolled all the way around.
After about a year tidal torques would have regained control and stopped this merry-go-round. But, as Wieczorek and Le Feuvre note, "It would be only a matter of chance as to whether the same face of the Moon would be directed towards the Earth as before the impact."
The French researchers aren't the first to examine the idea of a lunar flip-flop. Impact specialist H. Jay Melosh (University of Arizona) looked into it more than 30 years ago.
The problem then, as now, is that lunar geologists aren't sure they have a complete census of the most ancient impact basins. Many might lie buried under the vast lava plains that formed the maria; dozens more might be hidden under rubble from more recent strikes. That ambiguity should be resolved when scientists complete the improved topographic maps now being assembled from the Kaguya, Chang'e 1, and Chandrayaan 1 orbiters — fortified by NASA's Lunar Reconnaissance orbiter later this year.