Wieczorek: Moonface two-face

The man in the moon always presents us with the same mugshot, because the Earth's tides have locked the moon's spin to ours. But in a talk yesterday, Mark Wieczorek pointed out that not only did it not always have to be this way, but also that there is some evidence that the moon actually did swap its Earth-facing side at least once in the ancient past.

The work builds on a theoretical result in the 1970s from the University of Arizona's Jay Melosh, who showed that there were two equally stable ways in which the face of the moon could freeze toward Earth: the near side, and the far side. A glancing blow from a moderately big asteroid would be enough to do the job. Wieczorek, of the Institut de Physique du Globe de Paris, now shows that if that was the case, there would be a slight preponderance of big impacts on the moon's leading edge (marked 'apex' in the image here), since its orbiting velocity would be added to, rather than subtracted from, the impacting object. Lo and behold, he finds, the oldest impacts cluster around the moon's trailing face -- implying a flip-flop. "It's probably happened several times," he says. Most basin impacts would be big enough for the great switcheroo, but based on chronology, Wieczorek suggests that Smythii would be a likely candidate.

He says the process of a face switch could even start and stop temporary lunar dynamos -- which would be an interesting new mechanism for imprinting magnetic orientations onto lunar rocks.

Melosh was pleased that someone followed up on his theoretical idea, and says it needs to be tested on many of the Jovian and Saturnian satellites. "This suggests that this could be a common process with the other tidally locked satellites," he says.

LPS XL (2009)
Nature News