Around 3.9 billion years ago a massive asteroid may have slammed into the moon with such force that it changed the satellite's rotation, according to a new analysis by a pair of astrophysicists. The impact may have set the moon to spinning, so that it eventually settled down with a 180 rotation from its previous orientation. Currently, earthlings looking up at the moon always see its same side; the other "dark side" of the moon is pointed away
as a result of synchronous rotation, a sort of orbital lockstep that keeps the moon rotating once for every lap it takes around Earth [Scientific American].
The new findings suggest that Earth had a different view of the moon 3.9 billion years ago, although there was probably no life on the planet to take notice. The researchers came to this surprising conclusion by analyzing the moon's craters.
According to earlier computer simulations, the moon's western hemisphere as viewed from Earth should have about 30 per cent more craters than the eastern hemisphere. That's because the west always faces in the direction in which the moon orbits, which makes it more likely to be hit by debris, for the same reason that more raindrops strike a moving car's front windshield than its rear [New Scientist].
When researchers examined the age of the craters, however, they found a more complex scenario. The western hemisphere did have the greatest concentration of young impact basins, but the eastern hemisphere had most of the old craters. This suggests that the eastern hemisphere was once positioned to receive a heavy bombardment, and that the moon once had a different orientation. Researchers believe that our young solar system was a violent place around 4 billion years ago, when the Earth and moon were subject to a series of impacts known as the "late heavy bombardment." In the new study, to be published in the journal Icarus [subscription required], researchers suggest that one of these large impacts
would have put the satellite's rotation rate out of whack, so that for tens of thousands of years it would have appeared to slowly turn as viewed from Earth. Eventually, it would have settled into the current position [New Scientist].
Study coauthor Mark Wieczorek says that before the asteroid the moon was in a synchronous rotation, and that the altered orientation would likely have been a 180-degree-turn from its previous alignment. He notes
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.... "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 [Scientific American].
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