Everybody knows that people get crazier when the moon is full. And more babies get born then. And the moon is hollow, and full of aliens. OK, maybe none of that is true. But the moon surely makes the tides go in and out and helps birds and moths navigate, and tells grunion when to spawn, and encourages sweethearts to get all sappy. So the moon is a big influence on a lot of things, even if we’re only dimly aware of its presence most of the time. We certainly take it for granted as just another big thing in the sky that has always been there. But has it?
This was all before NASA sent some guys up to have a look-see, and they put a few rocks in their pockets before they came home. Those rocks showed that the earth and the moon have some very significant chemical match-ups, so the wandering-traveller-who-came-to-stay theory fell out of favor. An outlier wouldn’t have carried certain isotopes so closely mirroring Earth’s. Those moon rocks, similar as they are in some aspects, didn’t bear water, iron and some other minerals that should have been present if we had both formed from the very same glob of material whirling around the sun. And the spin-til-you-split theory suffered mathematically because if that had happened, the Earth/moon system would have a lot more angular momentum, meaning we would be spinning both on our axes and around each other faster than we actually do.
Scientists had to work up some new theories to account for the constraints that the new evidence imposed. And what better to pin it on than a good old-fashioned space collision! They figured it would take something about the size of Mars to bash Earth with enough oomph to tear off a moon-sized blob, which would then form a ring around Earth, and eventually coalesce into the moon. It would have to be less of a head-on collision and more likely a lower-angle crash, in order to break off a proper amount of Earth, but the models for such a scenario left too much of the intruder body in the soon-to-be-moon material. The moon rocks would have presented more foreign chemical content than they did. Other theories offer that a real head-on collision could have smooshed both Earth and its collision partner (which sometimes goes by the name of Theia) into a hot mush that gave Earth a new mix of ingredients—the same ingredients as the smaller chunk that coalesced into the moon. Another school of thought suggests that there were perhaps 20 or more impacts to the earth, each made by smaller bodies in the range of one to ten percent the size of Earth. The bits knocked off Earth each time would eventually gather themselves into the moon.
It’s extremely difficult to nail down the real story of the moon’s origin without more rock samples from more locations, and even samples from other planets, which might give clues to material sources and mechanisms of formation. Whatever the story, some fascinating process surely happened, because there is a moon out there in the sky tonight, and it didn’t just pop out of nowhere. Solving that mystery will probably keep astronomers and planetary physicists busy for some time to come.
Have a great look at the moon day or night! This beautiful info-packed National Geographic poster, available from Maps.com, shows both front and back sides of the moon and could be hanging on your wall! Click here:
source: Wikimedia Commons: NASA Goddard Space Flight Center/Scientific Visualization Studio (Public domain)
source: https://astrobiology.nasa.gov/news/support-for-a-catastrophic-formation-of-the-moon/: NASA/JPL-CalTech/T.Pyle (Public domain)