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Geo-Joint: Lunar Origins

Posted on March 13 2018

 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?

Our faithful companion, the moon.

There’s no recipe for planet-making that says each planet shall have one moon. The other planets in our solar system have multiple moons. Ours is pretty hefty—only four others are bigger, and at a quarter the size of Earth, it’s
the biggest moon in comparison to its planet. But the question remains: how did it get there? For a long time there were three competing theories. One held that the two of us formed at the same time, from the same mass of debris in solar orbit. Another posited that Earth snagged the moon as it ambled by on some random path, pulling the smaller body into its orbit. And others thought the Earth got to spinning so wildly fast that it ripped itself apart, and the moon is the piece that broke off.

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.

Earth and Theia mixing it up, resulting in our current Earth/moon situation.

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.

Far side of the moon—rougher and more elevated, possibly due to a thicker cheese layer.

These are all still just guesses, and finding the scenario that delivers just the right combination of mineral chemistry and gravitational result is tricky. Another interesting twist on the story is the fact that the far side of the moon, the side we don’t ever see from Earth, has some odd characteristics. It has a rougher surface than the near side, with fewer of the broad sea-like plains, or maria (singular, mare), that you can easily view through a telescope from here. The more mountainous terrain on the far side contributes to an average elevation that is about 1.2 miles higher than on the near side. Lunar thinkers posit that Earth may have once had two moons, both formed from the debris of Earth’s big collision with something. One moon would have been quite a bit smaller than the other, perhaps only 4 percent of its mass, and the two may have co-existed in a cosmic orbital balance with the Earth for millions of years before incremental changes brought the two lesser bodies into a sort of slow-motion crash. Slow, in this case, is still reckoned to be somewhere in the neighborhood of 5,000 miles per hour! Still, the concept is that such a collision just sort of pasted the small moon onto the big one rather than making a big crater in it. Like “a ball of Gruyere colliding into a ball of cheddar,” as one of the researchers put it. In so doing, the back side of the moon gained that mile or so of added average elevation, tens of miles thick in some places.

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.

However it got up there, the moon continues to affect life in many fascinating ways here on Earth.

Have a great look at the moon day or night! This beautiful info-packed National Geographic poster, available from, shows both front and back sides of the moon and could be hanging on your wall! Click here:




caption: Our faithful companion, the moon.

source: Wikimedia Commons: NASA Goddard Space Flight  Center/Scientific Visualization Studio (Public domain)

caption: Earth and Theia mixing it up, resulting in our current Earth/moon situation.

source: NASA/JPL-CalTech/T.Pyle (Public domain)

caption: Far side of the moon—rougher and more elevated, possibly due to a thicker cheese layer.

source: Wikimedia Commons: NASA/Apollo 16 astronauts (Public domain)

caption:  However it got up there, the moon continues to affect life in many fascinating ways here on Earth.
source: Pixabay: unknown (CC0 – Public domain)

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