8: Bad Moon Rising
Neil Armstrong: “I’ll step off the LM now...That’s one small step for man, one giant leap for mankind.”
It’s July 21, 1969. It has been one hour since Neil Armstrong and Buzz Aldrin became the first humans to land on the Moon. The famous 10 words have been spoken, flags have been placed, and President Nixon has sent his congratulations.
One hour later, another 10 words ring out from Houston, Texas. They are less poetic than Neil’s words, but for geologists, they are just as important.
Mission Control: “Neil Armstrong has the scoop for the bulk sample collection.”
For the next 15 minutes, using scoops, tongs, and hammers, Neil and Buzz collect 21 kilograms, 47 pounds of Moon rock from the Sea of Tranquility. Beyond the politics, beyond the space race, beyond the truly monumental achievement of the entire NASA team, Apollo 11 was a geological field trip. Once the Moon rocks returned safely to Earth, they were sent to laboratories around the world- the first time humans could really study our closest neighbor up close and personal.
The first question on everyone’s minds, the question we’ll tackle today:
Where did the moon come from?
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Part 1: Gathering Evidence
Every culture has a story that involves the moon. Sometimes the moon is a deity, running from or chasing after something. Sometimes the Moon is an island, a prison, or a refuge. The Moon is our closest companion, always present but just out of reach, always changing yet never permanently.
Today, we will learn the scientific story of how the Moon formed, 4.5 billion years ago. It is as dramatic and apocalyptic as any ancient tale, and even better- we have evidence for it.
Before the Apollo missions, there were three competing ideas on the Moon’s birth. Let’s line them up like contestants on a quiz show, and see which one will take home the grand prize. It’s time to play: Is That Our Moon?
1: Our first contestant is the Moon-Capture Hypothesis
This idea is straightforward- the Moon formed very far away in the solar system and was captured later by Earth’s gravity like a lasso on a bull. Similar events happened many times around Jupiter and Saturn- many of their moons used to be free-range asteroids.
Do the Apollo samples that Neil and Buzz collected fit this idea?
Sorry, but rocks from the moon have similar chemical fingerprints to rocks on Earth. In fact, Moon rocks are more like Earth than any other body in the Solar System. If the Moon formed much closer or farther from us, it should have a chemical signature closer to Venus or Mars. But instead, the Moon looks like Earth.
There are a few major chemical differences between the Earth and Moon, but their chemistries are too similar to be a coincidence. Our Moon was not pulled from somewhere else, so we have to look closer to home for an answer.
2: Our second contestant is The Double Planet hypothesis.
What if the moon formed right next to Earth? A constant companion, slowly building itself up over time just like the Earth. If the Moon and Earth formed from the same batch of asteroids, you would expect similar ingredients, just like the Apollo samples. Jackpot!
Well… there’s a bigger problem: momentum. In space, momentum is what makes a planet spin. On a playground, momentum is what makes a merry-go-round spin after you push it. You can calculate the momentum of a planet or a merry-go-round using mass and speed: we know how much a planet weighs and how fast it moves. Looking at every other planet in the solar system, their momentums, mass and speed make sense… all except the Earth and Moon. Our two-body system has too much momentum for our mass.
For a comparison, let’s return to the playground. You see eight merry-go-rounds spinning in the field. One of them has two small kids on it, but is spinning very fast- too fast for those kids to have pushed it themselves. Logically, we conclude that an adult gave those kids an extra big push. So what pushed out the Moon in Earth’s ancient solar system?
3: Our last contestant is the Fission hypothesis.
The last two ideas had the Moon form separately from the Earth, but this next idea is completely different. What if the Moon split away from our planet like a hunk of clay? This might sound ludicrous, but the idea isn’t completely out to lunch. We already know that the Earth-Moon system has too much momentum: If you spin that merry-go-round fast enough, kids will eventually fly off. If you spin a potter’s wheel too fast, gobs of clay will also go flying out in all directions. That clay will have the same chemistry as the original pot, just like Earth and Moon rocks are similar.
In fact, this idea was put forth by Darwin in 1879, almost 100 years before the Moon landings. Sorry, not Charles Darwin, but his son George. George Darwin did some biology work like his famous father, but went deeper into geology, especially planetary geology. The idea I just described, of the Moon splitting off from Earth, was one of George’s biggest hypotheses. He called it “Fission Theory”- fission meaning “to split apart”. After developing Fission Theory, George became a fellow of the Royal Society, the Royal Astronomical Society, and a professor at Cambridge, winning awards left and right. So George has to be our big winner, right?
Wrong. For all his accolades and pedigree, George’s Fission Theory is probably not correct. He was right that the Earth-Moon system has too much momentum, but it’s not enough to break away the moon. Even if the Earth was completely liquid and spinning 6 times faster than today, no pieces would split off. It appears that simply spinning the Earth faster cannot account for the Moon’s birth.
I’m not trying to rag on George Darwin here. Was he a fraud? No! George’s Fission Theory was based on the best knowledge he had at the time, and it lasted for several decades before new data came in. This is how a lot of science works. For example, I would be shocked if all my papers completely hold up in 100 years. New observations and technology will test all of our ideas, and hopefully improve upon them.
For example, I’ve just shown you three ways the Moon wasn’t formed. What was the point of that, why didn’t I just skip to the modern idea? Because each time we question an old idea and test it properly, we learn something, and can form new ideas. We’ve learned that the Moon and Earth have very similar chemistries, so the Moon probably formed from Earth material, but not from fission like George Darwin thought- the spinning clay option. We’ve learned that our Earth-Moon system is spinning much faster than it should, but not fast enough to break apart on its own. That can only mean that something outside of Earth gave a boost of energy to our system, like the adult on the playground.
And so, through a careful process of elimination, we come to the wildest hypothesis yet, the hypothesis currently most accepted as the Moon’ origin story: planetary collision.
Part 2: The Moon Mother
For its first 100 million years, the first week of the Earth calendar, our baby planet avoided being obliterated by larger objects. All the other planets have also settled into their orbits, but there are still one or two wild cards floating around.
In the ‘70’s, scientists realized that if just one of these rogue planets hit the Earth, that impact could provide enough energy to explain our outsized momentum, the dizzying merry-go-round between Earth and the Moon. As outlandish as this idea sounds, it is not the only evidence that the early solar system was a cosmic billiards table. Uranus, the third-largest planet, was knocked 90 degrees on its side, so that its south pole is always bathed in sunlight. I’ve said it before, and I’ll say it again: the early solar system was a dangerous and violent place to be.
Using computer models, scientists can recreate this deadly billiards game, just like we saw in Episode 5 when the Earth avoided being swallowed by the sun. Inside computers, we can toss imaginary planets at the Earth and see what happens when they collide. If the rogue planet is too large, Earth is obliterated, but if it’s too small, the Moon doesn’t form. The perfect size is in between, around the size of Mars. Mars only has 10% of Earth’s mass. You might think “That’s not too bad, I could take something 10% of my weight.” You might think differently if I started shooting bowling balls at your head, ~10% the weight of an average adult.
Eventually, it grew kind of awkward to keep calling this object the “Mars-sized impactor”, and around 2000, this mystery planet was given an unofficial name: Theia. In ancient Greek mythology, Theia was one of the original twelve Titans, gods hanging around long Zeus, Hera, and their famous friends. Theia was the old goddess of sight and precious minerals, but more importantly, she was the mother of the moon-goddess Selene.
Unfortunately for us, Theia was not a very friendly mother.
When the planet Theia hit the Earth, debris flew out in all directions. Earth’s gravity swept up all of this material, which clumped together into a smaller body. If this sounds familiar, it’s essentially how the Earth formed around the Sun, except without the giant impact.
In one stroke, Theia solves the major problems addressed last section: The collision between Earth and Theia was like a huge push on the merry-go-round, adding extra momentum to the system. The impact also sprayed out tons of Earth material which clumped together and formed the Moon. This is why the Moon rocks collected by Neil and Buzz were so similar to Earth rocks- they formed from the broken teeth of our shattered world.
But wait, there’s more. You see, Neil and Buzz didn’t just take material from the Moon- they left things as well, and not just the American flag. They left something that would make our old friend Inge Lehmann very proud: a seismometer.
Just like Earth has earthquakes, the Moon has moonquakes- no lie, that’s the literal word for them. As we learned in Episode 6, if you can measure earthquakes, you can see inside a planet, or moon. Just like the Earth and all the rocky planets, the Moon has a crust, mantle and core. But the Moon’s iron core is much smaller, even for its size. Why do we care? This tells us that the Moon formed from iron-poor ingredients. If there was more iron around, the Moon’s core would be much larger.
The closest pool of iron-poor material would be the early Earth’s magma ocean, which we explored last episode. In fact, the Moon’s weird internal structure is good evidence for Earth’s separated mantle and crust. If Earth was more well-mixed when Theia hit, the Moon would have a larger iron core. In other words, forming the moon was essentially like grabbing a handful of cake, but only getting the frosting on top.
So we’ve got the rocks, the momentum, and even the Moon’s core on our side. One last time: Is This Your Moon?
Well, yes and no. You see, while Theia answers a lot of important questions, there are still a few big ones left. Most of them boil down to one problem.
If the Earth was hit by an alien planet the size of Mars, why isn’t there any trace of it left?
For Earth, that’s an easy answer: it’s the same reason we don’t have any rocks this old: Earth has recycled all the evidence through plate tectonics. But the Moon should be a giant freeze-dried time capsule: there’s no wind, water, or plate tectonics to change up Moon rocks. If our computer models are correct, there should be some weird chemical trace of Theia still surviving on the Moon.
Does this mean that Theia never existed? That the impact never happened? That this whole episode has been a lie and a cover-up funded by Big Moon?
Probably not. So many of the puzzle pieces we’ve gathered point to a giant impact. Unless extremely convincing new evidence comes up, dismissing Theia would be like throwing out the baby with the bathwater. Nearly every scientist accepts the impact hypothesis, we’re just nitpicking the details. Two or three major papers come out every year chipping away at major Moon questions, especially the problem of Theia’s lost pieces.
More evidence is showing that the Moon is not exactly the same as Earth. Some elements appear to have distinct fingerprints, possibly from realms beyond Mars or Jupiter. Closer to home, some models suggest that the Earth and Moon intermingled before settling back into well-behaved spheres, increasing their similarity. Or perhaps Theia was just similar to Earth, though statistics argue against this idea.
As we round out this episode, let’s summarize what we’ve learned.
We take the Moon for granted, but looking at it from the outside, we live in a strange two-body system. Clearly, something weird happened here a long time ago.
Our Moon was not captured from outer space, it did not peacefully grow alongside us, and it was not spat out like a seed. A planet called Theia struck a glancing blow, spraying Earth material in all directions. The Earth recovered from this catastrophe, picking up the pieces to form something new, strange, and beautiful: the Moon.
The story of the Moon is also the story of humans who have explored and studied its secrets: the Apollo astronauts, the Mission Control members who guided them, and generations of scientists studying chemistry, earthquakes, and computer models. The story is not over yet, and I’m excited to give you more updates as they come.
Next time, we’ll return to the Earth and look at the aftermath of the Theia impact. We may be bruised, but we’re still standing. It’s time to begin the second week of the Earth Calendar.
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Thank you for listening to Bedrock, a part of Be Giants Media. As the show takes off, I would love to hear your input on style, topics, and people to interview- you can drop me a line at bedrock.mailbox@gmail.com. See you next time.
Images:
Apollo 11: https://commons.wikimedia.org/wiki/File:Aldrin_Apollo_11.jpg
Merry-go-round: https://commons.wikimedia.org/wiki/File:Homemade-merry-go-round.jpg
George Darwin: https://commons.wikimedia.org/wiki/File:George_Darwin_SI.jpg
Theia Clip: https://commons.wikimedia.org/wiki/File:Big_Splash_Theia.gif
Music:
Claire de Lune by Claude Debussy: https://commons.wikimedia.org/wiki/File:Clair_de_lune_(Claude_Debussy)_Suite_bergamasque.ogg
Buzzer: https://commons.wikimedia.org/wiki/File:Aggressive_electric_buzzing.ogg
Moonlight Sonata by Johann Sebastian Bach: https://commons.wikimedia.org/wiki/File:Moonlight_Sonata.ogg
Song to the Moon by Antonin Dvorak: https://commons.wikimedia.org/wiki/File:%22Song_to_the_Moon%22_from_Dvořák%27s_Rusalka_-_United_States_Navy_Band.ogg
Cold String by Tiny Music
Murder Case Investigation by High Street Music