You know, if you think about it, and trust me we’re about to, the Moon is kind of weird. Of all the terrestrial worlds of the solar system, we’re the only one with a substantial natural satellite. Mercury and Venus have nothing. And while Mars technically has two moons, they’re really just captured asteroids and don’t really count. Sorry Phobos and Deimos, but that’s the way it is.
Our Moon is huge, at least, relatively speaking. It’s 1.5% the mass of the Earth, which is bigger than any other ratio with the exception of the Pluto-Charon system. So we gotta ask: how did we get our big ol’ moon?
In the early 20th century, there were dozens of ideas floating around academic circles. Maybe the Moon was a still-forming protoplanet that we captured. Maybe when the Earth was still molten we were spinning so quickly that we flung off a chunk, like when you stir cookie batter too hard, and that became the Moon. Maybe the Moon formed on its own from the collision of several smaller objects in Earth’s orbit.
Or maybe none of the above. “All those ideas are wrong” is a perfectly acceptable statement. And what did the trick was the Apollo mission. While there are a lot of observations we can make about the Moon from the comfort of our own homes or observatories, or from orbiting satellites, that’s not enough to resolve the question of where the Moon came from.
The Apollo missions hit paydirt. Or pay-regolith, whatever you want to call it. With the samples those astronauts collected, we found one key piece of information.
One was the unmistakable evidence that the moon was once highly molten in its past. You know, the whole globe-spanning magma ocean thing. Many of the rocks on the lunar surface are igneous – they’re solidified magma. And plus that, they’re creepy.
Hold on, I’m serious, this is a real thing. Many lunar samples are called KREEP-rich. Where KREEP is, of course, an acronym. The “K” is the symbol for potassium. REE stands for rare-Earth elements. And P is for phosphorous. See, KREEP. It totally makes sense and I swear was not made up on the spot as a joke.
The point is that these elements don’t usually hang out together, and the only way to get them in high concentrations in the same rock is to liquify the whole dang thing for quite a long time. So yet another piece of evidence that the Moon was molten, which requires a lot – and I mean a lot – of energy.
The second major piece of evidence is in the oxygen. Yes, the Moon has oxygen, a lot of it in fact, but it’s bound up in rocks (just like it is on the Earth, by the way). And there are some isotopes of oxygen that are stable, meaning that they don’t experience radioactive decay. Amazingly, the Moon has the same proportion of these stable isotopes as the Earth does, which is way more than a coincidence.
But there are also some key differences between the Moon and the Earth. The Moon has a lot fewer heavier elements, and like absolutely zero super-light elements. Just the middle stuff.
How do all these pieces fit together?
They fit together with the giant impact hypothesis. This is the idea that when the Earth was still forming, we were struck by a Mars-sized protoplanet named Theia. Theia, by the way, was named after one of the Titans of Greek mythology, the daughter of the Sky and the Earth, who eventually got imprisoned by Zeus deep within the Earth for the crime of…checks notes…right, being a Titan. The collision released not a little bit of energy, and sent a chunk of the Earth and some bits of Theia into orbit around us.
This explains the common isotope ratios: we’re made out of the same fundamental stuff. And it explains why the Moon had deep magma oceans (our favorite kind of magma oceans): because the whole scene just kinda blew up. And it explains why the Moon doesn’t have a lot of heavier elements like iron, because all that sunk to the center of the Earth (and we have an extra-large core thanks to that). And it explains why the Moon lacks any really light elements, because all that got vaporized in the collision and it didn’t have enough gravity to hold onto it.
Phew! The giant impact hypothesis is now the most common belief amongst scientists as to the origins of the Moon. Without the Apollo missions, we would have a much less complete picture, and be much less sure of our conclusions.
Any one of these advances in science – the composition and structure of the moon, the calibration of cratering rates, and the linchpin evidence for the giant impact hypothesis – could have justified the Apollo missions. But we got all three – plus more that I haven’t even talked about.
And that’s just the science! There’s also the enormous technological progress, the spinoff technologies, the advances in engineering, materials, orbital dynamics. The list goes on.
Scientifically speaking, the Apollo missions were an unqualified expense. Plus we got to land on the Moon, which is incidentally pretty cool in its own right.
The next time we start debating the value of future crewed missions to the Moon or Mars, I want you to keep all this mind. We know a lot more about how the universe works thanks to the Apollo missions. All for a measly 12 and a half days on the surface. Imagine what we could learn if we stayed there.
