AaS! 245: What Made the Moon?

Ask a Spaceman!

Paul M. Sutter

Astrophysics, Science, Cosmos, Holes, Black, Astronomy, Natural Sciences, Universe, Cosmology, Space, Physics

4.8 • 853 Ratings

🗓️ 8 April 2025

⏱️ 25 minutes

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Summary

Why is the moon so special? Why do we think a giant impact created it? Why didn’t the same thing happen to other planets? I discuss these questions and more in today’s Ask a Spaceman!

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Transcript

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0:00.0	The moon is weird. It's weird. Look around at the rest of the solar system. And I dare you to find
0:15.8	something as weird as the moon. First off, it's big. It's huge. I mean, it's small compared to the Earth.
0:23.4	It's just around 1% of our mass, but as solar system objects go, it's on the big side.
0:29.7	Heck, Mercury is only a few times more massive than the moon, and it gets to be its own planet.
0:35.7	And yes, there are other large moons out there,
0:38.5	but those are all around the giant planets like Jupiter and Saturn.
0:43.3	The moon is big relative to the Earth.
0:47.4	Like there are larger moons like Ganymede and Europa,
0:50.3	but those are tiny, minuscule, nothing at all compared to their parent planets.
0:56.2	The moon compared to us is huge. There is no other system like this in the solar system,
1:01.5	except for Pluto and Sharon, and we all know what happened to Pluto. And just look at the
1:06.1	inner solar system, our nearest comparables. Mercury, no moons. Venus, no moons. Mars has two moons, but come
1:15.0	on, they're really just captured asteroids and they're not likely to stay in orbit for long,
1:20.0	astronomically speaking. So those don't really count. Of all the rocky planets, we're the only
1:26.4	one with a substantial moon. But that's not the only one with a substantial moon.
1:29.3	But that's not the only weird thing about the moon.
1:32.0	The moon orbits us really quickly, especially given its size.
1:37.9	In fact, if you add together the Earth's own spin, add in the spin of the moon, and add in the orbit of the moon, you get the total angular momentum of the Earth's own spin, add in the spin of the moon, and add in the orbit of the moon. You get the
1:45.8	total angular momentum of the Earth moon system. And when you take that total, we blow the other
1:53.0	rocky planets out of the water. How did we get so much spin? It gets weirder. The moon is creepy. Yes, that's an actual science term,
2:03.7	but it's an acronym, K-R-E-P. K is the symbol for potassium. R-E-E-E-E stands for rare earth
2:11.8	elements, and P is for phosphorus. Creep. Yeah, that's a real stretch even by my standards, which are admittedly pretty low, but the idea
	...

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