Our moon being formed.

[u/howimetyourcakeshop]

[u/slavaukrainifp2]

Always been fascinated by how we know so much about our own history - Even before writing and all the way back to pre historic events.

But this… shit, how 🤯

[u/grigby → u/slavaukrainifp2]

There were a few clues that led people to this idea. Btw this is called the Giant-Impact Hypothesis, where earth was hit by a Mars-sized planet named Theia.

First, the moon is weird. Why do we even have it? Mercury and Venus don't have any. Mars's moons are tiny. The gas giants have similar sized moon's but the planets are so large that they captured them easily. Our moon is defined as a planetary-mass object. It's bigger and heavier than any dwarf planet (including Pluto) and almost as large as mercury. Most models of solar system formation could not explain how a modest planet the size of earth in the inner solar system could just have a moon this large in relation to the size of the earth. So people thought for a long time something special was going on. Maybe it was a dwarf planet that somehow didn't get captured by Jupiter and found its way to earth. Problem with that though is that the orbit of the moon is almost parallel with the earth's rotation. What are the chances that the captured moon just so happened to be alogned with the equator?

Two, the moon is made up of rock that is very similar to earth rock. Same proportion of elements. That is strange. Every "naturally forming" planet or moon have different proportions of elements based off where in the solar accretion disk the planet formed in the early solar system. We've known this for a long time as we have moon meteorites (meteor hits the moon and ejects a bunch of rock debris, some lands on earth), and was confirmed by the Apollo missions.

Three, testing the igneous rock on the moon's surface which was created from cooling magma allowed us to date when the moon's crust formed. It formed "late". Later than the other planets and moons according to our best estimates. Also the fact that the moon was this much magma doesn't align with how much heat should have been there if it formed on its own.

Four, the earth-moon system has way higher relative angular momentum than other planet-moon systems in our solar system. Where did that momentum come from?

Five, the moon is much less dense than the earth. If it just formed from gravitational effects then it's density should be higher. The small iron core of the moon would fit with the idea of it being primarily created from the blown-off parts of earth's (and theia's) mantel(s).

Lastly, inside the earth the shape of the inner mantel is weird. There are two continent-sized bulges, one under Africa and one under the Pacific. This is very difficult to explain other than them being the remnants of Theia's core/mantel.

Ultimately it's still just a hypothesis as every piece of evidence can be explained in other ways. Taken together though everyone is pretty sure this is what happened. And then to test it, we've made hyper accurate models to simulate the impact. Most simulations do not predict the moon forming, but some do. Those few that do have Theia at a very specific angle and very specific speed and do result in the moon forming, at exactly the parameters that we measure today. This post is showing one of those simulations.

[u/Haunting-Vanilla1624 → u/grigby]

Lastly, inside the earth the shape of the inner mantel is weird. There are two continent-sized bulges, one under Africa and one under the Pacific. This is very difficult to explain other than them being the remnants of Theia's core/mantel.

This has fascinated me ever since I can remember. How do we know what the earth looks like inside without actually being inside? Genuinely. Like, I know the general idea of the science behind it and and whatnot, but the fact that we somehow figured it out because of earthquakes and shit is absolutely wild to me. And I still don't fully understand it.

[u/RollinThundaga → u/Haunting-Vanilla1624]

To expand on earthquakes; we know how fast the speed of sound through homogenous rock should be, and thereofre how fast earthquake tremors should propagate through an undifferentiated Earth.

However, we know that the Earth isn't undifferentiated. Knowing where the earthquakes occur, we can use the data points of when the shockwaves traveled through the Earth to be measured by different seismographs to give us a picture of the layers that it traveled through, a more clear picture forming the more earthquakes we measure.

Kind of like the ultrasounds given to pregnant women.