Genuine question, how do they figure stuff like this out? I don't understand the science behind it or even the logical approach for such an event even happening
If you know the current position and movement of both bodies, and the laws of motion, you can wind the clock back and predict what probably happened. Fun fact, the moon moves further away from the earth every year even now.
Yeah, sort of. We know the moon is made of mostly the same stuff as earth, with some differences. And on earth we see some similarities in different proportions. It's those slight differences that lead to the theory of another celestial body colliding. Then they run millions of different models with varying sizes, material compositions, initial conditions (spin, momentum, direction, etc.) and then look for convergence of the models with current or known conditions. If you've got a number of models that tend to show the conditions we see, then you can theorize that you're on the right track.
The earth's spin is only one of the aspects. The moon doesn't spin, but rather wobbles slightly, as it's tidally locked. I'd imagine it was spinning atbone point and because tidally locked due to proximity and gravitational effects (this is fairly common with systems with a larger and a smaller body).
I thought tidally locked meant it spins at the same rate of its orbit. So it does spin but also moves around earth at the same rate. Thus the slight wobble…is my understanding incorrect?
Yes, it spins once per orbit, but does not spin relative to earth. It likely used to spin much more, but gravitational forces over time slow that to lock it with the earth's rotation. My point wat that it's not spinning the same as it would have when it was formed.
Another fun fact, the moon moves further away from the Earth because angular momentum is being transferred from the Earth to the moon. Angular momentum transfer is due to the tides. As a result, the Earth’s rotation is slowing. According to Wikipedia, 620 million years ago, the day was 21.9 hours long and there were 400 days a year.
same, mine always thinks we should be up an extra hour or so no matter what we did yesterday. it's miserable trying to keep it on earth schedule. maybe i really am the alien i feel like sometimes.
Fun fact: while everything in space has a terrifyingly huge scale, the moon is only moving away less distance than the average human finger every year.
That’s not how this was figured out. Astronomers cant just wind the clock back and find what happened second by second after the big bang or even when the moon was formed.
Here's a sampling of some of the evidence that supports this, from Wikipedia:
The Moon's orbit has a similar orientation to Earth's rotation,[8] both of which are at a similar angle to the ecliptic plane of the Solar System.
The stable isotope ratios of lunar and terrestrial rock are identical, implying a common origin.[9]
The Earth–Moon system contains an anomalously high angular momentum, meaning the momentum contained in Earth's rotation, the Moon's rotation and the Moon's orbit around Earth is significantly higher than those of the other terrestrial planets. A giant impact could supply this excess momentum.
Moon samples indicate that the Moon was once molten to a substantial, but unknown, depth. This may have required much more energy than predicted to have been available from the accretion of a celestial body of the Moon's size and mass. An extremely energetic process, such as a giant impact, could provide this energy.
The Moon has a relatively small iron core, which gives it a much lower density than Earth. Computer models of a giant impact of a Mars-sized body with Earth indicate the impactor's core would likely penetrate deep into Earth and fuse with its own core. If this occurred, it would leave the Moon, which was formed from coalesced ejectae of lighter crustal and mantle fragments that went far enough beyond the Roche limit and thus were not pulled back by Earth's gravity to re-fuse with Earth, with less remaining metallic iron than other planetary bodies.
The Moon is depleted in volatile substances compared to Earth. Vaporizing at comparably lower temperatures, they could be lost in a high-energy event, with the Moon's smaller gravity unable to recapture them while Earth did.
There is evidence in other star systems of similar collisions, resulting in debris discs.
Giant collisions are consistent with the leading theory of the formation of the Solar System.
Depending on your point of view, maybe none of these are a smoking gun. But it's an educated guess that explains a lot of little odd things. And then you can run simulations like the one pictured above and see that with the right initial conditions for the impact, the results are plausible.
It's pretty cool I can't explain well but they did mapping of the earth core and realized it's made up of theia/moon material and the moon has earth material and they deduced this is what must of happened
In addition to things other people have mentioned, our moon is "too big". Mars has two moons that are basically little lopsided rocks. Mercury and Venus have no moons at all. And here we are with a big ol' moon that has more than enough mass to go sphere-shaped. So that's how we got to "something interesting must have happened" in the first place.
If youre interested, the below is review paper on how we run cosmological simulations. These are simulations which evolve extremely large chunks of the universe from start to finish (its not planet collisions but alot of the underlying codes are the same e.g. SWIFT is a smoothed particle hydrodynamics code that then has planet collisions and cosmology versions on top).
Lots of it comes from observation and prediction. Then suggesting ways to prove the prediction.
And then involving a TON of math that the vast majority of people would not have the faintest idea of understanding.
One of my favorite examples is how they identified that Earth's core is made up of molten iron.
(I don't have the exact details, so I am heavily paraphrasing here)...
When scientists figured out how big earth was, they first assumed it was a solid mass. But then they realized (based on Earth's orbit around the sun, the velocity of that orbit, and its gravitational field) that there must be a deposit of iron at the core to account for the weight. But then they figured that, if it was solid iron, the rotation wouldn't be what it is. So they theorized that it would be a core of molten iron.
Then, all that math came into play, and they figured that, based on Earth's size, rotation, velocity, orbit, gravitational field, and probably a lot of other factors, how large the molten iron deposit is.
Then they devised an experiment to test it. Vibrations move through solid material faster than they do through liquid. So they said that, if what they assume is true, then if there's an earthquake on one part of the earth, knowing how fast vibrations move through solids and liquids, they would expect toeasure the vibrations in the other side of the earth at two different times. One time, based on how fast the vibrations would travel along the surface. And a second, later, time, based on the vibrations going through Earth's core.
If the predicted times were correct, it was because the vibrations passed through a core of molten iron of an estimated size.
And it was true.
Now...there's still no 100% way to know this, as we can't access Earth's core. However, if all those calculations and models predict and confirm that Earth's core is made up of molten iron, then it's safe to accept it as fact.
If at some future point scientists drill down to the core and find out it's actually some mysterious, unknown, material that behaves exactly like molten iron, then they'll update the understanding.
But until that time...molten iron is good enough.
I know this is unrelated to the topic...I just find it fascinating that humans are able to figure stuff like this out.
so they make a guess or 20 and then they look at all the evidence and data they can collect.
and then they try to agree on which guess holds the most water based on what is actually known.
then once more information becomes available people might revisit it and maybe agree on something else.
but usually once an idea becomes popular it becomes entrenched.
A big part of it is trial and error. If you're studying the big impact hypothesis, you build models and vary the size of Theia, Earth, and the speed and angles of the collision and see what happens.
We've been making these models for decades, so they're getting pretty refined.
The fact that the moon recedes indicates that it used to be a lot closer - close enough to make its natural accretion in parallel with earth a bit troublesome. Then, back in the 60's when we brought samples back from the moon's surface, we found that they share certain chemical traces with earth material that indicate a common catastrophic origin. Since then we've found a lot of other indicators that point towards the hypothesis, including what appears to be the remains of Theia's core deep within the mantle mapped out with seismic scanning technology. It's fascinating stuff, it's called the Giant Impact Hypothesis if you want to read more about it
We make a best guess based on what we see today, and then the person who made that guess published it and says “I dare you to find a problem with this or make a better guess” and we see what happens. That’s science
It’s just the most prevailing theory right now, and it’s not even that old, and it’s constantly “evolving”. There are other theories, but this is the one that makes the most sense in our current model. The problem is that simulations of the collision that best explain the Moon’s chemical similarity to Earth can produce too much angular momentum, while simulations that match the angular momentum often predict a Moon with a different chemical composition. Scientists are still trying to explain how the collision could produce both the chemistry and orbital motion we observe today. Our moon is truly a wonder; it is certainly a difficult thing to explain in entirety.
One way is "inverse simulation" where you can try a lot of different simulations that get the same end result we see.
The simulations would use known laws of physics.
Then as you gain more data about the past, you can rule out the possibilities.
For instance, by gathering data on the composition of the moon you can rule out a lot of possibilities. If there are pieces of ancient earth in both places - THAT tells you a lot.
Now if we find a rusty beer can somewhere, THAT would be awesome.
Literally the same way you’d look at the evidence of a car crash and say “yea looks like this one slammed into the back of that one”. Just on a cosmic level.
Essentially using the facts today to make assumptions on what happened before. The chances you can be fairly accurate are good. The chance that you are completely wrong still exists.
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u/Universe_XoX 9h ago
Genuine question, how do they figure stuff like this out? I don't understand the science behind it or even the logical approach for such an event even happening