Those who cannot remember the past are condemned to repeat it (because they went extinct from their economy being wiped out by an asteroid) - George Santayana
What I would give to be able to see how that all came together over that timespan in person. Have to imagine observing something as dramatic as this kind of planetary collision over just half a day or so would have been awe inspiring.
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.
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.
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.
I really hope you write and/or teach because I'm having a pretty shit day and really enjoyed reading this. It made me think of Kurzgeagt's 4.5 Billion Years In An Hour video.
It's amazing how much you can prove with simple math. In fact, off topic but, there's a whole philosophical dilemma called Laplace's demon that questions the nature of freewill when we could hypothetically calculate all events past and future.
Earth is a strange planet for this reason. Because Earth's core is so large in proportion to it's diameter it has a stronger magnetic field at the surface and is denser than a planet formed through normal gravitational accretion. If Earth was any lighter, it would have ended up like Mars, unable to hold it's atmosphere.
Earth is probably at the very low end of habitability due to its size. Most exobiologists look at what we call super Earth's because they are more likely to have complex life.
Another cool fact is that the further you go back in time the shorter the day lasted on Earth and the closer the moon was. 65 million years ago a day was around 23 to 23.5 hours long and the moon was 2 to 3 thousand km closer.
And do you know there’s a very narrow window where planets have the right mass/density to allow for escape velocity?
So maybe larger worlds are more habitable but life on it is trapped in their gravity wells. And lighter planets don’t normally have cores large enough to allow for magnetic fields strong enough to protect their atmospheres.
I'm pretty sure it's in Asimov's Foundation series where a character is actively looking for Earth, which has long been abandoned and lost, and the character sees Earth's statistics ie moons, size, density etc and exclaims that there is no such planet, no planet that small has a moon that big etc. it very much highlights how rare our planet could be.
There are many other ways to leave the planet, it's not just chemical rockets, there is Project Orion which has the concept of using nuclear bombs detonating behind rockets in a controlled manner to send them to space for example, if they are intelligent life desperate to leave their star system knowing that they are going to die in it's red giant state, they would go out by even doing that!.
Not entirely. The reason we look at superearths, is because they are a) much more common than truly earth like planets b) much easier to detect.
We guess that superearths might be more likely to have complex life but we also can’t say much for sure, given we have only guesses at what spurs the development of complex life.
Well the reason super Earth's are looked at is simply because of detection bias. Planets the size of Earth are harder to detect and then you have to try and read a planet's atmosphere by the starlight that shines through it. That is right on the edge of our current technology. So a bigger earth is both easier to detect and establish as an 'Earth'.
The reason for super Earth's being thought of as being more habitable just comes from hypotheses around atmosphere retention. You go slightly less dense with earth, weaken the magnetic field a bit you end up with Mars. So a bigger 'earth' is safer from this fate.
Mercury is still a bit of a mystery. It's thought that mercury initially formed further out, was hit (like earth) that stripped off a bunch of the outer crust exposing the core and migrates towards the sun. Back then the sun was far more unstable and gave off way more hard radiation and blasted away most of the rest of the softer material leaving basically a great big cannonball iron core.
The scary thing about the rare earth hypothesis is, that if we fuck it up, that might be it for the prospect of an intelligent space fairing species exploring the universe
I can give some background here! The moon orbits the earth every 27.3 days, whereas the Earth rotates once every 24 hours. Because of this, the earth drags the tidal bulge in the direction of its rotation, slightly ahead of the moon instead of pointing straight at it.
This in turn pulls on the moon, increasing its orbital angular momentum relative to the earth ever so slightly. More angular momentum means a larger orbit, and the moon’s orbital distance increases by roughly 3.8 centimeters per year.
This also means that, while the moon gains orbital angular momentum, the earth loses rotational angular momentum, so while the moon gains distance from the earth, the earth actually slows down its rotation.
Theoretically, in the eventual equilibrium state the other commenter noted, the earth and moon are both tidally locked to each other, earth completing one rotation and the moon completing one orbit in exactly the same amount of time, roughly once every 47 years.
Obviously this assumes a closed system with no influence from the sun, so it wouldn’t look exactly like this, and as the other commenter said it would take such a ridiculously long time (almost triple the current age of the universe) the sun will have expanded and destroyed the earth and moon far before this theoretical equilibrium state would ever occur
Okay I have one more cool earth moon factoid. They Earth moon system is gradually trying to reach orbital resonance, which will mean they will be doubly tidally locked. So at from the closest moon side of the Earth the moon will never appear to move. Unfortunately this bleed off of orbital energy will take about 37 billion more years. Before that happens the Sun goes red giant and burns us up.
Actually no! The selenium in your body probably comes from meteors that hit afterwards. (Search for the Veneer hypothesis)
The phone you’re probably holding right now is only possible because of post-Theia meteorites. The early planet did have gold and other metals but they sank down to the core, never to be seen again.
I love this simulation. I makes me deeply uncomfortable in a weird way. Not because of a future concern, but the memory that our planet was once jiggly molten chaos after a collision with a similarly sized body. I studied astrophysics... I rationally know this. But seeing it makes my tummy feel bad. Love it.
It's kinda like the visualisation of what would happen to a small object (like a human) passing through the atmosphere of Jupiter towards the core... The compression as you pass through thicker and thicker layers of matter and closer to the center of gravity. I picture it more often than most, because it makes me feel icky and I'm grim I guess.
Even without perfectly understanding the physics behind such events, just the sheer VIOLENCE of this impact is insane
I mean we live on that ball, it's quite literally our whole world and it's absolutely massive from our perspective, to just see it being it with such force that it basically liquefies is almost incomprehensible
Not just the violence, but the speed of that violence. Like in my head, despite knowing how fast these things are moving, the collision is slow - Theia over time scraping against the surface and crushing things. But that’s absolutely not what happens. This is all over in less than a day. That’s insane.
There are articles floating around that talk about evidence from seismic observations that the Earth's core layers aren't as clean as we see drawn in the science books. According to what I read, there appears to be density anomalies (the core is lumpy) that lend credence to this theory.
Probably more because the early Moon's close influence kept the Earth's interior liquid for longer, at a point where other planets' interiors were cooling off and solidifying. It's the liquid interior sloshing around that sustains our magnetic field.
But yeah, the Moon increasingly seems tied up pretty closely in what makes Earth special.
It's impossible to know for sure but we can make fairly good guesses by looking at the composition of the Earth and the Moon and the running simulations of what we think might have happened. Then we can see if the results are similar or if there's something we're missing
This is SUPER cool, and timely as well. My daughter came home from school yesterday and told me that the moon was formed when a larger object hit the Earth, and being able to show her this visualization of it today was just perfect.
Yes, directly. I heard that Theia is one of the reasons we have so much water on our Earth. It was a water/ice planet. Trout population before the collision was 0.
Could be hyper intelligent trout evolved, advanced their technology so far until, fulled with ennui for the distant stars they could never reach, they magnetised Theia to make sure no trout would ever have to feel that depression again.
Of course, years later we put singing ones on walls, so they should have planned ahead better.
I vote we take a moon of mars turn it into a ship and send it to tau ceti to test it out. Let's also put a few AIs on it for running the ships doors and stuff
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).
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
New high-resolution supercomputer simulations by NASA and Southwest Research Institute researchers suggest the Moon may have formed intact in about 5 hours after a Mars-sized protoplanet named Theia slammed into the early Earth, rather than coalescing from a debris disk over months or years.
I’ll be diving for my computer mid air as I’m thrown into the seismic wave of magma and debris so I can click “deny” on one last health insurance claim and die in peace.
“Judgement day either is or is not coming. If it is not coming, I don’t want to lose a day of productivity, if it is coming, I’d rather be found working.”
I had to Google it cause the amount of time in the video seemed wild to me, but apparently new models put it at taking around 5 hours for the moon to form after the initial impact. That seems so crazy to me.
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u/redditsucksass69765 7h ago edited 6h ago
Imagine this happens again today. It will devastate the economy.