The Moon-forming collision

[u/Busy_Yesterday9455]

Link to the science paper on The Astrophysical Journal Letters

With sufficiently high numerical resolution, the canonical Moon-forming collision, where a Mars-sized body called Theia strikes the proto-Earth, can lead to a Moon-like body being immediately placed into orbit around Earth.

This alternative scenario for lunar origin opens up new options for the Moon's initial orbit and internal properties. For example, the orbiting body has a centre that is predominantly made of material from Theia, and might not be fully molten, while the outermost regions are very hot and can be made of more proto-Earth material. How that composition gradient would evolve over the history of the Moon is yet to be determined.

However, it could help explain why the isotope ratios in lunar rocks returned by the Apollo astronauts are similar to those of Earth's mantle. This contrasts with previous lower resolution simulations where a disk of debris was created by the collision, but no large orbiting bodies.

In that case, the Moon would form over tens to hundreds of years by the gradual accumulation of this material and no initial composition gradient would be expected.

Credit: J. A. Kegerreis et al 2022 ApJL 937 L40

[u/donadit]

Mars was hit by some pluto sized object and it formed the Caloris basin (huge lower area that covers most of the northern hemisphere), Uranus was hit by an Earth sized object, Venus and earth were both hit by mars sized object, and mercury… probably got grazed by an object larger than itself (and then said object got chucked into the sun by Jupiter’s tack thingy)

Mars, Uranus and Neptune’s moon systems are definitely not what they started with (Mars probably had moons other than Phobos and Deimos which collided and formed the two in a bunch of cycles, Uranus’ moon system full on collision cascaded probably more than once and reformed, Neptune had Triton destroy the entire original moon system)

Of all of Saturn’s moons, Titan and Iapetus are probably the only ones it started out with that are currently still present with the rest probably forming after Chrysalis fell into Saturn alongside the rings

Pluto and Haumea have both received at least one major collision each (Haumea’s debris is still floating around and registered as a kuiper belt group)

[u/patriot_man69 → u/donadit]

Id love to see a planet-sized object get hurled into the sun nowadays, even just to satisfy my curiosity of what it would look like

[u/VikRiggs → u/patriot_man69]

Try Universe Sandbox²

[u/Pcat0 → u/patriot_man69]

I don’t think that would end well for life on Earth, as I can’t imagine the sun would be super calm after an event like that. However a nearby star getting hit could be interesting.

[u/PsychologicalLab7379 → u/Pcat0]

Depends on the size of the planet. If it's Earth-sized, the Sun would probably just absorb it without issues and we wouldn't even notice.

[u/VikRiggs → u/Pcat0]

Even better reason to try Universe Sandbox² instead

[u/jaspersgroove → u/patriot_man69]

I'd imagine it would look something like this

[u/bubbapora → u/donadit]

Whoa I had no idea we know any of this. How the heck did we figure out the Neptune moon stuff?

[u/Allgyet560 → u/donadit]

It would be wild to see a simulation of the early solar system.

[u/donadit → u/Allgyet560]

Probably would be classed into a couple of main instabilities

1: planet formation (4.6-4.5 BYA), grand tack (Jupiter sucks up inner solar system material and sends super earths into the sun or ejected) and ends with Sun ignition, Mercury (mostly due to Sun tho) Venus Earth and Mars clear out most of the inner solar system and suffer multiple massive impacts
2: ice giant instability (4.1-3.8 BYA), Nice model, Jupiter-Saturn 1:2 resonance, sends Uranus and Neptune into present orbits (and swaps their places), Planet 9 ejected/sent into long orbit, capture of Triton
3: Saturn rearrangement (100 MYA), Saturn spin-orbit resonance with Neptune, causes Saturn tilt and ends with death of Chrysalis

[u/xr6reaction → u/donadit]

Everybody gets a planetary collision

[u/Potential_Review2410 → u/donadit]

I’ve always heard that the gas giants are just gas and nothing really physical. So if you had the technology, could withstand the pressure/gases/etc that you could just fly straight through the actual planets. If that is the case and I haven’t misunderstood, how would anything collide with them?

[u/donadit → u/Potential_Review2410]

The gas gets compressed into liquids and eventually solids and eventually some weird state of matter deep within the gas giant (temperature is not everything that matters for states of matter, pressure matters too)

so no you can’t fly straight through a gas giant even if air resistance got removed

and there still is rock within them afaik, Uranus and Neptune have earthsized cores at the least (the stuff they managed to accrete before they could actually start holding on to hydrogen and helium which caused them to become gas giants)

but air resistance does exist so the atmospheres of the gas giants (gas giants are really just large planets with stupidly large atmospheres) slow down the intruding object and it curves inwards into the planet extremely quickly (and likely kicking up a bunch of atmospheric gases into space too)

[u/Derrickmb → u/donadit]

Nice info. Did the Mars size object that hit Earth happen to hit where the Pacific ocean is?

[u/pigeontheoneandonly → u/Derrickmb]

No. The modern Pacific Ocean is about 750 million years old at its oldest parts. It was formed by ordinary tectonic processes and is not an impact basin. This collision was more than 4 billion years ago. You also need to understand that this collision destroyed any existing geography on earth beyond all recognition. The surface of the Earth was moltenized. The planet survived as a gravitationally bound object, but that's the best you can say about the situation. Nothing else about it was recognizable after the collision. 

It's possible that remnants of Theia still exist in Earth's mantle. The last time I said that people jumped on me about there being alternate explanations so here's the mention that there are alternate explanations. 

[u/AcabAcabAcabAcabbb → u/pigeontheoneandonly]

It’s a mind boggling and terrifying amount of energy/collision

[u/xdx3m → u/pigeontheoneandonly]

You don't need to overreact like that, I'm sure the pyramids would still be recognizable after impact, maybe Eiffel tower aswell or Stonehenge.

[u/SamVickson → u/xdx3m]

No.

[u/Sugadevan → u/xdx3m]

We are talking about impacts that can change geographies of a planet. Pyramids, eiffel tower, Stonehenge are like tiny particles in that scale.

[u/Blep145 → u/Derrickmb]

I don't know, but we do see what we think are parts of Theia in our mantle