Dynamical origin of Theia, the last giant impactor on Earth
arxiv.org96 points by bikenaga 22 days ago
96 points by bikenaga 22 days ago
So the boil-down on this is "Here's a theory that says about 5-10% of Earth's mass was mostly carbon and came all at once, like if Theia was mostly carbon and we got hit by it, so we did some simulation and the idea Theia was mostly carbon isn't ruled out by our current understanding of how our solar system might have formed?"
If so, cool. It's a wise step to check the hypothesis to make sure it isn't immediately contradicting what we already understand.
No, the 5-10% mass was not mostly carbon. It was also mostly oxygen with smaller amounts of iron, silicon and magnesium, like in any parts of the Solar System where the gravity is not so strong as to retain most of the hydrogen and helium.
That mass has come from bodies which have collided with Earth and which had a chemical composition similar to that of "carbonaceous chondrites". (That is a kind of meteorites; most of the Earth had a composition similar to "enstatite chondrites", another kind of meteorites.)
The carbonaceous chondrites are not made of carbon, they only have more carbon than the Earth and similar planets, because in the early Solar System the carbon was present mostly as carbon dioxide, which is volatile so that most of it does not condense into the planets at the higher temperatures of the zones closer to the Sun, where the inner planets have condensed, but only at lower temperatures, farther from the Sun, where it reacts with metallic oxides, forming solid carbonates, like limestone.
The last giant impactor so far.
The moon has 1.2% the mass of earth, so earth still got embiggened
This is true, but it was surprising to me because I know that the gravity on the moon is 17% of the gravity on earth.
It turns out that what I was missing is that an object on the surface of the moon is much closer to the center of the moon than the same object would be on earth, because the radius of the moon is much less than the radius of the earth, and gravity is inversely proportional to the square of the distance between the two objects.
mass_ratio / radius_ratio^2 = gravity_ratio
0.0123 / 0.273^2 = 0.17
https://www.wolframalpha.com/input?i=mass+of+moon+%2F+mass+o...
https://www.wolframalpha.com/input?i=gravity+on+moon+%2F+gra...
https://en.wikipedia.org/wiki/Gravity#Characterization
https://www.wolframalpha.com/input?i=radius+of+the+moon+%2F+...
What an epically cromulent day it was.
and thank you for letting me learn of an entirely new and delightful word today. Why you were downvoted for this harmless bit of dry-cut wit, I haven't a clue.
Thanks for the inline dictionary lookup, I TIL'd that both of those words were popularized by the Simpsons.
Imagine if life had evolved on Earth or Theia prior to impact. Imagine if it was intelligent and played witness to the giant cataclysm.
Given that intelligence took an awfully long time to emerge from LUCA, that seems implausible. But it's fun to imagine pre-Theia "Silurians". That sort of impact would have scorched earth of any trace or remnant of their existence. It feels as though there must be sufficiently advanced civilizations out there witnessing this exact scenario play out without the necessary technology to stop it. Though that fate would be horrifying.
Another thing to think about is that shortly after the Big Bang (if there was one, Lamda-CDM or similar models holding up), was that shortly after the Big Bang the temperature of the early universe was uniformly 0-100 degrees Celsius. It may have been possible for life to originated in this primordial interstellar medium without even so much as needing a host planet or star! Just life coalescing in space itself.
That early primordial soup, if it existed, could have seeded the whole universe. Most aliens might have matching molecules and chirality if those decisions predate our galaxy.
That early warm interval would have been a soup of 75% H, 25% He, and 0.0000000% or so Li, with nothing heavier.
Not much to start life with.
Also more importantly: it was uniformly warm. No gradients.
Life on earth doesn't work because we get energy from the sun. It works because we get low entropy energy from the sun and can radiate high entropy energy into cold space.
(There's approximately no net energy inflow.)
You are right about gradients, but while the energy from the Sun is what sustains most of the life today, it played no role in the appearance of life (because capturing solar energy requires exceedingly complex structures).
For the appearance of life it is necessary for the planet to have a much warmer interior than its surface (i.e. a radial gradient of temperature). In that case, volcanism and related phenomena bring to the surface chemical substances that have formed at higher temperatures and which are no longer in chemical equilibrium at the cooler planet surface, providing the chemical energy for the synthesis of the complex organic substances.
Some bacteria and archaea (belonging to the so-called acetogens and methanogens) still exploit the inner heat source of the Earth, living completely independently of the solar energy, in the same way like the first living beings. (However, in many popular science publications one can see frequently wrong claims about various organisms, including some animals, that they do not depend on solar energy, but those claims are false, because those living beings depend on using free oxygen for the oxidation of various substances, like hydrogen sulfide from oceanic vents, and the free oxygen comes from algae and plants that have used solar energy to separate it from water.)
> You are right about gradients, but while the energy from the Sun is what sustains most of the life today, it played no role in the appearance of life (because capturing solar energy requires exceedingly complex structures).
Yes, agreed!
My point was merely that the lukewarm phase of the universe didn't really have easily available temperature / energy gradients available. Even though the temperature itself was fine, but that's not enough.
No metals whatsoever. That's unfortunate.
Metal and hard rock are essential for life.
I think they were referring to metals like carbon and oxygen, but I’m sure Led Zeppelin is important too.
Carbon and Oxygen are not metals though, right?
According to the definition used by astronomers, all elements heavier than hydrogen and helium are metals.
> It feels as though there must be sufficiently advanced civilizations out there witnessing this exact scenario play out without the necessary technology to stop it. Though that fate would be horrifying.
I suspect this is not actually that common. Giant impacts are more common in early solar systems; things eventually settle into nice circular orbits like we have now. Whereas intelligent life does seem to take a while to evolve, so probably more common later in a solar system's life cycle.
Uncommon for sure.
Our sun and earth won't last long enough, but Mercury's orbit is potentially unstable.
A red dwarf might harbor live bearing planets long enough to see its long-lived orbits eventually destabilize. Or perhaps witness the even rarer interstellar collision or destabilization from rogue planets, etc.
> Whereas intelligent life does seem to take a while to evolve
basing that theory on an anecdotal story of 1.
Depends how you count it. One planet, but a solid handful of mass extinctions with big adaptive radiations, most with several million years of development, and tons of reasonably intelligent social animals, only one of which produced industrial civilization. But yes, working with the best evidence we have...
While true, the opposite is true that we have zero evidence of any life whatsoever, anywhere, ever, except with us.
This animation makes it appear that proto-Earth was very comprehensibly torn apart, stunningly so
https://en.wikipedia.org/wiki/Theia_(hypothetical_planet)#Co...
I remember there being some modeling done to determine whether the Theia impact blew a chunk off Earth or basically re-liquified the planet. If I recall correctly, the resulting hypothesis was that the thermal load would have re-melted at least the crust (evidence for this was stacking of density in the moon, suggesting it formed out of a basically completely-liquified ball, which would have implied the crust was also liquified).
There is some interesting evidence suggesting the deeper layers remained intact, in the form of a region under the Pacific that might be the impact scar. It's an inexplicably-dense zone that causes hot-spots at its corners resulting in increased surface volcanism, like how the edges of a leaf burn before the middle in a fire.
... but on the surface? Yeah, no hiding place.
Would oceans have remained at all?
According to physics simulations, the atmosphere was composed of vaporised rock for several centuries after the impact until it cooled down below 1000 degrees. The entire surface was magma. https://en.wikipedia.org/wiki/Synestia
No; I don't remember the article saying specifically but I would assume if there is no solid land left, there is no liquid water left either. Water molecules would have been blasted into the "crust soup" and eventually re-condensed into gaseous water and eventually liquid water via atmospheric regeneration after the surface settled down a bit (because the chemicals that could be gaseous would have tended to float to the top of the soup as it settled down).
If life had appeared before the Earth-Theia collision, it could have been terminated by any of the earlier impacts.
The Earth-Theia collision has been only the last of a great number of collisions.
The original Earth had been much smaller and it had grown by colliding with many similar bodies.
The reason why the last collision is likely to have been also the greatest is that both Earth and Theia had grown previously through many collisions.
This has been similar to the "Highlander" movies, where each victorious immortal took the force of the decapitated one, until only 2 remained for the final confrontation, which were stronger than any of the previous immortals, because these 2 had gathered the forces of all the others.
In the same way the planets close to Earth have grown after each collision, at the expense of the smaller impactor, so the impacts have been between bigger and bigger planets, until the final collision between Earth and Theia, which both were bigger than the participants to earlier impacts from their history.
Before this final impact, the time intervals between 2 successive big impacts might have been too short for life to appear.
The pre-Theia "Silurians" as you call them, depending on technological level could have left traces in the solar system like our Parker Solar Probe or something in the Lagrange points.
Then again, how well do we know of stuff in these spaces today? It seems to me we barely have a clue of the space junk we ourselves sent up orbiting in our backyard.
Temperature on its own wouldn't be enough for life would it? Isn't everything moving around way too fast after the Big bang and therefore too far apart for whatever life there would be to find food (or whatever equivalent source of energy)
Temperature isn’t even close to being enough. If we didn’t have a moon, despite everything else being so good for life, we may have been stuck at the bacterial phase if we didn’t have tides, or life may have never formed at all due to minerals not being recycled, tide pools not concentrating amino acids, and constant wet-dry phases driving evolutionary pushes.
Edit: beyond that, there’s the need for a stable orbit, a stable axial tilt, a stable star (few mega flares), some kind of galactic shield a la Jupiter, and more.
"Need" may be overstating it. Those things certainly all seem to have helped, though.
I don’t believe it’s an overstatement. We have thousands upon thousands of data points showing life doesn’t appear outside of absolutely perfect conditions. There is not one single piece of proof anywhere which runs against this idea, it’s just the extremely outdated concept that earth isn’t special which makes it seem wrong. That concept hasn’t been valid for probably decades now, but it hangs on due to scientists hoping against all odds that we aren’t alone.
We certainly cannot know if a previous intelligent civilization was present on earth before the impact.
And is not even that the impact erased their trace from earth, is that if they got extinct long enough (more than 100 million years ago), with today's technology, we cannot infer their presence, given the fossil (and other types of) record.
A great write up about this is: https://archive.is/https://pacificklaus.com/the-silurian-hyp...
Gurdjieff takes a different view on the cause and formation of the moon, which in some sense lines up with this - https://www.gurdjieff.org/beelzebub9.htm
He makes no claim to what life was before the formation of the moon, but rather than the cataclysm of formation of the moon is what changed life on Earth - of course he wraps the whole idea up in his mysticism - but his 1950s writing on this was not far off what happened.
Damn I'd like to know in a -lot- more detail what other cultures GG might have bumped into in his travels. Meetings falls way short there. Else: what an imagination.
That second idea (cosmic primordial soup causing universal similarity / compatibility of life) could be a great component of a sci-fi story.
The early universe was actually billions of degrees immediately post-Big Bang and remained far too hot for liquid water for hundreds of thousands of years, only reaching "habitable" temperatures long after matter had already begun forming into structures.
> Imagine if life had evolved on Earth or Theia prior to impact.
There's good reason why this first era of Earth's history is called the "Hadean eon" - as in the fires of hell.
> Throughout part of the eon, impacts from extraterrestrial bodies released enormous amounts of heat that likely prevented much of the rock from solidifying at the surface. As such, the name of the interval is a reference to Hades, a Greek translation of the Hebrew word for hell.
>Imagine if it was intelligent and played witness to the giant cataclysm.
They'd have had to be watching from one very high orbit, and even then I wouldn't bet favorably on their chances considering the sheer gargantuan volume of debris Theia tossed into the space above earth.
Anywhere terrestrial and they'd be dead far too quickly to watch much of anything. I've seen some fairly detailed models on the presumed effects of this collision and it would have rapidly super-heated the whole Earth's atmosphere while vaporizing or melting the crust of the entire world down to a depth of at least several hundred meters. Good luck finding a bunker that can handle that.