[HN Gopher] Dynamical origin of Theia, the last giant impactor o...
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Dynamical origin of Theia, the last giant impactor on Earth
Author : bikenaga
Score : 94 points
Date : 2025-07-08 18:10 UTC (1 days ago)
(HTM) web link (arxiv.org)
(TXT) w3m dump (arxiv.org)
| echelon wrote:
| 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.
| hbrav wrote:
| That second idea (cosmic primordial soup causing universal
| similarity / compatibility of life) could be a great component
| of a sci-fi story.
| MarkusQ wrote:
| 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.
| echelon wrote:
| No metals whatsoever. That's unfortunate.
| GuB-42 wrote:
| Metal and hard rock are essential for life.
| labster wrote:
| I think they were referring to metals like carbon and
| oxygen, but I'm sure Led Zeppelin is important too.
| 1-more wrote:
| Carbon and Oxygen are not metals though, right?
| hollerith wrote:
| According to the definition used by astronomers, all
| elements heavier than hydrogen and helium are metals.
| 1-more wrote:
| fascinating!!
| Towaway69 wrote:
| As Douglas Adams wrote, life is about banging the rocks
| together.
| eru wrote:
| 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.)
| adrian_b wrote:
| 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.)
| belinder wrote:
| 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)
| kulahan wrote:
| 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.
| kbelder wrote:
| "Need" may be overstating it. Those things certainly all
| seem to have helped, though.
| kulahan wrote:
| 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.
| andrewflnr wrote:
| > 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.
| echelon wrote:
| 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.
| dylan604 wrote:
| > Whereas intelligent life does seem to take a while to
| evolve
|
| basing that theory on an anecdotal story of 1.
| kaliqt wrote:
| While true, the opposite is true that we have zero evidence
| of any life whatsoever, anywhere, ever, except with us.
| nkrisc wrote:
| You got better evidence?
| andrewflnr wrote:
| 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...
| somanyphotons wrote:
| This animation makes it appear that proto-Earth was _very_
| comprehensibly torn apart, stunningly so
|
| https://en.wikipedia.org/wiki/Theia_(hypothetical_planet)#Co...
| shadowgovt wrote:
| 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.
| whycome wrote:
| Would oceans have remained at all?
| shadowgovt wrote:
| 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).
| ahazred8ta wrote:
| 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
| nntwozz wrote:
| 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.
| motoboi wrote:
| 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...
| ethan_smith wrote:
| 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.
| southernplaces7 wrote:
| >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.
| tanepiper wrote:
| 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.
| SideburnsOfDoom wrote:
| > 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.
|
| https://en.wikipedia.org/wiki/Hadean
|
| https://www.britannica.com/science/Hadean-Eon
| adrian_b wrote:
| 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.
| readthenotes1 wrote:
| The moon has 1.2% the mass of earth, so earth still got
| embiggened
| kridsdale1 wrote:
| What an epically cromulent day it was.
| southernplaces7 wrote:
| 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.
| GolDDranks wrote:
| Thanks for the inline dictionary lookup, I TIL'd that both
| of those words were popularized by the Simpsons.
| tim333 wrote:
| Invented by the Simpsons.
| shadowgovt wrote:
| 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.
| adrian_b wrote:
| 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.
| nehal3m wrote:
| The last giant impactor so far.
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