Dynamical origin of Theia, the last giant impactor on Earth
arxiv.org
arxiv.org
If so, cool. It's a wise step to check the hypothesis to make sure it isn't immediately contradicting what we already understand.
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.
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+...
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.
Not much to start life with.
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.)
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.)
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.
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.
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.
basing that theory on an anecdotal story of 1.
https://en.wikipedia.org/wiki/Theia_(hypothetical_planet)#Co...
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.
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.
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.
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.
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...
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.
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.
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.