Evidence of Protoplanet Found On Moon

mrspoonsi (2955715) writes 'Researchers have found evidence of the world that crashed into the Earth billions of years ago to form the Moon. Analysis of lunar rock brought back by Apollo astronauts shows traces of the "planet" called Theia. The researchers claim that their discovery confirms the theory that the Moon was created by just such a cataclysmic collision. The accepted theory since the 1980s is that the Moon arose as a result of a collision between the Earth and Theia 4.5bn years ago. It is the simplest explanation, and fits in well with computer simulations. The main drawback with the theory is that no one had found any evidence of Theia in lunar rock samples. Earlier analyses had shown Moon rock to have originated entirely from the Earth whereas computer simulations had shown that the Moon ought to have been mostly derived from Theia. Now a more refined analysis of Moon rock has found evidence of material thought to have an alien origin.'

Comments

Somewhat confused

[ocean_soul]

I haven't read the Science article yet, but from the BBC report it seems that the differences between the isotope ratios in moon rocks and earth are still a lot smaller than expected. This would suggest the Theia hypotheses to not be true, contrary to what the title says. I'm going to track down the original paper, because this BBC article has me somewhat confused.

Re:Somewhat confused

[PhilHibbs]

I haven't read the Science article yet, but from the BBC report it seems that the differences between the isotope ratios in moon rocks and earth are still a lot smaller than expected. This would suggest the Theia hypotheses to not be true, contrary to what the title says. I'm going to track down the original paper, because this BBC article has me somewhat confused.

The absolute terms "true" and "not true" are not appropriate for a hypothesis like this. There may be some parts of it that are accurate, but for instance the size, mass, velocity, density distribution etc. of the Theia might be wrong, or the physics in the simulation might be wrong, etc.

Re:Somewhat confused

[Sockatume]

The isotope ratio is more different than you'd expect for formation without an impact by a third body, but it's less different than you'd expect from an impact given what we know about the distributions of oxygen isotopes in the solar system. So both hypotheses need revision: for the Theia hypothesis, they suppose that it was an inner planet with a remarkably similar composition to Earth, and for the non-impact hypothesis, they suppose that the Earth's isotope ratio diverged from that of the moon due to later (small) impacts delivering different compositions.

Skeptics

[Dan East]

And the rest of the article discusses the skepticism of this "evidence". To sum it all up, the evidence is the different ratios of oxygen isotopes found between 3 moon rocks and Earth. Most experts are saying the difference in the ratios should be much, much larger, because of how different the ratios of isotopes are in meteorites and other outer solar system bodies. The difference between the earth and moon is so small that other theories are just as likely for explaining it. The counter argument is that maybe all of the inner planets have the same ratios of oxygen isotopes as one another, and it was an inner planet that struck Earth and basically everything involved was made of the same stuff so the differences are small.

I think that until we have actually measured the ratios from Mecury or Venus, we can't assume that every inner planet is exactly the same in that regard, and thus the "evidence" this study has found is actual evidence one way or another. The only thing we know for certain is all the extraterrestrial material we have analyzed so far from the rest of the solar system has had very different ratios of the isotopes, and so this evidence requires a whole new theory about the homogeneousness of the solar system to be true.

Re:Skeptics

[C0R1D4N]

Considering the nature of the impact it is also possible that most of Theia is here on Earth under the Pacific or something while the moon is made up more of jettisoned Earth pieces. Or that the original Theia pieces make up the core/underground bits of the Moon with a tasty Earth frosting.

Re:Skeptics

[Charliemopps]

The impact simulations kind of rule that out. I'd say the sample size is a real issue though. Also, there are quite a few assumptions that I'd say are questionable. We've no idea where this protoplanet came from. It could have been from outside the solar system... or it could have been part of earth at one time, jettisoned in a previous impact and came back for revenge. The fact that we have as much information as we do blows my mind. Science is amazing.

Re:Skeptics

[radtea]

The only thing we know for certain is all the extraterrestrial material we have analyzed so far from the rest of the solar system has had very different ratios of the isotopes, and so this evidence requires a whole new theory about the homogeneousness of the solar system to be true.

Not exactly. One thing missing from the popular discussions of this question is why we believe that isotope ratios necessarily vary across all larger bodies in the solar system.

It is true that measurements on meteorites show different ratios from what we see on Earth, but no particular conclusion can be drawn from that. It certainly does not follow from "None of the people I measure are the same height" that "No two people anywhere are the same height", so it would be bizarre in the extreme to go from a sample of fairly odd, mostly non-planetary, space rocks to a sweeping generalization about what is necessarily the case across the whole solar system. There may be some theoretical reason for believing this to be the case, but I've never seen it mentioned in any of the articles on this subject.

Furthermore, Theia has a very, very special property: its orbit intersected that of Earth's almost instantly after its formation. This is not the case with meteors, which have been wandering the solar system for more than four billion years, and therefore likely formed in very distant regions. Theia almost by necessity formed in a similar orbit to that of Earth. We know this, because only a body that formed in a similar orbit would likely find itself in a collision with Earth almost immediately after formation.

None of this "proves" or "disproves" anything, mind, because we're talking about knowledge here, not faith. Knowledge is by its nature uncertain, and the quest for certainty is simply an error pursued by pre-scientific peoples (philosophers), no different from the alchemical pursuit of transmutation of base metals into gold, or attempts to build perpetual motion machines, or attempts to trisect angles with nothing but straight-edge and compass.

Re:Theia

[Sockatume]

That's like asking, "how do we know that the Cretaceous period was called the Cretaceous period, the dinosaurs didn't have written language".

And now we know

[DrXym]

... where all those pesky Thetans came from

Re:I'm ignorant

[Anonymous Coward]

I'm ignorant, but today's science seems like "Tell me what is your theory, and i will find data to prove it's true"...

Yes, like that famous current scientist Einstein produced a theory of gravitation and then several years later they found data to prove it was true.

Or as a law predicts, "Given an enough amount of data, ANY theory can be proved"... :-)

There is no law that predicts such a thing. Given enough data, almost all theories are disproven. The only ones that remain are the ones that fit the data.

Re:I'm ignorant

[paiute]

Given enough data, almost all theories are disproven. The only ones that remain are the ones that fit the data.

Given enough data, almost all hypotheses are disproven. The ones which remain and have not yet been disproven by evidence become theories.

Re:"Simplest explanation"

[modmans2ndcoming]

Earth isn't large enough to capture an object the size of the moon in such a close orbit. and the moon was orbiting in a much much closer orbit 4 billion years ago.So, no, it was the most complex explanation.

BBC - Who

[rossdee]

Well they have The Doctor and I am sure he knows, he probably watched it happen

Re:"Simplest explanation"

[Rockoon]

I thought the simplest explanation was that it was captured by Earths Gravity

Thats only simple until you figure out that that sort of thing isnt actually possible. In cases where gravitational capture is possible, it is the gravity of a 2nd body (such as a moon) that enables a 3rd body (such as an asteroid) to lose enough velocity to orbit a 1st body (such as a planet.) Conservation of energy means any body that wasnt in a planets orbit will by default have escape velocity if it ever approaches that planet, and this is true unless it is acted upon by a force external to the mutual gravity of the planet and would-be capturer.

Re:"Simplest explanation"

[Ecuador]

Right, while the simplest explanation was "That's no moon..."

Re:"Simplest explanation"

[rgbatduke]

Damn, I had to give up modding this to answer, but I can't leave this.

One cannot "capture" a body the size of the moon by any two body elastic (e.g. gravitational) interaction. Within irrelevant perturbations such as gravitational wave radiation (presuming such a thing to exist), energy is conserved, and if it starts out unbound to the Earth it will end up unbound to the Earth.

One can capture in a three (or more) body interaction, but in that case the missing energy has to go someplace, and we are talking about a LOT of energy in the case of an orbiting moon. Enough energy to basically melt the moon and the earth and then some. One would expect to see some sort of orbital remnants of such a many-body event, and all of the other bodies in the solar system are a bit too far away to be good candidates in terms of the forces needed, and show none of the orbital perturbation one would expect as a consequence.

That leaves inelastic events. Tidal interaction is inelastic over time, but to make it strong enough to mediate a "capture" it would damn near be a collision anyway, brushing up on Roche's Limit (look that up). Also, that too would leave the nascent moon in an orbit much closer than the initial radius of its apparent orbit. Also, it wouldn't explain the apparent deficit of heavier elements and an iron core in the moon (thought to have been literally blown out of the incoming body in the collision and either ejected altogether to carry away the missing energy and momentum needed to leave the remnant in orbit or absorbed into the Earth) and a bunch of other things.

So really, the collision hypothesis makes "enough" sense and is consistent with enough data that it is AFAIK the "accepted" explanation of the moon's origin, with the usual caveat that contrary evidence or a better argument in the future might change that as we cannot easily be certain about events 4.5 billion years ago.

rgb