Evidence of Protoplanet Found On Moon 105
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.'
Re:Somewhat confused (Score:5, Informative)
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 (Score:5, Informative)
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.
Re:Theia (Score:0, Informative)
Yea but why are they giving a specific name to something only hypothesized to exist? Sure we can give names to imaginary things like Santa Clause, but its odd the scientists are doing it. Wikipedia gives this reason:
"Theia's mythological role as the mother of the Moon goddess Selene is alluded to in the application of the name to a hypothetical planet which, according to the giant impact hypothesis, collided with the Earth, resulting in the Moon's creation."
https://en.wikipedia.org/wiki/Theia
Re:"Simplest explanation" (Score:5, Informative)
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.
Re:"Simplest explanation" (Score:4, Informative)
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" (Score:5, Informative)
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.
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