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Origin of the moon

  1. Dec 14, 2007 #1
    Hi,

    I have recently been reading that the currently favoured theory for the origin of the moon is that a Mars sized object smashed into an almost finished earth, ripped off large parts of proto-earths mantle and the result debris coallesced into the earth and moon again.

    I know the universe is big and 4 billion years is beyond human comprehension, but boy, that does seem like a bit of jolly good luck to me.

    a) First of all that two such large bodies were in the same vicinity of each other
    b) That the they clipped each other and didnt bang into each other like marbles
    c) That the core that banged into us stayed around and didnt bugger off somewhere else
    d) That the earths orbit was pushed towards the sun or made elongated
    e) That the resulting debris coallesced back around these two large bodies and didnt get smacked into other orbits.
    f) That the mass of the two large space bodies was right just after to the collision to form bodies that would orbit each other and then later after the coallescing, still just right to orbit each other.

    As far as I've read, the main reason for this theory being preferred over co-forming is the large angular momentum of the earth moon system. But like I said 4 billion years is a long time and who knows how many collisions we've had in this time with other solar systems, or for that matter, what the energy level was in the forming universe.

    Anybody else there find this collision theory just a little too convenient?

    conan
     
  2. jcsd
  3. Dec 14, 2007 #2

    HallsofIvy

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    What is the probability of something happening AFTER IT HAS HAPPENED?
    There is no point in arguing the "low probability" something will happen if the evidence indicates that did happen. If the probability that something happens is very, very low, it can still happen. If the probability something will happen in 4 billion years is very, very low, it can still happen in the first year. It might make sense to argue about how many times it has happened in those 4 billion years. Have you noticed that there are NO OTHER large moons in the solar system?

    Finally, one can appeal to the anthropic principle: there is no point in arguing the low probability of something happening when, if it had not happened, we would not be here to ask about it!
     
    Last edited: Dec 14, 2007
  4. Dec 14, 2007 #3

    DaveC426913

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    I see your point but I think the idea here is that the unlikeliness of the events should factor in to the plausibility of the theory.
     
  5. Dec 14, 2007 #4
    I was just thinking that myself. I think the idea is a little far fetched myself. I assume a body of mass that large to get stuck in the gravitation pull of the Earth would have to have a small velocity. So the only logical explanation (as of now) is that it came off the Earth in a big chuck.
     
  6. Dec 14, 2007 #5

    rbj

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    so does the fact that the fine-structure constant is within 2% of what it is. it's a fundamental constant that Nature seems to have taken on and we have no idea how it came to be that it has value that it does, but if it were significantly different, we would be around to be talking about it.

    how about the jolly good luck we mammels benefited from when the Yucatan Asteroid came blasting down, executing judgement upon the nasty dinosaurs, allowing whatever was the proto-mammel to come forth and thrive in the absense of the nasty bully dinos that were dominating everything until then.

    revolution, baby! mammels of the world unite!

    i actually found it quite believable, both as the science was presented (in some NOVA Origins program) and satisfying from my world-view of things.


    not as uncommon for that epoch.

    from what i saw on NOVA, the depictation was more like the head-on collision (where the big asteroid or proto-planet got completely absorbed in the Earth, but a tremendous amout of matter, and more of it silicates, was tossed out the other side), but it certainly seems more likely that, if a collision happened, it would not be perfectly head-on.

    the planet is still pretty soft. dig a whole 1000 km deep and see what you get. it was even softer way back then.

    if it's gonna tip the axis (which it would do if the collision was not head-on) that tipping will, during some season of the year, expose the axis or pole toward the sun. as for the orbit, one would expect the collision to make it more elliptical, not less. it would have to be a very lucky collision, both in magnitude and timing, to cause an already elliptical orbit to become less elliptical (more circular).

    well, it possibly did pick up other debris, since this was so early in the life of the planet that our orbital territory hadn't yet been completely "cleared" or "mopped up" by the young Earth. after the collision, the Earth was still mostly a blob and soft (and liquid) enough that gravity would force it back to a spherical shape.

    that's just what you get when the initial spin or swirl has a resulting angular momentum. it's similar to what happens with the primordial swirls that become galaxies or solar systems. why is it that we just "luckily" have a sufficient momentum to orbit around the Sun or that our system has just the right momentum to orbit around the galactic center and yet the contents inside can array themselves? that "luck" appears to have been repeated a zillion times in our (or any) galaxy.


    yup. rather than a large main planet and a much smaller satellite "moon", it is more akin to twin planets whirling around their common center of mass.

    there was a whole lotta smashin' goin' on back then. this big mondo collision might have happened toward the tail end of that epoch.

    but, if it's after the fact, then it's more like the unlikliness should factor into the wonder of the fact that we're even here to notice it. very similar to the issue of the fine-tuned universe. science should search for natural causality (of the amazingly unlikely event), but when such knowledge seems to be lacking, it seems appropriate to me to exclaim "holy crap!".

    another event that the NOVA program pointed out was the Iron Catastrophe. as the Earth was being formed by clearing out its orbital realm by attracting globs and sticking to them (thus getting bigger and even more attractive), the composition, lotsa iron but also a bunch of other elements including silicates and what eventually formed the atmosphere, was roughly homogeneous, not (yet) stratified. but the molten silicates and stuff were less dense the liquid iron and nickel. in a single event, 99%+ of the iron decided to "go south" (down, way down) causing the lighter silicates to bubble up to eventually form the crust. but the iron core, remaining molten, spun due to the initial angular momentum it had and still is rotating down there today, creating this huge magnetic field that traps the nasty charged particles from the solar wind and flairs (the Van Allen radiation belt) and prevents that nasty solar blast from stripping our atmosphere as it had to Mars. Mars, being smaller, has a larger surface area to volume ratio and it's core may have cooled sufficiently to harden a bit, not spin (transferring the angular momentum to the rest of the planet), lose its magnetic field, then the nasty Sun spits out wind and solar flares that strip off the Martian atmosphere.

    there are lots of lucky breaks (or providential, for some that have sympathies for a notion of "intelligent design" with a small "i" and small "d", not that bullsh1t that spews from the Discovery Institute). more than you will ever know.

    that's my story and i'm sticking to it.
     
  7. Dec 14, 2007 #6

    rbj

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    it wouldn't have to come off in a mostly singular big chunk. it could be sorta like in the movie The Godfather when he gives someone "an offer he can't refuse", when Don Corlione and attached thug hold a gun to the head of some victim and assure him that either his brains or his signature will appear on the contract.

    it could have been that just splatter came out the back end of the collision, forming a ring around the Earth sorta like Saturn and the other gas giants, but the mass ratio of ring to planet is much larger this time. the developing moon coalesces and sweeps out matter from the ring as it orbits and grows, much like the Earth (or other planets) did in it's orbital domain around the Sun.
     
  8. Dec 14, 2007 #7

    SpaceTiger

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    Compared to what? The biases that HallsofIvy mentions make it almost impossible to judge the plausibility of the argument based on its raw probability. It's not enough to just wave one's hands and say something seems implausible, we need to come up with alternatives if we wish to use Occam's Razor in a meaningful way.
     
  9. Dec 14, 2007 #8

    DaveC426913

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    The alternatives are theories that don't depend on the Earth's relatively small gravity well capturing a large object that just happened to be moving slow enough.
     
  10. Dec 14, 2007 #9

    SpaceTiger

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    I don't think that's a requirement of the collision theory. Are you referring to the capture of the debris from the collision?
     
  11. Dec 14, 2007 #10

    russ_watters

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    Setting aside the anthropic principle, you can still say that such events were not too surprising. Most of your observations about what happened are just a result of the fact that the early solar system was in fact, a cosmic pinball machine, so it shouldn't be too surprising that we got "lucky" enough to get our moon. Ie:
    No luck required due to the amount of crap flying throughout the solar system as I said above.
    An off-axis hit is far more likely than a direct one.
    No more or less likely than capturing a random asteroid, which many planets have done (and Earth actually did for a few months last year).
    Not sure what you mean by that one.
    The coalescing is guaranteed by gravity, but we most certainly did get smacked-into by other debris. The side of the moon we see is all the debris that didn't hit the Earth instead of the moon (the other side is much more pockmarked with craters).
    Already covered above. The odds of it happening are probably no more or less likely than capturing an asteroid, which was a relatively common occurrence.
    Well, there's also the geological evidence for it.
     
  12. Dec 14, 2007 #11

    malty

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    Cool, would love to ask how this occured, would love to know when this occurred and if there any pics, vids etc of it. More importantly though I would LOVE to know how I did not know about this until now :frown:
     
  13. Dec 21, 2007 #12
    Thanks very much for your answers. I actually have a statistics major, so the arguments regarding probability were nice to see. Yes, it happened, we have a moon, but as the other posters posted, you have to also weigh up how likely a potential answer to a question is.

    @Mark, thanks for your detailed answer, I enjoyed it the most. It did however answer questions with further questions, which is kind of normal seeming that none of us are 4 billion years old and were there to see it. Take the cosmic pinball machine for a start. I think there are about 15 objects in the solar system of earth/moon size or only about 30 that are bigger than 1000k diameter. I think they occupy about 1E-20 of the volume of their orbits. That's an awfully big pinball machine with very small balls in it. Although I'm prepared to believe in the aggregation theories, and I'm sure there were a lot of clumps smashing into each other, I find it substantially harder to believe that in this huge pinball machine there were two massive orbitting bodies so close to each other that one day they decided to bump into each other and stay linked.

    Co-evolution seems so much more natural to me. One of the "ah-ah" theories that grabbed my attention last year was the big dust ball analogy for forming planets. That they start out as huge statically charge dustballs. That sounds so logical and probable to me. Instead of having hard rocks bouncing off each other into space again, you have a loosely spread junk collecting and collecting and eventually it collapses a bit under it's own gravity. And if you've ever observed dustballs under a sofa, they are never alone. There are always a number of them. That seems like a much more plausible reasoning for why there are multi body planetary systems.

    Of course observation tells us many other things. Mars's moons are more likely to be captured asteroids. And comets race around the place waiting to bump into things. And of course, the biggest contradiction to my beliefs is the asteroid belt with Ceres and Vesta and friends. All circling around in the vicinity of each other. Given enough time, two large objects there might bump into each, just as a proto-earth and proto moon may have done in our past.

    cheers

    conan
     
  14. Dec 21, 2007 #13

    russ_watters

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    Sorry I missed your post before. Here's the article:
    http://www.skyandtelescope.com/observing/highlights/7067527.html

    It wasn't very big, so I guess the major news outlets didn't pick up the story because they didn't think it was important.
     
  15. Dec 31, 2007 #14
    I don't see how such a large object as the moon could have had such a small effect on the orbit of the Earth around the sun. Our planet's eccentricity is so tiny; only Venus and Neptune have more perfect circular orbits. A collision would have to have been a cataclysmic event, yet our orbit is still very close to a circle.
     
  16. Dec 31, 2007 #15
    i reckon there are some planets out there that had maybe more or maybe less of a clip. and these are the planets which dont have stable enough orbits to hold life.
    its just like one of the millions of coincidences
     
  17. Dec 31, 2007 #16

    baywax

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    Its a distant probablility that the moon was deliberately placed where it is today by some intelligent beings, thus providing the "tidal" conditions for terrestrial life on our planet. If true the moon would also serve as a monitoring source for the same "intelligent beings", alerting them to our progress. The statistical odds for this being true may actually be higher than those governing the probablility of a collision resulting in the moon being where it is now.
     
  18. Dec 31, 2007 #17

    DaveC426913

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    Because, our "statistical" data shows, the instances of super-intelligent moon-tossing alien races far outweighs the instances of planets with moons...:rolleyes:
     
  19. Jan 1, 2008 #18

    baywax

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    Hee hee! Happy new year... OK then... ! (Moon tossing alien races!!)
     
  20. Jan 9, 2008 #19

    baywax

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  21. Jan 15, 2011 #20
    There is no point in arguing the "low probability" something will happen if the evidence indicates that did happen.

    Just what is the evidence you are citing??
     
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