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Thread: Been Mooning Over This

  1. #1 Been Mooning Over This 
    Time Lord zinjanthropos's Avatar
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    Planet Number of Moons
    Mercury 0
    Venus 0
    Earth 1
    Mars 2
    Jupiter 63
    Saturn 61
    Uranus 27
    Neptune 13


    Pasted these statistics off of a science site.

    This is probably nothing but I notice the greater percentage of moons are more concentrated around Jupiter and Saturn, the largest planets in the solar system. With the exception of Mars having two (I've read that they might not truly belong to Mars) and Earth having one the number of moons seems to correspond to the size of the planet. Are these just coincidences or is there something else at work here?


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    When the solar system formed maybe 4.6 billion years ago, it was rather chaotic. Jupiter and Saturn became the biggest planets so attracted much of the debris floating around loose in those years.

    While we sorted out planets so Pluto became a dwarf planet, the same has not been done for moons. Earth has a very large moon at 2,160 miles in diameter (Mercury is only 1,516 miles in diameter). Mars' two moons are just 5 and 10 miles across. The gas giants have a number of large moons but most of what are called moons going around them are little more than lumps of large rock similar to what is in some planet's rings or the asteroid belts.

    Gravity is the coincidence you are looking for, in that they attracted more rocks.


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  4. #3 Re: Been Mooning Over This 
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    Quote Originally Posted by zinjanthropos
    Planet Number of Moons
    Mercury 0
    Venus 0
    Earth 1
    Mars 2
    Jupiter 63
    Saturn 61
    Uranus 27
    Neptune 13


    Pasted these statistics off of a science site.

    This is probably nothing but I notice the greater percentage of moons are more concentrated around Jupiter and Saturn, the largest planets in the solar system. With the exception of Mars having two (I've read that they might not truly belong to Mars) and Earth having one the number of moons seems to correspond to the size of the planet. Are these just coincidences or is there something else at work here?
    Planets with more mass also have stronger gravity, allowing them to more easily capture smaller objects. Since satellites (moons) can either form naturally around a planet or consist of captured objects, it would make sense that the larger gas giants would have more moons.

    Or did I completely misunderstand the question?
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  5. #4 Re: Been Mooning Over This 
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    Quote Originally Posted by PhoenixG

    Planets with more mass also have stronger gravity, allowing them to more easily capture smaller objects. Since satellites (moons) can either form naturally around a planet or consist of captured objects, it would make sense that the larger gas giants would have more moons.

    Or did I completely misunderstand the question?
    No, I figured gravity was a good reason. I find the disparity between the inner 4 and the outer four quite disparaging. I know Mars has those two little ones so if it wasn't for Earth then the inner planets would be basically shut out. I find it hard to believe that the outer planets pretty much have them all. Could the Sun have taken moons that were destined for the inner planets? What is more likely...inner planet moons head towards the gas giants or to the Sun?
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  6. #5 Re: Been Mooning Over This 
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    Quote Originally Posted by zinjanthropos
    I find it hard to believe that the outer planets pretty much have them all.
    We can count them, so I don't know that incredulity with get you very far here

    Quote Originally Posted by zinjanthropos
    Could the Sun have taken moons that were destined for the inner planets?
    First, I'm not sure that any of the planets were "destined" to have any moons. Some moons are captured objects but not all of them are. It would make sense that the "heavier" outer planets would capture more the moons that were though. You wouldn't expect Earth to beat Jupiter in a gravitational tug-of-war would you?

    If it does get past the gas giants, then the next heavy hitter is the sun, so yes it would make sense that Sol would "eat" some of the objects that did get slung its way.

    Quote Originally Posted by zinjanthropos
    What is more likely...inner planet moons head towards the gas giants or to the Sun?
    I guess I don't understand the question. If the bodies we're referring to are already orbiting one of the rocky inner planets, why are they "heading" anywhere? I'm not debating that something would happen to our moon if the Earth suddenly disappeared, but I guess I'm wondering under which scenario we're assuming that it's going to be leaving us in the first place.
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    Quote Originally Posted by Cyberia
    Earth has a very large moon at 2,160 miles in diameter (Mercury is only 1,516 miles in diameter). Mars' two moons are just 5 and 10 miles across. The gas giants have a number of large moons but most of what are called moons going around them are little more than lumps of large rock similar to what is in some planet's rings or the asteroid belts.
    I always thought Mercury was larger than our Moon.
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    Quote Originally Posted by Halliday
    I always thought Mercury was larger than our Moon.
    I do enjoy a good joke Halliday and I quite missed poor old cyberia's one till you pointed it out. 'course the joke is on him. He's taken the diameter of the Moon, but the radius of Mercury. Let's hope he can multiply by two.

    No offence intended cyberia, but you got to admit its pretty funny. It would be like someone on the biology forum talking about the evolution of prokaryote plants. :wink:

    You know, as to what happened to all the moons that should have been in the inner solar system, why bless you, the buggers are still there, or at least some of them are, deeply embedded in our remaining planets. It surely was a chaotic place to be 4.5 billion years ago when collisions between moons and small planets was the norm. Good thing it quitened down. 8)
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  9. #8 Re: Been Mooning Over This 
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    Quote Originally Posted by PhoenixG
    We can count them, so I don't know that incredulity with get you very far here
    I can see where my wording is skewed. At the time I was more concerned with why there were next to no moons for the inner planets. Only a couple of derelicts it seems that must have avoided the gas giants' gravitational pull.

    First, I'm not sure that any of the planets were "destined" to have any moons. Some moons are captured objects but not all of them are. It would make sense that the "heavier" outer planets would capture more the moons that were though. You wouldn't expect Earth to beat Jupiter in a gravitational tug-of-war would you?
    No I would not. By destined I meant heading for one of the inner planets but intercepted along the way. And to clear up the last part of my blubbering I was just wondering if the Sun would have grabbed more potential inner planet moons than all the gas giants combined.

    If as they say it rained comets on the Earth for a time, why was it struck so many times in the past? Was it just a numbers game, too many for the gas giants to provide Earth close to full protection from?
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    Quote Originally Posted by zinjanthropos
    At the time I was more concerned with why there were next to no moons for the inner planets. Only a couple of derelicts it seems that must have avoided the gas giants' gravitational pull.
    Forgive me for asking, but I still get the impression that you're assuming that all moons are captured objects. Am I correct or am I missing something?

    By destined I meant heading for one of the inner planets but intercepted along the way.
    If something was on it's way to the inner planets, it was mostly likely heading for the sun...

    If as they say it rained comets on the Earth for a time, why was it struck so many times in the past? Was it just a numbers game, too many for the gas giants to provide Earth close to full protection from?
    ...now if a smaller object (or a bunch of smaller objects) were on its (their) way to the sun and we got in the way, then we would get hit.

    Keep in mind that all the planets orbit the sun at different rates. Mercury's year is much shorter than Neptune's, etc. Therefore unless the gas giants are "right there" when something passes through it's orbit, it's like they aren't there at all.

    I hope that helps.
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    Quote Originally Posted by PhoenixG
    Forgive me for asking, but I still get the impression that you're assuming that all moons are captured objects. Am I correct or am I missing something?
    I assume most are.

    Sorry if the questions sound amateurish but I need to take advantage of being in a place where people in the know can help me figure things out. I think a science forum is great for professionals who can share or argue a point but for guys like me it is a chance to further our knowledge. The trick is to not be embarrassed or intimidated to ask. Just so you know I appreciate it.


    Are we more vulnerable to an impact or to perhaps snag a moon when the gas giants have little or no affect. When was the last time that the outer planets were arranged in such a way? Is there a name for it?
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    Quote Originally Posted by zinjanthropos
    Sorry if the questions sound amateurish but I need to take advantage of being in a place where people in the know can help me figure things out.
    hehe, I'm a hobbyist at best (my son wants to be an astronomer, so I've kicked my passing interest into high gear in hopes of helping him to prepare for a career in the field).

    The impression that I've gotten is that while many moons are captured objects, the conditions have to be just so, therefore most moons are not. Again, I am hardly an expert and my position is based on what I've been lead to believe rather than what is empirically true.

    So while I am absolutely advising that you take my posts with a huge grain of salt, I also don't see why we should assume that most moons are captured objects. I hope that helps to clarify my position.

    Quote Originally Posted by zinjanthropos
    I think a science forum is great for professionals who can share or argue a point but for guys like me it is a chance to further our knowledge. The trick is to not be embarrassed or intimidated to ask. Just so you know I appreciate it.
    I agree and very much appreciate having this forum as well.

    Quote Originally Posted by zinjanthropos
    Are we more vulnerable to an impact or to perhaps snag a moon when the gas giants have little or no affect.
    I would say so

    But again this is all predicated on their being something headed our way in the first place.

    Quote Originally Posted by zinjanthropos
    When was the last time that the outer planets were arranged in such a way? Is there a name for it?
    It's not so much that a special arrangement is necessary. When you're traveling down the freeway, you're less likely to crash into another car when there are fewer of them. By the same token if something gets jerked out of the Oort cloud and sent hurtling toward the sun, it's not going to be affected by the gas giants unless it passes close enough to have it's trajectory changed by the planet's gravitation.

    Since you appear to be learning this just as I am, might I suggest a resource that I've found incredibly valuable? It's called AstronomyCast. The hosts are very knowledgeable, they frequently do "Question Shows" and they are fantastic about keeping the content accessible. I cannot recommend the show enough.

    I hope that helps.

    P.S. Also, Bad Astronomy.
    "PhoenixG makes me puke that why I quoted him." - esbo
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  13. #12 Re: Been Mooning Over This 
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    Quote Originally Posted by zinjanthropos
    Quote Originally Posted by PhoenixG

    Planets with more mass also have stronger gravity, allowing them to more easily capture smaller objects. Since satellites (moons) can either form naturally around a planet or consist of captured objects, it would make sense that the larger gas giants would have more moons.

    Or did I completely misunderstand the question?
    No, I figured gravity was a good reason. I find the disparity between the inner 4 and the outer four quite disparaging. I know Mars has those two little ones so if it wasn't for Earth then the inner planets would be basically shut out. I find it hard to believe that the outer planets pretty much have them all. Could the Sun have taken moons that were destined for the inner planets? What is more likely...inner planet moons head towards the gas giants or to the Sun?
    Yes the Sun does have a strong influence. Every planet has a "Hill sphere", which reperesents the maximum distance at which a moon could orbit without the Sun pulling it away into an independent orbit.

    It depends on the relative mass of the planet and Sun and the distance that the Planet orbits from the Sun.

    For Mercury is has a radius of 220,190 km.
    For Venus it is 1,009.298 km.
    For Earth it is 1,493,502 km.

    For contrast,
    Jupiter's equals 53,051,526 km
    and Saturn's equals 65.069,209 km.

    Jupiter's is slightly smaller, but being a larger planet, it most likely had more material left over to form moons and it is closer to the main asteroid belt giving it more chances to capture moons.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    Quote Originally Posted by Halliday
    Quote Originally Posted by Cyberia
    Earth has a very large moon at 2,160 miles in diameter (Mercury is only 1,516 miles in diameter). Mars' two moons are just 5 and 10 miles across. The gas giants have a number of large moons but most of what are called moons going around them are little more than lumps of large rock similar to what is in some planet's rings or the asteroid belts.
    I always thought Mercury was larger than our Moon.

    That'll teach me to post while watching TV Just a minute while I turn down "Charles in Charge".
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    Quote Originally Posted by TruePath
    It surely was a chaotic place to be 4.5 billion years ago when collisions between moons and small planets was the norm. Good thing it quitened down. 8)

    Just a thought. The material for our solar system came from a super-nova and at some point another super-nova exploded, caused shock waves in the material which led to the formation of the accretion disk which eventually formed the sun, planets and moons. We date U 238 changing to Pb 207 to get a date of nearly 4.6 billion years but that is from the creation of the uranium which would have been in the super-nova. It possibly took some hundreds of millions of years before it ended up in the newly formed planet Earth, making Earth maybe 4.1 to 4.5 billion years old?
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    Quote Originally Posted by Cyberia
    Quote Originally Posted by TruePath
    It surely was a chaotic place to be 4.5 billion years ago when collisions between moons and small planets was the norm. Good thing it quitened down. 8)

    Just a thought. The material for our solar system came from a super-nova and at some point another super-nova exploded, caused shock waves in the material which led to the formation of the accretion disk which eventually formed the sun, planets and moons. We date U 238 changing to Pb 207 to get a date of nearly 4.6 billion years but that is from the creation of the uranium which would have been in the super-nova. It possibly took some hundreds of millions of years before it ended up in the newly formed planet Earth, making Earth maybe 4.1 to 4.5 billion years old?
    No, We look for the ratio of U 238 to Pb 207 in a given rock. Due to convective mixing while the Earth was molten (and in the magma now), these two elements would be separated from each other due to their different densities, under these conditions they would also chemical combine with other elements to form separate ores.

    Once Uranium containing rock cools and solidifies, any Pb 207 formed in the sample from decay will remain in the sample. Thus such radio-dating tells us the oldest date at which the sample had cooled to a solid. So when we get dates of 4.6 billion years, that is the shortest amount of time that has passed since the Earth cooled from its molten state.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    Quote Originally Posted by Cyberia
    The material for our solar system came from a super-nova and at some point another super-nova exploded, caused shock waves in the material which led to the formation of the accretion disk ...etc
    Now I'm not what you'd call an expert on these things, or even much of an armchair amateur, but I do believe the boys (and girls) who study this sort of stuff have concluded that the collapse occured because of those pesky 'gravitational instabilities' and was not initiated by a supernova shock wave.
    But then again, what do I know? :wink:
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    Quote Originally Posted by TruePath
    Quote Originally Posted by Cyberia
    The material for our solar system came from a super-nova and at some point another super-nova exploded, caused shock waves in the material which led to the formation of the accretion disk ...etc
    Now I'm not what you'd call an expert on these things, or even much of an armchair amateur, but I do believe the boys (and girls) who study this sort of stuff have concluded that the collapse occured because of those pesky 'gravitational instabilities' and was not initiated by a supernova shock wave.
    But then again, what do I know? :wink:
    Shock waves like the ones of supernovae could induce instabilities in a stable cloud of dust gas which then might cause the cloud to collapse gravitationally. But this concept only exists theoretically and has not yet been proved being viable. Consider the SN being too close; then the shock wave would disrupt the cloud thus preventing the collapse forever. But there are actually indications (isotopes in rocks) that some of the material that formed the solar system once was expelled by a similar process, maybe a nova or a collapsing red giant star.
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    Mighty interesting Dishmaster, but as I understand it the evidence for 'contamination' of the nebula with fresh supernova material rests on evidence for short lived radionuclides, pretty much as you say. Of course them being short lived we can only detect their decay products now.

    Anyhow a gal and a couple of guys at the Natural History Museum in London did a little study and they concluded that those radionuclides had most likely been generated by some process called spallation close into the new sun. So if you take away the 'foreign' material, you take away a bunch of the evidence for a triggering supernova.

    You can read all about it in Origin of Radionuclides by Sara Russel and two guys in The Philosophical Transactions of the Royal Society London (A) in 2001.

    So maybe someone else has done some more fancy calculations and proved them wrong. I'll go and have a look see.

    Edit:
    Well, gee whiz, the boss man he says "These calculations imply that the supernova trigger hypothesis is the most likely mechanism for delivering the SLRIs present during the formation of the solar system." Alan Boss at the Carnegie Institute in Washington had his team do these fancy finite element analysis deals and found a way to make it plausibly work even when the nebula didn't stay isothermal. And that's in The Astrophysical Journal, Volume 686, Issue 2, pp. L119-L122, 2008. So maybe, just maybe it was a triggered collapse. We'll see.

    Edit 2:
    And this team of busy bunnies in Hawaii reckon they've proved it pretty damn unlikley the nuclides originated from a nearby triggering supernova. Instead they are invoking Wolf-Rayet winds (Oh, learn something every darn day!) and some complicated scenarios to produce an unlikely, but still most likely outcome.
    Eric Gaidos is the lucky guy soaking up the sun and surf and publishing in The Astrophysical Journal, Volume 696, Issue 2, pp. 1854-1863 (2009).

    So I guess the jury is out.
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    What's with the sarcasm?
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
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    What sarcasm?
    Just because I tend to write the way I tend to talk because that's the way I tend to think you want to call it sarcasm.:?
    I reckon I've put more effort, more work, more time into digging out some interesting - well I find 'em interesting - facts about this supernova business and you point a finger and say I'm being sarcastic.

    So why shouldn't I mention that some gal at the Natural History Museum was the prime author of the first paper. You may not be aware that there is still a 'glass ceiling' for women in the sciences, so I think we should celbrate every time a gal is lead author in some research.

    I don't know if you've ever been to the Natural History Museum in London, but it surely is a pretty building, quite apart from all the exhibits and all the scientists it contains. If I got carried away mentioning it well I'll stand corrected and I'll try not to be so enthusiastic in future, but I don't see how that's sarcastic.

    And Hawaii. I don't know if you've ever been there, but it's even prettier than the Natural History Museum and I can't help being hell of a jealous about a bunch of scientists who get to study all this interesting stuff and live in such a beautiful place at the same time. So I still don't see what's sarcastic about that.

    I'm hoping your going to enlighten me as to where this sarcasm is because it ain't put there by me. If I was wanting to insult someone, which is what I think sarcasm is generally used for, I'd just call them a scrawny, weasel sized pig and leave it at that. I reckon sarcasm is for pussy cats, not real debaters and I've not used it and I won't use it and you can ban my ass if I do use it.
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    Moderator comment:
    Welcome, TruePath. I think the way you write could be misunderstood, because most people here usually emphasise the facts. You seem to embed them in some flowery language. As long as the facts are clear and support a scientific discussion, that's absolutely fine. This is a science forum, but I cannot see what damage some spirit could make. So, if you continue to post, we will eventually get accustomed to it.

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    Quote Originally Posted by TruePath
    Mighty interesting Dishmaster, but as I understand it the evidence for 'contamination' of the nebula with fresh supernova material rests on evidence for short lived radionuclides, pretty much as you say.
    Mixing with SN material does not necessarily mean the this hypothetical SN did trigger the star formation process. That is what I was concentrating on. Eventually, all matter is enriched by material that once was cooked up by SNe.

    Your quotes clearly demonstrate that there is quite some discussion going on about the history of the solar neighbourhood. Even though one group thinks it to be the "most likely mechanism" another group brings forward alternative ideas. The "finite element" approach is an "a posteriori" method which can be dangerous. This means, you have an empiric result and you try to fit a mathematical model to it. A better way would be to derive predictions from a model and later try to verify them by observation. In this way, any possible model derived in such a way proves to be more robust than its application to the facts that are already there.
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    Quote Originally Posted by Dishmaster
    [color=green]Moderator comment:
    Welcome, TruePath. I think the way you write could be misunderstood, because most people here usually emphasise the facts. You seem to embed them in some flowery language. As long as the facts are clear and support a scientific discussion, that's absolutely fine. This is a science forum, but I cannot see what damage some spirit could make. So, if you continue to post, we will eventually get accustomed to it.
    Thank you kindly. I'm also interested in the facts and I can see that my particular way of saying things (which I thought was kind of cute) might not sit well with everyone. I'll try to be mindful of that in the future, but rest assured its the facts, or what we think are the facts, that I'm interested in.
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    What sarcasm?
    Please excuse me for overreacting then? I just mistook your way of speaking for sarcasm, as if you don't agree with what the "gals" are saying. Again, sorry.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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    Thank you for your kind response. Just a little misunderstanding, which can be the way of talking without seeing the person you're talking to. You take care now.
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    Can anybody tell me if there is a relationship between the rotational spin of a planet and the rate of revolution for any satellite it may have? Will a planet that rotates very rapidly have a moon(s) with a slower orbital speed than a planet of equal size that spins slower with a moon of equal size? Or does fast planet rotation mean fast revolution by its moon? Just curious.
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