The Moon: What is it's purpose?

I believe the prevailing theory is that there was an impact with another planet (Theia) the size of Mars. Out of the debris the moon formed.

 

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Thanks. I have never heard the before. I read that the impact was with earth. I will look into it. A quick search show it is certainly a theory.

http://en.wikipedia.org/wiki/Theia_(planet)

 

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Of course, I thought that was evident.

 

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The Earth probably had many small "moons" relatively shortly after a mass of Earth (and whatever collided with Earth) were ejected by the impact. The current moon is receding but was never in low Earth orbit - the gravity gradient would break it into smaller pieces. (The tides may have been nearly100 meters high when the moon was first in it lower orbit as they are directly related to the gravity gradient, which goes as the inverse cube. - I am sure one can find more accurate guesses by searching.)

I.e. many of these smaller moon pieces probably did form after the impact in orbits much lower than the moon has now. They were not, however in stable orbits. Gravitational interactions probably threw some back into the Earth and others into solar orbit. Still others may have collided with the moon. It would be very interesting to see if any of the Earth crossing asteroids have the same composition as the moon.

I have never read any discussion of this - perhaps it is wrong, but I think it is not. Hope someone will confirm or refute my beliefs stated here.

 

Interesting Billy.

 

That's not true. The Earth's Roche Limit is bellow the surface. In order to break up it would have had to hit the surface of the Planet. Only swith Planets of extreme size will large asteroids, comets and other debris break up before hitting the surface as the Shoemaker comet.

 

You are mistaken, not me.

Body Satellite ...Roche limit (km) ..R (km)
Earth Moon ........9,496 ...................1.49
I.e. the moon would break up when it is 49% of an Earth radius ABOVE THE SURFACE of the Earth.

From second table at: http://en.wikipedia.org/wiki/Roche_Limit

If you work backwards the changing rate of separation between the Earth and moon you will find that it is less than the age of the Earth when the moon would have been at the Roche limit. - Proof that the Earth initially did not have the moon. I.e. the Earth acquired its moon many years after it was solid.

You are mistaken here about Shoemaker also. It did not break up inside any planet’s Roche limit. Some planet it passed relative near, but outside of its Roche limit, disturbed its orbit about the sun enough so that near the next perigee (with the sun, not another planet), the original single Shoemaker body was inside the sun’s Roche limit for it.

It then broke into many smaller pieces. (As I recall, 23 pieces were large enough to have been observed years before the collision with the planet but there were many dozen smaller ones in all probability.) Each of these pieces was in essentially the same orbit but they did get very slightly different velocities during the break up. These small speed differences caused the 23+ separate pieces to become strung out in a long “chain” along the orbit trajectory as time passed. This chain was about 100km (or more, I forget) long before each piece separately crashed into the planet.

Typically the 23 major collisions were separated by hours, one from another. I think it took more than a day for the entire Shoemaker/planet collision event to be over. The planet’s Roche limit had nothing to do with this interesting event, which unfortunately happen when the Shoemaker chain of pieces was headed back towards the sun so the collision occurred on the far side of the planet from the sun and was not directly visible from Earth. Fortunately, it was visible to a satellite the Earth had launched earlier. As we knew in advance that the chain of pieces was going to hit the plant, that satellite was re-programmed to watch the event. After the impact area had rotated around to be visible directly from Earth the surface cooling at one impact site due to the added cold mass was still detectible with careful measurements.

Next time, check your "facts" before “correcting” me. I do make mistakes and welcome valid corrections. - I always thank for them.

In your brief post you have two errors, which I normally would not have bothered to point out.

 

Thanks. I was only guessing that the break up was inside the sun's Roche limit as that is more probable as a much larger volume than inside a planets limit. My main point remains vaild - Namely Shoemaker was a chain of pieces long before it collided with a Planet. - You have the facts about how it broke up and I was only assuming them based on the probability of a solar break up being more likely. Again thanks for the correction. (I am very lazy and rarely search for facts. I tend to rely on my memory and understanding of physics and probabilities - that latter tripped me up this time.)

PS -What do you think about my speculation in post 105 that the Earth once and relatively briefly had many moons? Do you have any facts on this?

 

I am not entirely convinced about the impact theory. For some reason i believe that impact fragments would be smaller due to the explosive nature or much less likely become embedded but this would only be possible if the impact were to be in deep water but i guess that is not likely considering the conditions. That part i am not certain of as i guess it is assumed the earth was an entirely solid object and perhaps devoid of water.

Another question and an important one is what happened to these other objects?

 

What i dont understand is the concept of a perfect sphere being ejected.

 

It surely was not a sphere when first ejected, but all planetary masses take that shape (if moon sized or bigger) as materials cannot sustain the gravitational stresses when not in approximately that shape.
For example, Everest is nearly as tall as a mountain can be on Earth. The weight of the elevated part pressing down on the base if much higher would make the base material "squeeze out" towards the sides. Rocks, in general "flow" under non uniform pressure, and more rapidly the greater the pressure is. That is to say, Earth can have some “out-of-round bumps,” but not much bigger than Everest is.

 

I will review the link.
However your claim of no moon formed with Earth isn't rational. Math only shows the eventuality between two positions. You have not accounted for the very probably effect of perturbations in the moons orbit between it's formation and now which could drasticly alter your conclusions.

As it happens the moon shows a great amount distress on it's Earth-facing side, magma release, massive cratering and fisures where as the "dark side" only has cratering. The moon isn't volcanic. The magma release must be from the gravitational influence of a large body, possibly passing between Earth and the Moon. This would have an effect on Earth's axis more than it's orbit and more of an effect on the moon's orbit. Even if this evidence did not exist it would be unreasonable to assume our values were and ranging were constant through out the time period in question.

Indeed, I see you have made errors yourself. Rather than arrogantly ascerting my advice on research practices, I will say only that Next time, I will do what I deem propper. Being corrected is part of my learning process. I speak from memory as do most of us. When memory does not serve it is logical to be corrected.

I don't believe the Earth colision moon creation theory.
But if I did I'd say it's possible that Mars and Jupiter have a portion of that debris but that would mean that the approaching object had considerable size and it's trajectory was outbound which would mean that it's velocity of couse would continue to decrease untill it's return trip...but that's not to say it rules out the possibility because it doesn't.

 

Do you think that the moon and Earth formed at essentially the same time? Despite the back calculation analysis of the moon / Earth separation placing the moon within the Roche limit more recently than the Earth formation time?

No one is assuming that "our values were and ranging were constant throughout the time period in question." Quite the contrary: Years ago, when the moon was closer to the Earth the tides were larger, the dissipation larger and the velocity of separation larger, all three dramatically larger even when the Earth moon separation was only 1/3 of what it is today. If the moon/ Earth distance were only 10% of today's value, then roughly speaking this 'larger" would on the order of 1000 times larger as the gradient, which drives all three, goes as the inverse cube.

I suspect that one or more of the relatively smooth "seas" on the side of the moon facing Earth could be caused by a collision of a briefly existing smaller moon (made during the ejection collision) with the larger ejection moon to make the current moon mass shortly after the moon was ejected by a impact collision. A large mass, passing between the Earth and moon, as you suggest, would not make the localized seas on the surface of the moon we see. Impacts or volcanoes do that, but I tend to agree that the moon was never with much volcanic activity. If your large mass passed quite close to the moon, it could flex the moon with two tides, one on each side, and of course give a strong gravitational impulse which would drastically change the moon's orbit but not make the localized “seas” we see.

I claimed, base on the Math PROOF mentioned in my prior post, that the current moon is younger than the Earth. Although I think it true, I did not exactly claim that no moon formed with the Earth. If it did, it was much smaller than the current moon and when the current moon was ejected it and probably many other smaller moons, which formed from the ejected material were in unstable orbits.

They all meet one of the three fates I discussed earlier in post 105. We know that any moon comparable in size to the current moon that formed when the Earth formed would have been destroyed within the Roche limit, if it was not significantly farther from earth than the present moon when formed. If, it formed when the Earth formed and it was farther away than the moon is currently, it would very likely still be near the Earth or at least in roughly the Earth's orbit about the sun if a passing mass scattered it from its gravitational binding to the Earth.

It is very improbable a large passing mass would come close enough to throw it into an entirely different orbit. Just as you idea of a large mass passing between Earth and the moon is very improbable. It is possible, but very unlikely, that a large mass could have passed close to the Earth, without contact, and pulled a chunk of the Earth free from the rest to form the main moon mass (and probably many smaller moons that briefly existed as discussed in post 105.)

 

The most widely accepted hypothesis (and it does not have enough behind it to qualify as a theory -- yet) is that a planetoid formed in the vicinity of the Earth-Sun L4 or L5 point.

You are ignoring that Jupiter has a very significant perturbative effect on the orbits of the inner planers. See the references in [post=2148290]post #27[/post].

You are (1) ignoring that the solar system, over long periods of time, is chaotic, and (2) you are ignoring that the probability of an event known to have happened is exactly 1, regardless of how improbable it appears to be.

 

I am not sure what you are thinking of here, but believe that you think a large third mass passing by the Earth moon system would pull the moon from the Earth (or at least significantly modify the backwards in time calculations of the Earth moon separation). That is certainly possible if it passes close to the moon; however the acceleration it gives to the Earth and moon would tend to be the same if it is a more probably miss distance.

I.e. its gravitational force on the Earth would be ~80 times larger than on the moon, but the Earth's inertia is also ~80 times larger so to first order, the motion of the earth wrt the moon (and conversely) would not change. Why Jupitor is not significantly distrubing the moon Earth orbit about their common center of mass even though it has been trying to for millions of years and is not just the one time passing event you speak of. Jupitor is no doubt slowly moving the center of mass's (of the Earth moon system) orbit about the sun. Mars is closer to Jupitor and it has more effect on Mars. I beleive that is why the excentricity of Mars'orbit is so much larger than the Earth's

 

No I even discussed and posted the Jupiter effect while you were posting (prior to reading your post). I know the solar system is unstable, but fortunately not coming apart (as far as we know) quickly enough to be a concern. I think in most cases only two mutually orbiting bodies are long term stable. Jupiter and the sun will be all the solar system is some day, if the sun were to last long enough.

Certainly the probability of a rare event that did happen is unity, but one is not sure how the moon formed or that the back projection of the separation is wrong. I believe that most think the most likely error in the calculation of how long ago the moon would have been inside the Roche limit is plate tectonic uncertainties – I.e. the dissipation in the changing shape and depth of the oceans errors.

BTW the exact location of where 1000s of year old total solar eclipse were seen is the most accurate measure of how much the Earth spin rate has changed in the last few millennia.

 

I do not believe the Moon formed seperately from the Earth. I "believe" they had the same start. The reason I have for this "belief" is the moon's shape which is mostly spherical. To me this means it's formation was complete during cooling. Any number of events could have occured during the solar systems's initial formation including multiple moons which eventually combined. Too me that's highly speculative in nature. Just because of the amount of debris and ambient temperture. I do understand why the speculation exist I merely think there is evidence to theorize a later disturbance rather than an early disturbance.

That being said. You are entirely over my head in your reply.
You say Years ago. How many Years ago?

I think there was a collision aswell. Multiple collisions. I think cometary debris accompanied by a large mass passed between Earth and Luna. Considering the sheer amount water in this solar system, Earth, Mars, Jupiter, Saturn, and almost every other Outter Planet I think the probability is high, it would explain why there is water on the moon which should not be there if. (I understand the formation correctly.) The time period, I'm not sure about but it would seem to be after formation.

What do you think occured to create the asteroid belt?