Formation of the Moon

[Orca]

So I have been pondering the current theories on the formation of the Moon. Apparently the prevailing hypothesis suggests something very large, "Mars-sized" I think is the phrase they use, impacted the Earth in its very early days. This catastrophic impact sent large amounts of material into space; the material eventually condensed into a body orbiting the Earth. There is also the thought that the Earth and Moon formed independently from the primordial cloud.

My instinct tells me that the Mars-crash is unlikely. I realize that in science my instinct isn't worth a flying dingo's kidney; however, I just can't shake the feeling. But there seem to be a few good indications that the Mars idea is the most likely candidate. For instance, the Moon is on average far less dense than the Earth; this could be explained by a collision throwing surface material off the Earth after chemical separation had taken place due to gravity (ie, the more dense material sinks to the core).

But honestly, how many Mars-like objects could just be flying around? Anything that large, I would think, would have had to have formed in a stable orbit from which it would not likely stray too far.

[makc]

with that many moons orbiting other planets, just how many mars-like bodies you need

[The Code]

But honestly, how many Mars-like objects could just be flying around? Anything that large, I would think, would have had to have formed in a stable orbit from which it would not likely stray too far.

You might like to read this.

http://www.msnbc.msn.com/id/31208155/

[Chris Peterson]

My instinct tells me that the Mars-crash is unlikely. I realize that in science my instinct isn't worth a flying dingo's kidney; however, I just can't shake the feeling.

I would think a flying dingo would be worth a lot- even just its kidney, maybe.

But honestly, how many Mars-like objects could just be flying around? Anything that large, I would think, would have had to have formed in a stable orbit from which it would not likely stray too far.

In the early Solar System nothing was in stable orbits (nothing is in a truly stable orbit even now). There is strong evidence that planets were moving in an out, swapping positions, and that collisions between large bodies were common. The idea that a Mars-sized planet collided with the Earth during this early chaotic period is pretty reasonable.

[neufer]

My instinct tells me that the Mars-crash is unlikely. I realize that in science my instinct isn't worth a flying dingo's kidney; however, I just can't shake the feeling.

I would think a flying dingo would be worth a lot- even just its kidney, maybe.

Dingo: French name of Goofy.

<<The Goofy holler is a stock sound effect that is used frequently in Disney cartoons and films.

It is the cry Goofy makes when falling or being launched into the air,
which could be transcribed as "yaaaaaaa-hoo-hoo-hoo-hooey!!"

The holler was originally recorded by yodeller Hannès Schrolle for the 1941 short The Art of Skiing.
Some sources claim that Schrolle was not paid for the recording.>>

http://www.youtube.com/watch?v=YOjOD1OmioY

[Orca]

In the early Solar System nothing was in stable orbits (nothing is in a truly stable orbit even now). There is strong evidence that planets were moving in an out, swapping positions, and that collisions between large bodies were common. The idea that a Mars-sized planet collided with the Earth during this early chaotic period is pretty reasonable.

Well perhaps "stable" was not a good word. I meant, "formed and solid enough that they'd [the planets] collected the local material around them. The point that they wouldn't be crossing orbits with other similar-sized objects.

But then it all goes back to the amount of mass in the early-formed solar system compared with the mass of the system today. Of course the current total has to be much smaller than the initial nebula and also smaller in the total number of objects in the early years after formation compared to today. I guess what makes things smell funny in Denmark, in my mind, is the idea of a theory that requires the summoning of a Mars-sized object out of no where to solve the problem. We don't know how many such objects existed back then; once the collision had occurred and our New Mars had careened out of the system we just can't account for it in any real way. It...strikes...me as a sort of Deus Ex.

Of course there is plenty of evidence of large impacts through out the solar system. The tilt of Uranus' rotational axis, Iapetus' Death Star crater, Miranda's apparent shatter-and-reassembly, ect. I just can't shake the feeling that when we don't know what happened we fall too easily on our default solution..."somethin' hit it."

Aren't there examples of moons forming along side their parent planets...such as in the Jovian and Saturnian systems? It would appear that there are several mechanisms that can create moons.


And of course the Kidney comment was a Douglas Adams reference. I try to work that one into conversation when ever possible.

[Chris Peterson]

Well perhaps "stable" was not a good word. I meant, "formed and solid enough that they'd [the planets] collected the local material around them. The point that they wouldn't be crossing orbits with other similar-sized objects.

But that's just what orbital simulations show: large bodies (the gas giants) which substantially change their orbits, resulting in various resonances that then destabilize smaller (terrestrial) bodies. From a theoretical standpoint this is known to occur, and from an empirical standpoint, seems to have happened in our own system.

But then it all goes back to the amount of mass in the early-formed solar system compared with the mass of the system today. Of course the current total has to be much smaller than the initial nebula and also smaller in the total number of objects in the early years after formation compared to today.

Indeed, it seems likely that substantial amount of material was jettisoned from the Solar System, or into the Sun, in its early history.

Aren't there examples of moons forming along side their parent planets...such as in the Jovian and Saturnian systems? It would appear that there are several mechanisms that can create moons.

But those are very different than the Earth-Moon double planet. Moons around gas giants are explained by a combination of capture and/or local accretion, which is made possible by the much stronger gravitational fields of these bodies. It would be very difficult for an object like the Earth to capture a body as massive as the Moon.

The Moon's mass, composition, orbital dynamics, and age all make the theory that it was formed from a planetary collision between the Earth and another body very plausible.

[bystander]

Age of the Moon
Universe Today - October 13, 2008

How old is the Moon? Almost the entire Solar System formed 4.6 billion years ago, when the solar nebula collapsed. But astronomers think that the Moon formed later than that, when a Mars-sized protoplanet smashed into the Earth. The debris from the collision splashed into orbit around the Earth and then reformed into the Moon, which still orbits us today.

So when did this happen?

Astronomers think this collision happened about 4.53 billion years ago, about 30-50 million years after the rest of the Solar System formed. This was relatively soon after the formation of the Solar System, and well before the time when life formed on Earth. Our planet was probably still mostly a molten ball of rock, and the impact of the Moon did little to change that. This is the dominant theory of how the Moon formed, but there are others. It's possible that the Moon was captured by the Earth's gravity, or it just formed in place around the Earth after the formation of the Solar System.

[Orca]

An early collision would explain the lack of physical evidence of the massive destruction such an impact would have caused; I imagine a mostly-liquid planet would have just resurfaced over time.

However, what about the argument of the density of the Moon? The idea is, if I understand it, that the since the Moon has a lower average density, it might be made of the surface material ejecta from the Earth during the impact. However, wouldn't the density of the still-liquid Earth be more uniform than it is today, what with all the churning convection going on throughout the planet? I am sure much of the heavy material would have sank to the center, but still, I'd guess that the Moon would have actually had an average density closer to our own.

As for the idea that the Moon might have formed simultaneously, wouldn't the local cloud from which they both formed lean in density toward the biggest pocket - the one that formed the Earth - thus making its average density higher? In other words, the big bit would get most of the heavy material and thus form a planet with higher average density. But then, Chris, you'd guess the Earth and Moon would not have formed that way? Granted, we don't see any other examples...

[Chris Peterson]

However, what about the argument of the density of the Moon? The idea is, if I understand it, that the since the Moon has a lower average density, it might be made of the surface material ejecta from the Earth during the impact. However, wouldn't the density of the still-liquid Earth be more uniform than it is today, what with all the churning convection going on throughout the planet? I am sure much of the heavy material would have sank to the center, but still, I'd guess that the Moon would have actually had an average density closer to our own.

"Fluid" would be a better term than "liquid" to describe the Earth back then- not unlike much of the inner planet still. It would have been highly differentiated 50 million years after its formation, so a glancing collision would splash material that had substantially less iron than the Earth's central zone.

As for the idea that the Moon might have formed simultaneously, wouldn't the local cloud from which they both formed lean in density toward the biggest pocket - the one that formed the Earth - thus making its average density higher? In other words, the big bit would get most of the heavy material and thus form a planet with higher average density. But then, Chris, you'd guess the Earth and Moon would not have formed that way? Granted, we don't see any other examples...

The lack of examples is a problem. And I've never seen a simulation or other theoretical treatment for the formation of binary planets, only for capture scenarios. It's entirely possible that the Moon didn't come from the Earth, that's just the theory that best fits the available evidence at the moment (and the theory has gotten stronger over the last ten years, as dynamical models of the early Solar System have been developed).

[neufer]

Age of the Moon
Universe Today - October 13, 2008

How old is the Moon? Almost the entire Solar System formed 4.6 billion years ago, when the solar nebula collapsed. But astronomers think that the Moon formed later than that, when a Mars-sized protoplanet smashed into the Earth. The debris from the collision splashed into orbit around the Earth and then reformed into the Moon, which still orbits us today.

So when did this happen?

Astronomers think this collision happened about 4.53 billion years ago, about 30-50 million years after the rest of the Solar System formed. This was relatively soon after the formation of the Solar System, and well before the time when life formed on Earth. Our planet was probably still mostly a molten ball of rock, and the impact of the Moon did little to change that. This is the dominant theory of how the Moon formed, but there are others. It's possible that the Moon was captured by the Earth's gravity, or it just formed in place around the Earth after the formation of the Solar System.

http://www.youtube.com/watch?v=_drW76wUuD4
http://www.youtube.com/watch?v=tJGx7zU11Qk
http://www.youtube.com/watch?v=_3PRsaaVMfU

[geckzilla]

Age of the Moon
Universe Today - October 13, 2008

I've always thought this kind of illustrations looks wrong. I have no idea how a planet would look while colliding with another planet. It bothers me that I can't imagine how it might look. Cosmic illustrations almost always bother me, though. Like those pictures of matter swirling into a black hole or a star sucking gasses off a smaller companion star? I'm sure the illustrators are doing their best, though.

[astrolabe]

Hello All,

A senario that seems plausible to me:

It has been stated in an older post that the early Earth had a faster rotation. IMHO the earth and moon resulted from the separation of a single molten/liquid sphere that had been accreting more and more material from the billions of objects flying around the Solar System (and of course beyond) and gradually reached a point in its molten state when it's Gravity could no longer hold a spherical shape for a body in such an elastic state and Earth developed a lopsided rotation. The process (the same as for liquids in rotation- even water) of mass verse density verses rotatational rate, perhaps helped along by an interloper, started the the gradual creation of the binary system we know today. I picture it like a stretching at first. then elongation, then as material flowed along the connecting bridge there came a point when Earth's Gravity physically balanced to the point where molten bridge became thinner and separated from it's tether to our future Moon.

Speculation? at this juncture, yes but it is an alternate possibility.

[dougettinger]

How is it possible that the collisional material created by the Earth and a hypothetical body the size of Mars could possibly accrete to form the Moon? Would not most of this material eventually fall back to Earth, be collected at Lagrangian points, or simply remain as collisional material in orbit much like the Asteroid Belt?

[RJN]

Doug, Yours is an interesting question. Wikipedia has some good entries that address this including its entry on the Giant Impact Hypothesis. Basically, much matter does fall back to Earth, but also much of it clumps due to its own growing gravitationally attraction. Like with the formation of the Solar System's planets initially, the growing Moon soon swept up most of the remaining matter. - RJN

[dougettinger]

One can safely assume that two or more blasted chunks of collisional material are unusually large and may create separate point sources for new gravity fields for a new forming satellite. It is difficult to conceive only one point source for a new gravity field being created after this hypothetical collision.

A then two or more accreted orbs will blast each other apart when they collide. I do not see how only one body can form and then orbit Earth.

[Chris Peterson]

How is it possible that the collisional material created by the Earth and a hypothetical body the size of Mars could possibly accrete to form the Moon? Would not most of this material eventually fall back to Earth, be collected at Lagrangian points, or simply remain as collisional material in orbit much like the Asteroid Belt?

This is an area of study which is highly dependent on computer simulations. And what simulations show is that physics allows for the formation of something like the Earth-Moon system following a massive collision if a narrow range of constraints are met. In particular, the collision seems to require a glancing blow. This can knock enough material into its own orbit to gravitationally recombine into a moon before the individual bits have time to fall back. Of course, a lot of material does fall back, and a lot escapes the system completely.

[Wayne]

Would not most of this material eventually fall back to Earth,

Yes, most models show over 80% of ejecta falling back to Earth. This is why the Moon has such a low density and low mass.

be collected at Lagrangian points

It would need to reach escape velocity to reach any of the Earth-Sun Lagrangian points. If it's at escape velocity, it naturally won't stop at a Lagragian point!

or simply remain as collisional material in orbit much like the Asteroid Belt?

The asteroid belt has two things going for it:
1. There's hardly anything there. Seriously, about 20% of the mass of the Moon (itself an undermassive object) is in the entire asteroid belt.
2. Jupiter disturbed the asteroid belt by ejecting its planetesimals before they could accrete into a planet. All that's left are failed planetary cores and some bits of rubble.

[dougettinger]

Thank you, RJN, Chris, and Wayne, for your thoughtful and insightful replies. I did not know that Wikipedia wrote a synopsis for the "giant impact hypothesis".

I wish to address issues with the Asteroid Belt once again. If Jupiter's gravity field perturbs and prevents the asteroids from combining, then why did Saturn in the next adjacent orbit form without being disturbed by Jupiter's gravity field? Also, if the all the planets formed at nearly the same time in a brief span of thousands of years as some protostar disk observations suggest, then the Asteroid planet should have had a head start before Jupiter grew to its present size. Many asteroids are known to be composed of hydrated materials. Hence, the lighter volatile materials were available to acrete with the heavier metallic materials before solar winds drove them away or before Jupiter's gravity field became a factor.

[Chris Peterson]

I wish to address issues with the Asteroid Belt once again. If Jupiter's gravity field perturbs and prevents the asteroids from combining, then why did Saturn in the next adjacent orbit form without being disturbed by Jupiter's gravity field?

It isn't just Jupiter's gravity that's the factor. What you get with a complex system of orbiting planets is resonance zones- specific orbital regions that are stable or unstable, depending on the distance from the Sun, distance from other planets, and the orbital periods of those planets. Even within the asteroid belt today it is possible to see radial zones that are completely clear of debris, and other zones where debris accumulates- all because of resonances with Jupiter.

The situation is further confused by the fact that planetary orbits appear not to have been constant over the lifetime of the Solar System. The planets have moved considerably from where they were originally formed- including extreme situations such as the possible switching of position of planetary orbits (see the Nice Model). All of this early action involved planets transferring angular momentum between each other, changing their orbital radii, and creating and breaking various resonances.

[dougettinger]

Any impact hypothesis needs an origin for the impactor. This impact hypothesis proposes a secondary planet formed at one of the Earth's Lagrangian points and then collided with Earth. So questions do arise. Why did not other secondary planets occur at other Lagrangian points - especially Jupiter and Saturn? Why have not the asteriods at Jupiter's Lagrangian points been perturbed and collided with Jupiter? What was the nature of the perturbation that caused the impactor to move toward and collide with Earth? Would not the impactor's glancing blow have knocked Earth away from its orbit as predicted by Bode-Titius Rule? Why are the bulk composites of the Moon and Earth quite different when they supposely formed from the same region of the solar nebula?

Obviously, I am having difficulties with the impactor's origin. Another possible orgin is the Kuiper Belt of which I have even more difficulties.

The hypothesis states that other alternative hypotheses for the Moon formation cannot address the high angular momentum of the Moon-Earth system. The subject hypothesis did not state how well it fared with the angular momentum problem. I thought all current hypotheses did not adequately address the angular momentum of the system. What is the current status of this problem? Does the computer simulation provide a complete orbit of the Moon around the Earth after striking the Earth?

[Wayne]

Why did not other secondary planets occur at other Lagrangian points - especially Jupiter and Saturn? Why have not the asteriods at Jupiter's Lagrangian points been perturbed and collided with Jupiter?

The 60 degree fore and aft Lagrange points are stable only when the object in it is of insignificant mass. Compared to Jupiter and the Sun, the Trojan asteroids are insignificant.

What was the nature of the perturbation that caused the impactor to move toward and collide with Earth?

It became too large.

Would not the impactor's glancing blow have knocked Earth away from its orbit as predicted by Bode-Titius Rule?

Bode's Law is empirical and most likely coincidental. It has little sway in celestial mechanics and no support from extrasolar discoveries. It is also, even if it WERE valid science (which it is not), has no relevance here. It "predicts" nothing.

Why are the bulk composites of the Moon and Earth quite different when they supposely formed from the same region of the solar nebula?

This is the biggest support of the giant impact hypothesis. When the impactor hit, its dense core merged with Earth's core, giving Earth the over-massive core it has today. The Moon was made from mantle and crust material, lighter silicates which would be blasted off into a stable enough orbit for secondary accretion.

This is why the Moon is depleted in heavier elements compared to Earth and giant impact is the only theory which can account for it.

[Chris Peterson]

Obviously, I am having difficulties with the impactor's origin.

It is likely that there were other planets formed early in the development of the Solar System. This was a chaotic environment, with planets perturbing each other, transferring angular momentum, shifting orbital radii, changing position, and being flung out of the Solar System entirely. I think there was plenty of material to allow for collisions like that believed to have created the Moon.

[Wayne]

It is likely that there were other planets formed early in the development of the Solar System. This was a chaotic environment, with planets perturbing each other, transferring angular momentum, shifting orbital radii, changing position, and being flung out of the Solar System entirely. I think there was plenty of material to allow for collisions like that believed to have created the Moon.

The leading hypothesis is a formation in L4 or L5 of the Earth-Sun system. No chaotic orbits needed. Just a simple case of a body becoming too large to be stable in L4/L5.

[Chris Peterson]

The leading hypothesis is a formation in L4 or L5 of the Earth-Sun system. No chaotic orbits needed. Just a simple case of a body becoming too large to be stable in L4/L5.

I can't say I've seen anything to suggest that this is the leading hypothesis. It is one of several, and IMO not the best. That the Solar System was a very chaotic place when the Moon formed is all but certain. That large collisions were common in the first few hundred million years of the Solar System is also virtually certain. So it is perfectly reasonable to argue that a collision happened between bodies that initially formed at different solar distances. The idea that you could actually form two planets in the same orbit strikes me as questionable, given the environment of the early system. I think stability would have been lost long before the Moon was actually formed (something that appears to have happened well after the Earth had accreted and cooled).