The Moon's Origin - What's your Theory?

[Mr Q]

OK, there are a lot of theories on the origin of the moon in the scientific community so which one do you believe is the most likely? Or, do you have your own theory?

Recently, I heard that the Moon is slowly nearing the Earth (an inch or so every year) so could this have been happening since the beginnings of the Solar System? Maybe the Moon in its early life was close enough to the Earth to be pulled to it by gravitational forces and is still today being pulled?

It's the Moon nearing the Earth slowly that leads me to the above possibility, otherwise, I don't have a clue as to where it came from (captured after the formation of the Solar System, an ancient comet, part of the Earth,etc?)

So what's your theory, if you have one?

Oh, no! Just saw a post below saying the Moon is moving away from the Earth at 4cm per year. Geeze, this is getting confusing already

[Qualia]

I was under the impression that the moon was gradually spiraling away from Earth at just under 4cm a year. Although I have read a number of hypothesis, the prevalent idea is that the moon was more than likely formed when a planet about the size of Mars collided with Earth a long, long time ago.

[nmoushon]

I thought that the moon was moving away from the earth not towards it.

[sologuitarist61]

Yes, the moon is certainly pulling away from the Earth and was much nearer in the past. The latest I have heard is that, if the moon was formed when a third body collided with the Earth and the moon formed from the debris in orbit, then the moon rocks should contain both the Earths material and the third parties material in fairly equal proportions. However, research into lunar rock samples seem to show that they are entirely Earth-like, so where is the missing material from the planet that hit us? It appears that ideas as to have the mon formed will have to be rethought unless someone can come up with a believable solution, but what that solution may be is anyones guess at th moment.

[nmoushon]

One theory I remember reading about a long time ago was that the earth had broken apart due to massive volcanic activity and thus creating the two bodies that would later form over time to be what they are today. Whether thats a serious theory or not I have know idea just remember reading somewhere a while ago. I doubt that happened but hey just putting it out there.

[Naemeth]

The Moon is indeed moving away from the Earth, as it does so, it's gravitational influence decreases and our Earth's rotation slows down (due to various forces) at a rate of a few seconds every decade (or something like that, not sure on the exact figures). The generally accepted view is that a Mars sized object collided with the Earth, causing a large amount of Earths' material to coalesce outside Earths' orbit to form the Moon. At this time the Moon was completely molten, which explains the almost complete lack of water, and the fact that most of the Moons' surface is the same age, about 3.8 - 4 billion years I believe.

[Capricorn]

However, research into lunar rock samples seem to show that they are entirely Earth-like, so where is the missing material from the planet that hit us? It appears that ideas as to have the mon formed will have to be rethought unless someone can come up with a believable solution, but what that solution may be is anyones guess at th moment.

Wouldn't the planet that hit us be part of the earth now, in effect the earth we have now and the moon came from the collision and combination of the 2 bodies. So both earth and moon would be products of the combined bodies, and hence the same.

There seems more chance that the moon could/would be fractionally different as I could see it possible that the mass that makes the moon may have come in the majority from the original earth body or from the body that hit it, thus being slightly different from the combined colliding bodies that we now would be. We would be nicely mixed owing to tectonic action over the past billions of years.

And yes the moon is moving away, get out there and see a total eclipse, one day they will be no more.

[Naemeth]

Wouldn't the planet that hit us be part of the earth now, in effect the earth we have now and the moon came from the collision and combination of the 2 bodies. So both earth and moon would be products of the combined bodies, and hence the same.

There seems more chance that the moon could/would be fractionally different as I could see it possible that the mass that makes the moon may have come in the majority from the original earth body or from the body that hit it, thus being slightly different from the combined colliding bodies that we now would be. We would be nicely mixed owing to tectonic action over the past billions of years.

In addition, isn't it likely an impact this size would have caused the Earth to become almost entirely molten (again), and so all the elements from this object would be all mixed into the Earth?

Evidence for the Earth being entirely molten at this stage is proven by the fact there is no Mars sized crater anywhere, because Earth itself had a resurfacing event then, when it became molten.

[sologuitarist61]

Wouldn't the planet that hit us be part of the earth now, in effect the earth we have now and the moon came from the collision and combination of the 2 bodies. So both earth and moon would be products of the combined bodies, and hence the same.

There seems more chance that the moon could/would be fractionally different as I could see it possible that the mass that makes the moon may have come in the majority from the original earth body or from the body that hit it, thus being slightly different from the combined colliding bodies that we now would be. We would be nicely mixed owing to tectonic action over the past billions of years.

This certainly sounds plausibe, but I am sure this would have been taken into account by the scientists involved. I think the problem comes from the fact that the debris thrown off during the collision would have done so straight away, so there would not have been time for the mixing of the materials as you suggest. so in theory what was ejected should have been half Earth, half the other body involved.

[stephenwilliamwatts]

Cheese Gromit!,,,

[Ronnie67]

I have always wondered, if the earth had a collition with a mars size planet, bear in mind the size of mars, as it was supose to have colided and part of the earth was ripped away, probably a part of the mars size planet broke away also, so! what happed to the rest of the mars size planet, where did that go?

Another question with an impact on that scale, how did the earth suvive such a force, I though it would obliterate earth also, especially if we see what an impact from a mile size metorite would do.

My theory is the moon was always here and was pulled gradually towards th earth bilions of years ago my the gravitational force of both combined earth and the sun, and now its slowly movind back, maybe to its orginal orbit those many billions of years ago.

[Naemeth]

I have always wondered, if the earth had a collition with a mars size planet, bear in mind the size of mars, as it was supose to have colided and part of the earth was ripped away, probably a part of the mars size planet broke away also, so! what happed to the rest of the mars size planet, where did that go?

Another question with an impact on that scale, how did the earth suvive such a force, I though it would obliterate earth also, especially if we see what an impact from a mile size metorite would do.

My theory is the moon was always here and was pulled gradually towards th earth bilions of years ago my the gravitational force of both combined earth and the sun, and now its slowly movind back, maybe to its orginal orbit those many billions of years ago.

First of all, yes, small meteorites do do a lot of damage. But it takes a lot of energy to obliterate a planet (killing all life on the planets is not the same thing, for instance, we wouldn't survive a collision with Mars today, but the Earth would). A quick google says it would take roughly 1.8 x 10³² Joules to blow up the planet. (Or a ball of uranium that is 84 miles in diameter).

Another google says the gravitational energy that binds the Earth is 2.24 · 10³²J, or 53.5 quadrillion megatons......

The Earth will have absorbed a lot of the Mars sized object, and remember, the Earth would probably have been slightly less massive and smaller in diameter than it is today.

[cantab]

Another question with an impact on that scale, how did the earth suvive such a force, I though it would obliterate earth also, especially if we see what an impact from a mile size metorite would do.

It's supposed to have been a glancing low-speed blow, explaining how the Earth survived. The impactor is thought to have originally formed at Earth's L4 or L5 point, but when it got too massive to be stable there it drifted closer to Earth, ultimately impacting.

I wonder if the similar composition of Earth and Moon is not to be expected given the impactor formed at the same distance from the Sun as Earth, and would thus probably have had an Earthlike composition itself.

[Black Knight]

A new theory suggests that the moon may have been essentially "cleaved" from the earth when "a far larger and faster-moving body made an even more glancing blow with the young Earth". This impactor would only have lost a small amount of material in the process meaning that the moon was effectively constructed almost entirely from material from the earth, and not from both bodies as previously proposed. This would go a long way to explain why the ratio of oxygen and titanium isotopes from both the earth and the moon are almost identical.

http://www.bbc.co.uk/news/science-environment-19011013

[sologuitarist61]

It's supposed to have been a glancing low-speed blow, explaining how the Earth survived. The impactor is thought to have originally formed at Earth's L4 or L5 point, but when it got too massive to be stable there it drifted closer to Earth, ultimately impacting.

I wonder if the similar composition of Earth and Moon is not to be expected given the impactor formed at the same distance from the Sun as Earth, and would thus probably have had an Earthlike composition itself.

This is a possibility but would a Mars sized object form that close to Earth (L4/L5 point)? Seems unlikely and usually it is smaller stuff that tends to accumulate at those points - and also we do not see this happening anywhere else in the solar system (though that is not to say that it hasn't). The other point to consider is just how fast the body needed to be travelling to knock off a moon sized chunk of the earth and put it into orbit? I would have thought it would have to be fairly fast to do so, otherwise a 'merger' would have been more likely at slow speed.

The other thing I wonder about is what is the likelihood that such a collision would have happened at all? There is an awful lot of space out there and so it would be unlikely that a direct hit would have happened, and any two objects in similar orbits would have passed each other many times before with the chance that the smaller would have been ejected from the orbit after even a close encounter with the larger Earth.

Questions, questions ..........

[ollypenrice]

The Big Splash theory (I only know the Mars sized impactor version, not the interesting 'larger, more glancing' version) explains why the moon has such a low density, since the more superficial layers of the earth (less dense than the core) were liquified and expelled into a ring system which then coalesced into the moon. The moon has an insignificant magnetic field because it lacks a dense iron core. This too accords with a superficial impact. (Not that superficial if you were sunbathing in the area, mind!!!)

The earth has no (almost no) craters because it has a system of plate tectonics which erase the older cratered surfaces as these are over-ridden by younger, un-cratered plates.

When observing the moon try looking for large craters whose walls are indented by small ones. I don't think you'll find any, which suggests that the impactors were getting smaller over time in accordance with the modern theory of planetary accretion. It's a nice bit of 'instant science' to do at the eyepiece. If you find a large crater pinged* by a smaller one let me know!

Olly

*A key term in physics. For a full definition send a postal order for £100 to....

[Tinker1947]

The Big Splash theory (I only know the Mars sized impactor version, not the interesting 'larger, more glancing' version) explains why the moon has such a low density, since the more superficial layers of the earth (less dense than the core) were liquified and expelled into a ring system which then coalesced into the moon. The moon has an insignificant magnetic field because it lacks a dense iron core. This too accords with a superficial impact. (Not that superficial if you were sunbathing in the area, mind!!!)

The earth has no (almost no) craters because it has a system of plate tectonics which erase the older cratered surfaces as these are over-ridden by younger, un-cratered plates.

When observing the moon try looking for large craters whose walls are indented by small ones. I don't think you'll find any, which suggests that the impactors were getting smaller over time in accordance with the modern theory of planetary accretion. It's a nice bit of 'instant science' to do at the eyepiece. If you find a large crater pinged* by a smaller one let me know!

Olly

*A key term in physics. For a full definition send a postal order for £100 to....

Only a £100, your so cheap.....

[stephenwilliamwatts]

Cheese Grommit!!!

[NeilFawcett]

Isn't there something extremely odd about the structure off the moon, in the sense that when impactors are fired at it, the resulting 'ring' almost implies the moon doesn't have a solid interior?

[xarcane]

I believe the object that hit the earth then gradually became our moon is often called 'Theia'

[symesie04]

I knew i had read somewhere that there was a new theory on the reason the moon has identical chemistry to earth.

http://www.bbc.co.uk/news/science-environment-19992233

[Ganymede12]

A documentary called How the Universe Works said that there was this period in the Solar Systems history called the Titan Marmakia (War of the Titans) where hundreds of proto planets collided. The smaller ones where destroyed and the larger ones grew. One of the last collisions of the Titan Marmakia was the collision between proto earth and Thea (a Mars sized planet). This collision was a glancing blow which knocked debris into space. Gravity collected some back to earth (making it round again) whilst the remaining Debris became 2 moons which then collided to become the moon we know and love.

[Mr Q]

A documentary called How the Universe Works said that there was this period in the Solar Systems history called the Titan Marmakia (War of the Titans) where hundreds of proto planets collided. The smaller ones where destroyed and the larger ones grew. One of the last collisions of the Titan Marmakia was the collision between proto earth and Thea (a Mars sized planet). This collision was a glancing blow which knocked debris into space. Gravity collected some back to earth (making it round again) whilst the remaining Debris became 2 moons which then collided to become the moon we know and love.

Well, most of us DSO observers don't love the Moon when it hinders observations - only love it when observing it Otherwise when we are not Kinda like clouds? Nice to look at during the day but not at night?

[cowasaki]

A documentary called How the Universe Works said that there was this period in the Solar Systems history called the Titan Marmakia (War of the Titans) where hundreds of proto planets collided. The smaller ones where destroyed and the larger ones grew. One of the last collisions of the Titan Marmakia was the collision between proto earth and Thea (a Mars sized planet). This collision was a glancing blow which knocked debris into space. Gravity collected some back to earth (making it round again) whilst the remaining Debris became 2 moons which then collided to become the moon we know and love.

This is the theory I would quote if someone asked me but there are now newer theories which don't quite fit with that one apparently. I've not had time to look into them in much detail but that theory did seem to fit with the facts.

[jnb]

It's supposed to have been a glancing low-speed blow, explaining how the Earth survived. The impactor is thought to have originally formed at Earth's L4 or L5 point, but when it got too massive to be stable there it drifted closer to Earth, ultimately impacting.

I wonder if the similar composition of Earth and Moon is not to be expected given the impactor formed at the same distance from the Sun as Earth, and would thus probably have had an Earthlike composition itself.

That would explain one of the questions I had about that theory, namely that if the Earth had been hit by such a body in the past then why is the Earth's orbit not more elliptical than it is?

But it raises another question because the Lagrangian points are only stable for small objects so that would presumably put an upper limit on the size of any such object. Why would a second planet sized body have formed, as a single body, 150 million km away at L4 / L5 thus creatng a single massive impact event rather than as collection of smaller objects migrating toward the early Earth as a succession of smaller (albeit pretty damned large) impacts?