formation of the moon

[damienpaul]

I have been reading about the theory that the moon was formed from a collision between earth and a mars sized object early in the formation of the solar system, can other people in the forum shed some light onto this?

[joetommasi]

That's probably brought fourth by the same people who believe that Venus came from Jupiter (or Saturn or from outer space).
There are one time events in the history of the solar system, but nothing the size of mars could have hit the earth and leave the earth as is. Besides, where's the culprit?
The moon did come out of the earth, but the modus operanti is different.

[DippyHippy]

Actually jtommasi, I think you'll find it is (or was) a very serious scientific theory - the important point to remember is that this collision was supposed to have happened during the very early stages of the solar system's formation: ie, when the inner planets were nothing but molten balls of lava and when there were a lot more planetesimals flying around because the planets were still forming.

The last I heard serious mention of it was in the 1999 A&E / BBC co-production, The Planets so I don't know if it's still a theory that under consideration.

[joetommasi]

I also read the same story about mars' sized planet hitting the earth and blasting the moon out.
There is no question the earth was molten at one time, although I haven't heard a good reason for the melting (regardless of how many meteorites hit it). I can only imagine what it would look like if a liquid mars was to hit a liquid earth.
Neverthless, liquid or solid, where is the mars that hit the earth? Did it turn into the moon?
It took a long time for a planet to form which means if there was a collision between earth and this planet, both planets must have crossed the same orbit. Well why did it take so long for the two planets to collide? Why didn't they collide long before they were fully formed and melted? I would expect this "collision" to have happened way before the planets were fully formed and melted.

[DippyHippy]

You're not reading it properly. It DID happen long before the planets were fully formed - that's what I've been saying. That's what damienpaul said in his original post.

I have been reading about the theory that the moon was formed from a collision between earth and a mars sized object early in the formation of the solar system

This theory proposes that the collision took place during the formation of the solar system and it's pretty much widely accepted that all the rocky inner planets were molten at that time because the material that went to form the planets was still cooling after the birth of the Sun. Nothing *melted* at all - we're talking about embryonic planets.

Likewise, there was an awful lot of material around and a lot more planets were forming. As with everything else in nature, it's a case of survival of the fittest - the largest planets drew in the smaller ones, thereby adding the smaller planet's mass to their own and increasing their size.

The Moon formation theory states that a planet - specifically a planet still in a molten state - hit the Earth - which was also still in a molten state - and that the material thrown out from the collision came together to form the Moon. Hence, no more Mars sized world (other than the Mar-sized world that actually is Mars) The Mars-sized object didn't have to be in the same orbit necessarily - the two worlds could have formed some distance apart but be slowly drawn towards a collision as their respective masses increased and their gravitational pull on one another got stronger.

I seriously suggest you do a quick search online on the theory of planetary formation. This is no crackpot idea. If you think it's lunacy, you might want to try telling the world's geologists and planetary scientists that.

[Dan Luna]

The idea is that the second planet hit Earth and was vaporised along with a big chunk of the Earth's outer layers. The iron core of this planet went into the Earth and the rest formed a big cloud which then condensed to form the Moon, but the lightest elements got lost to space. This explains why the Moon is made of material very similar to the outer layers of the Earth but lacks lighter elements such as hydrogen.

[joetommasi]

I find a consistency problem here.
The earth was molten. Fine. The moon came off the earth. Fine.
Problem.
The earth was molten.
A molten earth is almost a sphere. It has to be, but soon after freezing and water pooled into the first ocean, a continent called Pangea emerged.
I'm not into geology, but how would Pangea rise out of a molten earth?
Problem
Pangea was made up of rocks that were lighter than the rocks below
But why would those rocks congregate in one spot only and not spread all over the earth leaving it round?
Problem
The moon formed from liquid or gasseous rocks
The moon should have formed round and since no geologically noteceable forces have ever been at work on the moon the moon should have no mountains or seas.

[DippyHippy]

Thank you, Dan

jtommasi, you need to do a Google search.

[Dan Luna]

The Planetary Science Institute have a nice simple summary page on the impact theory:

http://www.psi.edu/projects/moon/moon.html

[Dan Luna]

Without going out and buying a geology book, if there was a continent and a sea on Earth, the continent must have bulged out more from the center of the Earth. This wouldn't happen while Earth was completely molten, so must have been after it had cooled enough to form a crust. The molten material underneath would then circulate in currents, rising in some places and sinking in others, and where it rose under a particular part of the crust it would push it up, break through and erupt, or both. This still happens today producing volcanic islands as in Hawaii and Iceland. If you look at Mars there is one massive volcano (Olympus Mons) with 3 smaller ones in a line near it. This suggests a big hot spot concentrated in one place. If the Earth had started with one big bulge like that and then had cooled enough for water to come out of the atmosphere and cover the lower land with a sea, then you would have your single continent.

[Dan Luna]

And finally (I hope), the Moon would also have been more or less spherical when it cooled and formed a crust. The "mountains" are not the same as on Earth, but are the result of asteroids hitting the Moon, forming craters, throwing lumps of rock about, and pushing parts of the crust up and down due to the shock waves.

The "seas" started as very large craters and you can see many of the mountains are round their edges (see

http://www.lpi.usra.edu/research/lunar_orbiter/

and search for "Mare Orientale" which is on the far side of the Moon, particularly looking at

http://www.lpi.usra.edu/research/lunar_orb...mg/iv_187_m.jpg

).

The crust on the near side is thinner (no, I don't know why yet) and because of this these big holes have been filled in with molten rock (lava) that came up through cracks from below and smoothed them out. Later impacts have also helpled destroy the nice bulls eye patterns, but parts of the mountain rings can still be seen (see

http://www.lpi.usra.edu/research/cla/

and look round Mare Imbrium in Part 1 sections B and C).

[joetommasi]

A couple of years ago I was asked to teach a basic course on geology/solar system and since my degree is in physics I went through a crash course on the internet to get some data.
What I found was a lot of good data and a lot of good guesses. Some of the simplest questions seemed to attract simple answers which I thought were not quite accurate. For example:
Why is most of the mass in the solar system concentrated in the sun while most of the momentum is concentrated in the planets?
Why is the moon facing only one side to earth (Mercury seems to do the same with the sun and gravity is not a cultprit)?
Why is the far side of the moon different from the near side?
What allowed planets to grow?
What melted the inner planets?
Why are the inner planets around the sun small while other star systems have large planets close to the star (I know, that's all we can detect with today's technology)?
How can pangea form if the earth was molten?
These are a few questions which I couldn't find a satisfactory answer to. If you know a website that can help, please let me know.

[lazserus]

jtommasi, you have a degree in physics and you don't know any of these answers?

I'll give you just a few answers, but you should already know most. Like Dippy says, you're in serious need of a Google search. Hell, just go to http://www.sciam.com and go to the "Ask the Experts" page - most of the answers lie there.

How can pangea form if the earth was molten?

This has been explained several times. One aspect of the entire model you're missing (which is the most important aspect) is time. None of this happened instantly, but over billions of years. When the collision took place the Earth was molten. Billions of years later the Earth eventually cooled, forming the outer lying layers. Pangaea formed AFTER the cooling, not during the molten stages.

Why is most of the mass in the solar system concentrated in the sun while most of the momentum is concentrated in the planets?

For starters, if Sun contains the most mass in the solar system why would you think it should be concentrated elsewhere? The momentum lies within the planets because of spin/orbit - it's basic Newtonian physics.

Why is the moon facing only one side to earth (Mercury seems to do the same with the sun and gravity is not a cultprit)?

What makes you think gravity isn't involved? That's exactly what causes it. Alan P. Boss of the Carnegie Institution of Washington says, "The moon keeps the same face pointing towards the Earth because its rate of spin is tidally locked so that it is synchronized with its rate of revolution (the time needed to complete one orbit). In other words, the moon rotates exactly once every time it circles the Earth."

Why is the far side of the moon different from the near side?

Why is the far side of the Earth different from the other side? Or any other body, for that matter?

[joetommasi]

Ok, let me explain my questions a bit more fully.
Momentum depends on mass and velocity. Since the mass of the planets (combined) is 1 or 2% of the sun's mass, it stands to reason that where the mass is the momentum is there also. That is not the case with the solar system. Most of the mass is in the sun while most of the momentum is in the planets. Now, the sun probably formed before the planets so the nebula that formed the sun should have concentrated it's momentum on the sun not the planets. Why?
The moon has two different sides much as the earth or any other object we can think of (except a neutron star). Problem is the side facing us has a lot of "seas" while the side that's not facing us has very few if any seas. It would be the same as if all the continents on the earth are on one side of the earth and the oceans are on the other side. You would figure that both sides would have the same or similar amount of cracks through which underlying magma would flow through but that is not the case. Why?
The earth was melted and the lighter lithosphere rose above the asthenosphere simply because it was less dense. A molten earth would have a layer of liquid granite surrounding it and as the earth cooled (let's skip for the moment the problem with the mars sized object since it's really irrelevant for our discussion) the earth should have been round and granite enclosed. Instead the earth sported pangea (granite continent) surrounded by a single ocean. How could the lithosphere have concentrated itself on one spot? Underground magma currents and time can't really explain that.
Tides can probably still a planet or satellite but then there is no reason why only one satellite and one moon in the solar system seem to have lost their spin. Indeed any satellite placed in orbit will end up with a spin. Why?
I can get into more details with these questions.

[DippyHippy]

I think you'll find as well that many many many moons in the solar system are tidally locked to their parent worlds - I think Saturn only has 2 moons that *don't* keep the same face turned towards it.

Perhaps the question should be "why are there moons that *don't* keep the same face turned towards it's parent planet?"

[TheThorn]

Let me take a shot at the pangea thing.

You don't seem to recognize that there are two types of rock (actually lots more than two, but two main igneous types). They refer to them as SIMA and SIAL for silica-magnesium and silica-alumina. Granite is one type of sial, basalt is one common type of sima. The key point is that sial is less dense than sima, so it "floats" on it. You could think of it as the crust is sial and the mantle is sima (except that then you'd have to consider the thin crust under the oceans as really just the upper part of the mantle). The continental part of the crust is made of sial, it's thicker than the oceanic crust - it sticks down into the sima a lot farther than it sticks up out of it (like an iceberg).

The sima of the mantle is plastic. Over long time frames it circulates. The icebergs circulate with the top layer.

Ever cook something that had a skim of impurities on top of a bubbling and convecting pot? They tend to collect together in clumps on the surface, right? Sometimes in one big clump, sometimes broken up into several clumps, moving around with the underlying convection. Froth on you sphagetti pot, foam on a batch of jelly that you're cooking, pangea on the surface of the earth, all the same thing.

Now as for the moon's two sides being so different, it isn't the only moon in the solar system like that. Iapetus is one of Saturn's moons, is tidally locked, and has one side very differnt than the other. In its case it's the leading and trailing sides of the moon that are different, not the front and back.

Iapetus


OTOH, our moon is very different than other moons in the solar system. For instance, it's the only one that is more strongly bound, gravitationally, to the sun than it is to its planet (yep, the gravitational force that the sun exerts on our moon is stronger than the gravitational force that the earth exerts onthe moon). It is also less inclined to the ecliptic (the sun's equator) than it is to it's planet's equator. Don't ask me why.

[Tinaa]

I think I can help with the Pangea thing. At one time I thought plate tectonics were very interesting and did some self-education. The Earth's crust moves around. During the time the dinosaurs roamed the Earth, Pangea was the "supercontinent," it then broke up into two large continents and moved until we see what we have today. The crust of the Earth has gone through many continent changes in its long life. At one time all the continents could have been covering the whole northern hemisphere or it could have covered the equater. This site has animation and lets you see where the continents where during a particular geologic age. The collision would have had to have happened way before there were any distinguishable landforms, during the accretion phase.

[damienpaul]

my understanding is that the collision occured sometime before there was appreciable water and possibly before most of the differentiation of the minerals of the earth occurred.

By the way, nice site tinaa

[Guest]

Dan
I read the article you pointed out about the formation of the moon. My understanding is that an impact with a liquid earth should form one or more rings around the earth similar to saturn's rings. The problem with these rings is that gravitationally it is very difficult for these rings to coalesce into one body and the tendency is for the debris to come back to earth. Indeed in the website there's an understanding of this and the website qualifies itself by stating that

Canup's early work, presented in July 1997, suggested the debris from an impact might not make a moon, but only a swarm of moonlets. Her later work (fall 1997) led to more "success" in aggregating the debris into a single moon.

Leading to more "success" makes the statement far from definitive, it may hint that the numbers were slightly bent.

By the way, I'm sure you noticed on that website's graphics that the "molten" earth is of the wrong color.

[Guest]

Tinaa
I understand that the continents move on plates and this motion appeard to be due to convection below the plates.
However the question still stands. What would cause the continents to originally congregate on one spot. The earth was molten, therefore it must have differentiated itself depending on density. The denser elements (and compounds) should have sank to the center of the earth and the lighter components should have floated above the core, and this is what we see.
In the cooling process, this segregation should have manifested itself in creating an earth that was like an "orange" where each layer is complete and surrounds the entire lower level (the orange sections envelop the core, the section's skin enclose the sections, the pulp surrounds the section's skin, and the outer "orangy" skin envelops everything). That being the case, the lightest type of rocks should have enclosed the entire earth just like the "orangy" skin surrounds the orange.
Instead most of the "skin" was removed from the earth (after it cooled so it couldn't have been the mars' sized object) and only one section of "skin" was left behind.
You see, convection has nothing to do with this left over "skin", convection came in afterwards to break up the piece of leftover "skin".

[VanderL]

Couldn't gravitational lock while the Earth was cooling be responsible for the Pangea supercontinent?
I'm not sure which moon (Titan?) seems to have continent-like elevation in one place due to gravitational lock of the moon with Saturn.
Of course we need to revise the history of the solar system, but that's ok.
Cheers.

[Tinaa]

Pangea was not the first continent on Earth. It is only the supercontinent that was present during the Mesozoic Era from 248 to 65 million years ago. The Mesozoic Era includes the Triassic, Jurassic and Cretateous periods (Dinosaurs). Pangea began to break up during the Mesozoic Era. During the Precambian Era, 4500 to 542 million years ago, is when the first continent formed. The oldest known rocks are from 3800 million years ago. I doubt we know where or how big the first continent was. There were probably little continents all over the place, the top of the froth as TheThorn suggested. They began moving and eventually got together, then split apart, then got together again.

[VanderL]

So there were plates drifting around, continents on top of them, that came together once upon a time, and drifting apart again some time later. I don't know, but simple drifting seems an unlikely force to do all that. Where is the logic, if we think of "unguided" drift as the only force working?

[thomastech]

There are 2 main reasons I think the "Collision Theory" is probably the best explanation.

1. Gravity

2. Time

We are just learning that "Galaxies" are colliding all over the universe...
...not to mention all of the small collisions that occur when this happens...
I am sure that somewhere in the universe there are collisions between newly forming planets that could produce "Earth and Moon" like environments.

We have not been on this planet but a very short "Time"...

...and I think its safe to say that "Gravity" has been around forever!

[VanderL]

Thomastech, you've stated exactly what the current thoughts are about the Universe; give gravity enough time and all can be explained. My guess is that we need more than that. Galaxies colliding seems an unlikely scenario, it takes an enormous amount of time and energy to get galaxies moving, let alone towards each other. We need something that can move matter more efficiently, like the magnetic force in electric currents in space (Birkeland currents) that can concentrate matter very efficiently. After that, gravity can take over.
Cheers.

[Guest]

So basically we have not records of earth prior to 3.8 billion years ago


"The oldest known rocks are from 3800 million years ago. "

"During the Precambian Era, 4500 to 542 million years ago, is when the first continent formed."

If, by our reckoning, the Mesozoic ranges only from .25 billion to .07 billion years ago, and during this period of time Pangea was there (intact or otherwise) can we assume that from 3.8 billion years through the early stages of the Mesozoic era Pangea was intact?

If that's the case there an astounding amount of time through which Pangea was intact. Considering the possibility that the earth must have been hotter, why this lack of movement? Did something speed up the plates?

Let's look at a boiling pot with froth on the surface. Yes the froth seems to concentrate itself in one spot so we could assume the same for the earth. The problem with this is that a pot has a bottom where the heat is and metal around it which is a good conductor of heat (better than water anyway). The water in contact with the pot's side is hotter than the water around it and gets pushed away from the sides and toward the center where it pools. The earth has no such sides therefore one blob of froth in one spot is rather unlikely.

[Guest]

Thomastech
collisions between galaxies do happen although there is a major difference between a Mars sized object colliding with the earth and two galaxies colliding together. Although time and gravity are main ingredients in both collisions, because of the inersellar distances the odds of two stars from the two colliding galaxies bumping into each other are slim. Of course if the two galactic center collide the odds increase.

[Guest]

Vanderl
I'n not familiar with Birkeland currents but please note that magnetic fields and electric fields do not range as far as gravity. There's no appreciable magnetic or electric field (or current or whatever) effect from the milky way on our sun. However there is an obvious gravitational field holding our sun around the milky way.

[thomastech]

The reining theory of the extinction of the dinosaurs is that a very large asteroid struck the southern Gulf of Mexico creating unimaginable conditions!

If you look at the area of impact from space, you can not even tell that an impact happened there!

In the geological timeline of the earth, the dinosaurs became extinct not too long ago.

There is no telling how many large asteroids or small planets that have hit the earth before the age of the dinosaurs.

Time & Gravity have made the evidence of past impacts hard to find on earth.

[thomastech]

I was just using the example of the Galaxies colliding as an example.

I was not implying that our Galaxy was colliding with another…

…although… it is possible that it could have happen in the past?

If it is happening in other parts of the Universe…

Why could it not have happened here in the past?