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damienpaul
2004-Jan-06, 11:38 PM
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
2004-Jan-07, 08:02 PM
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
2004-Jan-08, 12:49 AM
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
2004-Jan-08, 02:52 PM
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. :o

DippyHippy
2004-Jan-08, 03:22 PM
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
2004-Jan-08, 05:45 PM
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
2004-Jan-08, 06:17 PM
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
2004-Jan-08, 10:21 PM
Thank you, Dan :)

jtommasi, you need to do a Google search.

Dan Luna
2004-Jan-09, 05:46 AM
The Planetary Science Institute have a nice simple summary page on the impact theory:

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

Dan Luna
2004-Jan-09, 06:11 AM
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
2004-Jan-09, 06:51 AM
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 (http://www.lpi.usra.edu/research/lunar_orbiter/images/img/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
2004-Jan-09, 12:22 PM
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
2004-Jan-09, 05:29 PM
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
2004-Jan-09, 06:27 PM
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
2004-Jan-09, 11:17 PM
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
2004-Jan-09, 11:58 PM
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 (http://www.solarviews.com/eng/iapetus.htm)


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
2004-Jan-10, 03:42 AM
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 (http://www.ucmp.berkeley.edu/geology/tectonics.html) 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
2004-Jan-10, 05:12 PM
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
2004-Jan-11, 05:14 AM
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
2004-Jan-11, 05:30 AM
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
2004-Jan-11, 01:47 PM
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
2004-Jan-11, 06:02 PM
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
2004-Jan-11, 07:09 PM
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
2004-Jan-11, 07:33 PM
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
2004-Jan-11, 07:56 PM
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
2004-Jan-11, 09:52 PM
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
2004-Jan-11, 10:01 PM
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
2004-Jan-11, 10:11 PM
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
2004-Jan-11, 10:20 PM
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
2004-Jan-11, 10:32 PM
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?

Tinaa
2004-Jan-11, 11:20 PM
Before the Mesozoic Era, the continents were drifting towards each other, I think this was one of the reasons Earth had a global tropical climate. Millions of years in the future, we may see another supercontinent where China butts up against California (just an example).

TheThorn
2004-Jan-11, 11:34 PM
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?

No we can't. In fact, the evidence indicates that Pangaea was formed from smaller continents colliding together. Those collisions threw up some of the older mountain ranges on the planet - the ones that are a long ways from any current plate boundaries, like the Urals and the Appalachians.

Pangaea did not exist before Mesozoic era 250 to 65 million years ago. Before Pangaea came into existence, there were separate continents (Laurentia, Baltica, Gondwanaland and others) that collided to form Pangaea. Before that there may have been other stages where there was one super continent, and stages where there were many continents - possibly several such cycles given that before the Mesozoic we have 3,550 billion years to work with. We just can't tell (yet) because the farther back you go the more of the evidence has been erased by more recent events (erosion and mountain building).

Check out Formation of Pangaea (http://webspinners.com/dlblanc/tectonic/pangea.shtml) for more details.

JESMKS
2004-Jan-11, 11:56 PM
Hi
Tinaa asked me to visit this Question and Answer section and explain when and how the first continents were formed. I am a retired geologist, but my area of study was ground water which is far afield from the study of plate tectonics. I have no direct knowledge in this field, but I'll make an educated guess for what its worth:
When the earth was a molten orb, the heavier iron and nickel settled near the center of the earth forming what is now the core. The surrounding layer, which was lighter, had a basaltic composition, and the surficial layer, which was still lighter, was a viscus crust of silicic rock. This crust, which more or less floated on the less viscus basaltic rock, tended to break up into plates that could float around, like ice breaking up on a lake and moving before the wind. Movement of these plates was probably caused by centrifugal force of the revolving earth. Being almost molten, the period of revolution of the earth was probably different at the equator than at higher latitudes (Our sun takes 25.6 days to rotate at it's equator and a longer perior to rotate as you approach it's poles). These floating plates tended to pile up and formed the ancestral continents, which are still floating around in a sea of basalt like ice cubes floating in a glass of water.
I would guess that the continents would have formed sometime in the first billion years of earth's existence.
I hope this is of help.
Jack

joetommasi
2004-Jan-12, 08:25 PM
TheThorn

can you please identify the source of your statement

(yep, the gravitational force that the sun exerts on our moon is stronger than the gravitational force that the earth exerts onthe moon)

According to my calculations, the earth-moon attraction is a couple of magnitudes larger than the sun-earth attraction

joetommasi
2004-Jan-12, 08:29 PM
Tinaa
Thank you for your assistance. I did find at least a prior supercontinent. I'm looking into it ;)

TheThorn
2004-Jan-13, 12:54 AM
Originally posted by jtommasi@Jan 12 2004, 08:25 PM
TheThorn

can you please identify the source of your statement

(yep, the gravitational force that the sun exerts on our moon is stronger than the gravitational force that the earth exerts onthe moon)

According to my calculations, the earth-moon attraction is a couple of magnitudes larger than the sun-earth attraction
No problem. My original source was an essay by Asimov (sorry I can't recall which one, in which of his essay collections) that I read WAY back in the 70's or maybe even the '60s. But I also confirmed it for myself before posting that statement here.

Gravitational force is GMm/r^2 where M and m are the masses of the two bodies, and r is the distance between them.

Mass of the sun is 2.0 X 10^30 kg. (2 significant digits is more than enough for this.)

Call the mass of the moon L (it's going too fall out of the calculation at the end, and I'm too lazy to look it up).

Mass of the Earth is 6.0 X 10^24 kg.

Distance from Sun to Moon = 1.5 x 10^8 km (same as sun-earth distance).

Distance from Earth to Moon = 3.9 x 10^5 km

G is thegravitational constant which serves to get the units right, and like L it's going to fall out of this comparison.

Sun-Moon gravitational force = (G X L X 2.0 X 10^30) / ((1.5 X 10^8)^2)
= (G X L X 2.0 X 10^30) / (2.25 X 10^16)
= G X L X 9.0 X 10^13

Earth - Moon gravitational force = (G X L X 6.0 X 10^24) / ((3.9 X 10^5)^2)
= (G X L X 6.0 X 10^24) / (1.5 X 10^11)
= G X L X 4.0 X 10^13

So the ratio of the two is a little over 2:1 in favour of the sun.

Unless I screwed up the math, but this is the same conclusion that I remember Asimov coming to, so I'm confident that I got it right.

Geometrically, what it means is that the path that the moon takes through space is always concave towards the sun. I guess you could say that the moon really orbits the sun, co-orbital with the earth.

joetommasi
2004-Jan-13, 12:32 PM
TheThorn
You're quite right, I worked out the same numbers and the attraction between sun-moon is greater than earth-moon

Guest_jtommasi
2004-Jan-13, 04:27 PM
Tinaa
Let's go back to the original problem about the moon being formed by a metal rich mars sized planet (MSP) plunging into a metal poor liquid earth and blasting liquid rock into orbit where the moon eventually formed.

For the MSP to have formed to a planet's size, it must have been in a stable orbit for quite a while. No mechanism is proposed for moving the planet from its orbit to a collision with the earth. Gravity and time are not viable for a number of reasons.

Considering that mercury is virtually iron-nickel, the earth was poor in iron-nickel and jupiter is extremely deficient in iron-nickel we must assume that the MSP must have come from an orbit somewhere between mercury and venus. If both mercury and venus are stable in their orbit, why would the MSP be unstable? Besides, we said that it must have been in a stable orbit to allow the planet to form.

If we choose to make the MSP's orbit an elongated elipse, we run into a problem with it's composition. It will collect a great deal of rocks and less iron-nickel which will void the proposition that the center of the earth is the MSP.

Can you use your contact to answer these questions?

Thanks

billy jay
2004-Jan-13, 05:27 PM
One reason I heard why the side of the moon facing the earth has more Mares than the Moon's farside is the Earth acted to increase the velocity of meteors passing it. It was like the slingshot affect Jupiter had on the Voyager spacecraft so it more quickly reached Saturn, Uranus, & Neptune. As the Moon was solidifying, meteors passing Earth which then slammed into the Moon had greater velocity than the general velocity of meteors near the Earth-Moon system. As the Moon's surface cooled & solidified, the extra velocity of Earth speeded meteors penetrated the Moon's crust deeper to the hotter subsurface lavas. Thus, lavas were able to sprew up & over the Moon's near solid surface to a much greater extent than the Moon's farside. The earth does have 80 times the mass of the Moon which could account for the lunar far & near side differences.

Tinaa
2004-Jan-13, 06:57 PM
j-tommasi, I'll see what I can do and get back ASAP!

joetommasi
2004-Jan-13, 07:43 PM
jesmks

I don't want to get off the topic but I have a question that has to do with geology. Convections refers to heat transfer by rising fluid. In a classical pot with water boiling, heat is transfered from the bottom of the pot to the top by convection currents. In the pot there is a constant source of heat (at the bottom) and a constant source of cold (at the top) and fluid rises from bottom to top. If the heat is removed, convection currents stop.
Why does the earth's center continues to create currents. I mean is there a source of heat (apart from thorium or any other radioactive source) that keeps the current flowing? Could this heat be a left over from the earth melting? And if it is why does it seem to be concentrated in the core? Is the earth loosing that much heat?

TheThorn
2004-Jan-13, 11:29 PM
I mean is there a source of heat (apart from thorium or any other radioactive source) that keeps the current flowing?

Why do you say "apart from thorium or any other radioactive decay"? If I understand correctly, that source is enough to provide the heat flux that we observe. Nothing else is required.

Tinaa
2004-Jan-14, 02:03 PM
Dr. Wood came through again. I hope this answers your questions:

"I am not sure where the idea of Earth being metal-poor and the Mars-size impactor being metal rich came from. Both worlds were terrestrial planets and both possessed a significant metal content as they were forming.
*
There also seems to be some confusion regarding timescales as well. Planetesimal growth is roughly exponential, and you go from large grains to Earth-size objects in ~30 million years (< 1% of solar system history). It not only is plausible, it is even likely that several objects could maintain eccentric, planet-crossing orbits without collision over this timescale. The Moon was probably formed in a collision between Earth and a Mars-sized planetesimal within the first 100 million years (2% of solar system history) of Earth’s history. This is certainly consistent with numerical simulations of planet formation. Only those objects with low eccentricities would remain after ~ 500 million years (10% of solar system history) which is consistent with the period of heavy bombardment reflected in the lunar highland craters. Only after the heavy bombardment ends do you end up with a solarsystem that looks like the solar system we have today.
*
The general theory of impact origin of the Moon is nearly unanimously accepted by planetary scientists. The debate we have in the community is only in the details of the process. It sounds like your debate is more general. If someone is going to propose a theory to supercede the impact hypothesis, then the new theory must account for all of the other oddities in the solar system that the giant impact theory already handles so well."

joetommasi
2004-Jan-14, 03:45 PM
Tinaa.
Again you&#39;re coming through with good answers.
Again I must ask for help.
1) This bombardment. Is it needed for the sake of the moon or is there other reasons why we need it? In other words, does any other planet show signs of this bombardment?
2) How did the planets managed to heat up to the melting point? I was under the impression that it was because of the bombardment but if it happened afterwards then something else melted the planets. Radioactivity?
3) Any way I can get a short list of oddities in the solar system (restricted only to the inner planets)?

VanderL
2004-Jan-14, 04:52 PM
Thanks Tinaa, dr Woods,
True that a new theory should explain all that we can see, but there are indications that all is not well with impact theory. On the Moon we can see canyons, sinuous rilles, rayed craters, terraced craters, craters with flat floors and craters with central peaks. For every feature a different explanation is needed and most of them are not experimentally verified. Ralph Juergens and Wallace Thornhill (at www.holoscience.org you can find details) argue that all these features can be explained in one electrical model and they are verifiable in a laboratory. This model also explains why asteroids and comets (Borrelly and Wild 2) can look so pitted and cratered without falling apart (as is probable when only impacts are considered).
Cheers.

Tinaa
2004-Jan-14, 07:16 PM
Let me see if I can help you on my own with these questions. Oh yes, most every solid surfaced planet shows signs of the every heavy and continuing lighter bombardment. Check out these websites and see of they help.

Here (http://earthsci.terc.edu/content/data_centers/es2507.cfm?chapter_no=25) and here (http://earthview.sdsu.edu/trees/craters.html) are some interesting sites.

joetommasi
2004-Jan-14, 09:41 PM
Tinaa
I&#39;m aware of cratering etc. I understand that the planets were formed by accreting small and large bodies therefore there was a constant bombardment. My understanding also is that such bombardment petered off after the planets were formed (and the moon was blasted out of the earth). my question is this: was there a sudden pick up in the bombardment of the moon (earth, etc)? And if there was, what need is there (or proof is there) apart from explaining what happened to the moon?
I&#39;m having a difficult time asking the questions, ain&#39;t I?

Tiny
2004-Jan-14, 10:36 PM
I did a little research about the formation of the moon :ph34r:
There are 4 theories that explain the formation of the Moon

1. Capture Theory

The Moon was formed elsewhere in the Solar System and was captured by the Earth.

2. Formation of the Moon in Orbit Theory.

The Moon was formed from material that had been captured into orbit around the Earth after the Earth was mostly formed.

3. Fission Theory

The Earth spun fast enough so that it became deformed and a piece broke off to become the Moon.

4. Giant Imapct Theory

Collision of a Mars-size object

Here&#39;s the newest explaination :
http://www.ucsc.edu/currents/01-02/08-20/moon.html

joetommasi
2004-Jan-14, 11:26 PM
Wait a second. A couple of years ago a comet hit jupiter but before getting too close jupiter ripped it apart. Wouldn&#39;t a mars sized object suffer some kind of damage if it got too close to the earth (especially if it was liquid).

Well......... maybe they were both solids.....who knows... maybe it happened before they melted........but then, the earth would not have been differentiated......and the moon is made up of light rocks with no volatiles..........yep, it happened after the earth was melted........that means the earth was fully formed and got remelted in the impact......no that&#39;s not right.......I give up. All websites I get pointed at give slightly different variations on the same theme..... Something else must have been going on we&#39;re missing <_< .

Tinaa
2004-Jan-15, 12:27 AM
Try this site (http://seds.lpl.arizona.edu/nineplanets/nineplanets/origin.html). I think it answers your questions. If not, we&#39;ll keep trying&#33;

Remember too, Jupiter can fit like 1300 Earth inside it so its gravity is immense compared to ours.

Tiny
2004-Jan-15, 12:46 AM
so does formation of the Moon in Orbit Theory match the site u just post? (sry didn&#39;t read them all) The Moon was form after the Earth like 1 million yrs, and capture by Earth&#39;s gravity?

Tinaa
2004-Jan-15, 02:05 AM
One hundred million years may seem like a long time, but it is only 2% of the 4500 Billion years since the earth formed. The moon formed from the blow off from the impact of the mars sized "asteroid." Earth had enough gravity to hold it close. A side note: the moon is moving away from Earth a tiny bit every year. Someday it won&#39;t even cover the whole sun during an eclipse.

Josh
2004-Jan-15, 02:10 AM
4500 Billion?? That&#39;s like 4.5 Trillion. That&#39;s a long time. I thought the entire universe was only 13.7 Billion years old?

Tinaa
2004-Jan-15, 02:15 AM
Forgive me, I meant 4500 million. These numbers are so big and the time so vast, I get confused&#33; :blink:

TheThorn
2004-Jan-15, 02:28 AM
Originally posted by jtommasi@Jan 14 2004, 11:26 PM
Wait a second. A couple of years ago a comet hit jupiter but before getting too close jupiter ripped it apart. Wouldn&#39;t a mars sized object suffer some kind of damage if it got too close to the earth (especially if it was liquid).

Well......... maybe they were both solids.....who knows... maybe it happened before they melted........but then, the earth would not have been differentiated......and the moon is made up of light rocks with no volatiles..........yep, it happened after the earth was melted........that means the earth was fully formed and got remelted in the impact......no that&#39;s not right.......I give up. All websites I get pointed at give slightly different variations on the same theme..... Something else must have been going on we&#39;re missing <_< .
There&#39;s stuff here that I don&#39;t understand as well, but some of it is different stuff than you are struggling with.

I don&#39;t think the earth&#39;s tidal forces would be large enough to disrupt a Mars sized object the way Jupiter disrupted Shoemaker-Levy. Jupiter is way bigger than the earth, and that comet was way smaller than Mars.

For the impact theory to work, clearly this major impact had to happen late in the Earth&#39;s development, after the Earth was differentiated, but not necessarily while it was molten. I doubt that the surface of the earth was ever molten anyway, and parts of the core still are, so really we&#39;re just talking about how thick the solid layer was at the time. And it probably doesn&#39;t matter anyway. If the ejected material was solid it could still form a single body over time through gravity (or VanderL&#39;s electric forces if necessary). So ther isn&#39;t really a tight time window that this had to happen in.

The link that Tiny posted as the "newest theory" shows how a model can simulate the moon originating in such a collision. It leaves a moon sized object orbiting around the earth&#39;s equator. Most of the large, close moons in the solar system (rather than the small distant moons that appear to be captured asteroids) rotate in their planet&#39;s equator. Even Uranus&#39; moons orbit within a couple of degrees of it&#39;s equator, which is tipped over 90 degrees to the ecliptic.

But our moon doesn&#39;t. It orbits at an angle to the equator that varies from 18.28 to 28.58 degrees, but which is a constant 5.14 degrees from the ecliptic (the sun&#39;s equator). Strange, eh? And hard to explain with a collision. Maybe it evolved that way over time and perhaps someone else can explain how.

Another thing I don&#39;t understand is why the spins of most of the planets line up with the direction of rotation of the sun and the direction of their orbits around the sun. Is that just a co-incidence? If they all formed out of a vast number of accretionary collisions, their spins should be in a direction set by the last major glancing collision in their history, which would be a random direction. Yet all but Uranus, and Venus rotate directly, with an axial tilt less than 30 degrees. That&#39;s not random. If we use the collision theory to explain the two that don&#39;t fit, then we&#39;re left struggling to explain the 6 that do fit.

Triton, Neptune&#39;s largest moon is another anomaly. It&#39;s large and near to Neptune, so it doesn&#39;t appear to be a captured object. Yet its orbit is wildly tilted to Neptune&#39;s equator - even worse than our moon. It doesn&#39;t appear to be caused by a collision either. But if it isn&#39;t caused by a collision, and isn&#39;t captured, then what the heck is it?

That&#39;s the wonderful thing about science. There are always unanswered puzzles out there.

VanderL
2004-Jan-16, 08:40 AM
OK, my vote goes to the capture theory, we might not yet understand the capture mechanism (imo due to the fact we haven&#39;t considered electrical explanations yet), but it would explain the apparent diffences between the composition of the Moon&#39;s and the Earth&#39;s surface, and it would probably account for the angle of the Moon&#39;s orbit compared to the solar system&#39;s ecliptic.

Guest_jtommasi
2004-Jan-16, 06:26 PM
Tinaa
I looked at the site you pointed out. It&#39;s very general and doesn&#39;t quite answers the question as to why was there a big bombardment after the planets were formed. I can find no rational for it except for explaining the moon&#39;s cratering.

"But our moon doesn&#39;t. It orbits at an angle to the equator that varies from 18.28 to 28.58 degrees, but which is a constant 5.14 degrees from the ecliptic (the sun&#39;s equator). Strange, eh? And hard to explain with a collision. Maybe it evolved that way over time and perhaps someone else can explain how. "


TheThorn
I really believe the earth was totally molten at one time, it&#39;s the reason for the melting that bothers me.
And I really have a problem with a fully formed planet in a orbit for a number of years and never colliding with the earth until the most appropriate time to form a moon. I also have difficulties visualizing the orbit this planet would have, never mind the tremendous tidal force between the two planets all those times when they were at their closest.
As for the moon&#39;s orbital inclination, I wasn&#39;t aware of it. I wonder if the greater solar attraction toward the forming moon had any effect on that inclination. All the websites I look at (except the exotic ones), tell the same story
Maybe I should look into some other reason for the formation of the moon.

alpha
2004-Jan-19, 08:32 PM
As far as I am aware the moon is rather unique in terms of its size in relationship to the earth.

Could the moon have been captured by the earthe gravitational field and the tidal effects stopped its effective rotation,if so maybee the cratering on the visible side of the moon is due to plasma/electrical interchanges at that time??

VanderL
2004-Jan-19, 09:02 PM
Not necessarily, but it&#39;s not impossible. Electrical forces can make a capture possible that is impossible when only gravity is at work. So the discharges would actually help the capture. I don&#39;t know if it happened this way, maybe I&#39;ll ask the experts.
Cheers.

Tinaa
2004-Jan-19, 11:18 PM
Earth doesn&#39;t have the mass to capture an object as big as the moon, unless it was formed very near by.

Tiny
2004-Jan-19, 11:58 PM
In that case, the moon should be a Planet not a moon like Mars if the moon was form like 20 million mile from Earth...
I wonder with 6 x 10*24 Kg as Mass of Earth and an object with the range about 10 million mile, what will happen to the Object Capture by Earth&#39;s Mass or?

Tinaa
2004-Jan-20, 12:37 AM
The distance to the moon is only about 384,000 km. Mars at its closest was over 34,000,000 km away from Earth. Now, what question are you asking?

TheThorn
2004-Jan-20, 03:07 AM
Originally posted by Guest_jtommasi@Jan 16 2004, 06:26 PM
TheThorn

I really believe the earth was totally molten at one time, it&#39;s the reason for the melting that bothers me.

And I really have a problem with a fully formed planet in a orbit for a number of years and never colliding with the earth until the most appropriate time to form a moon. I also have difficulties visualizing the orbit this planet would have, never mind the tremendous tidal force between the two planets all those times when they were at their closest.
As for the moon&#39;s orbital inclination, I wasn&#39;t aware of it. I wonder if the greater solar attraction toward the forming moon had any effect on that inclination. All the websites I look at (except the exotic ones), tell the same story
Maybe I should look into some other reason for the formation of the moon.
I really have only a gut feel about the molten earth issue.

Seems to me that if the earth ever was totally molten, the surface would solidify very quickly into at least a thin skin of rock unless there was some sort of heat input from outside (like a very hot sun, or the planet being a lot closer to the sun than it currently is).

Consider how quickly lava freezes now when it gets exposed to the surface. It wouldn&#39;t be much different on an early Earth than it is right now - space would still be black at night, and the surface of the molten rock would still be radiating heat into it at a high rate. It wouldn&#39;t take long for that surface to freeze. It would take a lot longer for that solid surface layer to thicken, because heat would have to conduct through the solid layer to get to the surface and radiate away. That&#39;s just my speculation, and I&#39;m sure others know better than I.

As for a planet sized object hanging around for hundreds of millions of years before impacting the Earth, why not? Asteroid sized ones are still capable of getting here after billions of years. I think you may be considering crossing orbits and thinking that it would take a miracle for that situation to last hundreds of millions of years, and in that you are right. Once orbits have the potential to collide, they probably do collide rather in rather short order. But orbits can be chaotic, and what look like stable non crossing orbits can evolve into ones that do cross over time. That&#39;s why asteroids can still get us.

Here is a great link Orrery (http://www.cuug.ab.ca/~kmcclary/ORRERY/index.html) that really helped me to understand how chaotic orbits can be. Be ready to waste some time there because it is REAL neat.

It models the motions of planets, moons, asteroids, etc. It has a bunch of examples that demonstrate neat things about our solar system and about orbital mechanics in general. Check out the scenarios under "What If The Planets Were Heavier". One shows our current solar system with planetary masses raised 6 times, and the asteroid belt gets thoroughly disrupted. The one with masses raised 45 times has Saturn getting ejected from the system in a matter of a few dozen orbits. I wouldn&#39;t have thought changing the masses of the planets and leaving the orbits the same could do that.

Tiny
2004-Jan-20, 03:13 AM
If the Moon is 10 million mile from Earth, then do we still call it the moon or a planet?

Guest_jtommasi
2004-Jan-20, 04:30 PM
Consider how quickly lava freezes now when it gets exposed to the surface. It wouldn&#39;t be much different on an early Earth than it is right now - space would still be black at night, and the surface of the molten rock would still be radiating heat into it at a high rate. It wouldn&#39;t take long for that surface to freeze

I agree it wouldn&#39;t take too long for the surface of the earth to freeze although it would take a lot longer than it takes lava to freeze. You must remember that a liquid earth would not have an atmosphere to absorb heat heat so the only loss is infrared radiation which is not the optimal way to transfer heat. Considering that the earth is still molten in its core, it stands to reason that whatever melted the earth must have put in a great deal of energy into the earth (again, I&#39;m minimizing radioactive decay as a factor in melting the earth).

I doubt that the earth was any closer to the sun but I have no indication of how hot the sun was back then.


Asteroid sized ones are still capable of getting here after billions of years. I think you may be considering crossing orbits and thinking that it would take a miracle for that situation to last hundreds of millions of years, and in that you are right



It is true that there are asteroids that are coming down on the planets on a regular basis, the question is, how long have these asteroids been around? Are they relative newcomers to the solar system? Apparently there&#39;s a supply of asteroid sized bodies orbiting the outer reaches of the solar system and periodically they are attracted to the inner solar system where they eventually collide with planets. I have no information of how long these comets, or apollo object or asteroids have been in their respective orbits although I doubt that comets have been going around the sun too many times.

xstream
2004-Sep-04, 07:35 AM
:huh: For one We have mapped the Universe with Radiation. For another Mars like size? come on. Look at the curvature of the moon and look at the yucatan geez people do the math. the new neutron map of Mars shows its was once just like us. A possiblity of Mars being the victim and our moon being the culprit is what makes since and the early stages.

Callisto
2004-Sep-04, 01:57 PM
Well I think since the moon is orbiting around the Earth it should be considered a moon I guess. How was the moon formed? I think a pretty big object hit the Earth all that land clumped up into the moon. That land that is the moon used to be where the Pacific Ocean is now. <_<

Duane
2004-Sep-04, 07:07 PM
No Callisto, the Pacific Ocean basin has nothing to do with the moon.

The look of the Pacific basin is an accidental alignment of the moving continents, nothing more. Before we came to understand plate tectonics many scientists argued that the Pacific basin would seem to be the right size to be the area from which the moon split off from the Earth. Once tectonic movement was understood, it became clear that the area looks right only by the accidental placement of the continents today.

In a few million years, the Pacific basin will look very different than it does today.

Out of the many theories I have seen for the moon&#39;s formation, the proto-earth being struck at a glancing angle by a Mars-sized body answers most of the puzzles left over from studies of moon rocks brought back by Apollo astronauts. All of the other theories leave more questions than answers--which is not to say there aren&#39;t still alot of questions.

Callisto
2004-Sep-05, 05:28 AM
I didn&#39;t know that Duane, thanks for sharing that with me, I never knew that. So where did the moon come from.

eburacum45
2004-Sep-05, 12:22 PM
One very persuasive argument about the collision of the primeval Earth with the Primeval Mars-sized Impactor is that the Impactor was formed from trojan asteroids in the Earth&#39;s L4 or L5 Lagrange point;

a planet in this location would remain stable for millions, perhaps hundreds of millions of years; but eventually it could migrate out of the Lagrange point, and orbiting in nearly the same orbit as Earth it would creep round the Earth&#39;s orbit and impact at a relatively low relative speed after a decade or so.

The resultant splash would melt the Earth and also throw part of the crust high emough for it to collapse in orbit under the influence of its own gravity; no other factors are required in this explanation as far as I know, but that does not mean it has been proven correct.

Duane
2004-Sep-05, 06:03 PM
Excellent answer eburacum, couldn&#39;t have said it better myself :)

antoniseb
2004-Sep-07, 07:55 PM
Originally posted by eburacum45@Sep 5 2004, 12:22 PM
the Impactor was formed from trojan asteroids in the Earth&#39;s L4 or L5 Lagrange point
I&#39;m pretty familiar with the Mars-sized impactor theory, but I hadn&#39;t heard the L4 or L5 part of the theory before, and doubt that part very much for a few reasons:

1. L4 and L5 are the two stable Lagrange points for any sized body.
2. A Mars sized impactor would not have been formed simply from an assemblage of asteroids. It would need to have formed the same way the other planets formed.
3. If a large number of trojan asteroids could have collected at L5, we&#39;d see the remains of a second batch of them today. So far, we only see Trojan asteroids near Jupiter, and they are too wide spred to coalesce into a larger body.

Duane
2004-Sep-07, 09:36 PM
I don&#39;t know about that antoniseb, I have also seen something about the lagrange point theory. Essentially, there were 2 bodies forming in the region of the proto-earth, one which became this planet and the second being a mars-sized body acreating at one of the lagrange points. Over time, the larger body affected the smaller one, which moved out of the larger&#39;s L-point.

Remember that the L-point is not stable over long periods--thats why they have to keep adjusting the orbit of satilites that are there. Further, Earth&#39;s L-points are much closer to the planet and much smaller than Jupitor&#39;s.

I will do some checking and see if I can find where I read it.

Duane
2004-Sep-07, 09:43 PM
Ok, the theory was outlined by astrophysicist J. Richard Gott and mathematician Edward Belbruno, both of Princeton University. They did some modelling and discovered that a smaller body in a similar orbit could form at one of the l-points. Eventually it escaped, and thereby became a low-speed impactor with the larger protoearth.

This is the space.com article: Earth Hit by Neighbor in Making of Moon (http://www.space.com/scienceastronomy/moon_formation_040621.html)

antoniseb
2004-Sep-08, 12:57 PM
Originally posted by Duane@Sep 7 2004, 09:36 PM
Remember that the L-point is not stable over long periods--thats why they have to keep adjusting the orbit of satilites that are there.
There are two sets of Lagrange points that we could be discussing. Earth-Moon, and Earth Sun. The two we use for satelites are Earth-Sun L2 and L1. These are both unstable. The L4 and L5 positions are very stable in both cases, though the Earth-Moon Lagrangian points did not exist prior to the formation of the moon.

If there was an L4 or L5 Mars-sized co-Earth formed, it would be something that shared Earth&#39;s orbit but was two months [ignore moon reference] ahead or behind us. I need a lot of convincing to believe that such a lucky arrangement could have happened.

Guest_abyssalroamer
2004-Sep-08, 01:43 PM
The full paper by Belbruno and Gott has been available at arXiv:astro-ph/0405372v1. It&#39;s a 74 page report that details their work. The title is "Where did the moon come from?" and was published May18, 2004.

eburacum45
2004-Sep-09, 10:50 AM
Two planets in each other&#39;s lagrange points is a common science fiction fantasy in worldbuilding;

I think such an arrangement might be possible, but only in the short term-
the Lagrange points are much more stable when one object is several times as big as the other.
Trojan asteroids, yes; trojan planets, no.

So you would not get two planets of approximate equal size in the same orbit; eventually the smaller one would creep round and impact the larger.

Greg
2004-Sep-12, 07:05 AM
I would say that if something is possible then it likely has occured somewhere in the universe. I think the theory of the impacting object being at the Lagrange point in order to accumulate sufficient mass to achieve the size of this proported object is quite plausible. The notion also clears up the big controversy of how such a large object could have hit Earth slowly enough and at a low enough angle to form the moon. Nearly every simulation of a chance collision of a wayward planetoid (with a much faster impact speed at a sharper angle) wind up not forming a moon. It also offers a possible solution to what in hades happened to Uranus to knock it on its side. I don&#39;t think it helps much to explain how Venus came to rotate so slowly and in the wrong direction, though.

anneliese
2004-Sep-15, 03:22 AM
Originally posted by Greg@Sep 12 2004, 07:05 AM
It also offers a possible solution to what in hades happened to Uranus to knock it on its side. I don&#39;t think it helps much to explain how Venus came to rotate so slowly and in the wrong direction, though.
i disagree with Greg here... my understanding is that it is collisional theory that explains not just Uranus&#39; rotation, but also that of Venus, and in fact every body in the solar system&#33; if a mars-sized object can impact Earth, then why not Venus? and if an impactor of the right size collides with venus at the right angle, then it is entirely plausible that the planets rotation could end up as it is now... ;)

tony873004
2004-Sep-15, 08:44 AM
Originally posted by eburacum45@Sep 9 2004, 10:50 AM
...the Lagrange points are much more stable when one object is several times as big as the other....

So you would not get two planets of approximate equal size in the same orbit; eventually the smaller one would creep round and impact the larger.
The Lagrange 4 & 5 points are stable if the planet is less than about ~4% than the mass of the Parent object. The object in the Lagrange point can be any size, including equal to or larger than the planet, or can be as small you want too, as long as the combined mass of the planet and Lagrange objects < 4% the mass of the primary. So a planet can have a planet in its L4 or L5 point.

Duane
2004-Sep-15, 02:06 PM
Interesting discussion :).

As to Venus, and Uranus for that matter, something obviously happened to them both. With Venus, perhaps it was a sharp angled impact from the opposite side, such that the spin of the planet was reversed. There is no moon because the two objects merged--unlike the Earth, where the impact threw debris into orbit.

There is evidence of a major catastrophic resurfacing of Venus about 500,000,000 years ago. I wonder if that could have arisen in such a collision?