Formation of the moon theory - Fromation of the moon theory

Ian · 2003-06-22 07:02:07

Hi. I would like somebody to clear this up for me. I heard of a theory that the moon was formed when an asteriod impacted the Earth. I just want to say that if that happened, there should still be a crater somewhere on this planet that has the diameter of the moon. I would just like to know where that crater could be? Does anyone have any theorys?

Ian · 2003-06-22 15:25:14

"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."

What if the moon impacted the earth and while this was happening had some debris come off of it and what if it is still in orbit around the earth? What if also some of the debris could have fallen into earth's atmosphere as meteorites? Could that have happened?

Shaun Barrett · 2003-06-22 19:46:22

Assuming this isn't a private discussion (? ):-

Ian, what do you mean when you ask: "What if the moon impacted the Earth .. ?"
The current theory is that the Moon resulted from an impact between Earth and a proto-planet at least the size of present day Mars. (I read somewhere that the impactor may have to have been even bigger than that in order to result in today's Earth-Moon system.)

I don't understand what it is you're asking. Can you be more specific?

Ian · 2003-06-22 21:04:06

What I'm asking is whether when the Mars sized asteroid impacted the earth debris could have been left over and orbited the earth. If that happened, then there could still be some fragments of that Mars sized asteroid still orbiting the earth or perhaps some fragments of that Mars sized asteroid may have fallen into the earth's atmosphere and became meteroites. Could that have happened?

Shaun Barrett · 2003-06-23 06:30:07

As I understand it, a protoplanet somewhere between 1/10th and 1/5th the mass of proto-Earth struck our planet a little off-centre and at a considerable closing speed.
The kinetic energy released on impact was enough to liquify the impactor and proto-Earth's crust. The impactor's iron core sank into proto-Earth's interior and combined with its core while some of the impact energy caused a plume of liquified and vapourised mantle/crustal material to spew out into space.
Whatever proto-Earth's rate of rotation was before the event, we know that its rotation period (or day) after the impact was approximately 5 hours. i.e. 2.5 hours daylight and 2.5 hours darkness.
This was soon to be modified because the ring of material orbiting quite close to what we can now call Earth proper coalesced into our satellite, the Moon. The huge tidal forces resulting from the Moon's proximity to Earth acted quickly (in geological terms) to simultaneously slow Earth's rotation and shift the Moon further and further away.
Although a certain amount of the mixture of impactor and proto-Earth mantle/crust, which had been 'splashed' into space, would most probably have fallen back onto Earth's new surface, it would have been immediately assimilated into the roiling magma ocean.
Any material orbiting within Earth's Roche Limit (too close to accrete into a small moon), would have formed rings. Such rings, in common with those of Saturn today, are transient things lasting 'only' some tens of millions of years at best. And we have to remember the cataclysmic impact event occurred about 4.4 billion years ago.
In that time, our Moon has been moving steadily further away and would have 'mopped up' thousands of tiny moonlets, most quite early in the piece - contributing to its crater saturated appearance.

I very much doubt there's still some of the original impact ejecta independently orbiting Earth somewhere. There's been ample time for the Moon to 'acquire' it or, if it were in the form of small particles, for the solar wind to dispatch it! Besides, anything substantial would have been picked up by astronomers or on radar by now.

Ian, I've rambled on a bit here .. sorry! Does it answer your question or have I missed the point?

Ian · 2003-06-24 10:27:38

Maybe there are very very small pieces of that debris orbiting the earth that are too small to detect but could only be detected by very sensitive equipment If there aren't, then I'm wrong and the current theory of how the moon formed might be correct. Then again it might not be correct. I might be wrong about that.

dickbill · 2003-06-24 15:35:07

Whatever proto-Earth's rate of rotation was before the event, we know that its rotation period (or day) after the impact was approximately 5 hours. i.e. 2.5 hours daylight and 2.5 hours darkness.

that is very fast, should we not expect some equatorial bulge with such a fast speed, especially with an earth crust still melted ? If so, why this bulge has disapeared ?

same question for the moon or moonlets remants, before the moon was tidally locked on a synchronous rotation with earth, about 28 days now, do we know the early moon period of rotation ?

Shaun Barrett · 2003-06-24 18:48:13

Even today, Earth bulges a little at the equator due to its rotation; its equatorial diameter is a little larger than its polar diameter.
When the rotation period was 5 hours, I'm sure the equatorial bulge would have been very much more pronounced, as you suggest, Dickbill.
But Earth has always been flexible due to its molten interior. Even as we speak, after a 4.5 billion year opportunity for cooling and crust thickening, Earth is susceptible to flexing. The surface of the planet itself, as well as its veneer of water, rises and falls every day in response to the Moon's gravitational pull.
There was never any possibility of Earth retaining the large bulge which would have existed in its earliest times. Once the rate of rotation had decreased, Earth's structure was incapable of sustaining such a bulge against the pull of its own gravity.

In the same vein, it's interesting to consider the height of Earth's tallest mountain in relation to the highest Martian peak. Mauna Kea is about 10 kms high, while Olympus Mons reaches 27 kms above datum. The ratio of the heights is almost exactly the same as the inverse ratio of the gravitational accelerations of the two planets. This suggests that perhaps each mountain is at or near the maximum height its planet's gravity will allow before the crust sags under the weight.
One more indication that planets are more like balls of viscous liquid than solid bodies.

[ P.S. I can't remember reading anything about the Moon's original rate of rotation before it began to slow. It may be that it's impossible to know simply because its rotation is now 'captured' and we can never find out when it finally stopped spinning, from an Earthly observer's point of view. If it were still not completely 'spin-captured' today, or if we knew exactly when it became fully 'captured', I'm sure we would be able to calculate its initial rate of spin. ]

RobertDyck · 2003-06-24 20:19:52

I had heard that Earth had a rotational period of 8 hours after the impact. Whether it was 5 or 8 hours makes little difference, it was slowed by tides from the Moon. Today Earth's diameter is 12,756.3 km at the equator, but the crust is on average 15 miles (24 km) thick. Where the ocean is 5 miles deep the crust beneath is only 10 miles, for a total of 15 miles. Mountains can cause a local thickness up to 40 km, but I am told the average is 15 miles for continents. That means the solid crust is only 0.376% of the radius of the Earth. The Earth today is a ball of liquid magma with a solid crust as thin as an egg shell. Furthermore, rock will become plastic and squish like plastecine if you apply enough pressure. I am told that Mount Everest is approximately the maximum height on Earth before its own weight squishes the mountain down. The summit is 29,028 feet (8,848 metres) above sea level. The crust must be almost that distance deeper than usual to cause it to float on the magma. Yup, rock floats on magma.

Ian · 2003-06-25 09:19:09

"spreaded around the solar system..." So there might still be pieces of the early moon floating around the inner solar system. The question is. Where are they now?

RobertDyck · 2003-06-25 12:13:42

Oops, Mount Everest was re-measured using GPS in the 1990s and was determined to be 29,035 feet (8,850 metres) high. This is due to continuing uplift of the mountain. Mauna Kea is 13,796 feet (4,205 metres) above sea level, but 32,000 feet (9,800 metres) above its base on the ocean floor. Don't you love on-line encyclopaedias.

If you tried to find debris spread around the solar system, how would you identify it? Meteorites on Earth are believed to be from Mars if they have trapped bubbles with the exact same gas mixture as the atmosphere on Mars. What was the gas mixture of the planet that impacted Earth billions of years ago? Did it have an atmosphere at all? How much debris would be from its surface vs. from its interior? The only way to identify a rock as coming from the collision would be to match it with lunar rocks. But lunar rocks have undergone separation due to the Moon melting during formation. It had to have been liquid at one point to become tide locked with the Earth. There are "seas" on the Moon today that appear to be ancient volcanic outflows. How would you identify a rock that came from the same SOURCE material that formed the Moon?

RobS · 2003-06-25 17:46:44

Regarding the question when the moon's rotation was tidally locked to match its revolution around the Earth, the answer is: very early. The moon's crust today is not uniformly thick; it is thinnest on the Earth facing side and much thicker on the side facing away from the Earth. This is because the crust, being the rock of the lowest density, "floated" away from the pull of the Earth when it was forming, back when the moon was all molten after the impact. The moon's rotation must have been locked before the crust completely formed, which would have taken tens of millions of years, or maybe 100 million years. The oldest crust on the moon goes quite far back, close to the moon's origin.

Ian · 2003-06-26 19:31:30

Could some of the debris that was swept up by the moon during it's formation have led to the debris impacting the moon causing the craters to form?

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#1 2003-06-22 07:02:07

Re: Formation of the moon theory - Fromation of the moon theory

#2 2003-06-22 15:25:14

Re: Formation of the moon theory - Fromation of the moon theory

#3 2003-06-22 19:46:22

Re: Formation of the moon theory - Fromation of the moon theory

#4 2003-06-22 21:04:06

Re: Formation of the moon theory - Fromation of the moon theory

#5 2003-06-23 06:30:07

Re: Formation of the moon theory - Fromation of the moon theory

#6 2003-06-24 10:27:38

Re: Formation of the moon theory - Fromation of the moon theory

#7 2003-06-24 15:35:07

Re: Formation of the moon theory - Fromation of the moon theory

#8 2003-06-24 18:48:13

Re: Formation of the moon theory - Fromation of the moon theory

#9 2003-06-24 20:19:52

Re: Formation of the moon theory - Fromation of the moon theory

#10 2003-06-25 09:19:09

Re: Formation of the moon theory - Fromation of the moon theory

#11 2003-06-25 12:13:42

Re: Formation of the moon theory - Fromation of the moon theory

#12 2003-06-25 17:46:44

Re: Formation of the moon theory - Fromation of the moon theory

#13 2003-06-26 19:31:30

Re: Formation of the moon theory - Fromation of the moon theory

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