Moon creation theory question

[jmercer]

Ok - I understand that there are a number of theories about the formation of the moon, but the one currently in favor is that a collision between the Earth and a Mars-sized body may have been the cause.

The thing that bothers me about this is that theory also suggests (according to The History Channel's "Universe" series) that the Earth's rotation was imparted by that impact... yet there are other terrestrial-type planets - Mars, for example - that also have rotations. (Not to mention large moons with rotations as well!)

So... why would that particular bit of the theory hold water? Surely all the bodies with rotation didn't gain that from large-scale impacts?

[Mojo]

Ok - I understand that there are a number of theories about the formation of the moon, but the one currently in favor is that a collision between the Earth and a Mars-sized body may have been the cause.

The thing that bothers me about this is that theory also suggests (according to The History Channel's "Universe" series) that the Earth's rotation was imparted by that impact... yet there are other terrestrial-type planets - Mars, for example - that also have rotations. (Not to mention large moons with rotations as well!)

So... why would that particular bit of the theory hold water? Surely all the bodies with rotation didn't gain that from large-scale impacts?


The rotation is (on the whole) the result of the conservation of angular momentum as the gas/dust cloud that formed the Solar System collapsed under gravity to form smaller bodies.

[Dancing David]

I suppose that some portion of momentum is imparted. It would vary from lots and lots in the early formation to less and less. Some theories suggest Uranus was knocked head for heels at some point.

[Damien Evans]

I know that theory fairly well, but I've never heard of that claim before. As Mojo said, it's mostly angular momentum,

[shadron]

There are several hypotheses about the moon's creation, but the most widely accepted at this point in time is the Big Splash. It involves a growing (by accretion) earth and a similar but smaller body accreting at one of the stable Earth/Sun Lagrangian points, either 30 degree ahead of earth on its orbit or 30 degrees behind. Stability at those points depends on the mass of the body being negligible compared with Earth. As they both continued to increase in size, the Lagrangian point stability faded, resulting in he body moving in a motion back and forth along the orbit, getting farther and farther from the stable point and closer and closer to earth on each trip. Eventually it moved close enough to earth to be attracted away from the Lagrangian orbit, and it whacked earth with a skimming blow, but deep enough to leave its core behind. It had to be a skimming blow in order to get the rotational and orbital momentums right, and to assure the moon was mainly mantle material and not core, just as it is today. The actual dynamics of the encounter are being analyzed by a scientist in Boulder, Robin Canup of SRI, and she is getting closer and closer in her models to a good fit with all the variables.

The collision occurred 4.5 billion years ago. See the wiki article for illustrations of the orbital motions involved.

[shadron]

Ok - I understand that there are a number of theories about the formation of the moon, but the one currently in favor is that a collision between the Earth and a Mars-sized body may have been the cause.The thing that bothers me about this is that theory also suggests (according to The History Channel's "Universe" series) that the Earth's rotation was imparted by that impact... yet there are other terrestrial-type planets - Mars, for example - that also have rotations. (Not to mention large moons with rotations as well!)

So... why would that particular bit of the theory hold water? Surely all the bodies with rotation didn't gain that from large-scale impacts?

After the collision, earth's rotation was much increased from what it had been before, but that rotation was now coupled to the moon's orbital velocity (or, rather, it became coupled once the earth developed oceans with usefully braking tides). That coupling slowed the earth down and increased the moon's distance from earth, down to where the day is more or less equivalent to that of the other planets.

[jmercer]

Hmmm... lots of interesting answers. Thanks, folks. :) Looks like I'm not alone in wondering why "Universe" would make such a statement.

Interestingly enough, angular momentum was during formation was covered in one of the three theories put out about the formation. The braking aspect wasn't mentioned, but the tides were.

As usual, TV gets it *almost* right. :D

[Soapy Sam]

I recall an article from the 1970's in the Quarterly Journal of Geology by Runcorn and Moorbath on Devonian fossil corals. These had daily growth accretion rings related to the tides and so to the number of days in the year,- in turn related to both the Earth's orbital period and the diurnal rotation period- the year and number of days in the year.

They suggested a 400 day year , some 400+ million years ago.

The suggested reason was that the moon was closer and conservation of angular momentum (the ice skater effect) due to tidal drag, was pushing the Earth and Moon apart, giving us today's larger system with slower axial rotation, (a longer day) and a 365 day year.
But you can only push that back so far , before the Moon enters the Roche limit. Long before that, ocean tides would have made life problematic for littoral fauna. Yet precambrian algal banded ironstone formations and stromatolites suggest that tidal range was actually low .
Not sure if there's a solution to this apparent paradox?

[nathan]

stable Earth/Sun Lagrangian points, either 30 degree ahead of earth on its orbit or 30 degrees behind..

Correction: 60 degrees before or behind. Right?

[RecoveringYuppy]

Yes, 60 degrees.

[shadron]

Yup. My mistake.

[m_huber]

Minor aside: When I was in Africa, I got to stand on tidal deposits that were 3200 million years old. There were periodic thick bands in the deposits, interpreted to be from the Spring tide. By measuring the number of layers between these, it can be estimated that the Moon had a 22 day cycle in the Archean (now it's 28).

From what I have seen of BIF's in the Archean, they seem to have been formed below wave base. Possibly analogous to the modern deep ocean carbonates, they may have been siderite (FeCO3), and were likely a chemical precipitate. Not sure if that relates to the Roche limit or not.

[rjh01]

One effect of the moon being closer is that the tides would be much bigger. And tidal currents would be bigger too, both because of this and the shorter day.

[Macoy]

Here's another:

http://www.cosmosmagazine.com/features/print/2421/birth-moon?page=0%2C0

Where's Wollery when you need him?

[wollery]

I'm right here, and this isn't my field, but yes, the day was shorter, the year was longer, the Moon was closer, and the tides were higher.

Eventually the Earth will reach tidal lock, and will always present the same face to the Moon.

[nathan]

Eventually the Earth will reach tidal lock, and will always present the same face to the Moon.

Hot House, James Blish.

IIRC though, I calculated the time it would take for this to happen to be longer than the expected remaining life of the Sun (~5 billion years)

[Macoy]

Hot House, James Blish.

Shurely Brian Aldiss?

[wollery]

Hot House, James Blish.

IIRC though, I calculated the time it would take for this to happen to be longer than the expected remaining life of the Sun (~5 billion years)Possibly, but it's dependent on a lot of factors. E.g., if global warming melts the icecaps the tidal friction will be increased, which will slow the Earth's rotation down a bit faster.

[Miss_Kitt]

I love you guys!! Thanks for this thread. Wollery tell your hedgehogs I said, "Sniffle, sniff; puff-puff!!!"

I miss my hedgie, he was a great little ornery guy.

[nathan]

Shurely Brian Aldiss?

Er, yes. thanks for the correction. I should really have looked at my bookshelf :)