[meteorite-list] Lunar capture theory

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 10:21:03 2004
Message-ID: <3F0F5FE0.8EB94538_at_bhil.com>

Hi,

    Howard's already pointed how hard (and unlikely) capture is. Before the
capture theory was popular, the theory was fission: that the Moon split off from
a rapidly spinning early Earth. While this may sound whacky today, the theory
had a lot going for it.
    First, the density of the Moon as a whole is exactly the same as the bulk
density of the Earth's crust to within less than 1% difference. Wow, how could
that be coincidence?
    Second, the tidal action of the Moon is enlarging it's orbit, slowly moving
the Moon further away and lengthen the month. If you run this progression
backwards, you end up with a Moon in contact with the Earth's crust and the
Earth's equator spinning at a speed just a hair faster than orbital velocity!
    And third, there is this <bleep>ing big hole in the side of the Earth called
the Pacific Ocean whose excavated volume is roughly Moon-sized (1-1/4% of
Earth)!
    Hey, sounds pretty good, don't it?
    This theory was still going strong up to about 1950. The author of this
theory was Charles Darwin. No, not THAT Charles Darwin. His nephew (or was it
grandnephew? can't remember).
    The density WAS a coincidence.
    The tidal equations run backward, once you have a computer to do the
crunching, reveal that there are unstable oscillations in the Moon's orbit that
limit how close the Moon could ever have been.
    Since continents move and oceans spread, the Pacific Ocean basin is not an
ancient feature of the Earth.
    Bang! One Dead theory.

    The problem with James'"slow capture" theory is that the more we learn about
planetary accretion, the more it looks like the final stages of big
planetesimals (1000 km objects) growing to bigger planets at last goes wizz-bang
FAST. And, the Earth and the Moon are SO different, that it's impossible to
imagine them both forming in the same region of the solar nebula. One of us
doesn't belong here...

    The impact origin of the Moon, if true, is an unbelievably messy
complication. We would like to be able to extrapolate from the Earth's
composition the composition of the planetesimals at this distance from the Sun,
but if the Earth is partly made up of some rogue Mars-sized impactor that came
from somewhere else in the system, well, you can toss all that research out of
the window! Because the Earth would no longer be representative of this
neighborhood in the nebula.
    Well, why can't we figure out from the Moon's composition where in the solar
system it came from and learn about the composition of the solar nebula there?
O, yeah, take a planet and smash it into dust and gas and let recondense. Where
have all the volatiles gone? Were there any volatiles to begin with? Are all
those refractories in the Moon the result of them surviving the impact, or was
the Moon rich in those refractories to begin with? (The Moon should be called
Titaniumville, and I have no doubt that someday some burg on the Moon will be
named just that, maybe in Chinese...)
    Maybe the Earth is so rich in water (instead of being "normal" like Venus)
because it captured all the volatiles from the ur-Moon. Maybe the Earth had
oceans 80 kilometers deep BEFORE it got whacked with a <bleep>ing planet and
this little bit of water is all that's left!
    And then there's the core. We have always assumed that the Earth's core is a
native feature of the planet, but if the Moon impactor theory is correct, some
percentage of the Earth's core is really the Moon's core, captured by the
heavier Earth in the impact. That means it can't be a "normal" core (whatever
the H*** that is) and we can't know how "normal" the other terrestial planets'
cores are. Maybe the Earth is the only planet with tectonics because it has this
extra big core? (Personally, I think it's the extra water, but...)
    You see, this impact thing just messes up everything!


Sterling K. Webb
------------------------------------------------------------------------------------

Ken O'Neill wrote:

> Hi List,
>
> Would there be anything in the geological comparison between Earth and Moon
> that would lean toward the "capture" rather than "same debris" theory or
> vice versa ?
>
> Regards
>
> Ken O'Neill
>
> -----Original Message-----
> From: meteorite-list-admin_at_meteoritecentral.com
> [mailto:meteorite-list-admin_at_meteoritecentral.com]On Behalf Of Tom aka
> James Knudson
> Sent: 11 July 2003 21:48
> To: Matson, Robert; meteorite-list
> Subject: Re: [meteorite-list] Lunar capture theory
>
> Hello Robert and list, My understanding of planet formation in a nutshell is
> that debris orbiting the Sun gradually merged and formed planets. Right?
> Why couldn't two bodies have formed from the same debris in the same
> orbit and orbiting around the sun together in the same direction and the
> same orbit, Gradually the Moon slowly caught up with the earth and got
> caught up in the earths gravity? Or the earth came up behind the Moon and
> captured it?
>
> Thanks, Tom
> Peregrineflier
> The proudest member of the IMCA 6168
> ----- Original Message -----
> From: Matson, Robert <ROBERT.D.MATSON_at_saic.com>
> To: meteorite-list <meteorite-list_at_meteoritecentral.com>
> Sent: Friday, July 11, 2003 1:01 PM
> Subject: [meteorite-list] Lunar capture theory
>
> > Tom wrote:
> >
> > > I do not believe the moon was made by a asteroid impact on the
> > > earth.
> >
> > What, specifically, about this theory bothers you?
> >
> > > I would first stand by the theory that it was caught up in our
> > > gravity.
> >
> > While this is a ~possible~ scenario, you have to understand how
> > extremely unlikely graceful capture is compared to impact.
> > The capture idea also has a difficult time explaining why the
> > Moon doesn't have a normal-sized core for a body of its size,
> > which the impact theory explains nicely. Finally, why the
> > oxygen-isotope similarity of earth and the Moon if the two
> > bodies formed in different parts of the solar system?
> >
> > Prior to the Apollo sample return missions (and the discovery
> > of our beloved lunar meteorites), the capture theory at least
> > had some wobbly legs to stand on. But O-isotope analysis of
> > the moon rocks knocked one leg out, and the other leg was swept
> > away by Lunar Prospector's confirmation that the moon's core
> > comprises less than 3% of the moon's mass.
> >
> > --Rob
> >
> >
> >
> > ______________________________________________
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> >
>
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Received on Fri 11 Jul 2003 09:09:53 PM PDT