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Posted by N:dlzc D:aol T:com \(dlzc\) on December 2, 2007, 1:38 pm
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Dear Peter Munn:
...
> This leads me to propose scenarios like this: there
> were late Earth impactors that threw up a lot of
> mantle material, but the material coated (say 100km
> or more in depth) a satellite that was already there.
> I suspect such Earth mantle coating of any pre-
> existing satellite was a likely event; and that gaining
> a suitable pre-existing satellite (I guess by capture
> or partial capture) requires much more easily
> achievable constraints than the short story giant
> impact does.
This does not provide enough angular momentum to keep a
"significantly plated" Moon in orbit, certainly not a circular
one. The material would have to enter the Moon's orbit, then be
captured over some time.
> A summary of my favoured alternative hypothesis
> would be: the moon was captured; the giant impact
> thesis correctly fingers collision as the mechanism
> responsible for the moon's very small core, but
> collisions were common in the early solar system
> and the Earth may not have been party to this
> particular one; the giant impact thesis correctly
> identifies ejected Earth mantel as the source of
> known lunar materials, but this is a surface effect.
I like the spinning-but-mixed hypothesis. With gravitational
settling causing an increase in rotational speed, until
instability occurs. No significant collision required, and could
also occur during early solar system formation. With further
settling in the Earth occuring after "lobe off", would yield
sufficient additional boost to keep from capturing the Moon
again.
>>> So the report says that either:
>>> 1) giant impact does not occur to produce
>>> moons like ours, or
>>> 2) giant impact does not produce a great
>>> cloud of debris when making moons like ours, or
>>> 3) giant impact must produce a cloud of debris
>>> present for millions of years, and moons like
>>> ours, are very rare.
>>
>> I'd put it a little differently, given that your 2) is
>> almost inconceivable:
>>1) Moons like ours are rare, or
>>2) Moons like ours form by some process that
>>does not generate dust.
>
> There is also 3) Moons like ours can form early
> enough that, as the original article puts it, the
> "dust [that astronomers detect] could be left over
> from the planet-formation process", so the signal
> is unclear amongst the noise.
True.
Another option might be that dust distributions evoke "Dark
Matter"-type behavior, and tend to coallesce dust more rapidly
than for large bodies.
David A. Smith
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