

A new model – “The Big Splat” – explains the strange asymmetry of the moon - BobbyVsTheDevil
http://nautil.us/issue/13/symmetry/when-the-earth-had-two-moons

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Slackwise
While trying to find more information on this new model/theory, I found
another one:

[http://news.psu.edu/story/317841/2014/06/09/research/55-year...](http://news.psu.edu/story/317841/2014/06/09/research/55-year-
old-dark-side-moon-mystery-solved)

Which purports that the difference was created due to tidal locking and a
temperature gradient in the early formative years of the Earth and Moon.

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leeoniya
"Because the moon’s rotation is perfectly synchronized with its revolution..."

i'm not sure how i feel about this definition of "rotation". i would say that
"rotation" in the celestial body context would always refer to rotation about
an axis passing through the object. while "revolution" is rotation about an
axis external to the object.

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sixbrx
I think that's exactly what it means in this case also - the period of
rotation on its own axis is exactly matching the period of revolution of its
center of mass around the Earth. So that's why the same it is always
presenting the same face towards Earth. Did you understand this differently?

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bitslayer
That is the type of theory that when you hear it sounds so plausible that it
must be true.

~~~
phkahler
>> That is the type of theory that when you hear it sounds so plausible that
it must be true.

But when 2 bodies of that size come together, and one of them splats onto the
other, there is going to be a major change in rotation rate. There is no
reason given for the resulting rotation rate to match the orbital period. I
find that bothersome.

~~~
TheLoneWolfling
There is a potential reason:

There is "drag" resulting from tides whenever a body is rotating faster or
slower than the orbital period. (First approximation: actually there are weird
things like orbital resonances. Mercury for example.)

This drag becomes much more prevalent when the body is asymmetrical - say,
when a large body just hit it. (Again: first approximation.)

As such, one could conceivably say that the moon was rotating faster than it
is now, and the impact slowed it down enough that the drag in the meantime was
enough to tidally lock it to Earth sometime between the impact and now. (Or
slower, and sped up. Same thing.)

Although I don't know nearly enough to know how large the "window" would be of
rotation rates that would tidally lock in the amount of time between the
impact and now.

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kazinator
The thing that is a little hard to account for is the synchronized rotation:
how could the splat happen in exactly such a way that the rugged far side
remains out of Earth's view. Say that before the splat, the original Moon is
already synchronized so that it faces the Earth with one side at all times.

The popular hypothesis that the Moon fragmented away from the Earth does
contain an explanation for both phenomena: the synchronous rotation, as well
as the different characteristics of the far side.

Since the Moon is small and has no plate tectonic activity, it would tend to
"record" a permanent geological snapshot of the "gash" on the side where it
was separated from the larger object. The larger object with plate tectonic
activity would "heal" over many eons, since that activity basically re-
surfaces it.

The Wikipedia article
[[http://en.wikipedia.org/wiki/Giant_impact_hypothesis](http://en.wikipedia.org/wiki/Giant_impact_hypothesis)]
does not say, under Difficulties, anything about the difference in
characteristics between near and far side being specifically a problem. There
are difficulties stemming from the composition of the Moon as such.

