[extropy-chat] Water Marks the Asthenosphere
hkhenson at rogers.com
Sat Jan 27 17:37:56 UTC 2007
At 05:44 PM 1/27/2007 +0100, Amara wrote:
>The Giant Impact model explains some important observational points. It
>explains why the Moon has a lower density -- because the outer part of
>the Earth would have lower density material since it already
>differentiated, with iron already being formed in its (Earth's) core.
>It explains why so much of the angular momentum of the Earth-Moon system
>is in the Moon's rotation.
>It also explains why the Moon is is 'bone dry' -- because much of the
>material that shot into orbit was vaporized, with only the least
>volatile material remaining and condensing into solids.
>There is an observational 'sticking point' for the Giant Impact
>hypothesis. The oxygen isotopes from the Earth and the Moon are
>_identical_. About 80% of the Moon-forming material is estimated from
>theoretical work to originate from the Mars-sized impactor. Because of
>the stochastic collisions between embryos born in different parts of the
>inner solar system, one needs a consistent explanation for the oxygen
>isotope similarities between the Moon and Earth. The answer might be
>Pahlevan K., and Stevenson D. J. (2005) "The Oxygen Isotope Similarity
>of the Earth and Moon: Source Region or Formation Process?", Lunar
>Planet. Sci., XXXVI, #2382
>.. which says that an oxygen isotopic exchange occurred between the
>Earth's mantle and the circumterrestrial vapor-melt silicate disk that
>produced the Moon. In other words, that material that formed the Earth's
>mantle and the Moon was very hot and mixed.
If the two bodies had the same oxygen ratio before they hit, they must have
formed at the same distance from the sun.
"One hypothesis posits that Theia formed at a Lagrangian point relative to
Earth, that is, in about the same orbit and about 60° ahead or behind.
When the protoplanet Theia had grown to about the size of Mars, its size
made it too heavy for its status (comparable to the Trojan asteroids in the
orbit of Jupiter) to be stable. As a result, its angular distance from
Earth varied more and more until it finally hit the molten world."
Man, that would have been a sight to behold!
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