Expert: Tom Whiting - 3/7/2008

Question
According to the giant impact theory, the early Earth was hit by the planet
Thea (Theia).  The two planets broke apart and then fuzed together to form
our present Earth and the remains created our moon.  Is there any truth to
this theory?  If Thea (Theia) existed what can you tell me about the planet (I
know it has been said that it was Earths sister planet)?  Also, do you know
who came up with this theory or where to find out?

Answer
Hi Tim,
Before the moon landings, several theories accounted for our moon.
1. large asteroid captured by the Earth.
2. moon just formed in it's current position independent of the earth.
3. massive collision (Yeah, sure)! by another body early in the
Earth's history (~4.5 billion years ago) the material ejecta of which formed the moon.

Well, amazingly to all concerned, (especially me) after it was found that the compositions of the lunar surface were very close to the Earth's crust and mantle, #3 jumped quickly to the #1 hypothesis,
around 1972, because it best explains the similarity in compositions
of the Earth and moon.
See
http://en.wikipedia.org/wiki/Giant_impact_hypothesis

I think the actual theory is a grazing collision, not a break
apart and reform.  Since the colliding body in question was smaller
than the Earth (some estimates put it at Mars size, or about 1/4
the mass of the earth) and then the Earth just absorbed the
remaining material.  Remember, afterward, the Earth became completely
molten for a few million years, forming our nickel/iron core;
Differentiation occurred which destroys all the evidence.
(We have the same problem in determining the Late Devonian mass
extinction of 230 million years ago too, where 95% of lifeforms
bit the dust...crustal motion has probably destroyed the impact
crater, if there even was one...or was it a GRB? - Gamma Ray Burster, nearby?) No one knows, as yet.

Is there any truth to this theory?  Well, it provides for the best
fit solution to the observations, so until more information becomes
available, we have to assume that it's the #1 theory for now. Whether it's actually true or not, I don't think there is any way to tell, as obviously, there is no record, or pre-collision records, of
body in question.  So that answers the question about any pre-collison knowledge about Theia too. The body would have been either destroyed completely, or absorbed by the Earth, with no past records
available or existing today.
Without hard facts or actual observations, even science cannot know the unknown....as Mr. Spock said once on a Startrek episode.

As I said earlier, that's an old, old, back in antiquity theory, as are the other 2 moon hypotheses mentioned above...I don't know who first described it...but it didn't seem very plausible (considering the extreme rarity of major major collisions of big bodies) and wasn't taken very seriously... until after our 6 moon landings from 1969-1972.

So that's why it remains only a hypothesis, and probably always will
be, unless we can locate some very advanced "space aliens" someday who were around our immediate area some 4.5 billion years ago, and they took some fantastic video pictures of "our collision"...:-)
Hope this helps,
Clear Skies,
Tom Whiting
Erie PA