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The Question
(Submitted June 26, 1997)
I am a first year science teacher and I having been looking for the latest
theories on the origin of the moon. Can you provide me with some new
information?
The Answer
We are X-ray and gamma-ray astronomers in our group...not really active in
the "solar system" fields of studies. Please keep that in mind.
What we know (from tests on lunar samples brought back by the Apollo
astronauts) is that the Moon is about 4.6 billion years old. It formed at
about the same time as the rest of the solar system did.
There are 3 possibilities for its creation:
1. it formed near the Earth as a separate body
2. it formed as part of the Earth and separated from it
3. it formed somewhere else and was captured by the Earth
Given that we know the rough ages of the Earth and Moon are the same, we
can conclude the following: if it formed as part of Earth and separated, it
must have done so right at the beginning of the solar system. Also, the Moon
has a different chemical composition that the Earth...which some scientists
believe points to it having formed as a separate body, either near or far.
Last I heard, the 3 theories all had their strengths and weaknesses...but
none were definitive.
The latest theory we have heard given serious consideration of is
that a Mars-sized asteroid knocked a lot of the surface off
the just-formed Earth. A long string of rocky fragments would be blown out
from the Earth like a tail. All of the iron falls back onto the Earth and
settles in the core. Part of the rocky tail accretes to make the Moon.
That may be why the Moon doesn't have an iron core and is somewhat short on
certain other elements when compared to Earth.
There is also a Scientific American article by G. Jeffrey Taylor
in the July 1994 issue (pp 40-47). The article clearly states that the giant
impact theory is the current favorite; if there is a bias,
I think it's only a matter of degree (it may be less overwhelming
a favorite than this article makes it out to be).
The reasons why giant impact theory has become such a favorite is
listed on p43 of this article --- for one, such a collision is a
natural consequence of planet formation. For another, "it simply
explains too many observations", including the similar oxygen
isotope ratios between the Moon and the Earth, and the angular momentum
of the Earth-Moon system. The capture theory has difficulty with the
former, while the fission theory cannot explain the angular momentum.
You might also try contacting the folks at the Lunar and Planetary Institute to
see if they have strong opinions (and why!) about any particular theory.
The LPI web site will be found by any Search Engine.
Regards,
Laura Whitlock and Koji Mukai
for the Ask a High-Energy Astronomer Team
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