Joseph Lazio <jlazio@patriot.net>,

The Moon presents a curious problem.  Of the terrestrial planets
(Mercury, Venus, Earth, and Mars) only Earth and Mars have satellites.
Mars' satellites are much smaller than the Moon, both in absolute size
and in comparison to their primary.  (The Moon is 3476 km in diameter
while Phobos is 23 km in diameter; the Moon's diameter is 27% that of
the Earth while Phobos' diameter is 0.34% that of Mars.)

Furthermore, the Moon's chemical composition is peculiar.  In many
respects it is quite similar to the Earth's, except that the Moon
seems to have less iron (and similar elements like nickel) and
considerably less water (it's quite dry!).

Until recently there were three competing theories to explain the
Moon's origin.  (1) The Moon formed elsewhere in the solar system and
was captured eventually by the Earth.  (2) The Moon and Earth formed
together at the same time in essentially the same place.  (3) The
early Earth was spinning so fast that a portion of it broke off and
formed the Moon (possibly leaving the Pacific Ocean basin as a
result).  All theories had their difficulties, though.

If the Moon formed elsewhere in the solar system (like between the
orbits of Venus and Earth or between the orbits of Earth and Mars),
how did it get disturbed into the orbit that took it near the Earth?
Furthermore, it is actually quite difficult for an object that is not
initially orbiting the Earth to begin doing so.  The incoming object
must lose energy.  In the case of Mars, its small satellites could
have gotten close enough to skim the upper part of its atmosphere,
which would cause them to lose energy from air resistance.  Because
the Moon is so big, it probably would have hit the Earth rather than
passing just close enough to lose just enough energy to be captured
into orbit.

If the Earth and Moon formed simultaneously at nearly the same
location in the solar system, then the differing chemical compositions
of the two are quite difficult to understand.  Why are they similar
yet so different?

Finally, there isn't much evidence to suggest that the early Earth was
spinning anywhere near fast enough for it to break apart.

With the realization in the 1980s that impacts (of comets, asteroids,
etc.) have played a major role in the history of the solar system, a
new theory emerged:

The Moon was formed when a Mars-sized object collided with the Earth
when the Earth was very young, about 4.5 billion years ago.  Much of
the Earth's crust and mantle, along with most of the colliding object,
disintegrated and was blown into orbit thousands of kilometers high.
About half of this debris fell back to Earth.  The rest coalesced into
the Moon.  (Loose material in orbit can coalesce if it is outside the
"Roche limit," otherwise it will be pulled apart by tidal forces.  The
Roche limit for the Earth is approximately 3 Earth radii.  The
material outside this limit formed the Moon, the material inside the
limit fell back to Earth.)  Since the time of its original formation,
the Moon has slowly moved farther from the Earth to its present
position.

This theory does a good job of explaining why only the Earth has a
large moon and why the Moon's chemical composition is similar yet
different.  Impacts are random events, and there almost certainly were
not a lot of large objects left in the solar system as the planets
were nearly the end of their formation.  The Earth just happened to be
the planet struck by this large, rogue planetoid.  If we could start
over the formation of the solar system, it might be Venus or Mars that
would end up with a large moon.  The chemical composition of the Earth
and Moon are clearly predicted to be similar in this model, since a
portion of the Earth went into forming the Moon and a portion of the
impactor remained in the Earth.  The Moon would be deficient in iron
and similar metals if the impact occurred after those elements had
largely sunk to the center of the Earth (i.e., after the Earth
differentiated).  The Moon should also be quite dry because the
material from which the Moon formed was heated to a high temperature
in the impact, thereby evaporating all of the water.

Computer models of this event indicate that the Moon coalesced in only
about a year.  Also interesting is that a large percentage of
simulations result in the formation of two moons.  Some of the more
recent simulations suggest that the colliding object might have had to
have been much larger, about three times the size of Mars.

More information on this theory of Moon formation can be found at
<URL:http://www.earthsky.com/specials/moonformation.html>.