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The origin of the EarthMoon system is very much related to the origin
of the solar system as a whole. The ancient lunar surface has preserved a
record of events over the last four years. Astronomers obtain relative crater
ages from superimposition. For example, younger craters are found on top of
older craters. Ejecta rays from younger craters also fall over older craters.
Craters on lava flows maria similarly are younger than the lava. The purpose of
the Apollo lunar missions was to obtain rock samples from different regions so
that the relative age history of the lunar system could be translated into one
with absolute ages. The planet Mercury, which is also heavily cratered with an
apparently similar cratering history as the Moon, supplies additional evidence
to theorize the Moons history and origin. This, and other evidence, points to a
process by which smaller objects planetesimals,or little planets merged to form
the surviving planetary objects of todays solar system.
Earth and the Moon are so similar they can be thought of as forming abinary
planetary system.Study of their chemical makeup provides important information
on how these two objects became permanently associated with each other. The
Moon is relatively deficient in heavier elements mean density . gcmcompared to
. gcmfor Earth. More specific chemical analysis of Moon rocks shows that the
chemistry of the two objects is otherwise very similar, but not identical.
Traditionally, three theories explain the association of the two objects. The
theory ofcoeval formationargues that the Moon and Earth coalesced together out
of the same materials. The idea that their chemistry is not identical poses a
severe problem for this theory.Fissiontheorysuggests that a single, initially
rapidly rotating object broke apart. But this theory would require nearly
identical chemical composition for the surviving objects. Dynamical problems
also hinder this idea. Thecapture hypothesistheorizes that the Moon
formed elsewhere in the solar system and only later became bound to Earth. This
model allows for differences in the chemical composition of the two objects but
the problem is that their chemistry is too similar. Also, dynamical problems
exist involving a loss of orbital energy necessary to end up with the two
objects orbiting each other.
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