Theories of Formation
for the Moon
An extremely important question is that of how the Moon was formed and came to
have its present
orbit around the Earth.
Five Serious Theories
Five serious theories have been proposed for
the formation of the Moon (not counting the one involving green cheese):
The Fission Theory: The Moon was once part of the Earth and somehow
separated from the Earth early in the history of the Solar System. The present
Pacific Ocean basin is the most popular site for the part of the Earth from
which the Moon came.
The Capture Theory: The Moon was formed somewhere else, and was
later captured by the gravitational field of the Earth.
The Condensation Theory: The Moon and the Earth condensed together
from the original nebula that formed the Solar System.
The Colliding Planetesimals Theory:
The interaction of earth-orbiting and Sun-orbiting planetesimals (very large
chunks of rocks like asteroids) early in the history of the Solar System led
to their breakup. The Moon condensed from this debris.
The Ejected Ring Theory:
A planetesimal the size of Mars struck the earth, ejecting large volumes of
matter. A disk of orbiting material was formed, and this matter eventually
condensed to form the Moon in orbit around the Earth.
Constraints from Recent Data
A detailed comparison of the properties of Lunar and Earth rock samples has
placed very strong constraints on the possible validity of these hypotheses.
For example, if the Moon came from material that once made up the Earth, then
Lunar and Terrestrial rocks should be much more similar in composition than if
the Moon was formed somewhere else and only later was captured by the Earth.
These analyses indicate that the abundances of elements in Lunar and
Terrestrial material are sufficiently different to make it unlikely that the
Moon formed directly from the Earth. Generally, work over the last 10 years
has essentially ruled out the first two explanations and made the third one
rather unlikely. At present the fifth hypothesis, that the Moon was formed
from a ring of matter ejected by collision of a large object with the Earth, is
the favored hypothesis; however, the question is not completely settled and many
details remain to the accounted for.