Theories Of The Origin Of The Moon

The Moon is the only natural satellite of Earth. The distance from Earth
is about
384,400km with a diameter of 3476km and a mass of 7.35*1022kg. Through
history it has had many names: Called Luna by the Romans, Selene and
Artemis
by the Greeks. And of course, has been known through prehistoric times.
It is
the second brightest object in the sky after the Sun. Due to its size and
composition, the Moon is sometimes classified as a terrestrial "planet"
along with
Mercury, Venus, Earth and Mars.

Origin of the Moon
Before the modern age of space exploration, scientists had three major
theories for the origin of the moon: fission from the earth; formation in
earth
orbit; and formation far from earth. Then, in 1975, having studied moon
rocks
and close-up pictures of the moon, scientists proposed what has come to be
regarded as the most probable of the theories of formation, planetesimal
impact
or giant impact theory.

Formation by Fission from the Earth
The modern version of this theory proposes that the moon was spun off from
the earth when the earth was young and rotating rapidly on its axis. This
idea
gained support partly because the density of the moon is the same as that
of
the rocks just below the crust, or upper mantle, of the earth. A major
difficulty
with this theory is that the angular momentum of the earth, in order to
achieve
rotational instability, would have to have been much greater than the
angular
momentum of the present earth-moon system.

Formation in Orbit Near the Earth
This theory proposes that the earth and moon, and all other bodies of the
solar
system, condensed independently out of the huge cloud of cold gases and
solid
particles that constituted the primordial solar nebula. Much of this
material
finally collected at the center to form the sun.

Formation Far from Earth
According to this theory, independent formation of the earth and moon, as
in
the above theory, is assumed; but the moon is supposed to have formed at a
different place in the solar system, far from earth. The orbits of the
earth and
moon then, it is surmised, carried them near each other so that the moon
was
pulled into permanent orbit about the earth.

Planetesimal Impact
First published in 1975, this theory proposes that early in the earth's
history,
well over 4 billion years ago, the earth was struck by a large body called
a
planetesimal, about the size of Mars. The catastrophic impact blasted
portions
of the earth and the planetesimal into earth orbit, where debris from the
impact
eventually coalesced to form the moon. This theory, after years of research
on
moon rocks in the 1970s and 1980s, has become the most widely accepted
one for the moon's origin. The major problem with the theory is that it
would
seem to require that the earth melted throughout, following the impact,
whereas
the earth's geochemistry does not indicate such a radical melting.

Planetesimal Impact Theory (Giant Impact Theory)
As the Apollo project progressed, it became noteworthy that few scientists
working on the project were changing their minds about which of these three

theories they believed was most likely correct, and each of the theories
had its
vocal advocates. In the years immediately following the Apollo project,
this
division of opinion continued to exist. One observer of the scene, a
psychologist,
concluded that the scientists studying the Moon were extremely dogmatic and

largely immune to persuasion by scientific evidence. But the facts were
that the
scientific evidence did not single out any one of these theories. Each one
of them
had several grave difficulties as well as one or more points in its favor.

In the mid-1970s, other ideas began to emerge. William K. Hartmann and D.R.

Davis (Planetary Sciences Institute in Tucson AZ) pointed out that the
Earth, in
the course of its accumulation, would undergo some major collisions with
other
bodies that have a substantial fraction of its mass and that these
collision would
produce large vapor clouds that they believe might play a role in the
formation of
the Moon. A.G.W. Cameron and William R. Ward (Harvard University,
Cambridge MA) pointed out that a collision with a body having at least the
mass
of Mars would be needed to give the Earth the present angular momentum of
the
Earth-Moon system, and they also pointed out that such a collision would
produce a large vapor cloud that would leave a substantial amount of
material in
orbit about