: The Moon is the only natural satellite of Earth. The distancefrom 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
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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 within
earths 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.
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
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 the Earth, the dissipation of which could be expected to form the Moon.
The Giant Impact Theory of the origin of the Moon has emerged from these
These ideas attracted relatively little comment in the scientific community
during the next few years. However, in 1984, when a scientific conference on the
origin of the Moon was organized in Kona, Hawaii, a surprising number of papers
were submitted that discussed various aspects of the giant impact theory. At the
same meeting, the three classical theories of formation of the Moon were
discussed in depth, and it was clear that all continued to present grave
difficulties. The giant impact theory emerged as the “fashionable”
theory, but everyone agreed that it was relatively untested and that it would be
appropriate to reserve judgment on it until a lot of testing has been conducted.
The next step clearly called for numerical simulations on supercomputers.
“The author in collaboration with Willy Benz (Harvard), Wayne L.Slattery
at (Los Alamos National Laboratory, Los Alamos NM), and H. Jay Melosh
(University of Arizona, Tucson, AZ) undertook such simulations. They have used
an unconventional technique called smooth particle hydrodynamics to simulate the
planetary collision in three dimensions. With this technique, we have followed a
simulated collision (with some set of initial conditions) for many hours of real
time, determining the amount of mass that would escape from the Earth-Moon
system, the amount of mass that would be left in orbit, as well as the relative
amounts of rock and iron that would be in each of these different mass
We have carried out simulations for a variety of different initial conditions
and have shown that a “successful” simulation was possible if the
impacting body had a mass not very different from 1.2 Mars masses, that the
collision occurred with approximately the present angular momentum of the
Earth-Moon system, and that the impacting body was initially in an orbit not
very different from that of the Earth.
“The Moon is a compositionally unique body, having not more than 4% of
its mass in the form of an iron core (more likely only 2% of its mass in this
This contrasts with the Earth, a typical terrestrial planet in bulk
composition, which has about one-third of its mass in the form of the iron core.
Thus, a simulation could not be regarded as ‘successful’ unless the material
left in orbit was iron free or nearly so and was substantially in excess of the
mass of the Moon. This uniqueness highly constrains the conditions that must be
imposed on the planetary collision scenario. If the Moon had a composition
typical of other terrestrial planets, it would be far more difficult to
determine the conditions that led to its formation.
The early part of this work was done using Los Alamos Cray X-MP computers.
This work established that the giant impact theory was indeed promising and that
a collision of slightly more than a Mars mass with the Earth, with the
Earth-Moon angular momentum in the collision, would put almost 2 Moon masses of
rock into orbit, forming a disk of material that is a necessary precursor to the
formation of the Moon from much of this rock. Further development of the
hydrodynamics code made it possible to do the calculations on fast small
computers that are dedicated to them.
Subsequent calculations have been done at Harvard. The first set of
calculations was intended to determine whether the revised hydrodynamics code
reproduced previous results (and it did). Subsequent calculations have been
directed toward determining whether “successful” outcomes are possible
with a wider range of initial conditions than were first used. The results
indicate that the impactor must approach the Earth with a velocity (at large
distances) of not more than about 5 kilometers. This restricts the orbit of the
impactor to lie near that of the Earth. It has also been found that collisions
involving larger impactors with more than the Earth-Moon angular momentum can
give “successful” outcomes. This initial condition is reasonable
because it is known that the Earth-Moon system has lost angular momentum due to
solar tides, but the amount is uncertain. These calculations are still in
progress and will probably take 1 or 2 years more to complete
GIANT IMPACT THEORY OF THE ORIGIN OF THE MOON, A.G.W. Cameron,
Harvard-Smithsonian Center for Astrophysics, Cambridge MA 02138,
PLANETARY GEOSCIENCES-1988, NASA SP-498
EARTH’S ROTATION RATE MAY BE DUE TO EARLY COLLISIONS, Paula Cleggett-Haleim,
Michael Mewhinney, Ames Research Center, Mountain View, Calif. RELEASE: 93-012
Hartmann, W. K. 1969. “Terrestrial, Lunar, and Interplanetary Rock
Hartmann, W. K. 1977. “Large Planetesimals in the Early Solar
“Landmarks of the Moon,” Microsoft(r) Encarta(r) 96 Encyclopedia.
(c) 1993-1995 Microsoft Corporation. All rights reserved.
“Characteristics of the Moon,” Microsoft(r) Encarta(r) 96
Encyclopedia. (c) 1993-1995 Microsoft Corporation. All rights reserved.