Origin Of The Moon

The moon is the only natural satellite of earth and its
nearest neighbor in space. Its distance from the earth is
about 384,400km. It is the brightest object in the night
sky, bit it gives off no light of its own. The light that
is visible is what the moon is reflecting from the sun. It
has a diameter of 3476km and a mass of 7.35*1022kg.
Throughout history, it has had many names; Luna, as named
by the Romans, and Selene or Artemis by the Greeks. As a
result of 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: (1)
fission from the earth, (2) formation in earth orbit, and,
(3) 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.
 
(1) 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.
 
(2) 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. 

(3) 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. 

The Planetesimal Impact Theory, the modern version of the
fission theory, was 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. 

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 indicated 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 suggestions. 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. 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, they 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 fractions. 

Simulations for a variety of different initial conditions
were conducted and it was concluded 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 form). 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). Additional 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 
 
Bibliography
 
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 Fragmentation." Hartmann, W. K.
1977. "Large Planetesimals in the Early Solar System." 1
"Landmarks of the Moon," Microsoft® Encarta® 96
Encyclopedia. © 1993-1995 Microsoft Corporation. All rights
reserved. 2 "Characteristics of the Moon," Microsoft®
Encarta® 96 Encyclopedia. © 1993-1995 Microsoft
Corporation. All rights reserved.