ORIGIN AND REVOLUTION OF THE MOON
“Taking all the
evidence together, what conclusions can be made about the history of moon?
At some time more than 4.6 billion years ago, the moon began to be built
up from a swarm of smaller bodies. Either
these bodies had already been degassed in some great heating episode, or the
early moon contained volatiles that subsequently vanished from its crust.
Neither of these theories seems attractive.
If the volatiles were absent at the start, an implication would be that
the moon formed much closer to the sun than it is now, and somehow was
transferred out to a cooler part of the solar system where it became captured by
the earth. No plausible model for
such a chain of events has yet been put forward.
A related theory has the moon building up within a giant incandescent
silicate atmosphere of the proto-earth. This
model, too, encounters difficulties in meeting all the present data constraints.
If the early volatiles were not absent, then they were
present but have since been very thoroughly removed from at least the outer
equatorial parts of the moon. This
raises the prospect of a baked-Alaska moon (cool inside, hot outside) at the
outset, a possibility that would have required a very special series of events.
In any case, sometime after its initial formation the early lunar crust
was thoroughly heated (perhaps by short-lived radioactive elements), chemically
differentiated, and degassed.
giant bombardment by asteroid-sized objects went on, probably for millions of
years, forming the repeatedly smashed and pulverized highland rocks called
breccias, some of which include the oldest samples--4.6 billion years old--yet
recovered from the moon. The crust
then cooled enough to become rigid, preserving much of the topography of the
huge impact basins and their surrounding ejected matter.
A billion or more years later, the lava flooding of the
basins that are now on the moon’s earthward side began, creating the maria and
the mascons. Long-lived radioactive
elements may have provided the energy for this second and long-continued episode
of partial melting. Meanwhile, a
diminishing number of meteorites continued to fall on maria and highlands alike,
pulverizing the surface and creating the broken-up surface material called the
discovery made by the Apollo astronauts while walking on the moon is that the
regolith is rich in blobs and spherules of glass, formed by the rapid cooling of
materials melted by impact shock. An
important finding of the Apollo seismic experiments is that the regolith causes
great dispersion of seismic waves because of its chaotic structure. Also, the regolith has very low damping, when struck, the
moon “rings” much longer than the earth.
Only at great depths do the waves propagate as if through solid rock.
After the formation of the maria, it seems that the
moon has just been cooling and quieting down.
Three billion years ago it may have looked much as it does today.
The only more recent changes are those wrought by occasional volcanic
emanations and the continuing influx of meteorites, dust, elementary particles,
simplified picture ignores the variety of local tectonic and morphologic
features on the moon. It leaves
unsolved the problem of origin, and it is silent on such subjects as the
possibility of polar cold-trapped volatiles.
Truly, there is much more to be discovered by future explorers of the
See also SPACE EXPLORATION.
James D.Burke, Jet Propulsion Laboratory California Institute of
Alfven, Hannes, and Gustaf
Arrhenius, Evolution of the Solar System (USPGO 1976).
Baldwin, Ralph B., A Fundamental
Survey of the Moon (McGraw-Hill 1965).
Cherrington, Ernest H., Exploring
the Moon through Binoculars and Small Telescopes (Dover 1983).
Gully, Rosemary, Moonscapes: A
Celebration of Lunar Astronomy, Magic, Legend, and Lore (Prentice-Hall 1991).
Kopal, Zdenek, The Moon in the
Post-Apollo Era (Reidel 1974).
Runcorn, S. K., et al., eds., The
Moon: A New Appraisal from Space Missions and Laboratory Analyses (Royal Society
Taylor, Stuart R., Planetary
Science: A Lunar Perspective (Lunar and Planetary Inst. 1982)
Information acquired within the
quotes is from: Encyclopedia Americana-International Edition
Copyright 1994, Grolier Incorporated, Volume 19, page 437
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