Science Digest
April 1982, p. 82
Included as a substory to "Exploding Planets"


BREAKAWAY MOON

Could the moon have split off from the earth's mantle 4.5 billion years ago when
the planet was still semimolten? This old theory has been revived by new
evidence.

German astrochemist Heinrich Wiinke has recently shown that the moon shares
certain properties with the earth's mantle, such as a tungsten deficiency. When
the planet was semimolten, theorists suggest, tungsten (under high pressure)
combined chemically with the heavy element iron and sank toward the earth's
core, causing the planet to rotate so fast that a day may have lasted only two
and a half hours. A huge bulge then formed in the mantle and eventually tore
away. This new mass, like the earth's crust and mantle, would be low in
tungsten. Further, a deficiency in tungsten could not have developed
independently on the moon because the pressures within it aren't high enough.

Previously, this breakaway theory was condemned because of dynamical
difficulties. If the moon (about I percent of the earth's mass) broke off,
figured scientists, its angular momentum would have been so great that it would
have flown away. (The angular momentum of an orbiting object is the product of
its mass, speed and distance from the center of rotation.) NASA geophysicist
John O'Keefe has shown that more of the earth's mass-perhaps 10 percent-may have
split off. The bulk of this mass did escape into space. Only the moon remains.

Another criticism deals with the moon's orbit throughout its past. Since
scientists know the moon's current orbit and the rate at which the orbit is
slowing down and increasing in diameter, they can trace the evolution of its
orbit backward in time. By this method, it was once calculated that the moon's
initial orbit could not have been in the plane of the earth's Equator if the
earth were semimolten. But a chunk of the earth could have broken off only from
the Equator. Yet, recently, along with Robert Harrington at the U.S. Naval
Observatory, I traced the moon's orbit backward and discovered an instability
point. Before that time, the orbit could have been either in or out of the
equatorial plane. We then traced the orbit forward, assuming the moon broke off
from the Equator, and we arrived at the moon's current orbit.	-T.V.F.