Geoscience Reference
In-Depth Information
Loss of Momentum
Over the decades, the three theories waxed and waned as new methods and data
became available, but the flaws in each stubbornly refused to yield. If geology in
the early years of the twentieth century was a science without a theory, by midcen-
tury selenology was a science with too many theories and no way to choose among
them.
Beyond explaining the known angular momentum of the solar system, any the-
ory of the Moon's origin also had to account for the Moon's lower density, 3.3
grams per cubic centimeter, compared to 5.5 gm/cc for the Earth. This posed a
problem for the sister theory, as sibling planets born close to each other and made
from the same primordial material should hardly show such a large difference in
such a fundamental property as density.
On the other hand, the main advantage of the sister theory was that, unlike the
daughter and spouse (capture) models, it did not appeal to a one-off, improbable
event, instead viewing the formation of the Moon as a natural, perhaps inevitable,
step in solar system evolution, a process that might explain the moons around oth-
er planets. But once computers allowed scientists to construct and test models of
solar system formation, they soon found that the sister theory also failed to explain
the observed angular momentum of the Earth-Moon system.
The daughter theory suffered from the same problem. For the Earth to have spun
fast enough to fling off the Moon, our planet would have to have had four times
the angular momentum that the combined Earth-Moon system has today. No one
could explain how the proto-Earth acquired this extra angular momentum or, since
angular momentum is conserved, where it went. On the other hand, the density of
the Moon is about the same as the density of the Earth's mantle. If in spite of the
angular momentum problem, the Moon did somehow spin off from the Earth, per-
haps most of the Moon's material came from the Earth's mantle.
The capture model had its own problems. It demanded that the passing moon be
in just the right orbit and be braked by just the right amount so as neither to collide
with the Earth norshoot past it. Areturning spacecraft manages this task byhaving
small thrusters and astronauts and computers to control them. It is a tricky propos-
ition. If the reentry angle is too large, the spacecraft will burn its way through the
atmosphere and, if anything is left, crash into the Earth's surface. If the angle is too
shallow, the vehicle will bounce off the atmosphere and vanish into space. Capture
is inherently improbable. Yet the solar system has many moons.
A Buried Star
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