The farther we reach toward the stars, the more
questions we discover about our home. While scientists
have proposed many theories about the origin of the
Moon, deeper investigation into other planetary bodies
has complicated each hypothesis. At this time, three
major theories still exist to explain how Earth
obtained its satellite. Each theory attempts to explain
several factors, the primary ones being the Moon's
chemical composition, density, rotation, and orbit.
Recent investigation into samples from the Moon and
other extraterrestrial bodies have complicated each
theory.
The leading Moon origin theory since the 1970s
has been the giant impact hypothesis. According to this
theory, the Earth received a glancing blow from
another planetary body (referred to as Theia) billions
of years ago, and parts of the debris from Theia and
Earth merged to form the Moon. Astronomers have
simulated the impact using computer models that take
into account the gravitational force of the Earth. If
the Earth spun at approximately twelve times its
current rate at the time of impact, this theory would
account for the rotation and orbit of the Moon. The
giant impact hypothesis has been refined by many
scientists over the last few decades, until it fully
accounted for the Moon's chemical composition and
density. However, recent examination of rocks from
various locations in the solar system has revealed that
every extraterrestrial body has a distinct isotopic
signature, a ratio between stable and unstable isotopes
within each element. By contrast, close study shows
that the Earth and its Moon have roughly identical
isotopic signatures. This makes it virtually impossible
for the Moon to be made largely of material from
another planet.
Although support for the giant impact
hypothesis was weakened by this isotopic evidence,
scientists now seek to refine the hypothesis to take
the new information into account. One theory suggests
that two large planets collided directly, scattering
debris to form both Earth and the Moon. However, one of
the virtues of the giant impact hypothesis is its
well-developed explanation for the Moon's rotation and
orbit; this new theory dismisses that research. Another
theory suggests that, since Theia formed in
approximately the same orbit around the sun as the
Earth, they may both have formed from a single group of
loose, floating materials.
As much challenge as the findings regarding
isotopes pose for the giant impact hypothesis, they
provide even more problems for another leading theory
of the Moon's origin. This hypothesis, known as the
capture theory, suggests that the Moon formed
separately from the Earth and was later captured by
Earth's gravitational field. Like the giant impact
hypothesis, the capture theory was able to explain the
Moon's orbit and rotation. Also like the giant impact
hypothesis, the capture theory requires an adjustment
in our understanding of the early Earth. In order for
the Earth to have captured the Moon, its atmosphere
must have extended much further from the surface than
it does today. Such an atmosphere could slow the Moon
sufficiently for it to enter orbit rather than breaking
away from the Earth. The discovery that the Moon and
Earth have identical isotopic signatures makes this
theory highly unlikely.
The remaining major theory, the fission theory,
has no challenge explaining the isotopic similarity of
the Earth and Moon. According to this theory, the Moon
was created when the rapidly spinning Earth extended
portions of its outer layer far enough from the
planet's surface for them to become a satellite. Early
versions of this theory suggested that the material
came from the Pacific Ocean basin, which would explain
the massive indent in the Earth's surface at that site.
Analysis of moon rocks supported this theory by
revealing that the Moon contains many of the same
chemicals as the Earth. While the fission theory could
explain why the Earth and the Moon have identical
isotopic signatures, other aspects of the theory
require refinement. For instance, if the fission theory
were true, the Moon's orbit would be expected to
follow the Earth's equator. Additionally, it is highly
improbable that the Earth would have reached the
rotational speed required to throw that much material
from its surface while maintaining the rest of its
structure.
In light of the difficulties new discoveries
have presented to old hypotheses, it is important to
remember that contrary evidence benefits any scientific
theory. The more scientists learn about errors in
current models, the more accurate their models become
through revision. As astronomers learn more about the
isotopic signatures of various planets in our solar
system and beyond, the theories of the Moon's origin
are not damaged, but improved.
