An Unnatural Occurrence?
We think we know so much about our origins, yet science has yet to
provide a satisfactory answer to explain even the origin of our closest
neighbor in space, the Moon. It plays a critical role in the biosphere
of the earth. It's ability to create a total eclipse of the sun is the
result of its near perfect spherical shape AND its distance to the sun being PERFECTLY proportioned in
relation to size to the sun. Is this just one more coincidence or just
one more evidence of Divine Creation?
See more on the Moon
or on other unusual
relationships in the solar system or read the excerpt below from the PBS
site as you study and form your own conclusions.
Whence our moon? Was it a chunk of Earth flung off in
our planet's early history? Did the Earth capture a small, roaming
planet in its gravity grip? Or did the moon fashion itself
alongside our world from the same planetary batter? One of the
Apollo program's chief scientific goals was to give lunar
researchers the means to decide, once and for all, between these
three main theories of how the moon formed.
Three Theories of Origin
What transpired in this "battle of the Big Three"
after the last Apollo mission flew in 1972 surprised just about
everyone. The story provides a revealing glimpse of the workings
of the scientific process, while at the same time opening a window
on the origins of what one lunar researcher has called "one
of the most peculiar bodies in the solar system" -- the moon.
Theory 1 - Fission:
A chunk of the Earth spins off
The first moon-origin theory to gain a solid foothold was put
forth in 1878. That year, George Howard Darwin, son of the famous
evolutionist, proposed that Earth spun so rapidly in its early
years that the sun's gravity eventually yanked off a chunk of an
increasingly elongated Earth; that chunk became the moon. Four
years later, the geologist Osmond Fisher added a juicy addendum:
The Pacific ocean basin marks the scar left behind where our
future satellite ripped away. The so-called "fission"
theory became the accepted wisdom well into the 20th century, as
this quirky, 1936 U.S. Office of Education script for a children's
radio program attests:
Theory 2 - Capture:
The Earth captures a wandering body
The Darwin-Fisher model eventually met with competition from two
other theories. In 1909, an astronomer with the all-American name
of Thomas Jefferson Jackson See proposed that the moon was a
wandering planet that had been snared by Earth's gravity, like a
fly in a spider web.
Theory 3 - Coaccretion:
Side by side formation
The third theory, advocated by the astronomer
Edouard Roche among others, was coaccretion. In this model, the
Earth and the moon formed independently, side by side as it were,
from the same material that formed all the planets of our solar
By the end of the Apollo program, lunar scientists had elucidated
many aspects of the moon's history, giving them clues unavailable
to the likes of Darwin or See. Selenology, the study of the origin
of the moon, had taken off. Most of the new evidence came from the
more than 800 pounds of moon rocks retrieved by the American and
Russian lunar missions.
In many ways, the moon turned out to be quite different from
Mother Earth. Anybody can see that, of course: It's airless,
colorless, lifeless. But the differences run deeper. It is
compositionally different, with fewer volatile elements -- those
that tend to boil off at high temperature. The moon might have
inherited such differences -- maria rocks contain no water, for
instance, unlike volcanic rocks on our planet -- from the impactor.
The lunar samples also suggest that much of the moon may have once
been molten; no definitive evidence exists that the Earth ever
melted to such a degree. And while one-quarter its size, the moon
has but one percent of our planet's mass, and its density more
closely resembles that of Earth's mantle rather than the planet as
a whole. Lunar scientists in the immediate post-Apollo years
explained these discrepancies by postulating that the moon had but
a tiny core. In 1998, the Lunar Prospector, NASA's first mission
to the moon since Apollo, confirmed that the moon's core indeed
comprises less than three percent of its mass. (By contrast,
Earth's core represents 30 percent of its mass.)
In other ways, the Earth and moon have remarkably similar
characteristics. Studies of radiogenic elements and isotopes in
lunar rocks reveal that the two bodies are roughly the same age,
4.5 billion years old. They also came from the same neighborhood:
Unlike those in all meteorites ever analyzed, the nonradioactive,
stable isotopes of oxygen in moon and Earth rocks match like blood
types, implying the two spheres formed at the same radial distance
from the sun. Indeed, results from Apollo showed the pair to be
more intimately connected than previously thought. "Apollo
tied together for the first time the history of the moon with the
history of the Earth," says William Hartmann of the Planetary
Science Institute in Tucson, Arizona. "It showed us that we
live in a system, the Earth-moon system."
A Pairing Unlike Any Other in the Solar System
In fact, it's a pairing unlike any other in the solar system.
Our moon is far more massive relative to Earth, for example, than
the satellites of all other planets save Pluto (whose moon, Charon,
is half its size). The Earth-moon system also has an unusually
high angular momentum -- that is, the sum of the our planet's
rotational velocity and the speed at which the moon orbits the
So how do the Big Three stand up in the face of all the new
evidence? Not well, it turns out.
The fission theory might explain
the moon's lack of a large core and the oxygen-isotope similarity,
astronomers say, but calculations show that the Earth would have
to have had four times its present angular momentum -- a
lightning-fast rotational speed that astronomers cannot square in
their models. Add to that the understanding reached decades ago
that the Pacific basin formed less than 70 million years ago and
therefore could not possibly have spawned the moon, and the
Darwin-Fisher model suddenly comes up short.
See's capture theory suffers as well. The idea that Earth's
gravity caught a rogue planet might explain the compositional
differences between the two bodies. But, then, why doesn't the
moon have its own regular-sized core? And why the oxygen-isotope
similarity if the two formed in different parts of the solar
system? Finally, most modelers deem the chance that a speeding
planet would gracefully ease into Earth's embrace rather than slam
into it or career off into space too remote for consideration.
Coaccretion led the pack through the 1970s, because, for one
thing, it doesn't require a low-probability event like capture.
But today it faces the same problem regarding the core. As
Hartmann says, "It's very hard to imagine the two bodies
growing together but somehow the Earth magically gets all the
stuff with the iron in it and the moon doesn't get any." Even
more troublesome, experts say, the theory cannot account for the
enormous angular momentum we see in the Earth-moon system today.