Origins of the Moon

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 system.

Apollo's Impact

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 Earth.

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.