The Big Splat or How Our Moon Came to Be by Dana Mackenzie. Wiley, $24.95, 232 pages.



The Chapel Hill News


August 3, 2003

How Earth Got Its Moon


By Phillip Manning


Shakespeare, Byron, and hosts of song writers have written about the Moon, the brightest object in the night sky. It has inspired scientists, too, but not to pen dreamy verses. When scientists look at the Moon, they don’t see metaphors, they see a big question: Where did it come from?

The answer is not obvious. Until recently, no one had been to the Moon. Simple questions like, What is it made of? could not be answered. However, the Moon is our nearest neighbor in the cosmos, so the shortage of evidence did not stop scientists from speculating about its origin. Three competing theories developed. How these theories — condensation, splitting off, and capture — and a fourth one that was added later were winnowed down to a single, generally accepted hypothesis neatly illustrates how science progresses.

First up was the German philosopher Immanuel Kant, who said in 1755 that the Moon condensed out of the same swirling nebula that formed the Earth. The astronomer George Darwin (son of Charles) proposed in 1878 that the Earth and Moon were once a single body, a molten dervish that ejected the material that became the Moon. Another theory came a few decades later from astronomer Thomas Jefferson Jackson See, who postulated that the Moon was a body from outer space that had been captured by Earth’s gravity.

These three theories, refined over the years, dominated the thinking about the Moon’s birth until the early 1970s. By then scientists were finding problems with each of them. In his book “The Big Splat, or How Our Moon Came to Be,” science writer Dana Mackenzie details the reasons. Kant’s condensation theory was in trouble because the density of the Moon is lower than that of the Earth, which has a heavy iron core. Thus, scientists reasoned, the Moon must have a smaller, or nonexistent, iron core. If Earth and Moon condensed out of the same material, why should the Moon have less iron? Darwin’s splitting-off theory was questioned next. Using what he called “high school physics,” Harvard astrophysicist Alistair Cameron calculated that the early Earth could not have been spinning fast enough to throw off a piece of itself to form the Moon. See’s capture theory fell into disfavor for same reason as the condensation theory. Rocky bodies in space are rich in iron, which didn’t fit with the Moon’s low iron content. Nevertheless, astronomers needed a theory to hold on to and these three were still around in 1974 when William Hartman proposed something new. The Moon, he said, was formed when another planet struck the Earth. Hartman and a colleague published the “giant impact theory” in 1975. Cameron, who developed the same idea independently, and a postdoctoral student followed with their paper a year later. Nobody paid much attention. Scientists were busy analyzing Moon rocks brought back to Earth by Apollo astronauts and finding surprising results.

"If it disagrees with experiment, it is wrong,” observed Nobel laureate Richard Feynman. And each of the early three theories — already suspect — seemed to disagree with the accumulating evidence. The percentage of a rare, heavy form of oxygen found in Moon rocks was identical to that found on Earth. Asteroids and other planets have different percentages of heavy oxygen than Earth, which cast more doubt on the capture theory. Another analysis showed almost no volatiles (substances with low boiling points, such as water and nitrogen) were present in Moon rocks. This finding further eroded the condensation hypothesis. If Earth and Moon had both condensed out of the same matter, what happened to the Moon’s volatiles?

Moon scientists gathered in Hawaii in 1984 to discuss the four origin theories, which now included the giant impact hypothesis. It was a rarity among scientific meetings; the conferees approached a consensus. The only hypothesis that fit all the data was the giant impact theory.

The Moon’s story, as recapped by Mackenzie, starts with the formation of the Earth 4.57 billion years ago. Fifty million years later, a Mars-sized planet plowed “into Earth with the energy of trillions of hydrogen bombs.” Material from Earth’s mantle was blown into the sky. A ring of debris went into orbit and coalesced into the Moon. Because the Moon was formed primarily from the Earth’s mantle and not from its iron-rich core, the theory accounted for the iron-poor Moon and for the identical percentage of heavy oxygen. Furthermore, the heat of the explosion would have vaporized any water or nitrogen present, thus explaining why the Moon has no volatiles.

The giant impact theory seems sound, and most scientists now subscribe to it. Of course, new evidence could consign it to the same trash heap where the older theories are buried. New data sometimes leads to new theories; that is how science progresses.
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