The Big Splat, or How Our Moon Came to Be

by Dana Mackenzie
2003
hardcover - 232 pages
John Wiley

To look at the Moon sailing peacefully across our sky as it has for countless ages, it would not seem to be a place of great mystery. Surely the Moon, that world we once visited back when your parents (or was it grandparents?) were young, is just a dull, boring, dusty old rock. Nowadays some people even seem unclear as to whether we really went there or whether the trip was all a fake.

Well, if you think the Moon is boring and devoid of mystery, think again. The Moon has been a constant puzzle to astronomers right up to the present day. We didn't even know how the Moon got there in orbit around our Earth until 25 years ago. And the story is quite an adventure, involving Galileo, Sir Isaac Newton, Charles Darwin's son George, the Apollo astronauts, and an astronomer cum painter cum science fiction writer, plus (talk about special effects) the biggest impact ("splat") in Earth's history. The whole fascinating story is told in mathematician and science writer Dana Mackenzie's The Big Splat, or How Our Moon Came to Be.

Mackenzie begins his tale by reminding us of the enormous importance of the Moon through the ages, both as a timepiece and nightlight in the darkness, and later as an object close enough to be distinguished as a world. Roughly the first third of the book details the problems that the Moon posed for Greek, Mediaeval and Renaissance philosopher-scientists. For instance, while Galileo observed the Moon as a world with surface features somewhat like those of our Earth, its relation to the tides eluded him. Once selenologists (Moon scientists) got beyond simply working out the Moon's orbit and studying its obvious features, a larger question loomed: Why do we have a moon at all and how did it get there?

Since the 18th Century, various theorists argued over whether the Moon had been formed where it is, or captured from elsewhere in the solar system, or spun off from the rapidly rotating, early Earth. And so the debate continued, schools of selenology arguing back and forth, right up to the early 1980's when everything came together dramatically at a conference in Kona, Hawaii.

A group led by astronomer/artist/writer William K. Hartmann proposed that the Moon had been born out of the red-hot debris ejected from Earth's greatest catastrophe, the impact 4.5 billion years ago of another planet about the size of Mars. This new theory was born out of analysis of the 850 pounds of moon rock returned by the Apollo astronauts, computer simulations of planet-on-planet impacts, and a recognition that catastrophic impacts are not rare and unusual events, but rather "the norm" over astronomical timescales. The time was right and by the end of the Kona Conference, most members had discarded the traditional theories in favor of the new impact model.

So our serene, peaceful, sublimely beautiful Moon turns out to have been formed in the most violent event in Earth's history. This new vision of the Moon's origin, and the scientific evidence for it, is neatly laid out in the final two chapters of The Big Splat, or How Our Moon Came to Be. Mackenzie even has a name (suggested by one of the theorists) for the defunct planet whose impact demise gave birth to the Moon, Theia, from the mother of the Moon goddess Selene in Greek mythology. He includes an addendum on the silly Apollo Moon Hoax "theory," pointing out its flawed logic, contradictions, and mistakes in basic science. All in all, it's a fine book on a fascinating subject, though I don't think that "Big Splat" is evocative or euphonious enough to become as popular a phrase as "Big Bang."