How the Moon Was Born

New evidence solves an old mystery about the origins of Earth's largest satellite.

How did the moon become the moon? The most dominant theory, the Giant Impact Theory, has held that the moon resulted  from a cosmic collision: When Earth was still proto-Earth, a body called Theia (named for Greek mythology's mother of the moon Selene) collided with it. The resulting impact released so much energy that it vaporized Theia and much of the mantle of Earth -- and the matter that remained resulted in the beautiful but barren body that has shaped so much of what it means to be human.

The impact theory, first suggested by George Darwin in 1898 and reintroduced in 1974, gained traction in part because computer simulations demonstrated that a giant collision of the kind it described could have created a Earth-Moon system with the kind of orbital dynamics we have today. But the theory has, all this time, been just that: a theory -- a likely explanation, but with little physical evidence to back it up. 

Today, though, the theory has gotten one step closer to validation. A team at Washington University in St. Louis has uncovered evidence, to be published in the journal Nature, that our little moon is indeed the result of a long-ago galactic cataclysm. Their findings solve mysteries that scientists have been puzzling over for more than 30 years.

The key to those findings? Zinc. Or, more specifically, a heavier variant of zinc found in moon rocks. And key to that finding were samples taken from the moon -- basalts collected during the Apollo lunar landing missions -- as well as a lunar meteorite. (To ensure as much diversity as possible, the team analyzed 20 different samples of lunar rocks gathered from Apollo missions 11, 12, 15, and 17, which were gathered from different locations on the moon.) The researchers, a group of geochemists, compared the lunar samples to their terrestrial counterparts. 

Enter the zinc. While moon rocks are, in general, chemically similar to the Earthly versions, they are also relatively lacking, the researchers found, in the easily evaporated elements known as "volatiles." The cataclysmic impact resulting from the Theia/Earth collision, the Giant Impact Theory goes, would have created intense heat, which would in turn have created "an intense evaporation event" early in the moon's formation. Water on the moon would have boiled away -- and the zinc on the moon's surface would have been similarly vaporized. As condensation occurred, the heavier zinc atoms would have condensed out of the cloud of vaporized rock created by the collision, while the lighter zinc atoms would have escaped from the whole thing in vapor form.

What the researchers' found in the lunar rocks was a phenomenon called isotopic fractionation: a kind of elemental sorting by mass. Despite their similarity to moon rocks, the lunar basalts were short on volatiles -- elements that evaporate easily. The moon rocks, the researchers found, were extremely poor in sodium, potassium, zinc, and lead -- all of them the kind of moderately volatile elements that would have been vaporized in a cataclysm of the kind described by the Giant Impact Theory. Alternative theories for the moon's origin, crucially, wouldn't explain that fractionation. The isotopic composition data, study co-author Frédéric Moynier says, "provide the first physical evidence for wholesale vaporization event since the discovery of volatile depletion in moon rocks." 

Here's some amazing -- though artist-rendered, obviously -- video of what the moon's origin might have looked like.