Moon was created in giant smashup artist’s conception of a planetary smashup whose debris was spotted by NASA’s Spitzer Space Telescope three years ago gives an impression of the carnage that would have been wrought when a similar impact created Earth’s moon. A team at Washington University in St. Louis has uncovered evidence of this impact that scientists have been trying to find for more than 30 years. Credit: NASA/JPL-Caltech.

It’s a big claim, but Washington University in St. Louis planetary scientist Frédéric Moynier says his group has discovered evidence that the moon was born in a flaming blaze of glory when a body the size of Mars collided with the early Earth.

The evidence might not seem all that impressive to a nonscientist: a tiny excess of a heavier variant of the element zinc in moon rocks. But the enrichment probably arose because heavier zinc atoms condensed out of the roiling cloud of vaporized rock created by a catastrophic collision faster than lighter zinc atoms, and the remaining vapor escaped before it could condense.

Scientists have been looking for this kind of sorting by mass, called isotopic fractionation, since the Apollo missions first brought moon rocks to Earth in the 1970s. Moynier, PhD, assistant professor of earth and planetary sciences in Arts & Sciences — together with PhD student, Randal Paniello, and colleague James Day of the Scripps Institution of Oceanography — are the first to find it.

The moon rocks, geochemists discovered, while otherwise chemically similar to Earth rocks, were woefully short on volatiles (easily evaporated elements). A giant impact explained this depletion, whereas alternative theories for the moon’s origin did not.

But a creation event that allowed volatiles to slip away should also have produced isotopic fractionation. Scientists looked for fractionation but were unable to find it, leaving the impact theory of origin in limbo — neither proved nor disproved — for more than 30 years.

“The magnitude of the fractionation we measured in lunar rocks is 10 times larger than what we see in terrestrial and Martian rocks,” Moynier says, “so it’s an important difference.”

The data, published in the Oct. 18, 2012, issue of Nature, provide the first physical evidence for wholesale vaporization event since the discovery of volatile depletion in moon rocks, Moynier says.

The Giant Impact Theory

Cross-polarized, transmitted-light image of a lunar rock reveals its hidden beauty. In broad daylight, the rocks are an unprepossessing, even an ugly, gray. (Credit: J. Day)

According to the Giant Impact Theory, proposed in its modern form at a conference in 1975, Earth’s moon was created in an apocalyptic collision between a planetary body called Theia (in Greek mythology the mother of the moon Selene) and the early Earth.

This collision was so powerful it is hard for mere mortals to imagine, but the asteroid theorized to have killed the dinosaurs is thought to have been the size of Manhattan. Theia is thought to have been the size of the planet Mars.

The smashup released so much energy it melted and vaporized Theia and much of the proto-Earth’s mantle. The moon then condensed out of the cloud of rock vapor, some of which also re-accreted to the Earth.

This seemingly outlandish idea gained traction because computer simulations showed a giant collision could have created an Earth-moon system with the right orbital dynamics and because it explained a key characteristic of the moon rocks.

Once geochemists got moon rocks into the lab, they quickly realized that the rocks are depleted in what geochemists call “moderately volatile” elements. They are very poor in sodium, potassium, zinc and lead, Moynier says.

“But if the rocks were depleted in volatiles because they had been vaporized during a giant impact, we should also have seen isotopic fractionation,” he says. (Isotopes are variants of an element that have slightly different masses.)

“When a rock is melted and then evaporated, the light isotopes enter the vapor phase faster than the heavy isotopes, so you end up with a vapor enriched in the light isotopes and a solid residue enriched in the heavier isotopes. If you lose the vapor, the residue will be enriched in the heavy isotopes compared to the starting material,” Moynier says.

The trouble was that scientists who looked for isotopic fractionation couldn’t find it.

Extraordinary claims require extraordinary data

Asked how he felt when he saw the first results, Moynier says, “When you find something that is new and that has important ramifications, you want to be sure you haven’t gotten anything wrong.

Taken from Esfahan, Iran, this image captures Earthshine, light from Earth illuminating the moon’s night side. An observer on the moon looking our way at the same moment would have seen a brilliantly lit, nearly full Earth. (Credit: M. Taha Ghouchkanlu)

Compared to terrestrial or Martian rocks, the lunar rocks Moynier and his team analyzed have much lower concentrations of zinc but are enriched in the heavy isotopes of zinc.
Earth and Mars have isotopic compositions like those of chondritic meteorites, which are thought to represent the original composition of the cloud of gas and dust from which the solar system formed.

The simplest explanation for these differences is that conditions during or after the formation of the moon led to more extensive volatile loss and isotopic fractionation than was experienced by Earth or Mars.

The isotopic homogeneity of the lunar materials, in turn, suggests that isotopic fractionation resulted from a large-scale process rather than one that operated only locally.

Given these lines of evidence, the most likely large-scale event is wholesale melting during the formation of the moon. The zinc isotopic data therefore supports the theory that a giant impact gave rise to the Earth-moon system.

“The work also has implications for the origin of the Earth,” Moynier points out, “because the origin of the moon was a big part of the origin of the Earth.”

Without the stabilizing influence of the moon, the Earth would probably be a very different sort of place. Planetary scientists think the Earth would spin more rapidly, days would be shorter, weather more violent, and climate more chaotic and extreme. In fact, it might have been such a harsh world, that it would have been unfit for the evolution of our favorite species: us.