The Second-Moon Theory

Is Earth’s moon the product of a big splat as well as a big whack?

If the Earth started out with two moons, a slow collision between them could explain our remaining moon's appearance today. (M. Jutzi and E. Asphaug, UC Santa Cruz)
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Hartmann agrees that the giant impact theory has room for the idea of two moons. “Maybe there was a time when you could have stood on the surface of Earth and seen two moons in the sky, or three or four” he says, “along with the smaller remnants of a ring. It’s not a radical perturbation of the theory.” Even better, it potentially solves some mysteries about the moon’s appearance and composition.

“The lunar far side has a bulge, like it’s wearing a yarmulke,” says Asphaug. The far side’s mountainous crust is about 30 miles thicker than the near side’s. In addition, the far side is less rich in certain minerals, like potassium and phosphorus, than the near side, almost as if something pushed them from far to near.

Last year a theory proposed that the bulge in the moon’s crust was created by the tidal pull of Earth’s gravity, just as the moon’s pull creates tides in Earth’s oceans. If that were the case, however, the lunar crust should show two bulges, Asphaug says. In our oceans, the moon’s gravity pulls more strongly on the near side of Earth, producing a bulge; inertia from Earth’s rotation produces a second one to balance the first.

Instead, Asphaug and Jutzi hypothesized that the bulge formed from the impact of a second, much smaller moon. The gentler, relatively slow collision would have squeezed some of the larger moon’s still partially molten crust from the far side to the near side, resulting in the trademark bulge and the movement of minerals.

While some computer models show the growing moons colliding quickly to form a single body, the Asphaug-Jutzi “splat” model requires the second moon to stick around for tens of millions of years to give it time to cool and solidify. This model assumes, therefore, that the second moon—and any other bodies—existed for all that time at gravitationally stable points, locked in synchronicity with the larger moon in orbit around Earth.

Eventually, Earth’s gravity acted as a brake, slowing the moons’ orbits, which in turn caused them to move farther away. At some point, the smaller moon would no longer be in a stable point, setting up the slow-speed merger. Within 100 million years, this gravitational jostling caused the cosmically slow splat that would have left Earth with the single moon we see today.

A pair of NASA spacecraft launched last September 10 could help planetary scientists evaluate the scenario. The Gravity Recovery and Interior Laboratory (GRAIL) mission, which will begin observations this January, will make the most detailed measurements to date of the moon’s gravitational field. “GRAIL will show us the thickness of the crust and help us see if our model holds up,” Asphaug says.

“All of the aspects—the timing, the dynamics—they all fit together. Whether it’s right…” Asphaug draws out the last word, then lets his voice trail off for a moment. “I’m cautious about it, but my job now is to be the attorney for the idea and defend it to the best of my abilities.” And, of course, hope that the pie-in-the-face idea doesn’t leave Asphaug with egg on his.

Damond Benningfield is a freelance science writer and radio producer in Austin, Texas.

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