Two Moons in Earth's Past?

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Our Moon. It lights up our nights, governs our tides and has inspired millions — perhaps billions -– of people throughout history to contemplate its nature, its influence on our lives (if any) and, of course, where it may have come from.

The currently accepted theory is that over four and a half billion years ago, our newly formed planet was hit by a Mars-sized body, a catastrophic collision that flung molten bits of Earth's mantle into space and created a ring of debris.

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This ring gradually coalesced into the moon, which cooled and moved further and further away from Earth into its current position. This theory is widely accepted, and research on the composition of lunar samples seems to coordinate with an Earthly origin of the moon. But one thing never quite lined up perfectly with the whole scenario: the moon's far side.

The lunar far side looks very different from the face we see. It is much more heavily cratered, for one thing, in fact featuring one of the largest impact basins in our solar system. It lacks the features that create the "man-in-the-moon" patterns so familiar to us and while the near side is relatively flat, the far side is much more mountainous.

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These "lunar highlands" have been an enigma to scientists since they were discovered at the beginning of the Space Age. Thanks to some recent computer models by researchers at University of California, Santa Cruz, we may be a step closer to solving the mystery.

Earth may have once had two moons.

Erik Asphaug, professor of Earth and planetary sciences, along with postdoctoral researcher Martin Jutzi, have developed a model that fits in with the giant impact theory, but also explains the disparity of the lunar far side.

In this model, the remnants of the collision coalesced into two moons, one larger than the other. These primordial moons settled into orbit around Earth within the Lagrange points — that is, until the smaller one became dislodged and migrated toward its larger sibling, eventually colliding with it in a low-speed impact.

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Rather than smashing apart and starting the process all over again, the smaller moon got absorbed into the larger one, creating the far side highlands and making a single, "lopsided" moon that eventually oriented itself so one side continually faces Earth — exactly what we see today.

"Of course, impact modelers try to explain everything with collisions," said Asphaug. "In this case, it requires an odd collision: being slow, it does not form a crater, but splats material onto one side. It is something new to think about."

It's definitely an interesting thought!

(I wonder how having two moons might affect our werewolf population…)

Read more in the journal Geology here.