Fast-spinning Earth settles mystery of moon's make-up

For the first time, two new models of the giant collision that spawned the moon can explain why our satellite seems to be made of the same stuff as Earth. Both rely on the very young Earth spinning much faster than was thought possible ? resulting in a day lasting just 2.5 hours.

The moon is thought to have formed from the debris remaining after a planet-sized object collided with the early Earth some 4.5 billion years ago. The trouble is that until now, impact simulations have always resulted in a moon made mainly of material from the impactor.

That conflicts with analyses of lunar rocks carried out over the last 10 years. They show that the ratios of oxygen isotopes are almost identical on the Earth and the moon, suggesting the two bodies formed from the same material. Meteorites from Mars and asteroids have very different oxygen isotope compositions from the Earth, so the impacting proto-planet probably did, too.

"There's been this quandary," says Robin Canup of the Southwest Research Institute in Boulder, Colorado. "How do you end up with a moon and Earth that have identical oxygen isotope composition?"

Rotation clue

The two new models resolve the conundrum thanks to a new insight about the rate of Earth's rotation in the solar system's chaotic early days.

Earlier collision models all assumed that just after the moon formed, the Earth rotated once every 5 hours. That's because the moon started out much closer to the Earth than it is now, and has been drifting away slowly ever since (it is still receding by a few centimetres every year). Just as an ice skater that pulls her arms in will spin faster, because angular momentum is conserved, the Earth spun faster when the moon was closer ? and spun slower and slower the further out the moon moved. Taking this into account, a five-hour day at the time of the moon's formation results in the 24-hour day we have now.

But models featuring Earth spinning at this rate during the great collision always resulted in dynamics that produce a moon with the wrong composition.

So recently, Matija Cuk of the SETI Institute in Mountain View, California, and Sarah Stewart of Harvard University have suggested a new mechanism for slowing down Earth's spin. It meant Earth could have had a day as short as 2.5 hours when the great collision occurred ? and still have the 24-hour day we observe now. The mechanism involves gravitational tugs by the sun on the moon that, in turn, allow the moon to "steal" momentum from the Earth, slowing it more aggressively.

Smaller, faster impactor

Now, Canup, who saw Cuk and Stewart's work presented in February, has used the insight to create a full model of moon formation.

In her model, two bodies, each about half the current mass of the Earth, come together and stick. The energy from the impact melts them both. Most of the mass is merged into a central planet, but a bit streams out into a surrounding disc, which coalesces into the moon within 100 years. The resulting two objects have the same composition because of thorough mixing of the molten rocks following the impact.

Such a model seemed impossible previously because it spits out an Earth with a 2.5-hour day ? but thanks to Cuk and Stewart, that's no longer a problem.

Cuk and Stewart, meanwhile, have come up with a rival model. They find that if you assume that Earth already had a 2.5-hour day before the collision, a much smaller, faster impactor than previously thought possible can create a moon. Similar to Canup's model, the resulting planet and moon would both be about 90 per cent proto-Earth material, 10 per cent impactor material.

Cuk claims his model is more likely. "It's easier to have a smaller body, because there are more of them," he says. Based on simulation data, Canup says that the odds of the impactor being the same size as the proto-Earth are about 1 in 5. "It's not overly improbable," she says.

Neither model is the final word on the formation of the moon, says Erik Asphaug of the University of California, Santa Cruz, who was not involved in the new studies. But allowing the Earth to spin faster will open up a new set of ideas that couldn't be explored before. "There's a big new conversation beginning," he says.

Journal reference: Science, DOI: 10.1126/science.1225542 and 10.1126.science.1226073

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