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August 04, 2011

Moonstruck: The 'Big Splat Theory' on How Earth's Two Moons Became One



When we talk about things that go bump in the night, the moon turns out to be among them.  Eons ago when the Earth was formed, our moon had a clumsy sibling. They crashed into each other, in a “Big Splat,” and well, the rest is history.  

As reported by Nature on August 3, that’s the new scenario two astronomers have suggested, based on a sophisticated computer model of the ancient sky.

Erik Asphaug, a planetary scientist at the University of California-Santa Cruz and his colleague, Martin Jutzi, now of the University of Berne, have created a computer model showing that the Moon's current landscape can be explained by a collision with a sister moon about one-thirtieth the Moon's mass, or around 620 miles in diameter.

Both satellites would have formed from debris that was ejected when a Mars-size protoplanet bumped into the newborn Earth. Although conventional wisdom holds that our bigger Moon rapidly rejected any rivals – either absorbing them or gravitationally impelling them – the new theory suggests that one sibling moon survived, parked in a gravitationally stable point in the Earth–Moon system.

Meanwhile, tidal forces from Earth made both moons migrate outward. When they reached about one-third of the Moon's present distance (a process that would take tens of millions of years), the Sun's gravity become a player in their orbital dynamics.

The scars of that scuffle are still there to be seen. The Moon's visible side is dominated by low-lying lava plains, whereas its far side is composed of highlands. But the contrast is more than skin deep. The crust on the far side is 31 miles thicker than that on the nearside. The nearside is also richer in potassium, rare-earth elements, and phosphorus – components collectively known as KREEP. Crust-forming computer models show that these would have been concentrated in the last remnants of subsurface magma to crystallize as the Moon cooled.

What this suggests, Asphaug says, is that something “squished” the late-solidifying KREEP layer to one side of the Moon, well after the rest of the crust had hardened. An impact, he believes, is the most likely explanation.

“By definition, a big collision occurs only on one side,” he said, “and unless it globally melts the planet, it creates an asymmetry.”

“The physics is really surprisingly similar to a pie in the face,” Asphaug commented.

The theory was the buzz this week in Woods Hole, Massachusetts, at a conference of scientists working on NASA's next robotic mission to the moon, said H. Jay Melosh of Purdue University (News - Alert).

“We can't find anything wrong with it,” Melosh told the Associated Press. “It may or may not be right.”

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Cheryl Kaften is an accomplished communicator who has written for consumer and corporate audiences. She has worked extensively for MasterCard (News - Alert) Worldwide, Philip Morris USA (Altria), and KPMG, and has consulted for Estee Lauder and the Philadelphia Inquirer Newspapers. To read more of her articles, please visit her columnist page.

Edited by Jennifer Russell

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