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Earth’s Second Moon

August 3, 2011

Astronomers say that the Earth may have once had two moons, until the current moon devoured the other.

A new study by planetary scientists at the University of California – Santa Cruz said that the far side of the moon, known as the lunar farside highlands, may be the remains of a collision with a similar companion moon.

The near side of the moon is low and flat, while the far side is mountainous with a thicker crust. 

The new study suggests that Earth’s once second moon eventually fell back onto the current moon and coated one side with an extra layer of solid crust miles thick.

“Our model works well with models of the moon-forming giant impact, which predict there should be massive debris left in orbit about the Earth, besides the moon itself. It agrees with what is known about the dynamical stability of such a system, the timing of the cooling of the moon, and the ages of lunar rocks,” Erik Asphaug, professor of Earth and planetary sciences at UC Santa Cruz, said in a press release.

The team used computer simulations of an impact between the moon and a smaller companion to study the dynamics of the collision and track the evolution and distribution of lunar material in its aftermath.

The researchers said most of the colliding material is piled onto the impact hemisphere as a thick new layer of solid crust, forming a mountainous region comparable in extent to the lunar farside highlands.

Asphaug and his colleagues believe the companion moon was initially trapped at one of the gravitationally stable “Trojan points” sharing the moon’s orbit, and became destabilized after the moon’s orbit had expanded far from Earth.

“The collision could have happened anywhere on the moon,” UCSC postdoctoral researcher Martin Jutzi said in a press release. “The final body is lopsided and would reorient so that one side faces Earth.”

The model could explain the composition of the moon’s crust, which is dominated by potassium, rare-earth elements, and phosphorous. 

“The fact that the near side of the moon looks so different to the far side has been a puzzle since the dawn of the space age, perhaps second only to the origin of the moon itself,” Francis Nimmo, a professor of Earth and planetary sciences, said.

“One of the elegant aspects of Erik’s article is that it links these two puzzles together: perhaps the giant collision that formed the moon also spalled off some smaller bodies, one of which later fell back to the Moon to cause the dichotomy that we see today.”

The study will be published in the August 4 issue of Nature and was supported by NASA’s Planetary Geology and Geophysics Program

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