Second Moon May Have Orbited Earth Billions of Years Ago

by Nancy Atkinson on August 3, 2011

Four snapshots from the computer simulation of a collision between the moon and a smaller companion moon show most of the companion moon is accreted as a pancake-shaped layer, forming a mountainous region on one side of the moon. Credit: M. Jutzi and E. Asphaug, Nature.

It’s a view science fiction fans could only hope for: twin moons in the night sky above Earth. But it might have been reality about 4 billion years ago. A new model suggests the lunar farside highlands could have been created from a collision with a smaller companion moon in what scientists from the University of California, Santa Cruz are calling “the big splat.”

Why the near and far sides of the Moon are so different has long puzzled planetary scientists. The near side is relatively low and flat, while the topography of the far side is high and mountainous, with a much thicker crust.

We actually have a somewhat lopsided Moon.

The new study, published in the August 4 issue of Nature, builds on the “giant impact” model for the origin of the moon, in which a Mars-sized object collided with Earth early in the history of the solar system and ejected debris that coalesced to form the moon.

According to the new computer model, the second moon around Earth would have been about 1,200 kilometers (750 miles) wide and could have formed from the same collision. Later, the smaller moon fell back onto the bigger Moon and coated one side with an extra layer of solid crust tens of kilometers 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,” said Erik Asphaug, professor of Earth and planetary sciences at UC Santa Cruz. “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.”

Other computer models have suggested a companion moon, said Asphaug, who coauthored the paper with UCSC postdoctoral researcher Martin Jutzi.

A previous collision with a smaller companion could explain why the Moon's two sides look so different. Credit: Martin Jutzi and Erik Asphaug

Asphaug and Jutzi used computer simulations to study the dynamics of the collision between the Moon and a smaller companion, which was about one-thirtieth the mass of the “main” moon. They tracked the evolution and distribution of lunar material in its aftermath.

The impact between the two bodies would have been relatively slow, at about 8,000 kph (5,000 mph) which is slow enough for rocks not to melt and no impact crater to form. Instead, the rocks and crust from the smaller moon would have spread over and around the bigger moon.

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

He and Jutzi hypothesize that 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,” Jutzi said. “The final body is lopsided and would reorient so that one side faces Earth.”

The model may also explain variations in the composition of the moon’s crust, which is dominated on the near side by terrain comparatively rich in potassium, rare-earth elements, and phosphorus (KREEP). These elements, as well as uranium and thorium, are believed to have been concentrated in the magma ocean that remained as molten rock solidified under the moon’s thickening crust. In the simulations, the collision squishes this KREEP-rich layer onto the opposite hemisphere, setting the stage for the geology now seen on the near side of the moon.

While the model explains many things, the jury is still out among planetary scientists as to the full history of the Moon and what really happened. Scientists say the best way to figure out the Moon’s history is to get more data from lunar orbiting spacecraft and – even better – sample return missions or human missions to study the Moon.

Sources: Nature, UC Santa Cruz

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Nancy Atkinson is Universe Today's Senior Editor. She also works with Astronomy Cast, and is a NASA/JPL Solar System Ambassador.

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