Did The Earth Once Have Two Moons?

By James Donahue

Of all the fun theories being kicked around by astronomers concerning the possible history of our solar system, this latest idea, recently published in the journal Nature, suggests that the Earth once had two smaller moons orbiting around it.

Authors Erik Asphaug of the University of California, Santa Cruz, and Martin Jutzi of the University of Bern, Switzerland, say that the two moons may have collided. This, they say, would explain why our moon’s far side is bulked-up and slightly lopsided, and more hilly than the side that always faces Earth.

Astronomers have been busy utilizing all of the new data gained by the many NASA orbiters that have spent the past decades photographing, measuring and studying the planets and moons in our solar system. They have been trying to make sense out of all the oddities and surprise discoveries that have been collected.

There have been a lot of interesting theories, not all of them speaking or writing in unison. If there is anything they can agree on, however, is that nothing is what scientists once thought to be true is true.

The Asphaug/Jutzi theory suggests that sometime about 4.4 billion years ago, there were two moons, one much larger than the other, that were in orbit around Earth. They may have been formed by a collision of two hot planets that either became the Earth, or perhaps bounced off to form what we now know as Venus.

The collision tossed two large chunks of hot molten material from the super-heated new planet into orbit, the astronomers say. The larger piece was three times wider and 25 times heavier than the small moon. They were close enough together that the larger moon’s gravity eventually drew the two bodies together.

Asphaug says the collision was more of a slow-motion “splat” rather than a dramatic and fiery crash. Consequently there was no melting of the rocks, but rather a crumbling of the smaller moon into material that formed the rocky, hilly and lop-sided dark side of the moon that we know today.

Because the moon does not spin, and the face we see is always facing Earth, details as to the shape and content of the mystery side of the moon were not known to us until orbiter cameras and the Apollo astronauts began flying around it in space craft.

That odd shape may explain why the moon does not spin and always displays the same “face” to earthlings

Jutzi said the research conducted by team was an attempt to explain why the opposite side of the moon appeared so different from the familiar side astronomers have been looking at for years. He said they noticed that the landscape on the dark side looked as if something had been added to the surface.

They also noted that Earth is the only planet in the solar system with only a single moon. Venus and Mercury, orbiting closer to the Sun, have no moons. Mars has two moons. Saturn and Jupiter, the two gas giants, have more than 60 moons, each. Even icy little Pluto entertains two moons.

The team used computer simulations of cosmic collisions to test their theory of a collision of two moons. They found that a relatively slow collision could have produced a pile of rubble as appears on the back of the moon, without leaving a large crater.

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