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Water Cooler – Back When the Earth Had Two Moons


Just the other day I was talking about how science continues to look for answers, even when it has an apparent one that is nice and neat. And here we go again! This time it is not Paleobiology, but Astronomy.

As everyone but Bill O’Reilly knows our moon was formed about 4.5 billion years ago when a Mars size planet, which was probably co-orbiting with the Earth, slammed into our planet. Impact liquefied both bodies and threw off a big gobbet of matter that coalesced into the Moon.

This is a very good theory and as far as we can tell it is what happened, but there is a problem. If a big mass of molten rock was flung out from the Earth and then was brought together by gravity, why is the moon so lopsided?

It might surprise you to know that the moon is not a very good sphere, after all the part that we grow up seeing is very round and has lots of relatively smooth places. That is until you see the far side of the Moon. The picture above is of the far side of the Moon (called that because the Moon is tidally locked, meaning it dose not rotate and the same side always faces the Earth)

As you can see it is a very crated and mountainous place. The question is why? Well the L.A. Times is reporting that a scientist from the University of California, Santa Cruz, by the name of Erik Asphaug (you think he got a lot of teasing about that last name?) has come up with a pretty good idea. At one time there were two moons orbiting the Earth.

Ashpuag an his colleague Martin Jutzi of the University of Bern has postulated that when the molten rock that formed the Moon was flung out from the Earth, not all of it coalesced into a single body. They believe that the when the main body formed, there was a lot of a other debris that was not captured.

Another smaller body was able to form, at the Lagrange point between the Earth and the Moon. Lagrange points are around orbiting bodies that have a basic gravitational stability. They most well known ones are L4 and L5 which precede and follow the Earth (and every other body orbiting another) by 60 degrees. Below is a chart of the Earth’s Lagrange points

The Moon has similar points and it theorized that the second moon would have formed in the L2 position on the far side of the Moons orbit. The two scientists have developed a computer simulation of what they think happened next. For about a half billion years there were two moons in the sky, but the gravity from the Sun would have eventually disrupted a mass sitting at the Lunar L2 point and then it was just a matter of time until it deorbited and collided with the Moon.

The second moon would have hit relatively slowly, somewhere between 4,500 and 6,700 miles per hour, the astronomers estimate. At that speed it would have taken as long as three hours for the collision to be complete.

This kind of collision would explain the massive cratering on the Moon (which happened approximately 4 billion years ago) and the model also gets the thicker than average surface we’ve observed on the far side.

Is this the final word on Lunar formation? Of course not! It’s science after all! Still as we look for solutions to our observed data we gain a picture of the early Solar System that is the serene gathering of small particles into planets and moons. Instead there were planetary and sub-planetary bodies whizzing around and colliding. While I would not want to be on any of these stellar bodies, wouldn’t it be really cool to see a time lapse montage?

What is on your minds tonight Firedogs? The floor is yours!

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Bill Egnor

Bill Egnor

I am a life long Democrat from a political family. Work wise I am a Six Sigma Black Belt (process improvement project manager) and Freelance reporter for