Giant Impact Gave Birth to the Moon

Analysis of lunar samples returned by the Apollo astronauts confirms previous indications that the ratios of the different types, or isotopes, of oxygen in the Moon?s soil are similar to those of Earth. It is a finding that is consistent with the giant impact model for the Moon?s formation. Computer simulations offer additional proof of the theory that the Moon was born out of a big collision between the Earth and a Mars-sized planet.

This would have happened when the solar system was still very young. Computer simulations of the impact suggest that the Moon would have been formed mostly from material from the planet that hit the Earth, which was itself destroyed in the collision. But if this was the case, then scientists are puzzled as to why the ratios of isotopes of oxygen for Earth and Moon rocks are almost identical.

The best way to explain this, according to a team led by Ernst Wiechert of ETH Zentrium, Zurich, Switzerland, is that the impactor was almost identical in composition to the proto-Earth, having formed a similar distance from the Sun as our own planet.

This ?giant impact? was first proposed in the 1970s, but now scientists at the Southwest Research Institute and the University of California-Berkeley have put together a theory that accounts for the Moon?s creation as well as the fact that a day on Earth is 24 hours long.

?The previous models of the impact theory had identified impacts capable of producing the Moon, but they were unable to account for all of these features of the Earth-Moon system simultaneously,? says researcher Robin Canup. ?By showing that just one impact can do the job, what we?re doing in effect is demonstrating a more probable scenario.?

The new research is based on the idea of an enormously energetic crash between Earth and a planet the size of Mars, which is about half Earth?s size. The energy unleashed by this collision some 4.5 billion years ago would have been enough to destroy the incoming planet and melt Earth all the way through, Camp says. There would also have been some vaporized rock debris resulting from the crash, which would have started orbiting Earth.

?Once the orbiting debris cooled, it?s from that stuff that the Moon then coalesced,? she says. The whole process took less than 100 years, an incredibly short time in planetary terms.

The glancing angle of the collision — perhaps 40 degrees or so — caused Earth to start spinning, according to Canup, but much faster than it does now. In those early times, an Earth day would have lasted only five hours.

The Moon was also thought to be much closer to Earth in those times than it is now. The Earth and the Moon are continuing to drift apart, by several inches a year, Canup says. As the Moon moved away from Earth, the Earth?s rotation slowed down.

The planet that crashed into the Earth is long gone, one of a dozen or more mini-planets in the process of formation that never quite made it in our solar system. Most of these mini-planets were about the size of Mars, Canup says.

?Everything has really come together, because it looks like the type of impact you need to explain the Earth?s mass and initial spin rate also tends to naturally place a sufficient amount of material into orbit to form a Moon with the size of our Moon.? An animation of the crash can be seen here.

Insight: In his book the Secret School, Whitley Strieber described precisely this scenario. The book was published in 1997, and the information came from memories of his time in the secret school as a child. He learned this at about age ten, in 1955. The Secret School is now out of print, so we are providing the relevant excerpt. To read it, click here.

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