The moon may have been adopted by our planet instead of descended from it.
If a new twist on a decades-old theory is right, conditions in the early solar system suggest the moon formed inside Mercury's orbit and migrated out until it was roped into orbit around Earth.
The idea flies in the face of scientific consensus, known as the giant impact hypothesis, which holds that the moon formed from red-hot debris left over after a Mars-sized object collided with Earth around 4.5 billion years ago.
However, the moon has several curious traits that go unexplained with that theory, and Robert Malcuit of Denison University has argued for decades for an alternative view of our moon's history.
Malcuit's version of events is tantamount to cosmic blasphemy, but scientists have recently found 4 billion-year-old minerals in Australia that suggest our planet was too cool to have sustained a cataclysmic moon-forming impact early in its history.
"Everything in the giant impact model is hot, hot, hot," he said. "It's incompatible with what we see in the geologic record. Earth is cool enough at that time to have ocean water on its surface."
Malcuit's computer modeling studies, which he has worked on since the 1980s, show that it is possible for Earth's gravitational pull to capture the moon.
At first, the moon's orbits would have been highly elliptical, swinging close to Earth and then far away about eight times a year.
The gravitational pull from each pass would have stretched the planet 18 to 20 kilometers (11.2 to 12.4 miles) near the equator, churning the hot mantle and crust. Rocks closer to the poles, like those found today in Australia, would have been spared. The upper layers of the newly-captured moon would have melted from gravitational friction, until the satellite's orbit stabilized about 3 billion years ago.
Malcuit presented his theory at the annual meeting of the Geological Society of America in Portland, Ore.
Traditionally, scientists cite the moon's low density and a lack of iron as reasons why it came from Earth — the giant impact skimmed light material off Earth's upper layers and flung it into orbit.
"I think this it is highly unlikely," that Malcuit's idea is correct, Jack Lissauer of the National Aeronautics and Space Administration in Moffett Field, Calif., said. "Capture is very, very difficult. You have to have just the right velocity and very special parameters to all be just right."
Lissauer allowed that the current giant impact theory of the moon's formation may yet be revised, even replaced, but probably not by Malcuit's capture model. The fact that Earth was cool 4 billion years ago doesn't mean the moon was captured.
"Heat from the impact dissipated very quickly," he said. "It wouldn't take 100 million years, and it certainly wouldn't take 500 million. The impact is not going to affect Earth at 4 billion years ago."