A physicist, a chemist and a mathematician walk into a bar. It sounds like the start of a bad joke, but in my case, it was the start of an idea that could reshape how scientists think about the history of the Moon.
The three of us were all interested in the Moon, but from different perspectives: As a geophysicist, I thought about its interior; Thorsten Kleine studied its chemistry; and Alessandro Morbidelli wanted to know what the Moon’s formation could tell us about how the planets were assembled 4.5 billion years ago.
When we got together to discuss how old the Moon really was, having those multiple perspectives turned out to be crucial.
How did the Moon form?
At a conference in Hawaii in the late 1980s, a group of scientists solved the problem of how the Moon formed. Their research suggested that a Mars-size object crashed into the early Earth, jettisoning molten material into space. That glowing material coalesced into the body now called the Moon.
This story explained many things. For one, the Moon has very little material that evaporates easily, such as water, because it began life molten. It has only a tiny iron core, because it was mostly formed from the outer part of the Earth, which has very little iron. And it has a buoyant, white-colored crust made from minerals that floated to the surface as the molten Moon solidified.
The glowing, newly formed Moon was initially very close to the Earth, at roughly the distance that TV satellites orbit. The early Moon would have raised gigantic tides on the early Earth, which itself was mostly molten and spinning rapidly.
These tides took energy from the Earth’s spin and transferred some to the Moon’s orbit, slowly pushing the Moon away from the Earth and slowing the Earth’s spin as they did so. This motion continues today – the Moon still recedes from the Earth about 2 inches per year.
An artist’s impression of what the Moon looked like during the tidal heating event. There would have been intense volcanism everywhere. The early Earth would have loomed much larger in the sky because it was closer.
MPS/Alexey Chizhik
As the Moon moved away, it passed through particular points where its orbit temporarily became disturbed. These orbital disturbances were an important component of its history and are a key part of our hypothesis.
When did the Moon form?
When the Moon actually formed and receded away from the Earth is a thorny issue.
Thanks to the Apollo astronauts, scientists have a collection of Moon rocks, which they can measure the age of. The oldest rocks are all about 4.35 billion years old, which is roughly 200 million years after the birth of the solar system.
Many geochemists, like my colleague Thorsten Kleine, suggested (not unreasonably) that the age of these rocks is the same as the age of the Moon.
But people like Alessandro Morbidelli, who study planet formation, didn’t like this answer very much. In their…
