Scientists Propose New Model of Moon Formation

Two Harvard scientists proposed a new model to explain the formation of the Moon in a study published this Wednesday in Science Magazine.

The professors presented analysis that could rationalize the “giant impact model,” which hypothesizes that the Moon formed from the coalescing of a disk of debris following the collision of a small Mars-sized celestial body and the early proto-Earth.

While earlier models contend that the Moon mainly consists of material from the other non-Earth object, recent geochemical studies suggest that the isotopic fingerprint of the Moon and the Earth are far too similar for the Moon to have formed from another body.

The new model, developed by Earth and Planetary Science professor Sarah T. Stewart-Mukhopadhyay and EPS post-doctoral fellow Matija Cuk, indicates that the Earth may have been rotating faster than previously thought—possibly fast enough for the impact to eject enough Earth material to create the Moon.

“We were looking through all these papers and identifying things that we thought would be the constraints we could remove,” said Stewart-Mukhopadhyay, who started working on the question two years ago.


They settled on one constraint, changing the speed of the rotation of the Earth, tweaking the model from a five-hour day to a two to three-hour day.

“The main thing was to throw out conventional thinking and go with the fast-spinning earth and see where it took us,” Stewart-Mukhopadhyay said. “No one else had ever considered [this] case, and they weren’t allowed to because angular momentum would not be conserved to today.”

In their paper, the two said that the gravitational interactions of the Earth’s and the Moon’s orbits can slow the planet’s rotation faster than previously believed. “It was very embarrassing that we didn’t have a single working theory of how the Moon formed, and now we do,” Stewart-Mukhopadhyay pointed out.

Stewart-Mukhopadhyay and Cuk’s work could have significant implications for the field. “If we knew what forces had to work for this to work out, we can maybe reverse engineer,” Cuk said.

Understanding these forces can help scientists understand the processes that shaped early Earth, Stewart-Mukhopadhyay said. This research will also establish a foundation for understanding planet formation in general.

Other scientists are already building on Stewart-Mukhopadhyay and Cuk’s model, which Cuk said will require much more research to refine.

“The other thing that we didn’t do that people should be doing is forming the Moon from the disk,” Cuk said. “My simulation starts thousands of years later and assumes the Moon has already formed—we’re missing the middle.”