New Israeli Research Sheds Light on the Moon’s Origins

Study, published in Nature, proposes that the moon was formed as a result of a collision of the Earth and a major celestial body called Theia.

Ido Efrati
Ido Efrati
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A simulation of the collision that created the moon.
A simulation of the collision that created the moon.Credit: Haggai Peretz / Technion
Ido Efrati
Ido Efrati

The question of how the moon was created has occupied generations of researchers who study the origins of the solar system. The accepted theory has been that it was formed around the time of the creation of the Earth, about four and a half billion years ago, as part of the creation of our entire solar system. How that happened, however, has not been fully explained, and new research, conducted by Israeli scientists and a researcher in France, has shed new light on the creation of the moon.

The study, results from which appeared over the weekend in the publication “Nature,” was conducted by Asst. Prof. Hagai Perets and post-doctoral student Alessandra Mastrobuono-Battisti of the Technion Israel Institute of Technology in Haifa, in cooperation with Dr. Sean Raymond of the University of Bordeaux.

It proposes that the moon was formed as a result of a collision of the Earth and a major celestial body called Theia, which in turn was apparently created through a process similar to that which resulted in the Earth’s formation and therefore had a similar chemical composition.

“Since the 1980s, the giant impact hypothesis has taken hold in the scientific community,” Perets explained. “Based on this model, the moon was created through a collision between Theia and the primordial Earth. Theia is a body about half the size of Earth with a much smaller mass. Part of the matter emitted as a result of the collision settled back onto the Earth. A portion was shot out into space and the rest began to orbit in separate pieces around the Earth and gradually coalesced into one object, the moon.”

The moon remained trapped within the gravitational pull of the Earth, and like the Earth around the sun, the moon cooled and remains in an elliptical orbit around us, according to this theory.

One of the major questions that has occupied astrophysicists researching the moon’s creation is whether this newborn celestial body was more like Mom or Dad, in other words, whether it was from matter that broke off the Earth or from Theia. Simulations and research on collisions of this type gave rise to the assumption that moons are formed from the body that causes the collision, in this case Theia, and not the object of the collision, such as the Earth. But the similarity of the composition of the Earth and the moon, following analysis of samples taken from the moon by Apollo space missions, supported the belief that the moon was created from matter from the Earth, and not from Theia.

The question has vexed theoreticians for four decades, but the new research proposes a theory that would resolve the contradiction. Even before the giant impact, it is suggested, the Earth and Theia had similar composition.

The team including the Technion researchers conducted computer simulations in connection with the formation of about 40 solar systems around the galaxy and found that in many instances there was a high degree of similarity between planets and other bodies that collided with them. Therefore, the researchers suggest, the moon was created from Theia, but resembles the Earth only because of the similarity of the composition of Theia and the Earth itself.

“Theia and the Earth grew in the same area,” explained Mastrobuono-Battisti, “and therefore were formed from similar matter. At a certain stage they collided with one another, an event that caused matter to be projected from Theia, and some of this matter ultimately became the moon. We therefore actually settle what had been seen up to now as a contradiction between how moons are created and the similarity between the Earth and the moon, which erroneously caused the scientific community to believe that the moon was created from the Earth and not from Theia.”

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