How Many Moons Became One

An artistic depiction of a collision between two planetary bodies that will form a new moon, while a pre-existing moon already orbits the proto-Earth.” Credit: Hagai Perets. Real images of Mars and Ganymede and artist image of a planet courtesy of NASA were used in the picture construction.

The Moon, and the question of how it was formed, has long been a source of fascination and wonder. Now, a team of Israeli researchers suggests that the Moon we see every night is not Earth’s first moon, but rather the last in a series of moons that orbited the Earth in the past. The findings by the team of researchers from the Technion-Israel Institute of Technology and the Weizmann Institute of Science are published in Nature Geoscience.

The newly proposed theory by researchers Assistant Prof. Hagai Perets, of the Technion, and Weizmann Institute Raluca Rufu (lead author) and Prof. Oded Aharonson, runs counter to the commonly held “giant impact” paradigm that the moon is a single object that was formed following a single giant collision between a small Mars-like planet and the ancient Earth.

“Our model suggests that the ancient Earth once hosted a series of moons, each one formed from a different collision with the proto-Earth,” said co-author Assistant Prof. Perets. “It’s likely that such moonlets were later ejected, or collided with the Earth or with each other to form bigger moons.” To check the conditions for the formation of such mini-moons or moonlets the researchers ran 800 simulations of impacts with the Earth.

The new model is consistent with science’s current understanding of the formation of the Earth. In its last stages of the growth, the Earth experienced many giant impacts with other bodies. Each of these impacts contributed more material to the proto-Earth, until it reached its current size.

“We believe the Earth had many previous moons,” said Assistant Prof. Perets, who added that, “a previously formed moon could therefore already exist when another moon-forming giant impact occurs.”

The tidal forces from the Earth could cause moons to slowly migrate outwards (the current Moon is slowly doing that at a pace of about 1 cm a year). A pre-existing moon would slowly move out by the time another moon forms. However, their mutual gravitational attraction would eventually cause the moons to affect each other, and change their orbits.

”It’s likely that small moons formed through the process could cross orbits, collide and merge”, said Rufu, and Perets summarizes “A long series of such moon-moon collisions could gradually build-up a bigger moon – the Moon we see today.”

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