Moon may have formed from collision of tiny 'moonlets'

Small 'moonlets' may have collided to form the Moon as we see it today, according to a new study which contradicts the prevalent theory that our natural satellite resulted from a giant impact between a small Mars-like planet and the ancient Earth. The study also claims that the Moon we see now is not Earth's first moon, but rather the last in a series of moons that orbited our planet.

The newly proposed theory by researchers at the Technion – Israel Institute of Technology and Weizmann Institute of Science in Israel 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 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 Hagai Perets from the Technion.

"It is likely that such moonlets were later ejected, or collided with Earth or with each other to form bigger moons," said Perets. To check the conditions for the formation of such mini-moons or moonlets the researchers ran 800 simulations of impacts with Earth. The new model is consistent with science's current understanding of the formation of Earth.

In its last stages of the growth, 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 Earth had many previous moons, a previously formed moon could therefore already exist when another moon-forming giant impact occurs," said Perets.

The tidal forces from Earth could cause moons to slowly migrate outwards - the current Moon is slowly doing that at a pace of about one centimetre 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 is likely that small moons formed through the process could cross orbits, collide and merge," said lead author Raluca Rufo from Weizmann. "A long series of such moon-moon collisions could gradually build-up a bigger moon - the Moon we see today," said Rufo. The study was published in the journal Nature Geoscience.