How Earth's Previous Moons Collided to Form the Moon

The newly proposed theory by researchers Prof. Hagai Perets, of the Technion, and Weizmann Institute Profs. Raluca Rufo (lead author)and 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 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.

Credit  Weizmann Institute of Science

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 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.

Credit  Weizmann Institute of Science

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

 

Research student Raluca Rufu and Prof. Oded Aharonson of the Weizmann Institute’s Earth and Planetary Sciences Department point out that the accepted explanations for the formation of our Moon rely on highly specific initial conditions – for example, a collision with an object of a particular size traveling at a defined velocity and hitting Earth at a specific angle. 

 

Furthermore, in a typical impact different proportions of that object would have ended up in the Earth and the Moon, leaving a detectable difference between the bodies. But various chemical analyses of the Moon’s makeup, taken from samples returned by astronauts, reveal that it is nearly identical to that of Earth. In other words, there is no trace of the large body that supposedly hit Earth, and the theories, say the researchers, turn out to be improbable.

Rufu and Aharonson, together with Dr. Hagai Perets of the Technion – Israel Institute of Technology, asked whether a number of smaller collisions might better explain what happened several billion years ago, when the solar system was taking shape. Such smaller bodies would have been more prevalent in the system, and thus collisions with the smaller objects would have been more likely. Small, high-velocity collisions could also mine more material from Earth than a single, large one. 

 

In addition, explains Aharonson, if a number of different bodies collided with Earth over a period of millions of years, their different chemical signatures – for example, ratios of oxygen-16 to its heavier cousins, oxygen-17 and -18 – might even out, masking the traces of the various collisions.

The collisions – with small planets one tenth the mass of Earth to space rocks the size of the Moon, a hundredth the mass of Earth – would have sent clouds of rubble, melt and vapor into orbit around the early Earth. These, according the simulations the scientists created, would have cooled and agglomerated into small moonlets that, in time, could have merged into one.

To test this scenario, the group ran around 800 impact simulations on the Weizmann Institute of Science’s Chemfarm cluster, which has more than 5000 processor cores.

“The new scenario does not require finely tuned initial conditions,” says Rufu, “and if the smaller moonlets, as we think, were drawn into the same orbit, they could have merged over millions of years.”

“We are now running further simulations to try to understand how the smaller moonlets produced in these simulations might have coalesced to form our Moon,” adds Aharonson.

 

Contacts and sources:

Weizmann Institute of Science
The Technion-Israel Institute of Technology