January 12, 2017

It is known that Earth has just one Moon. But now Israeli scientists are raising the idea that this celestial orb may have actually had several moons that at some point in time mashed together to form the one Moon we now see.

A group at the Weizmann Institute of Science ran some 800 impact simulations on the institute’s Chemfarm computer cluster, which has more than 5000 processor cores.

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

Rufu and Prof. Oded Aharonson of the Weizmann Institute’s Earth and Planetary Sciences Department highlight 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.

They note that 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.

The scientists say smaller bodies would have been more prevalent in the system, and collisions with the smaller objects would have been more likely.

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 to the simulations the scientists created, would have cooled and agglomerated into small moonlets that, in time, could have merged into one.

“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,” says Aharonson.

The new study was recently published in the journal Nature Geosciences.

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Jason Harris

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