The moon rises over the Toronto city skyline as seen from Milton, Ontario.
(photo credit: REUTERS)
The moon was not created from a single cataclysmic crash between a young Earth and another object the size of Mars – the common belief among astronomers – but from several run-ins with smaller objects.
The idea was suggested by scientists from the Weizmann Institute of Science in Rehovot and the Technion-Israel Institute of Technology in Haifa, who performed hundreds of simulations and published their findings in the latest issue of Nature Geoscience.
This alternate scenario would have produced smaller “moonlets” that would have eventually coalesced into the single moon we have today, said research student Raluca Rufu and Prof. Oded Aharonson of Weizmann Institute’s earth and planetary sciences department and Dr. Hagai Perets of the Technion.
They pointed out that the accepted explanations for the moon’s formation 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 – thus the theories, argued the researchers – turn out to be “improbable.”
They wondered 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, they said. In addition, 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, about one-eightieth 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’s Chemfarm computer cluster, which has more than 5,000 processor cores.
“The new scenario does not require finely tuned initial conditions,” said Rufu, “and if the smaller moonlets, as we think, were drawn into the same orbit, they could have merged over millions of years.”
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 preexisting 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,” said Perets.
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