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Planetary science

Mars-sized object probably created the Moon

16 Aug 2001

Our moon almost certainly formed from matter ejected from the Earth after a collision with a large body. Previous studies have failed to determine what kind of object this might be, but a highly accurate model has now found that a collision with a body the size of Mars could account for the features of the Earth and the Moon. According to Robin Canup and Erik Asphaug, the discovery further strengthens the impact theory because objects of this size were reasonably abundant in the early solar system (R Canup and E Asphaug 2001 Nature 412 708).

Collision course

The large angular momentum of the Earth-Moon system suggests that the Earth rotated once every five hours before the Moon formed. The motion of the Moon also shows that it is relatively light, which means that it probably contains a smaller proportion of iron than the Earth. These widely accepted ideas limit the kind of impact that could have given rise to the Moon.

Earlier studies identified a class of bodies with a large angular momentum that could have produced a Moon with the mass we observe today. But such an object must have collided with a well-developed Earth, and would not have increased its rotation period to 24 hours. This theory requires a second impact – for which there is little evidence – to have removed angular momentum from the system.

A collision with a second type of object could have resulted in the current rotational period of the Earth, but would have taken place before the Earth was fully formed. This means the Earth and Moon must have continued to gather similar matter after the collision. But the matter must have been rich in iron to explain the composition of the Earth, and this cannot account for the low density of the Moon today.

The new simulations – carried out at the Southwest Research Institute in Colorado and the University of California at Santa Cruz – show that a collision with a body about the size of Mars could lead to an iron-poor Moon and the current dynamics of the Earth and the Moon. The 36 calculations were based on ‘smooth particle hydrodynamics’, which modelled the dynamics of different impacts and the tens of thousands of ejected fragments.

“Our model requires a smaller impactor than previous models, making it more likely that the Earth should have a Moon as large as ours”, says Asphaug.

“The type of impact we now find can best produce the Earth-Moon system was erroneously ruled out a decade ago”, Canup told PhysicsWeb, “but our studies have a resolution ten times better than those early works”.

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