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New clues to impact that created the Moon

Paul Sutherland, Feature writer
Sep 1, 2012, 7:00 UTC

Sen—Astronomers generally accept that the Moon was formed 4.5 billion years ago from a massive collision between Earth and another body the size of Mars.

This mysterious impactor has even been given a name, Theia, and planetary scientists have recreated the catastrophic event on their computers.

While these simulations managed to reproduce many of the known properties of the Earth-Moon system, they also threw up something of a puzzle.

It seemed that the Moon was made up of material different from that which would be expected from the collision theory as it stood. Now the so-called Lunar Paradox finally may have been explained thanks to a new study reported in the science journal Icarus.

Predictions of what the Moon should be made of, produced by computer modelling of the collision, suggest that it should mostly be formed of material from the impacting world Theia. However, studies of actual rocks found that material on the Earth and the Moon are remarkably similar and elements on the Moon show identical properties to those here on Earth.

The puzzle here is that it is thought extremely unlikely that Theia and Earth would have had identical compositions because that of our world differs from all other worlds other than our Moon. Also, as Theia must have been smaller than Earth, the isotopic composition of some of its elements, such as silicon, should certainly have been different because that depends on the body's size.

Now a team of scientists from the University of Bern, Switzerland, have now made a significant breakthrough in finding a plausible answer to this Lunar Paradox.

They looked at different models for the geometry of the collision between Earth and Theia than those previously considered. The new simulations also tested new impacts configurations such as those termed "hit-and-run collisions," where a significant amount of material is lost into space on orbits not bound to the Earth.

The results, though not perfect, gave a much closer match to the make-up of the Earth-Moon system that is found today, leading the scientists to think they are on the right track.

Lead author of the study, Andreas Reufer, said: "Our model considers new impact parameters, which were never tested before. Besides the implications for the Earth-Moon system itself, the considerably higher impact velocity opens up new possibilities for the origin of the impactor and therefore also for models of terrestrial planet formation."

Alessandro Morbidelli, one of the Icarus’ editors, adds: "While none of the simulations presented in their research provides a perfect match for the constraints from the actual Earth-Moon-system, several do come close.

"This work, therefore, suggests that a future exhaustive exploration of the vast collisional parameter space may finally lead to the long-searched solution of the Lunar Paradox."