Scientists have thought for many years that our Moon was formed four and a half billion years ago… when a young planet about the size of Mars collided with the infant Earth… throwing matter off into space… which conglomerated together in orbit around a re-forming Earth… But no one has been able to fully explain why the Moon and the Earth seem to be made of exactly the same materials – and especially that they show identical types of isotopic oxygen. Now, researchers at the Southwest Research Institute have found a plausible scenario. If the impactor was much larger – say 4 to 5 times the mass of Mars, in other words about the same as the young Earth – the titanic collision’s effects would produce a new planet Earth… and a Moon of identical composition coming together from debris in orbit around it. This would have made an Earth that rotates more than twice as fast as it actually does… …but another team - from the SETI Institute and Harvard University – show that a resonance effect of the Sun’s gravity would have slowed the Earth Moon system down to the angular velocity we now see… …and an Earth and Moon with the composition they now have. For Space.com. I’m Dave Brody