At www.sciencedaily.com/releases/2010/12/101209141130.htm …. worlds in collision – actually bashing up against each other – is one hypothesis of the early solar system now being floated. An object the size of Mars, it is said, smashed into the earth and ripped out its guts – producing the Moon. Until now it was thought the Moon owes its existence to one random collision event – but why be shy? It is now being suggested there were lots and lots of collisions between earth and pieces of space rock – but all this went on some 4.5 billion years ago. This change of opinion, it seems, came about as a result of computer modelling. The problem was that such a huge single event should have depleted the mantle of metal loving elements – but the mantle is full of them. Therefore, unwilling to let loose the idea of such a massive collision event, the answer to the anomaly must be that such elements were replenished by a bout of impacts by planetisimals – the building blocks of the planets (or at least, some of them). In other words, the facts did not fit the idea the Moon was ejected out of the earth so a series of lesser impacts must have occurred in order to explain the anomaly. Indeed, the model went on to show that the planetisimals that struck the earth would have had to be 1500 to 2000 miles in diameter while those that hit the Moon could have been no more than 150 to 200 miles across. The size of the impactor was deduced in order to account for the observed enrichment in elements, on earth, but the size of the impactor on the moon was restricted by the size of its craters. Further, models were created in order to explain how such planetisimals formed and evolved. This showed there was a propensity for the bigger ones to acrete the smaller ones. These are, they claim, the asteroids – such as Ceres, Pallas, and Vesta.
There is even more – the larger projectiles that struck the earth were capable of modifying its spin axis – by as much as 10 degrees. Wow. The full paper is available in Science December 10th issue, 'Stochastic Late Accretion to Earth, the Moon, and Mars'.