Why Earth and the Moon have similar compositions

Los Angeles Times : Based on the physics of the Earth–Moon system, it is generally believed that the Moon was created when another planetary body, called Theia, collided with Earth some 4.5 billion years ago. Yet if the Moon was formed from remnants of Theia, why are the chemical compositions of Earth and the Moon so similar? To explain that long-standing mystery, Alessandra Mastrobuono-Battisti of the Technion–Israel Institute of Technology and her colleagues ran computer simulations of the creation of the solar system. They found that while different planets formed in the same simulation will most likely have distinct compositions, the compositions of giant impactors, like Theia, are more likely to be similar to the planets they collide with. Therefore, the material that made up Theia and Earth may very well have been nearly identical.