DescriptionSmashups of long time: could explain the strange composition of the Earth.The Earth and the other rocky planets have a composition that is different from the building blocks that formed them a long time ago. Collisions between these coalescing blocks, known as planetesimals, generated enough heat to vaporize large amounts of rock, most of which escaped into space. This loss of vapor mass, in turn, drastically changes the composition of the planets, which in fact explains the different composition of the Earth, said Remco Hin of the University of Bristol in England who directed one of the two new studies.This composition is marked by depletion of volatile rock-forming elements, which have a lower boiling point, which are more easily lost compared to the planetesimals. Scientists have a good idea of the planetesimal composition based on their analysis of certain meteorites known as chondrites, which are believed to be remnants of these ancient bodies. Hin and his colleagues measured the abundance of two magnesium isotopes Mg 24 and Mg 25 in a variety of chondrites, Earth rocks and Mars meteorites. Isotopes are versions of an element that differ in the number of neutrons in their atomic nuclei.