By measuring the types and amounts of different isotopes present in a range of meteorites, Richard C. Greenwood of Open University in Milton Keynes, U.K. and his colleagues reconstructed their histories. The studied samples were formed from their parent asteroids--the 530-kilometer wide Vesta and a second unnamed asteroid--more than four billion years ago. The researchers determined that all the meteorites from Vesta exhibit the same ratio of oxygen isotopes, as did the meteorites from the second source. The find suggests that both asteroids experienced widespread melting with more than 50 percent of each object becoming liquid.
In the magma oceans, other elements in the asteroids would have separated out according to mass, the researchers report. The resulting layered composition of such an asteroid could have contributed to the uneven distribution of elements among the planets, they say, if developing protoplanets crashed into the asteroid once it had cooled. In this scenario, the elements abundant in the crust would be transferred to one planet and those present in its core would end up on another. According to the report, Earth's high magnesium to silicon ratio is one anomalous feature that could be explained under these circumstances.