Small differences in sameness

摘要

Writing in Earth and Planetary Science Letters, Craddock et al.1 provide strong evidence that iron-isotope differences between planetary samples reflect the origins of the samples themselves rather than isotopic fractionation during planet formation. Although this is a negative result, it says a lot about planet and core formation. Meteorites provide an invaluable archive of the circumstellar disk from which the terrestrial planets and asteroids formed. With the advent of accurate mass spectrometry and its application to meteorite samples, it was soon shown that this disk had relatively uniform isotopic compositions. For example, the uranium found on Earth has the same atomic mass as the uranium found in meteorites from the asteroid belt that lies between Mars and Jupiter, showing that the mix of isotopes from diverse primordial circumstellar-disk