Freie Universitaet Berlin: In Search of Earth’s Building Blocks

The most common type of meteorites that fall onto Earth are called chondrites, which represent little modified aggregates of dust from the early solar nebula. Therefore, it has long been assumed that chondrites represent the most plausible building material of the rocky planets. Evidence for a more complex composition of Earth’s building materials now comes from examining the isotopic abundances of the rare earth element neodymium in representative Earth rocks in comparison to data from meteorites. The abundances of the neodymium isotopes Nd-142 and Nd-143 vary in nature mainly because they are formed by the decay of the radioactive samarium isotopes Sm-146 and Sm-147, respectively. Because of the short half-life of Sm-146, Nd-142 only increased during Earth’s early history, but has not changed since then because all the atoms of Sm-146 have decayed. In contrast, new Nd-143 is still formed today through the slow decay of Sm-147. The different time dependence of the growth of Nd-142 and Nd-143 allows study of the time scales of chemical changes during the growth of the planets. Furthermore, with this method one can derive the average concentration ratio of samarium to neodymium in the Earth to compare it with the values in chondritic meteorites.

The new results from the two research teams are consistent with each other in that both studies show a small but solvable excess of Nd-142 for the Earth compared to chondrite meteorites, which is caused by the decay of excess Sm-146 – the excess resulting from a slightly higher concentration ratio of samarium to neodymium in the Earth compared to chondrites. Where the two studies differ, however, is in the preferred explanation of the chemical difference between Earth and meteorites, according to Professor Harry Becker of the Institute of Geological Sciences, one of the authors of the study in Nature. In this study, the authors argue that some of Earth’s building materials were formed in different areas of the inner solar nebula from those where chondrites were formed, which was also proposed by some astrophysical models. Subtle chemical variations in the abundances of samarium and neodymium in dust condensed from local gas may have been caused by changing proportions of specific minerals in the dust, according to Professor Alan Brandon, previously at the University of Houston and co-author of the study.

In contrast, the authors of the study in Science argue that the higher ratio of samarium to neodymium in Earth is the result of the loss of some of the crust from Earth progenitors. As the Earth grew as a result of the collision of smaller bodies, it is conceivable that early crust from these progenitor bodies was blown off and lost, which may also cause the observed chemical effects in the Earth. Further studies will show which interpretation is more likely, or whether both processes are responsible for the Earth’s particular chemical composition.