Planetary scientists have long believed that the Earth formed from planetary objects similar to meteorites. A decade ago, perplexing new measurements challenged that assumption by showing that the Earth and its supposed “building blocks” actually contain significantly different isotopic compositions.
For the past 10 years, scientists have been trying to understand why. Recent work by the University of Chicago’s Christoph Burkhardt and Nicolas Dauphas, together with their collaborators from Lawrence Livermore National Laboratory and the University of Münster, suggest a new explanation that may help illuminate both the composition of the Earth and the beginnings of the solar system.
“These recent measurements contribute to the growing evidence that the meteorites delivered to Earth provide an imperfect match to Earth’s composition,” said Richard Carlson, director of the Department of Terrestrial Magnetism at the Carnegie Institution for Science. Carlson was one of the scientists who found the compositional mismatch between meteorites and Earth 10 years ago. “This realization opens new views both to how Earth formed and to the bulk chemical composition of our home planet.” The study was published in the Sept. 15 issue of Nature.
Think of the nascent solar system as a soup, said Dauphas, the Louis Block Professor in Geophysical Sciences, who worked on the new measurements. The common notion is that the soup was a puree; its ingredients were thoroughly mixed and only separated later. The results of Burkhardt, Dauphas and their colleagues suggest instead that it was more like a minestrone, with clumps of elements scattered unpredictably here and there.
“The conclusion of our study is that the solar system was never completely homogenized,” Dauphas said. “It is made of the products of stars that never were mixed when the planets were formed. So it is very different from what people have proposed before.” This original clumpiness also has implications for the composition of the Earth.
Fingerprints of the solar system
Sifting out Earth’s constituents is tricky. The meteorites called chondrites that presumably made the planet have long since blended, and much of the deep Earth is not observable.
“We cannot distinguish now some of the components that made the Earth,” Dauphas said. “So we have to work with indirect methods to try to figure out what it was made of.“
The most powerful technique available is to look at the relative amounts of different isotopes—variant forms of elements—present in the observable Earth today.
“They are like fingerprints of the material that made the planet,” Dauphas said. One particular isotope of neodymium, a rare earth element used in electronics, is key. There is more neodynium-142 in the Earth’s mantle than there is in chondrites.
The prevailing explanation holds that the excess neodymium isotope was originally another element, samarium-146, which decays into neodymium-142, leaving more of it on the planet than there is in chondrites. Samarium-146 has a short half-life, and none of it is left on Earth today. But the theory proposes that it was abundant in the early life of the planet and that some of samarium and neodymium became part of a crust that was later buried deep underground where it can’t be seen or was kicked off into space by a meteorite impact.
Burkhardt, Dauphas and their colleagues pursued an explanation that didn’t require samarium: that the excess neodymium arrived as residue from a forming solar system that was never well mixed.
“When stars make elements, they make them with ratios that are very peculiar,” Dauphas said. “If the solar system was not a puree—if it was not completely uniform in its isotopic composition—then there should be other isotopes of neodymium in certain ratios. So we wanted to measure very precisely the isotopes of neodymium to see if there were these variations.” And that is what they found.
The group processed the largest chondrite samples they could get their hands on—two grams, as opposed to the hundreds of milligrams usually used for such measurements.
“That was part of the trick,” said Dauphas, “to use a lot of material so that there were enough atoms for our colleagues at Lawrence Livermore National Laboratory to do very precise measurements.” The samples were pulverized, “digested” in several batches of acid, and finally analyzed in a specialized mass spectrometer. “It was not sure that we would find anything,” Dauphas said. “We were pleasantly surprised when we could see those strange isotopic compositions.”
The group hopes to use the analysis of neodymium isotope ratios to look at other kinds of meteorites in scientific collections to try to figure out how they relate to the composition of the Earth.
“We have a new tool to understand how the Earth was formed,” Dauphas said. “The Earth is a strange object in many respects. Why is it different than Mars? There is a lot of diversity in the solar system among planets and meteorites. And we do not understand fully how that diversity was established, but these studies bring us closer to understanding why.”
Citation: “A nucleosynthetic origin of the Earth’s anomalous 142Nd composition,” by C. Burkhardt, L.E. Borg, G.A. Brennecka, Q.R. Shollenberger, N. Dauphas, and T. Kleine, Nature, Sept. 15, 2016, doi: 10.1038/nature18956.
Funding: National Science Foundation, National Aeronautics and Space Administration, and the Swiss National Science Foundation
Acknowledgement: The meteorites for this study were provided by Philipp Heck, curator of meteorites and director of the Robert A. Pritzker Center for Meteoritics and Polar Studies at the Field Museum in Chicago