New study in "Science": Planetary impacts changed the chemical composition of the Earth

More than 4.56 billion years ago, the first planetary bodies of the solar system formed from dust clouds created by the condensation of nebular gases. These asteroids subsequently coalesced into larger solid bodies called protoplanets, which researchers refer to as planetesimals. These planetesimals were the building blocks of Earth and other terrestrial planets. To learn more about the chemical composition of these early celestial bodies, Prof. Dr. Audrey Bouvier at the Bavarian Research Institute of Experimental Geochemistry and Geophysics (BGI) of the University of Bayreuth with former co-tutelle doctoral student Dr. Paul Frossard and research partners in Clermont-Ferrand studied a series of chondrite meteorites. These are primitive unmelted space rocks that have impacted the Earth and are, in terms of their chemical composition, probably similar to the early primitive asteroids from which planetesimals and the young Earth evolved. Mass spectrometric analyses were focussed, in particular, on neodymium (Nd) and samarium (Sm). These two elements, which belong to the rare-earth metals, are of special interest for geochemical research: the isotope ¹⁴⁶Sm is radioactive and decays with a short half-life of 103 million years into the isotope ¹⁴²Nd. It is therefore extinct in the Solar System for the last 4 billion years. The researchers first determined how the proportion of the isotope ¹⁴²Nd in the neodymium atoms – in short: the ¹⁴²Nd abundance – varies in the dust components of different chondrites. 

The results of the chondrite analyses specify a puzzling finding that has been discussed in research for some time: the ¹⁴²Nd abundance in the Earth's interior is significantly higher than in the chondrites. In this respect, the chemical composition of Earth does not match the chemical composition of primitive asteroids, which grew by gravitational forces into planetesimals – the building blocks of Earth. "By combining the results of our measurements with established astrophysical models which describe nucleosynthesis processes, we have now been able to find a plausible explanation for the different ¹⁴²Nd abundance in Earth and in primitive asteroids," says co-author Prof. Audrey Bouvier, Ph.D., co-author of the new study.

The starting point of the new explanation is the distribution of chemical elements in the early planetesimals that formed from primitive asteroids. Over time, progressive heating and chemical differentiation occurred: siderophile elements, which bind iron to themselves, concentrated in the center of the planetesimals, creating a metallic core. Lithophilic elements, on the other hand, which have an affinity for silicates, tended to accumulate in the upper layers – mantle and crust – of the planetesimals. This process produced two distinct reservoirs which differ in their Sm and Nd proportions. Over time, due to radioactive decay, the ¹⁴²Nd abundance on the Earth increased and became higher than the ¹⁴²Nd abundance in the primitive asteroids. The research team in Bayreuth and Clermont-Ferrand propose, on the basis of their measurements, a very clear scenario: The planetesimal that formed the precursor of the early Earth repeatedly collided with other planetesimals within the emerging solar system. Thereby, large amounts of material were blasted out of its crust again and again. Together, the authors of the new study conclude that planetary impacts blew out about 4 to 20 percent of the mass of the young Earth and other planetesimals.

"It is very likely that other key elements were affected by the enormous collision-induced loss of material. Therefore, the question arises again as to what quantities of radioactive elements such as uranium, potassium, and thorium are present in the Earth's interior today and contribute to its heat balance and physical evolution. Furthermore, the assumption that planetesimals repeatedly collided with each other in the early phase of their formation could also be informative for the chemical composition of further planets – be it inside or outside the Solar System," says Bayreuth cosmochemist Prof. Dr. Audrey Bouvier.

Publication:
Paul Frossard, Claudine Israel, Audrey Bouvier, Maud Boyet: Earth's composition was modi-fied by collisional erosion. Science Vol. 377, Issue 6614. DOI: https://dx.doi.org/10.1126/science.abq7351

The study includes research results obtained by first author Dr. Paul Frossard as part of his doctoral thesis supervised by the Universities of Bayreuth and Clermont-Ferrand.