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A new isotope effect opens up unexpected clues to the formation of the Solar System

Researchers from CNRS, Muséum National d'Histoire Naturelle de Paris, University of Manchester, Université Paris XIII, Sorbonne Université and Institut de Physique du Globe de Paris/Université de Paris, have just experimentally confirmed the existence of a new isotope effect, discovered 35 years ago during ozone synthesis.

A new isotope effect opens up unexpected clues to the formation of the Solar System

© NASA

Publication date: 20/04/2020

Press, Research

Related themes : Origins

This effect – by its magnitude and properties – violates the basic rules governing the distribution of isotopes during chemical reactions. It could affect many chemical elements and play a part in certain isotopic variations that have long been identified in the constituents of meteorites and generally attributed to nucleosynthesis processes in stars.

In the solar system, the isotopic compositions of most chemical elements show variations, or ‘anomalies’, that are not predicted by any theory and are never observed in the laboratory or in terrestrial rocks. These ‘anomalies’ are classically attributed to the presence, in the protosolar disc, of grains condensed in the envelopes of stars at the end of their lives, millions or billions of years before the formation of the solar system. These grains, known as presolarites, have been isolated in meteorites and their diversity of isotopic composition is thought to reflect the processes of nucleosynthesis of chemical elements in stars.

At its formation, the protosolar disc is essentially a very hot gas in the inner zone. It cooled slowly (over a few million years) and microscopic solids formed by condensation. These would later form the bulk of the matter in the terrestrial planets. In the disc, the turbulence of the protosolar gas mixes these condensates with the presolar grains on large scales. The constituents of this mixture have been preserved in certain meteorites, whose parent bodies have never been completely melted since their formation. This is a simplified image of the Standard Model that accounts for isotopic anomalies in meteorites and planets.

In the early 1980s, it was shown experimentally that ozone produced by electrical discharges in oxygen exhibited variations in oxygen isotopic composition similar to the anomalies discovered in meteorites. This effect is observed in atmospheric ozone. The reaction identified for ozone cannot be the cause of the variations observed in the solar system because ozone did not exist in the protosolar nebula. All the theoretical studies that have taken place since then have failed to identify the process at the origin of this isotopic effect for ozone.

Recent theoretical studies have proposed a new solution to the ozone effect. This approach has enabled us to sketch out the chemical reactions that could lead to anomalies: the surface of nanometric grains condensed directly from a plasma could favour two coupled reactions, one isotopic and the other chemical.

Following this lead, experiments were carried out with titanium, a refractory element that is emblematic of the isotopic anomalies that exist in meteorites. These experiments were carried out in a plasma produced by a high-frequency discharge inside a vapour of titanium chloride solubilised in an organic liquid.

Analysis of nano- to micro-metric condensates rich in titanium and organic carbon shows that they all carry titanium isotopic anomalies that are similar to those in pre-solar grains. These variations are well modelled by the theory developed to explain ozone. This suggests that isotopic anomalies of chemical origin could exist for all the elements. These isotopic variations would no longer be anomalies but the signature of very specific reactions in plasmas. This result opens the door to theoretical and experimental studies in a hitherto poorly understood area: chemical reactions in natural plasmas. New avenues are opening up concerning the formation of the solar system and even stellar processes.

Ref: Robert F., Tartèse R., Lombardi G., Reinhardt P., Roskosz M., Doisneau B., Deng Z. & Chaussidon M. (2020) Mass-independent fractionation of titanium isotopes and its cosmochemical consequences. Nature Astronomy – https://doi.org/10.1038/s41550-020-1043-1

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