The composition of the asteroid Ryugu revealed by Hayabusa-2

By recovering some 5.4 grams from a piece of asteroid on December 6, 2020, chemists did not expect that. “It’s a surprise! It’s extraordinary ! » appreciates Jérôme Aléon, CNRS researcher at the Institute of Mineralogy, Physics of Materials and Cosmochemistry in Paris. He is one of the 140 signatories of an article, published in Science on June 9, where the chemical composition of the first 95 milligrams of the precious samples taken from the asteroid Ryugu twice in 2019, then brought back to Earth by the Japanese Hayabusa-2 mission, a year and a half later, is finely analyzed.

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This top-shaped rocky body nearly a kilometer in diameter fills scientists who have been studying it since June 2021 with joy, for at least two reasons. The first is that the analyzes show that it is the oldest fossil in the Solar System to have been able to pass under terrestrial microscopes. Its material was formed about 5.2 million years after the Sun and has never been heated above 100°C. Furthermore, “We have sifted through the entire periodic table, and the quantities of iron, silicon, magnesium… are the same as for the Sun”, emphasizes Jérôme Aléon. This is proof that the matter of Ryugu must have been among the first to aggregate in the cloud of gas and dust which then gravitated around the young star.

A pebble of reference

The second reason is the discovery that this asteroid belongs to one of the rarest families of meteorites studied on Earth. This category contains less than a dozen specimens, the most famous of which, known as d’Orgueil, which fell in 1864 near Montauban, weighs 14 kilograms. “A lot of things we know about the early Solar System – its age, the amounts of water, its likely formation scenario, etc. – come from meteorites, which are pieces of asteroids that have fallen to Earth. But how can we be sure that this is not distorted by the transformations undergone when arriving on our planet or by their stay on the surface? » recalls Frédéric Moynier, professor at the University of Paris Cité and co-author of the study of Science. “When we analyze meteorites, we don’t know where they come from. One of the challenges is therefore to make the link between observations in the sky and analyzes of what we discover on Earth.adds Jérôme Aléon.

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It’s done now: Ryugu is in the same vein as Ivuna type meteorites, the learned name of this small family to which Orgueil belongs. “From now on Ryugu will be a reference stone to which we will compare all the other samples”insists Frédéric Moynier.

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