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The composition of the asteroid Ryugu revealed by the Japanese space mission Hayabusa-2

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File photo released by the Japan Aerospace Exploration Agency (JAXA) on December 24, 2020 showing soil samples from the asteroid Ryugu.

Recovering about 5.4 grams from a chunk of the asteroid on December 6, 2020, chemists weren’t expecting it. “It’s a surprise! It’s extraordinary!” appreciates Jérôme Aléon, researcher of the CNRS 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 is finely analyzed, then returned to Earth by the Japanese Hayabusa-2 mission, a year and a half later.

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This peak-shaped rock body nearly a kilometer in diameter has been filling scientists who have been studying it since June 2021 with joy, for at least two reasons. The first is that 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 was never heated above 100 ° C. More, “we sifted the entire periodic table and the quantities of iron, silicon, magnesium … are the same as the sun”, emphasizes Jérôme Aléon. This is proof that Ryugu’s matter must have been among the first to aggregate in the cloud of gas and dust that then gravitated around the young star.

A reference stone

The second reason is the discovery that this asteroid belongs to one of the rarest meteorite families 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. “Many things we know about the first Solar System: its age, the amount of water, its probable formation scenario, etc. – come from meteorites, which are pieces of asteroids that have fallen to Earth. But how to be sure that this is not distorted by the transformations undergone 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 by Science. “When we analyze meteorites, we don’t know where they come from. One of the challenges is therefore to make the connection between observations in the sky and the analyzes of what we discover on Earth.adds Jérôme Aléon.

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Now it’s done: Ryugu is in the same vein as Ivuna-type meteorites, the scientific name for this small family to which Orgueil belongs. “From now on Ryugu will be a benchmark against which we will compare all the other champions”insists Frédéric Moynier.

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