Speaker:　Prof. Katsura Kobayashi (Institute for Planetary Materials, Okayama University)

Title: Origin, evolution, and dynamics of the asteroid Ryugu~Perspectives from the comprehensive geochemical approach~

Abstract:

The asteroid Ryugu is a near-Earth asteroid classified as a C-type asteroid. C-type asteroids have been presumed to have a similar chemical composition to the so-called primordial carbonaceous chondrites that have fallen to Earth. In other words, they are considered to be "fossils" that retain information from the formation of planets in the early solar system, especially organic matter and water, without being exposed to a high-temperature environment after the birth of celestial bodies, melting, or even melting. Based on this scientific background, it was selected as a target for the Hayabusa2 project, an asteroid exploration mission in Japan, and as a result, in December 2021, samples collected from near the surface of Ryugu (total amount of 5.4 g) were successfully brought back to Earth.

The Institute for Planetary Materials, Okayama University, is a higher-level curation facility in charge of the initial analysis of recovered samples. Therefore, 16 particles (~55 mg) of the samples described by JAXA/ISAS, the primary curation facility, were transported to the institute for mineralogical description, elemental concentration, isotope analysis, and dating one particle at a time in our laboratory. In addition, solvent-soluble organic matter (SOM) and poorly soluble organic matter (IOM) in the particles were analyzed, and a variety of organic compounds including 23 kinds of amino acids were successfully detected.

From these basic data, the current asteroid Ryugu originated from an ice planet that existed in the outer solar system ~3 million years after the formation of the solar system (~4.565 Ga). In the interior of the ice planet, it is thought that the minerals observed in the recovered samples were formed by the alteration of the water quality between the fluid mainly water and inorganic minerals at a low temperature (~30°C) formed by the decay heat of the short-lived radiative element, such as 26Al. It is thought that various organic matter, including amino acids, were also formed by reactions mediated by this fluid. We believe that this icy planet was subsequently refrozen and became the current asteroid Ryugu by moving fragments into the solar system that were destroyed by some physical disturbance.

In this seminar, we would like to present the results of the initial analysis of the asteroid Ryugu and introduce our materials science approach used in the analysis.