Kyoto University

Kyushu University

Hiroshima University

Tohoku University

Hokkaido University

School of Science, The University of Tokyo

Osaka Metropolitan University

Japan Agency for Marine-Earth Science and Technology

Summary of Presentations

Icy bodies and comets born far from the sun store large amounts of nitrogen compounds such as ammonium salts. Such nitrogen-containing solids are thought to be very important as material substances for life, but no evidence of their transport to the Earth orbital region has been found. In this study, we examined sand from the asteroid Ryuguu, which orbits near Earth, using an electron microscope and discovered that the very surface of the sand is covered with nitrided iron (iron nitride: Fe₄N ). Iron nitride is found on the surface of a mineral of iron and oxygen atoms called magnetite. We hypothesized that the iron nitride was formed by a chemical reaction on the surface of magnetite when a small meteorite containing a large amount of ammonia compounds from an icy body hit Ryuguu. On the surface of the asteroid, oxygen is lost from the magnetite surface due to exposure to ionic winds (solar wind) from the sun, and metallic iron, which easily reacts with ammonia, forms on the very surface. This is assumed to have promoted the synthesis of iron nitride derived from ammonia on the surface of the magnetite. This micrometeorite may have come from an icy body in the distant solar system, and it is possible that a larger amount of nitrogen compounds than previously realized were transported to the solar system near Earth to provide the material for life.

The research was conducted by a group led by Dr. Toru Matsumoto, a specific Assistant Professor at the Hakubi Center, Kyoto University, Professor Takaaki Noguchi, Associate Professor Akira Miyake, and Assistant Professor Yohei Igami at the Graduate School of Science, Kyoto University, Associate Professor Mitsutaka Haruta at the Institute for Chemical Research, Kyoto University, and international collaborators, and was published online in the British international journal Nature Astronomy on November 30, 2023.

(A) Magnetite particles in a sample from the asteroid Ryuguu. They are round in shape because they grew in the water that existed inside the celestial body. The surface of magnetite is very porous and this feature is found only on surfaces exposed to space. (B) Cross-sectional image of round magnetite. The distribution of elements is colored. Iron and nitrogen-rich layers are seen on the surface (green areas). Iron nitride covers a thickness of several tens of nanometers on the very surface.

Professor Shogo Tachibana and Associate Professor Aki Takigawa of the Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo contributed to this research.

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Journal

Journal name	Nature Astronomy
Title of paper	Influx of nitrogen-rich material from the outer Solar System indicated by iron nitride in Ryugu samples