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micrometeorite particles
From left: electron image of a fine Ryugu grain and magnetite particles on Ryugu are porous due to harsh space environment. (Photo credit: Matsumoto et al, 2023)

Micrometeorites (small particles of rock in space, usually weighing less than a gram) originating from icy celestial bodies in the outer solar system may be responsible for transporting nitrogen to the near-Earth region in the early days of our solar system. That discovery, based on material gathered from an asteroid, was published on November 30 in Nature Astronomy by an international team of researchers, including University of Hawaiʻi at Mānoa scientists, led by Kyoto University.

Nitrogen compounds, such as ammonium salts, are abundant in material born in regions far from the sun, but evidence of their transport to Earth’s orbital region had been poorly understood.

“Our recent findings suggest that a greater amount of nitrogen compounds than previously recognized was transported near Earth where they potentially served as building blocks for life on our planet,” said Hope Ishii, study co-author and affiliate faculty at the Hawaiʻi Institute of Geophysics and Planetology in the UH Mānoa School of Ocean and Earth Science and Technology (SOEST).

iron nitride layer under microscope
Cross-section of magnetite with iron nitride layer. (Photo credit: Matsumoto et al, 2023)

The asteroid Ryugu is a small, rocky object that orbits the sun. The Japan Aerospace Exploration Agency’s Hayabusa2 spacecraft explored Ryugu and brought material from its surface back to Earth in 2020. This intriguing asteroid is rich in carbon and has undergone significant space weathering caused by micrometeorite collisions and exposure to charged ions streaming from the sun.

Looking for evidence of space weathering

In this study, the scientists aimed to discover clues about the materials arriving near Earth’s orbit, where Ryugu is currently located, by examining the evidence of space weathering in Ryugu samples. Using an electron microscope, they found that the surface of the Ryugu samples is covered with tiny minerals composed of iron and nitrogen.

“We proposed that tiny meteorites, called micrometeorites, containing ammonia compounds were delivered from icy celestial bodies and collided with Ryugu,” said Toru Matsumoto, lead author of the study and assistant professor at Kyoto University. “The micrometeorite collisions trigger chemical reactions on magnetite and lead to the formation of the iron nitride.”

The iron nitride was observed on the surface of magnetite, which consists of iron and oxygen atoms. When magnetite is exposed to the space environment, oxygen atoms are lost from the surface by the irradiation of hydrogen ions from the sun (solar wind) and by heating through micrometeorite impact. These processes form metallic iron on the very surface of the magnetite, which readily reacts with ammonia—creating ideal conditions for synthesis of iron nitride.

“Discovering iron nitride on the surfaces was a surprise,” said Ishii. “It indicates that significantly more nitrogen could have been delivered to early Earth than we previously thought, helping with the eventual development of life. Surprises like this are what make studying returned extraterrestrial samples so exciting!”

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