Micrometeorites 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 was published today in Nature Astronomy by an international team of researchers, including University of Hawai’i 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 suggests the possibility that a greater amount of nitrogen compounds than previously recognized was transported near Earth, potentially serving as building blocks for life on our planet,” says Hope Ishii, study co-author and affiliate faculty at the Hawai’i Institute of Geophysics and Planetology in the UH Mānoa School of Ocean and Earth Science and Technology (SOEST).

Like all asteroids, 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.

a, HAADF-STEM image of modified magnetite (Mgt). Pt-C is the protective platinum coat. b,c, STEM-EDX elemental maps of the modified magnetite. An RGB composite image of oxygen (red), iron (green) and silicon (blue) is shown in b and that of sulfur (red), nitrogen (green) and magnesium (blue) is in c. d,e, BF-TEM image (d) and DF-STEM image (e) of modified magnetite constructed from a STEM-NBD dataset. The DF-STEM image in e was produced by selecting a diffraction spot of roaldite, indicated in f. Dashed lines denote the boundaries of the iron-rich layer (I). The white areas in e correspond to high-intensity regions for the diffraction spot of roaldite. f, Position-averaged NBD pattern for the rectangular region covering the iron-rich layer and the substrate magnetite in e. Red circles indicate diffraction spots from roaldite (superscript R), which has a crystallographic relationship with magnetite (superscript M). The arrow shows the diffraction spot of roaldite chosen for producing the DF-STEM image in e. g, NBD pattern from the iron-rich layer in e. Diffraction spots of roaldite (arrowed) are dominant, and those of magnetite are weak in the layer. Sample ID is A0104-028098.

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 are covered with tiny minerals composed of iron and nitrogen (iron nitride: Fe4N).

“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.

Influx of nitrogen-rich material from the outer Solar System indicated by iron nitride in Ryugu samples, Nature Astronomy (open access)

Astrobiology