 |
Abstract EANA2026-23 |
|
Radiation-Driven Prebiotic Chemistry inside Asteroids: Formation of Amino Acid Precursors and Sugars with Implications for Molecular Chirality
A variety of organic compounds have been identified in carbonaceous chondrites and carbonaceous asteroids, suggesting that extraterrestrial organics delivered to the early Earth could have contributed to the emergence of life. However, it remains unclear where and how these compounds formed. We focused on two possible sites for prebiotic synthesis: interstellar ices and asteroids in the early solar system. Organics of interstellar origin, together with volatiles such as water, were likely incorporated into asteroids during the early stages of solar system formation. We experimentally investigated the formation of organic compounds in ices and aqueous environments within asteroids.
In asteroid interiors, water ice could have been thawed by heat generated from the decay of　Al-26. We experimentally examined organic synthesis in such aqueous environments. An aqueous solution of formaldehyde, methanol, and ammonia was irradiated with gamma rays from a Co-60 source. Hydrothermal reactions using the same solution were also performed. Mixtures of formaldehyde, methanol, ammonia, and water were subjected to heating and/or gamma-ray irradiation. Both energy sources yielded amino acids and sugars [1,2]. Glycolaldehyde (the formaldehyde dimer) was formed more efficiently by gamma-ray irradiation than by heating, resulting in higher yields of tetroses and pentoses. These results indicate that both gamma-ray and thermal energy contributed to organic synthesis within asteroid interiors, although radiation was particularly important as a trigger for chain reactions.
If asteroids were small, internal heating by radionuclide decay would have been insufficient to melt ice effectively. In such cases, galactic cosmic rays (GCRs) could have served as energy sources for organic synthesis in asteroid ices. Mixtures of formaldehyde, methanol, ammonia, and water with various mixing ratios were prepared as analogs of asteroid ices and irradiated with 290 MeV/u carbon ions at HIMAC (QST). Amino acid precursors were formed, with molecular weights estimated to be approximately 1000 using an ultrafiltration technique. Not only GCRs but also solar energetic particles [3] may have contributed to organic synthesis in small asteroid ices.
When high-energy protons interact with asteroid materials, spin-polarized muons can be generated. In addition, the decay of Al-26 produces spin-polarized positrons [4]. These spin-polarized particles may induce enantiomeric excesses in amino acids and sugars [5]. Experimental verification of the roles of muons in prebiotic stereochemistry is currently in progress.
Organic compounds in large asteroids were altered by radiation from long-lived radionuclides such as U-238, whereas those in small asteroids or meteorites were modified mainly by GCRs. Thus, ancient asteroids and meteorites (~4 Ga) may have contained larger amounts of bioorganic compounds, including amino acids and sugars, than present-day asteroids and meteorites. Reconstruction of organic chemistry in ancient asteroids is therefore important for evaluating the role of extraterrestrial organics in the origin of terrestrial life.
References
[1] N. Imai et al., Geochem. J., 68, 304 (2025).
[2] S. Abe et al., ACS Earth Space Chem., 9, 2115 (2025).
[3] K. Kobayashi et al., Astrophys. J. Lett., 1002, L12 (2026).
[4] N. Globus et al., Astrophys. J., 910, 85 (2021).
[5] J. Takahashi and K. Kobayashi, Symmetry, 11, 919 (2019).