Abstract EANA2025-60

Bioorganic compounds such as amino acids, nucleobases, and sugars have been identified in carbonaceous chondrites and on asteroids like Ryugu and Bennu [1,2]. These molecules may have been delivered to the early Earth via meteorites and cosmic dust before the emergence of life. However, the organic compounds currently detected in extraterrestrial materials have likely undergone extensive alteration due to long-term exposure to radiation, including from radioactive nuclides present in parent bodies. This raises the key question: How can we reconstruct the original extraterrestrial organic inventory that was delivered to the prebiotic Earth?

One plausible scenario involves a multistage synthesis and alteration process: (i) complex organic compounds formed from interstellar materials in molecular clouds, (ii) these materials were incorporated into planetesimals during Solar System formation, and (iii) subsequent aqueous alteration occurred within asteroids, where gamma radiation from short-lived radionuclides such as 26Al triggered chemical reactions. In this study, we simulated such aqueous alteration within asteroid interiors using laboratory analogs of interstellar organic matter, and examined the formation and stability of amino acids and nucleobases under these conditions.

A gas mixture of CO, NH₃, and H₂O was irradiated with high-energy protons to produce an interstellar organic analog (hereinafter abbreviated as CAW), which contains amino acid precursors [3]. Separately, an aqueous solution of HCHO, CH₃OH and NH₃ (hereinafter abbreviated as FMAW) was subjected to gamma irradiation, both with and without CAW. To assess the stability of extraterrestrial organic compounds, free amino acids, CAW, and nucleobases were also gamma-irradiated in aqueous environments. Amino acids were quantified after acid hydrolysis of the irradiated samples.

Gamma irradiation of FMAW produced a variety of amino acids [4]. When CAW was included, the yield of glycine from CAW decreased, but more complex amino acids such as aminobutyric acids increased. This suggests that interstellar organic matter imported into asteroids could have played a role in diversifying the organic inventory observed in meteorites. However, excessive irradiation led to a reduction in amino acid concentrations, indicating that prolonged aqueous alteration may degrade organics after an initial increase.

Amino acids lacking an α-hydrogen (e.g., isovaline) were more radiation-resistant than those with it (e.g., valine), and purines were more stable than pyrimidines. These trends may explain the compositional differences between meteoritic organics and those produced in laboratory simulations. Because meteorites have been exposed to radiation from medium- and long-lived radionuclides for ~4 billion years, such post-formation effects must be considered when reconstructing the nature of extraterrestrial organics originally delivered to the early Earth. Space experiments are also promising to examine the fate of extraterrestrial organics in deep space.

[1]         D. P. Glavin et al. (2020) Chem. Rev. 120, 4660–4689.

[2]         H. Naraoka et al. (2023) Science 379, 6634.

[3]         T. Kasamatsu et al. (1997) Bull. Chem. Soc. of Jpn. 70, 1021–1026.

[4]         Y. Kebukawa et al. (2022) ACS Cent. Sci. 8, 1664–1671.