[M-IS02] Astrobiology

[MIS02-P02] Possible roles of solar energetic particles in the formation of amino acids and their enantiomeric excesses in early Earth atmosphere

*Kensei Kobayashi¹, Koki Naito¹, Toshiki Sakamoto¹, Yoko Kebukawa¹, Jun-ichi Takahashi¹, Hiromi Shibata², Kenya Kubo³, Hitoshi Fukuda⁴, Yoshiyuki Oguri⁴, Vladimir Airapetian⁵ (1.Yokohama National University, 2.Osaka University, 3.International Christian University, 4.Tokyo Institute of Technology, 5.NASA Goddard Flight Center)

Keywords:Origins of life, The early Earth, Amino acids, Homochirality, Muon, Solar energetic particles

Solar energetic particles (SEPs) are high energy particles ejected from the Sun during solar flares and corona mass ejection, and their major constituents are protons. It was suggested that high-flux SEPs were generated from the young Sun and entered into the Earth atmosphere. Recently superflares were observed in solar-type stars [1]. Thus, we can estimate that high-flux of SEPs were shot out by the young Sun, which could have been important energy sources for prebiotic synthesis. It was estimated that the solar luminosity at 4 Ga was about 70 % of the present value. If so, the Earth should be frozen at that time (the faint Sun paradox). Airapetian et al. reported theoretically that SEPs could form N₂O in the primitive Earth atmosphere, which could work as a greenhouse gas to keep the Earth warm [2]. We performed experiments simulating the action of SEPs and discussed possible roles of SEPs in habitability of the early Earth and in formation of amino acids and their enantiomeric excesses.

In these days, it was considered that the early Earth atmosphere was only slightly reducing (e.g., N₂ + CO₂ + H₂O + trace of reducing gases) [3] rather than strongly reducing (e.g., CH₄ + NH₃ + H₂O) [4]. It is difficult to produce amino acids or N-containing organics from non-reducing gas mixtures (N₂ + CO₂ + H₂O) [5]. We compared the yields of amino acids from slightly reducing gas mixtures (N₂ + CO₂ + H₂O + CH₄) by spark discharges (using a Tesla coil), UV irradiation (using a Xe lamp) and proton irradiation (using a Tandem accelerator, Tokyo Institute of Technology), where the molar ratio of CH₄ was 0 – 50%. Amino acids were determined by HPLC after the products were acid-hydrolyzed. UV could not yield amino acids since it could not dissociate N₂. Proton irradiation could give amino acids even the molar ratio of CH₄ was as low as 0.5 %, while spark discharge gave them only when the molar ration of CH₄ was 15% or over. Thus, it was suggested that galactic cosmic rays (GCRs) and SEPs were most effective energies for prebiotic formation of amino acids from slightly reducing atmospheres. Since the energy flux of SEPs from the young Sun was expected to be much more than that of GCRs, it can be said that SEPs are most promising energy sources for prebiotic production of bioorganic compounds. In addition, the formation of N₂O was confirmed by GC/MS when a gas mixture of N₂, CO₂ and H₂O was irradiated by protons. Enhancement of amino acid yields by addition of N₂O to the gas mixtures were also observed.

SEPs could generate spin-polarized muons, which may have formed enantiomeric excesses of amino acids toward generation of their homochirality in the terrestrial biological system [6]. Such possibility is now being tested by spin-polarized muon (μ⁺ and μ^-) irradiation of amino acids in Materials and Life Science Experimental Facility (MLF) of Japan Proton Accelerator Research Complex (J-PARC).



[1] H. Maehara et al., Nature 485, 478-481 (2012).

[2] V. S. Airapetian et al., Nat. Geosci. 9, 452-455 (2016).

[3] D. C. Catling and J. F. Casting, Atmospheric Evolution on Inhabited and Lifeless Worlds, Cambridge University Press (2017).

[4] S. L. Miller, Science 117, 528-529 (1953).

[5] H. Kuwahara et al., Orig. Life Evol. Biosph. 42, 533-541 (2012).

[6] N. Globus and R. D. Brandford, Astrophys. J. Lett. 895, L11 (2020).