[U-08] Biogeochemistry of CO world

[U08-09] Prebiotic Synthesis of Amino Acids and Nucleic Acid Bases from CO-rich (Exo)Planetary Atmospheres

*Kensei Kobayashi¹, Vladimir S. Airapetian^2,3, Takumi Udo¹, Yoko Kebukawa¹, Jun-ichi Takahashi¹, Hiromi Shibata⁴, MITA Hajime⁵, Hitoshi Fukuda⁶, Yoshiyuki Oguri⁶ (1.Faculty of Engineering, Yokohama National University, 2.NASA Goddard Space Flight Center, 3.American University, 4.Osaka University, 5.Fukuoka Institute of Technology, 6.Tokyo Institute of Technology)

Keywords:Carbon monoxide, Amino acids, Nucleic acid bases, Solar energetic particles, Early planetary atmospheres

After Miller’s historic spark discharge experiments in 1953 [1], many experiments were conducted by using gas mixtures including methane as starting materials, and the formation of amino acids and some nucleic acids was reported. Later studies suggested that not methane, but carbon monoxide was a major reducing carbon species in the early Earth (and Mars) atmosphere [2]. Thus, it is important to examine whether carbon monoxide is a good starting material for the prebiotic synthesis of molecules of biological importance. Spark discharge was often used as an energy source to simulate chemical evolution in early atmospheres, which effectively produced amino acids from CH₄-NH₃ or CH₄-N₂ type gas mixtures, but it could not form them from CO-N₂ type gas mixtures [3].
We have been searching for efficient energy sources to produce bioorganic compounds including amino acids, carboxylic acids and nucleobases from CO-containing mixtures. When CO-N₂-type gas mixtures were irradiated with high-energy protons, amino acids were produced with the same energy yield as CH₄-N₂-type gas mixtures [4]. Amino acids were also formed by high energy photon (X-rays [5] or g-rays [6]) irradiation of CO-N₂-type gas mixtures. Amino acids and nucleobases were formed when high-temperature plasma of CO-N₂-H₂O was quenched [7,8]. Thus, CO represents a promising carbon source for prebiotic synthesis in an early (exo)planetary atmosphere. These experiments simulated Galactic Cosmic Rays (GCRs), solar and stellar cosmic rays, short-wavelength solar radiation, and bolide impacts of meteorites. When CO₂-CO-N₂ type gas mixtures were used in these experiments, the amino acid formation was still possible though the yield came down [9].
Observations of superflares in young and active solar-type stars [10, 11] suggest that the young Sun also had produced such energetic and frequent flares and associated coronal mass ejections, which would have accelerated high-flux of solar energetic particles (SEPs). Our recent proton irradiation experiments have also shown that SEP events from the young Sun could represent the most effective energy sources for the prebiotic formation of biologically important organic compounds from a weakly reducing atmosphere of the early Earth [12]. Since the energy flux of space weather generated frequent SEPs from the young Sun and other young solar-like stars in the first 600 million years after the birth of the Solar system is expected to be much greater than that of GCRs, we conclude that SEP-driven energetic protons were the most promising energy sources for prebiotic production of bioorganic compounds in CO-containing early Earth, and exoplanetary atmospheres [12].

References
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[12] Kobayashi, K. et al. Life, submitted.