[U-12] Biogeochemistry of CO world

[U12-08] Roles of CO in the formation of haze in early Earth, early Mars and Titan atmospheres

*Kensei Kobayashi^1,2,3, Vladimir S. Airapetian^4,5,6, Yuto Kajikiya^2,3, Takumi Udo¹, Ikeda Ibuki^1,2,3, Kanji Nakajima^2,3, Yoko Kebukawa², Hiromi Shibata⁷, Naoto Hagura⁸ (1.Yokohama National University, 2.Institute of Science Tokyo, 3.RIKEN, 4.NASA Goddard Space Flight Center, 5.American University, 6.Kyoto University, 7.Kobe University, 8.Tokyo City University)

Keywords:Carbon monoxide, Haze, Early Earth, Early Mars, Titan, Solar energetic particles

The composition of the atmospheres of early Earth and early Mars has been a topic of debate. However, it is now widely accepted that they were weakly reducing, consisting of a mixture of CO₂, CO, N₂, and H₂O. It has been reported that early Mars had a relatively high CO content [1]. Such weakly reducing gas mixtures containing CO can yield amino acids when exposed to high-energy proton irradiation [2]. Since the young Sun is believed to have frequently produced large flares [3], the resulting solar energetic particles (SEPs) could have generated a significant amount of amino acid precursors, surpassing the supply of extraterrestrial amino acids [4].
On the other hand, Titan, the largest moon of Saturn, has a dense atmosphere composed mainly of nitrogen and methane, where haze is observed. The Cassini-Huygens mission confirmed that this haze consists of complex organic compounds. It has been proposed that haze formation begins in the upper atmosphere and continues as the particles descend to the lower atmosphere [5]. However, we have found that haze can also form in the dense lower atmosphere when exposed to high-energy proton irradiation [6].
In this study, we irradiated gas mixtures simulating the atmospheres of early Earth, early Mars, and Titan to observe haze formation and analyze the irradiation products.
Experimental: Gas mixtures of CO₂, CO, N₂, and/or H₂ (simulating early Earth/Mars atmospheres) and those of N₂, CH₄, and/or CO (simulating Titan’s atmosphere) were irradiated with 3.0 MeV protons from a Pelletron accelerator (RIKEN). The resulting solid products (Titan simulation) were dissolved sequentially in tetrahydrofuran, acetonitrile, and H₂O. Each solution was acid-hydrolyzed and analyzed for amino acids. Additionally, gas mixtures of CO₂, CO, N₂, and H₂O (with 5 mL of liquid water) were irradiated with 2.0 MeV protons from a Tandem accelerator (TCU). The resulting products was acid-hydrolyzed and analyzed for amino acids.
Results and Discussion: Haze formation was observed in all proton-irradiated gas mixtures, suggesting that haze could form in the lower atmospheres of early Earth, early Mars, and Titan due to SEPs or galactic cosmic rays (GCRs). Various amino acids were detected in the hydrolysates of the proton irradiation products, indicating that each haze (solid organic matter) contained amino acid precursors. Notably, when CO was added to the Titan gas mixture, the amino acid yield in the water fraction increased. Considering possible chemical evolution in Titan’s subsurface ocean, atmospheric CO could enhance the supply of organic compounds to the ocean.
Haze formation in planetary atmospheres can influence not only prebiotic chemistry but also climate and habitability. We plan to further characterize the haze (solid complex organics) formed by irradiation and explore its potential roles.
We express our thanks to Dr. Toshiyuki Azuma (RIKEN) and Dr. Tokihiro Ikeda (RIKEN) for their kind help in the irradiation experiments using a Pelletron accelerator at RIKEN.
[1] Y. Ueno et al., Nat. Geosci., 17, 503-507 (2024).
[2] K. Kobayashi et al., Orig. Life Evol. Biosph., 28, 155-165 (1998).
[3] V. S. Airapetian et al., Nat. Geosci., 9, 452-455 (2016).
[4] K. Kobayashi et al., Life, 13, 1103 (2023).
[5] J. H. Waite et al., Science, 316, 870-875 (2007).
[6] T. Taniuchi et al., Anal. Sci., 29, 777-785 (2013).