Untangling the Cosmic Dust Catalyzed Synthesis of Complex Organic Molecules in the Interstellar Medium
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Final Report Abstract
In the project we were able to demonstrate that galactic cosmic rays (GCRs) can induce enolization of molecules by electronic excitations and intermolecular hydrogen transfer in hydrogen bonded dimers. Furthermore it was shown that the non-equilibrium reactions driven by GCRs can produce high energy isomers and tautomers at hundreds of orders of magnitude higher abundance than expected from their equilibrium value. Since the low temperatures in the ice and the low densities in the gas phase prevent the high energy molecules to overcome barriers to isomerization or tautomerization, they remain stable and available for further reactions. Since their energy is higher and, e.g., enols are strong nucleophiles, they are far more reactive than there less energetic isomers and can therefore play a crucial role in advancing the chemical complexity found in the interstellar medium. Most notably, 1,2-ethenediol, the enol of glycolaldehyde and crucial reaction intermediate in the formose cycle to ultimately form ribose and DNA nucleosides, was detected after irradiating simple interstellar analogue ices. Because prebiotic molecules formed in interstellar ices or later on asteroids can ultimately be delivered to planets like Earth, the results strengthen the theory of a possible extraterrestrial origin of life. Moreover, a new type of low-temperature reaction was revealed that was hitherto not considered in planetary and space science. Electronic excitations with low energy light, below the ionization threshold or bond strength of the participating reactants, can lead to barrierless reaction between an excited molecule with one or more ground state molecules, leading to the formation of benzene and higher PAHs in simple acetylene ices. On planets and planetoids with an atmosphere that is opaque to high energy photons such as Ly-α and an ionosphere that deflects solar winds, low energy photons therefore represent the main driver of chemical reactions, with doses exceeding those of GCRs by several orders of magnitude. Such reactions could for example be responsible for the coloration of the surface of Pluto, which cannot be caused by the low dose of GCRs it receives before the surface is covered with fresh, colorless precipitates. Therefore, low energy excitations have to be considered in planetary science as a driver of chemical complexity found on the surface of planets where ionization and radical formation play a less important role.
Publications
- “A Mechanistic Study on the Formation of Acetic Acid (CH3COOH) in Polar Interstellar Analog Ices Exploiting Photoionization Reflectron Time– of–flight Mass Spectrometry” , Astrophys. J., 901, 84 (2020)
N. F. Kleimeier, A. K. Eckhardt, R. I. Kaiser
(See online at https://doi.org/10.3847/1538-4357/abafa4) - “Interstellar Formation of Biorelevant Pyruvic Acid (CH3COCOOH)”, Chem, 6, 1-11 (2020)
N. F. Kleimeier, A. K. Eckhardt, P. R. Schreiner, R. I. Kaiser
(See online at https://doi.org/10.1016/j.chempr.2020.10.003) - “Identification of Glycolaldehyde Enol (HOHC═CHOH) in Interstellar Analogue Ices”, JACS, 143, 14009-14018 (2021)
N. F. Kleimeier, A. K. Eckhardt, R. I. Kaiser
(See online at https://doi.org/10.1021/jacs.1c07978) - “Bottom-Up Synthesis of 1,1-Ethenediol (H2CC(OH)2) - the Simplest Unsaturated Geminal Diol - in Interstellar Analogue Ices”, J. Phys. Chem. Lett., 24, 229-235 (2022)
N. F. Kleimeier, R. I. Kaiser
(See online at https://doi.org/10.1021/acs.jpclett.1c03515)
