Exploring impact vapor plume reactions from asteroidal impacts: Monte Carlo simulations and implications for biomolecules synthesis

Abstract

During a hypervelocity impact, both the impactor and target materials evaporate, generating an impact vapor plume with temperatures reaching several thousand K. As the plume cools through adiabatic expansion, chemical reactions are predicted to quench, leading to a non-equilibrium composition. Previous experiments simulating meteorite impacts on the early Earth have reported the formation of biomolecules such as amino acids and nucleobases, suggesting that the chemical reactions within impact vapor plumes may have contributed to the origins of the building blocks of life. However, it is still unclear how chemical reactions proceed during the cooling impact vapor plume and lead to the synthesis of such organic molecules. In this study, to investigate the evolution of chemical composition within impact vapor plumes, we conducted a Monte Carlo chemical reaction simulation for complex organic synthesis, developed in our previous work (Ochiai, Y., Ida, S., Shoji, D., [2024], Astron. Astrophys., 687, A232). In conventional kinetic model-based studies, chemical species and their associated reaction pathways are predefined to calculate the time evolution of chemical compositions using the thermodynamic data of these species and reaction rate coefficients. In contrast, our model does not rely on a predefined reaction network; instead, it utilizes imposed conditions for chemical changes and an approximate method for calculating reaction rates suited to our objectives. Additionally, we developed a new approach to couple these chemical reaction calculations with the rapid temperature and pressure decay in the vapor plume. Results show diverse organic molecule production depending on the impactor materials assumed in this study (LL, CI, and EL chondritic types). These products include important precursors to biomolecules such as amino acids, sugars, and nucleobases. On the other hand, for all impactor compositions, the abundance of biomolecules themselves remains extremely low throughout the reactions from an impact to quenching. Therefore, our results suggest that biomolecules are not directly produced in impact vapor plumes but rather synthesized through reactions of these precursor molecules in aqueous solutions, following H${}_2$O condensation as the vapor plume cools. Many of the detected organic compounds, including the precursor molecules such as imine compounds and formamide, are not included in the reaction networks of previous kinetic model simulations, and their formation has not been predicted. This demonstrates the effectiveness of our Monte Carlo simulation as a powerful tool for investigating the synthesis of low-abundance organic compounds, including biomolecules.