09:20 〜 09:35
[MIS07-02] コンドライト質天体の衝突によって形成された蒸気雲中での化学反応シミュレーション
In the study of the origin of life, it is important to understand how precursors to life were generated or supplied on Earth. During the period when life is thought to have originated on Earth, from 4.5 to about 3.8 billion years ago, it is likely that the Earth had an atmosphere rich in CO2. Such an atmospheric composition would have been less favorable for the synthesis of life precursors than a reducing atmosphere rich in CH4 and CO. Therefore, the existence of a locally reducing environment on the early Earth would be more convenient from the viewpoint of the origin of life.
Chemical reactions in impact vapor plume have attracted attention as one of the processes that produced a locally reducing environment on early Earth. Impact vapor plume can be formed by high-velocity impacts of small bodies on Earth and undergo a process of rapid adiabatic expansion from a high-temperature to a low-temperature state. During this process, the chemical composition in the vapor plume remains in equilibrium at high temperatures, but as the temperature decreases, the chemical reaction timescale becomes longer than the cooling timescale of the expansion, and the chemical composition shifts to a non-equilibrium state. This frozen chemical composition is called the quench composition. Such physical and chemical processes may have produced non-equilibrium chemical compositions that are stable at high temperatures and pressures, and may have released chemical species to the surface that would not occur in a planetary surface mean field.
Previous studies for quench compositions have mainly considered gas-phase burning reactions of H, C, N, and O for comet impacts rich in volatile elements (Ishimaru et al., 2010), but there are no studies on non-equilibrium chemical reactions for impacts of rocky bodies containing rocky and metallic materials. In this study, we try to estimate the quench composition based on the non-equilibrium chemical reaction rate calculations for the impact of rocky bodies.
We created a new chemical reaction network that incorporates sulfur S in addition to the 53 chemical species and 325 HCNOs in GRI Mech 3.0. To account for the effects of non-volatile elements, the initial elemental composition ratios of HCNOS in the gas phase from impactor were set based on the results of Schaefer and Fegley (2010), where they performed multiphase equilibrium calculations of about 930 condensed and gaseous compounds. In the subsequent chemical reaction rate calculations, only the gas-phase combustion reaction of HCNOS was considered, ignoring the effects of non-volatile elements, thereby simplifying the chemical reaction calculations.
The results show that the quench temperatures are around 1500 K for most of the chemical species in all the impactor materials considered (carbonaceous, ordinary, and enstatite chondrites). On the other hand, some organic compounds such as C2H6 and CH2O and some sulfur compounds such as HSSH and COS were found to quench at temperatures below 1000K. This indicates that the freeze-out model (Hashimoto et al., 2007; Schaefer and Fegley, 2010), which assumes that all chemical reactions stop as soon as the vapor cloud temperature falls below a certain threshold, does not explain the actual quench process. It is also clear that the quench composition differs greatly depending on the impactor material. In particular, the collision of ordinary and enstatite chondrites was shown to result in the formation of a locally reducing atmosphere. This suggests that the impact of reducing bodies such as ordinary and enstatite chondrites may have played an important role in the chemical evolution of life on the early Earth.
Chemical reactions in impact vapor plume have attracted attention as one of the processes that produced a locally reducing environment on early Earth. Impact vapor plume can be formed by high-velocity impacts of small bodies on Earth and undergo a process of rapid adiabatic expansion from a high-temperature to a low-temperature state. During this process, the chemical composition in the vapor plume remains in equilibrium at high temperatures, but as the temperature decreases, the chemical reaction timescale becomes longer than the cooling timescale of the expansion, and the chemical composition shifts to a non-equilibrium state. This frozen chemical composition is called the quench composition. Such physical and chemical processes may have produced non-equilibrium chemical compositions that are stable at high temperatures and pressures, and may have released chemical species to the surface that would not occur in a planetary surface mean field.
Previous studies for quench compositions have mainly considered gas-phase burning reactions of H, C, N, and O for comet impacts rich in volatile elements (Ishimaru et al., 2010), but there are no studies on non-equilibrium chemical reactions for impacts of rocky bodies containing rocky and metallic materials. In this study, we try to estimate the quench composition based on the non-equilibrium chemical reaction rate calculations for the impact of rocky bodies.
We created a new chemical reaction network that incorporates sulfur S in addition to the 53 chemical species and 325 HCNOs in GRI Mech 3.0. To account for the effects of non-volatile elements, the initial elemental composition ratios of HCNOS in the gas phase from impactor were set based on the results of Schaefer and Fegley (2010), where they performed multiphase equilibrium calculations of about 930 condensed and gaseous compounds. In the subsequent chemical reaction rate calculations, only the gas-phase combustion reaction of HCNOS was considered, ignoring the effects of non-volatile elements, thereby simplifying the chemical reaction calculations.
The results show that the quench temperatures are around 1500 K for most of the chemical species in all the impactor materials considered (carbonaceous, ordinary, and enstatite chondrites). On the other hand, some organic compounds such as C2H6 and CH2O and some sulfur compounds such as HSSH and COS were found to quench at temperatures below 1000K. This indicates that the freeze-out model (Hashimoto et al., 2007; Schaefer and Fegley, 2010), which assumes that all chemical reactions stop as soon as the vapor cloud temperature falls below a certain threshold, does not explain the actual quench process. It is also clear that the quench composition differs greatly depending on the impactor material. In particular, the collision of ordinary and enstatite chondrites was shown to result in the formation of a locally reducing atmosphere. This suggests that the impact of reducing bodies such as ordinary and enstatite chondrites may have played an important role in the chemical evolution of life on the early Earth.