4:30 PM - 4:45 PM
[MIS14-17] Possibility of formation of local reductive environment by cometary impact on the early Earth
Keywords:early Earth, impact vapor plume, life building block
When an impact of asteroids or comets in high velocity occurs, they evaporates due to shock heating, forming an impact vapor plume. Chemical reaction in the impact vapor plume is one of the processes that locally bring a reduced environment on the early Earth. Impact experiments show that some organic materials related to life building blocks are formed via chemical reactions in the impact vapor plume. On the other hand, because it is difficult to reproduce planetary-scale collisions in collision experiments, and it is difficult to achieve collision velocities exceeding 10 km/s , theoretical methods are also important.
Ishimaru et al. (2010), which is one of the theoretical methods estimated the quenched composition of impact vapor plume using numerical model that solves the motion of the plume and the chemical reactions simultaneously. However, this model simplifies the motion of vapor plumes based on a few assumptions.
In this study, we investigated the quench composition generated by cometary impacts on the ocean on the early Earth. Specifically, we first performed impact calculations using the impact code iSALE-2D(cylindrical coordinate system) to precisely follow the physical processes of the vapor plume, and to explore the temperature and pressure changes of the plume. Then, the chemical reaction of the HCNO system was calculated under the obtained temperature and pressure changes to estimate quenched composition.
The impact calculations showed the peak temperature and pressure of the plume are found to be consistent with Ishimaru et al. (2010) for the pressure, but about 1/2~1/3 for the temperature. Tis because the previous study estimated the peak temperature based on the planar impact assumption and did not consider the temperature dependence of the specific heat ratio and evaporation heat of ice. In addition, it was thought that the change in comet size did not affect the peak temperature and pressure of the plume, but it was found that if the comet size is larger than the thickness of the ocean, penetration into the oceanic crust occurs and the peak temperature and pressure increases due to the difference in shock wave characteristics with the ocean.
As a result of chemical reaction calculations, it is found that the quenched composition obtained by cometary impact on the early Earth is rich in H2, H2O, CO. Moreover, even at the same collision velocity (15 km/s), the compositions of strongly reducing species (CH4, NH3) and life building blocks (HCN) are 10 to 1000 times richer than those in Ishimaru et al.(2010). This is because Ishimaru et al.(2010) overestimated the entropy of the plume.
Considering the formation of vapor plume, which is a mixture of comets and oceans during the impact process, we calculated the chemical reaction when H2O is mixed in. The result shows, as the mixing ratio of the target ocean(H2O) in vapor plume increased, the molar fractions of reductive nitrogen and carbon compounds in the quench composition became smaller, while oxidative nitrogen, carbon compounds, and H2 became larger. This is considered to be due to the fact that the mixed target(H2O) was used to oxidize the reductive nitrogen and carbon.
It was found that a strongly reducing quench composition can be obtained at an impact velocity of ~15 km/s, which is favorable for the formation of life building blocks on the early Earth. On the other hand, since cometary impact velocities to the Earth are often much higher, it is difficult to create a strongly reducing environment on the early Earth by simple HCNO system reactions. However, by the use of metallic elements in the comet as catalysts (e.g., Sekine et al., 2006).it is possible that even a high-velocity cometary impact could have brought a strongly reducing environment.
Ishimaru et al. (2010), which is one of the theoretical methods estimated the quenched composition of impact vapor plume using numerical model that solves the motion of the plume and the chemical reactions simultaneously. However, this model simplifies the motion of vapor plumes based on a few assumptions.
In this study, we investigated the quench composition generated by cometary impacts on the ocean on the early Earth. Specifically, we first performed impact calculations using the impact code iSALE-2D(cylindrical coordinate system) to precisely follow the physical processes of the vapor plume, and to explore the temperature and pressure changes of the plume. Then, the chemical reaction of the HCNO system was calculated under the obtained temperature and pressure changes to estimate quenched composition.
The impact calculations showed the peak temperature and pressure of the plume are found to be consistent with Ishimaru et al. (2010) for the pressure, but about 1/2~1/3 for the temperature. Tis because the previous study estimated the peak temperature based on the planar impact assumption and did not consider the temperature dependence of the specific heat ratio and evaporation heat of ice. In addition, it was thought that the change in comet size did not affect the peak temperature and pressure of the plume, but it was found that if the comet size is larger than the thickness of the ocean, penetration into the oceanic crust occurs and the peak temperature and pressure increases due to the difference in shock wave characteristics with the ocean.
As a result of chemical reaction calculations, it is found that the quenched composition obtained by cometary impact on the early Earth is rich in H2, H2O, CO. Moreover, even at the same collision velocity (15 km/s), the compositions of strongly reducing species (CH4, NH3) and life building blocks (HCN) are 10 to 1000 times richer than those in Ishimaru et al.(2010). This is because Ishimaru et al.(2010) overestimated the entropy of the plume.
Considering the formation of vapor plume, which is a mixture of comets and oceans during the impact process, we calculated the chemical reaction when H2O is mixed in. The result shows, as the mixing ratio of the target ocean(H2O) in vapor plume increased, the molar fractions of reductive nitrogen and carbon compounds in the quench composition became smaller, while oxidative nitrogen, carbon compounds, and H2 became larger. This is considered to be due to the fact that the mixed target(H2O) was used to oxidize the reductive nitrogen and carbon.
It was found that a strongly reducing quench composition can be obtained at an impact velocity of ~15 km/s, which is favorable for the formation of life building blocks on the early Earth. On the other hand, since cometary impact velocities to the Earth are often much higher, it is difficult to create a strongly reducing environment on the early Earth by simple HCNO system reactions. However, by the use of metallic elements in the comet as catalysts (e.g., Sekine et al., 2006).it is possible that even a high-velocity cometary impact could have brought a strongly reducing environment.