10:45 AM - 12:15 PM
[PPS07-P17] Theoretical study on the evolution of the chemical and isotopic compositions of the Venusian atmosphere due to thermal escape
Keywords:Venus, Atmospheric loss
The chemical and isotopic compositions of the present Venus atmosphere are important indicators for understanding Venus in the past, because they are the result of the loss and supply of volatile elements. For example, D/H on Venusian atmosphere is about 100 times higher than that of the Earth’s ocean and is caused by the massive escape of hydrogen from Venus. Since the composition and isotopic ratios of noble gases also change due to large-scale loss of hydrogen, the measurement of noble gases in the present Venus atmosphere is a target of future Venus exploration missions, and provides clues to the surface evolution on Venus.
Previous studies of hydrogen loss from the Venusian atmosphere (Gillmann et al. 2022, Grinspoon 1993) have suggested that the current high D/H on Venus may be due to non-thermal escape of hydrogen, rather than the large-scale thermal escape that occurred in the early stages. However, the process of non-thermal escape is complex, the fractionation process is not well determined, and the parameters (fractionation factors and degassing rates) that can explain a high Venusian D/H are arbitrarily used.
In this study, we focus on hydrodynamic escape, which is one of the thermal escape whose physical process is well understood and calculate the large-scale escape of hydrogen from water acquired during Venus formation, as well as the steady-state process considering water supplied by meteorite impacts. We investigate which parameters control the current water vapor content and D/H on Venus and find the parameter sets that can explain the current observations.
We took a mixed atmosphere composed of CO2 and water vapor as initial conditions of Venusian atmosphere, based on Hamano et al. (2013). As for the water supply by meteorites, we considered the meteorite impact flux model of Neukum (1983), in which the amount of supply exponentially decreases with time. As the escape model, depending on the situation, we used the energy-limited escape (Watson et al. 1981), in which the amount of escape is defined by the XUV flux from the sun, and the diffusion-limited escape (Hunten 1973), in which the amount of escape is defined by the rate of hydrogen diffusion through the CO2 atmosphere. Fractionation processes caused by light hydrogen atoms dragging heavy deuterium atoms were also considered (Zahnle and Kasting 1986).
The results show that the initial water is lost in about 400 million years through an energy-limited escape. However, the increase in D/H is limited by a factor of ~ 2. After the initial water is lost, the escape process is dominated by diffusion-limited escape, and the balance between the escape and the supply by meteorites determines the amount of water vapor in the atmosphere and D/H. The parameters that explain the present Venus are found to be the atmospheric temperature of 1400 K and the total supply of water by meteorites of 3.4 terrestrial oceans.
In this study, we also calculated the loss of noble gases. The results show that about 30% of the initial amount of Ne and Ar acquired during the formation of Venus is lost, while Kr and Xe are not lost at all. The noble gas isotope ratios (20Ne/22Ne, 36Ar/38Ar) were found to be 3% lighter than those of the initial values. This suggests that the survival of solar components with higher isotopic ratios is necessary to explain the higher values of the Venus atmosphere than the noble gas isotopic ratios (especially Ne) of the CI chondrite meteorites.
These results indicate that when atmospheric compositions and isotopic ratios are measured by future Venus explorations, they will not provide information on water acquired during Venus formation. However, since the noble gases still have material acquired during formation, they are expected to provide strong constraints on the supply process of volatile elements during Venus formation.
Previous studies of hydrogen loss from the Venusian atmosphere (Gillmann et al. 2022, Grinspoon 1993) have suggested that the current high D/H on Venus may be due to non-thermal escape of hydrogen, rather than the large-scale thermal escape that occurred in the early stages. However, the process of non-thermal escape is complex, the fractionation process is not well determined, and the parameters (fractionation factors and degassing rates) that can explain a high Venusian D/H are arbitrarily used.
In this study, we focus on hydrodynamic escape, which is one of the thermal escape whose physical process is well understood and calculate the large-scale escape of hydrogen from water acquired during Venus formation, as well as the steady-state process considering water supplied by meteorite impacts. We investigate which parameters control the current water vapor content and D/H on Venus and find the parameter sets that can explain the current observations.
We took a mixed atmosphere composed of CO2 and water vapor as initial conditions of Venusian atmosphere, based on Hamano et al. (2013). As for the water supply by meteorites, we considered the meteorite impact flux model of Neukum (1983), in which the amount of supply exponentially decreases with time. As the escape model, depending on the situation, we used the energy-limited escape (Watson et al. 1981), in which the amount of escape is defined by the XUV flux from the sun, and the diffusion-limited escape (Hunten 1973), in which the amount of escape is defined by the rate of hydrogen diffusion through the CO2 atmosphere. Fractionation processes caused by light hydrogen atoms dragging heavy deuterium atoms were also considered (Zahnle and Kasting 1986).
The results show that the initial water is lost in about 400 million years through an energy-limited escape. However, the increase in D/H is limited by a factor of ~ 2. After the initial water is lost, the escape process is dominated by diffusion-limited escape, and the balance between the escape and the supply by meteorites determines the amount of water vapor in the atmosphere and D/H. The parameters that explain the present Venus are found to be the atmospheric temperature of 1400 K and the total supply of water by meteorites of 3.4 terrestrial oceans.
In this study, we also calculated the loss of noble gases. The results show that about 30% of the initial amount of Ne and Ar acquired during the formation of Venus is lost, while Kr and Xe are not lost at all. The noble gas isotope ratios (20Ne/22Ne, 36Ar/38Ar) were found to be 3% lighter than those of the initial values. This suggests that the survival of solar components with higher isotopic ratios is necessary to explain the higher values of the Venus atmosphere than the noble gas isotopic ratios (especially Ne) of the CI chondrite meteorites.
These results indicate that when atmospheric compositions and isotopic ratios are measured by future Venus explorations, they will not provide information on water acquired during Venus formation. However, since the noble gases still have material acquired during formation, they are expected to provide strong constraints on the supply process of volatile elements during Venus formation.