11:00 AM - 1:00 PM
[PCG19-P09] Comparison of nitrogen atmospheric escape between the ancient Earth and present Titan using the 3D global MHD simulation
Keywords:Titan, atmospheric escape, nitrogen
Titan is a unique satellite with an environment close to that of the early Earth: liquid on the surface (methane and ethane), nitrogen-dominated atmosphere, and tick atmospheric pressure around 1 atm on the surface. In order to unveil the atmospheric evolution of the early Earth, it is very important to understand that of Titan, especially the atmospheric escape. Titan's atmosphere is composed of 94% nitrogen, which is believed to escape from Titan mainly by the non-thermal escape processes (Michael et al. 2005; Shematovich et al. 2003; Cravens et al. 1997). However, the global spatiotemporal variations in Titan’s non-thermal atmospheric escape have not been investigated quantitatively.
Here we quantitatively model the global non-thermal escape of nitrogen atmosphere for Titan and the early Earth by using the global 3D MHD simulation. We investigated responses of the nitrogen atmospheric escape to different solar wind conditions and also surveyed the essential parameters that differentiate the atmospheric escape of Titan from Earth. The escape rate of Titan's nitrogen atmosphere was estimated to be 5.97×1023/s under a quiet solar wind condition (solar wind speed of 450 km, plasma density of 7 /cc, dynamic pressure of 1.19×10-9 Pa, magnetic field strength of 7 nT). While the escape rate of the early Earth under the same solar wind conditions was estimated to be 7.24×1025/s, which is about two orders of magnitude higher than the escape rate of the Titan. This difference was interpreted with the following two factors. (1) The solar XUV flux of the early Earth is 104 times greater than that of Titan, which likely makes the atmospheric ion production rate and escape rate about 104 times greater. (2) The Earth's gravity is about 10 times greater than Titan's, which reduces Earth’s atmospheric scale height by a factor of 1/10. The difference in the escape rate can be attributed to the factors (1) and (2). When the solar wind dynamic pressure was increased about 57 times, the nitrogen atmospheric escape rates of Titan and the early Earth were found to be increased by a factor of about 6 and 360, respectively.
These results demonstrate that the solar XUV, celestial gravity, and solar wind interaction are the dominant controlling factors for the nitrogen atmospheric escapes on Titan and the early Earth.
In this study, the inner boundary of the simulation for Titan was located at the upper ionosphere (above 900 km). We plan to include lower altitudes for estimation of a more realistic escape rate.
Here we quantitatively model the global non-thermal escape of nitrogen atmosphere for Titan and the early Earth by using the global 3D MHD simulation. We investigated responses of the nitrogen atmospheric escape to different solar wind conditions and also surveyed the essential parameters that differentiate the atmospheric escape of Titan from Earth. The escape rate of Titan's nitrogen atmosphere was estimated to be 5.97×1023/s under a quiet solar wind condition (solar wind speed of 450 km, plasma density of 7 /cc, dynamic pressure of 1.19×10-9 Pa, magnetic field strength of 7 nT). While the escape rate of the early Earth under the same solar wind conditions was estimated to be 7.24×1025/s, which is about two orders of magnitude higher than the escape rate of the Titan. This difference was interpreted with the following two factors. (1) The solar XUV flux of the early Earth is 104 times greater than that of Titan, which likely makes the atmospheric ion production rate and escape rate about 104 times greater. (2) The Earth's gravity is about 10 times greater than Titan's, which reduces Earth’s atmospheric scale height by a factor of 1/10. The difference in the escape rate can be attributed to the factors (1) and (2). When the solar wind dynamic pressure was increased about 57 times, the nitrogen atmospheric escape rates of Titan and the early Earth were found to be increased by a factor of about 6 and 360, respectively.
These results demonstrate that the solar XUV, celestial gravity, and solar wind interaction are the dominant controlling factors for the nitrogen atmospheric escapes on Titan and the early Earth.
In this study, the inner boundary of the simulation for Titan was located at the upper ionosphere (above 900 km). We plan to include lower altitudes for estimation of a more realistic escape rate.