5:15 PM - 7:15 PM
[PPS01-P07] Development of a gas system for modeling Titan's atmospheric chemistry driven by water group ion irradiation
Keywords:Titan, Saturn, atmosphere, organic aerosol, prebiotic substances
Titan, an icy satellite of Saturn, is the only body in our solar system with an abundant atmosphere of 1.5 atm pressure on the surface. Titan's atmosphere is chemically reducing and nitrogen-dominated (nitrogen: 94%, methane: 5%, hydrogen: 1% Magee et al., 2009), like the atmospheric composition of early Earth just before biotic oxygen began to increase. Titan's atmosphere also contains a trace amount of other oxygen compounds such as CO and CO2. Hydrocarbon polymers and nitriles are likely produced from atmospheric nitrogen and methane by incident energy such as solar ultraviolet radiation and Saturn's magnetospheric plasma [Israel et al., 2005]. Irradiation experiments have been performed with gas samples modeling Titan's atmosphere to reproduce the aerosol production process for each incident energy source [e.g., Peng et al., 2013; Liu et al., 2023]. Many of these experiments irradiated photons and charged particles modeling MeV cosmic rays, solar energetic particles, and solar ultraviolet radiation [cable et al., 2012]. Titan is typically located within Saturn's magnetosphere and is bombarded by keV oxygen and hydrogen ions originating in Enceladus' interior ocean. However, there is no previous experimental study for gas irradiation with keV hydrogen and oxygen ions, and only the MeV irradiation of protons and electrons has been conducted [Taniuchi et al., 2013; Liu et al., 2023]. Therefore, the chemical processes in Titan's atmosphere driven by keV hydrogen and oxygen ions delivered from Saturn's magnetosphere as energy and material sources have not been realistically reproduced by experiments, which makes the formation and growth processes of atmospheric organic aerosols not yet understood.
Here, we develop a gas system for irradiation of gas samples with keV hydrogen and oxygen ions to reproduce the formation process of Titan's atmospheric hydrocarbon aerosols driven by water group ions delivered from Saturn's magnetosphere. A gas cell and a gas mixing system are implemented on the plasma irradiation system Plasma Irradiation Emulator for Celestial Environments (PIECE, Kimura et al., 2023), which was originally a system for keV charged particle irradiation for solid samples. Our development enables keV ion irradiation of gas samples, which has been difficult in previous studies using accelerators and other devices. To summarize the gas system requirements, a charged particle simulation in a pure nitrogen atmosphere was conducted using Particle TRansport In Planetary atmospheres (PTRIP, Nakamura et al., 2021). The stopping column number density of an incident proton at 10 keV in the nitrogen atmosphere is found to be 1.E+18 /cm^2 for hydrogen ions. Also, Sillanpää et al. (2015) and MSTAR suggest that the stopping column number density is 1.9E+17-2.4E+17 /cm^2 for oxygen ions. To reproduce these column densities, a gas cell with a total length of about 0.4 m with an internal pressure of about 100 Pa was required. Because the incident particle energy is relatively low compared to big accelerators, the gas cell should not be sealed off, and a micro hole needs to be opened in the cell wall to penetrate the incident-charged particle beam. We designed that by installing a differential pumping chamber between the ion irradiation system (operating pressure of less than about 0.01 Pa) and the gas cell (operating pressure of about 100 Pa), each can be separated with a plate with a micro hole with a diameter of 0.7-10 mm maintaining the differential pressure. The design was implemented, and the differential pressure was confirmed to be maintained at 4.1E-3 Pa in the ion irradiation device, less than 0.01 Pa in the differential pumping chamber, and 100 Pa in the gas cell. The ion beam current injected into the gas cell was measured to be a maximum flux of 1.34E+14 /cm^2/s and a maximum current of 168 nA for hydrogen ions, and a maximum flux of 8.67E+13 /cm^2/s and a maximum current of 109 nA for oxygen ions. The obtained maximum fluxes can reasonably reproduce the real flux of hydrogen (6.3E+5 /cm^2/s) and oxygen ions (1.E+6 /cm^2/s) above Titan's atmosphere [Cravens et al., 2008; Hartle et al., 2006; Horst et al., 2008]. We are going to establish an experimental procedure for stable irradiation, conduct irradiation of gaseous samples, and then analyze irradiated products. In this presentation, we will report the current status of the above.
Here, we develop a gas system for irradiation of gas samples with keV hydrogen and oxygen ions to reproduce the formation process of Titan's atmospheric hydrocarbon aerosols driven by water group ions delivered from Saturn's magnetosphere. A gas cell and a gas mixing system are implemented on the plasma irradiation system Plasma Irradiation Emulator for Celestial Environments (PIECE, Kimura et al., 2023), which was originally a system for keV charged particle irradiation for solid samples. Our development enables keV ion irradiation of gas samples, which has been difficult in previous studies using accelerators and other devices. To summarize the gas system requirements, a charged particle simulation in a pure nitrogen atmosphere was conducted using Particle TRansport In Planetary atmospheres (PTRIP, Nakamura et al., 2021). The stopping column number density of an incident proton at 10 keV in the nitrogen atmosphere is found to be 1.E+18 /cm^2 for hydrogen ions. Also, Sillanpää et al. (2015) and MSTAR suggest that the stopping column number density is 1.9E+17-2.4E+17 /cm^2 for oxygen ions. To reproduce these column densities, a gas cell with a total length of about 0.4 m with an internal pressure of about 100 Pa was required. Because the incident particle energy is relatively low compared to big accelerators, the gas cell should not be sealed off, and a micro hole needs to be opened in the cell wall to penetrate the incident-charged particle beam. We designed that by installing a differential pumping chamber between the ion irradiation system (operating pressure of less than about 0.01 Pa) and the gas cell (operating pressure of about 100 Pa), each can be separated with a plate with a micro hole with a diameter of 0.7-10 mm maintaining the differential pressure. The design was implemented, and the differential pressure was confirmed to be maintained at 4.1E-3 Pa in the ion irradiation device, less than 0.01 Pa in the differential pumping chamber, and 100 Pa in the gas cell. The ion beam current injected into the gas cell was measured to be a maximum flux of 1.34E+14 /cm^2/s and a maximum current of 168 nA for hydrogen ions, and a maximum flux of 8.67E+13 /cm^2/s and a maximum current of 109 nA for oxygen ions. The obtained maximum fluxes can reasonably reproduce the real flux of hydrogen (6.3E+5 /cm^2/s) and oxygen ions (1.E+6 /cm^2/s) above Titan's atmosphere [Cravens et al., 2008; Hartle et al., 2006; Horst et al., 2008]. We are going to establish an experimental procedure for stable irradiation, conduct irradiation of gaseous samples, and then analyze irradiated products. In this presentation, we will report the current status of the above.