5:15 PM - 7:15 PM
[PPS06-P10] Time Variations of Titan's Atmosphere Probed by Visible and Near-Infrared Observations
Keywords:Titan, Ground-based Telescope, Atmosphere
Titan has a thick atmosphere that reaches 1.46 atm at the surface. The atmosphere, which is more than 95% nitrogen and 1-5% methane, resembles the early Earth's atmosphere before the oxidation event that led to the birth of life, and the chemical reactions that led to the birth of life are thought to continue even today. In the upper atmosphere, methane and nitrogen molecules are photodissociated and ionized to form organic aerosols (tholin), which form a global haze layer. The haze layer is optically thick in the visible wavelengths, and the wavelengths that reach the surface are limited to wavelengths longer than the infrared. Titan passes through Saturn's magnetospheric plasma sheet during its orbital motion (period of about 16 days), which increases the number of energetic particles falling into the atmosphere. This is thought to accelerate methane consumption and tholin formation reactions in Titan's atmosphere. The effect of Saturn's magnetosphere on Titan's atmosphere is reported to be that the methane mixing ratio in the Titan thermosphere varies between 1.1-2.4% (Vervack et al., 2004) and that the temperature in the Titan thermosphere is on average 29 K higher in the plasma sheet than in the plasma lobes (Westlake et al., 2011). However, monitoring that covers the entire orbital period is not possible. However, monitoring observations covering the entire orbital period have not yet been conducted, and the time variations of Titan's atmosphere within the orbital period are unknown. When solar wind dynamic pressure is high, Titan's orbit is observed to be outside of Saturn's magnetospheric bow shock (Feyerabend et al., 2015), at which time solar wind particles are thought to promote methane consumption and tholin production reactions, but no monitoring observations including before and after the increase in solar wind dynamic pressure have been conducted. However, there have been no monitoring observations including before and after the increase in solar wind dynamic pressure, and the time variation of Titan's atmosphere in relation to solar activity has not been clarified.
The purpose of this study is to elucidate the temporal variations of Titan's atmosphere related to Titan's orbital motion and solar activity. Using the Multispectral Imager (MSI) on the Pirka Telescope owned by Hokkaido University (Watanabe et al., 2012), we conducted multi-wavelength imaging observations including methane absorption wavelengths (727 and 889 nm) from 2021 to 2024. The reflectance and its temporal variation were derived from the observation data. The derived reflectance was compared with an atmospheric radiative transfer model to quantify the methane absorption and the optical thickness of the haze layer. The mean reflectance at 727 nm was 0.1549, which is consistent with the value (~0.16) observed in a previous study (Karkoschka, 1994). 727 nm reflectance varied with a standard deviation of 0.0151 and tended to be smaller at near-Saturn points. This may be due to an increase in the number of energetic particles falling from Saturn's magnetospheric plasma sheet or an increase in the number of solar wind particles falling due to strong solar wind dynamic pressure. When Titan is inside the plasma sheet or outside the bow shock, the number of energetic particles increases, methane is consumed, and tholin is produced, resulting in a decrease in methane column density and an increase in reflectance at methane absorption wavelengths. In this presentation, we will compare the derived reflectance variations and possible factors, and discuss the temporal variations of Titan's atmosphere related to the orbital motion.
The purpose of this study is to elucidate the temporal variations of Titan's atmosphere related to Titan's orbital motion and solar activity. Using the Multispectral Imager (MSI) on the Pirka Telescope owned by Hokkaido University (Watanabe et al., 2012), we conducted multi-wavelength imaging observations including methane absorption wavelengths (727 and 889 nm) from 2021 to 2024. The reflectance and its temporal variation were derived from the observation data. The derived reflectance was compared with an atmospheric radiative transfer model to quantify the methane absorption and the optical thickness of the haze layer. The mean reflectance at 727 nm was 0.1549, which is consistent with the value (~0.16) observed in a previous study (Karkoschka, 1994). 727 nm reflectance varied with a standard deviation of 0.0151 and tended to be smaller at near-Saturn points. This may be due to an increase in the number of energetic particles falling from Saturn's magnetospheric plasma sheet or an increase in the number of solar wind particles falling due to strong solar wind dynamic pressure. When Titan is inside the plasma sheet or outside the bow shock, the number of energetic particles increases, methane is consumed, and tholin is produced, resulting in a decrease in methane column density and an increase in reflectance at methane absorption wavelengths. In this presentation, we will compare the derived reflectance variations and possible factors, and discuss the temporal variations of Titan's atmosphere related to the orbital motion.