Presentation Information
[PPS01-P16]Time Variations of Titan's Atmosphere Proved by Ground-based Telescope
*Mizuho NAKAJIMA1, Seiko Takagi1, Mitsuteru Sato1, Hisayuki Kubota1, Yukihiro Takahashi1 (1.Hokkaido University)
Keywords:
Titan,Ground-based Telescope,Atmosphere
Saturn's moon Titan is the only satellite with a substantial atmosphere, composed primarily of nitrogen and methane. In the upper atmosphere, chemical reactions driven by ultraviolet radiation and high-energy particles produce complex organic molecules. These organic molecules condense and grow, forming haze layers in Titan's atmosphere. Ground-based telescopes and the Cassini-Huygens mission have observed temporal variations in haze layers and their precursor methane. During the winter season, the optical thickness of the haze increases [Karkoschka, 2022], and the amount of methane present also increases [Vinatier et al., 2015]. These variations are thought to result from changes in haze particle production and transport rates. However, many aspects of the haze layer's formation and maintenance mechanisms, as well as its temporal variations, remain poorly understood [Hörst, 2017]. Furthermore, while it has been suggested that the haze particle production rate may be influenced by the ionospheric plasma environment [Lavvas et al., 2013], knowledge of the Saturn magnetosphere's impact on Titan's atmosphere and haze layer remains limited due to insufficient observations. This study aimed to elucidate the formation and maintenance mechanisms of the haze layer.
Long-term continuous observations of Titan were conducted from 2023 to 2025 using the Pirika telescope and its instruments, which are owned by the Hokkaido University Observatory. Analysis of the observational data yielded reflectance and its temporal variations. The results show that the average equivalent widths in the methane absorption bands at 727 nm and 889 nm were 6.87 and 8.97, respectively, with an increasing trend in the annual average values. Furthermore, by comparing observational results with numerical calculations from radiative transfer models, we quantitatively evaluated variations in haze and methane optical thickness and derived their temporal variations. The results indicated that to match the observed equivalent width, the haze optical thickness needed to be adjusted by a factor of 1.12, or the methane optical thickness by a factor of 0.43 to 0.92. Moreover, to explain the observed increasing trend in equivalent width, the haze optical thickness must decrease from 1.20 times to 1.15 times in the 727 nm band and from 1.11 times to 0.56 times in the 889 nm band. This is consistent with the seasonal variation in the global average optical thickness of the haze layer, which passes through the vernal equinox in 2025. When explaining the temporal variation in equivalent width by the variation in methane optical thickness, the methane optical thickness must increase, which is the opposite trend to the seasonal variation. On the other hand, no correlation was found between Titan's position within Saturn's magnetosphere and the variation in haze and methane optical thickness.
Therefore, this study concludes that the observed temporal variations in Titan's reflectance spectrum can be explained by variations in the haze layer's optical thickness. The results suggest that the mechanism for generating and maintaining the haze layer is related to variations in the amount of incident sunlight. On the other hand, it is suggested that the influence of Saturn's magnetosphere on the mechanism for generating and maintaining the haze layer is likely small.
Long-term continuous observations of Titan were conducted from 2023 to 2025 using the Pirika telescope and its instruments, which are owned by the Hokkaido University Observatory. Analysis of the observational data yielded reflectance and its temporal variations. The results show that the average equivalent widths in the methane absorption bands at 727 nm and 889 nm were 6.87 and 8.97, respectively, with an increasing trend in the annual average values. Furthermore, by comparing observational results with numerical calculations from radiative transfer models, we quantitatively evaluated variations in haze and methane optical thickness and derived their temporal variations. The results indicated that to match the observed equivalent width, the haze optical thickness needed to be adjusted by a factor of 1.12, or the methane optical thickness by a factor of 0.43 to 0.92. Moreover, to explain the observed increasing trend in equivalent width, the haze optical thickness must decrease from 1.20 times to 1.15 times in the 727 nm band and from 1.11 times to 0.56 times in the 889 nm band. This is consistent with the seasonal variation in the global average optical thickness of the haze layer, which passes through the vernal equinox in 2025. When explaining the temporal variation in equivalent width by the variation in methane optical thickness, the methane optical thickness must increase, which is the opposite trend to the seasonal variation. On the other hand, no correlation was found between Titan's position within Saturn's magnetosphere and the variation in haze and methane optical thickness.
Therefore, this study concludes that the observed temporal variations in Titan's reflectance spectrum can be explained by variations in the haze layer's optical thickness. The results suggest that the mechanism for generating and maintaining the haze layer is related to variations in the amount of incident sunlight. On the other hand, it is suggested that the influence of Saturn's magnetosphere on the mechanism for generating and maintaining the haze layer is likely small.