1,2,3,2,- 1Kyoto Sangyo University, Kyoto, Japan
- 2Koyama Astronomical Observatory, Koyama Space Science Institute, Kyoto, Japan
- 3Photocross. Co. Ltd., Kyoto, Japan
- 4American University, Washington, D.C., USA
- 5Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA
- 6Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
Titan, Saturn's largest moon, possesses a dense atmosphere (1.5 bar at the surface) rich in nitrogen—unique among the moons of our solar system. Dominated by nitrogen with a methane fraction of several percent, this atmosphere hosts remarkably complex photochemical processes [e.g., 1]. Ultraviolet radiation and energetic particles trigger a cascade of reactions in the upper atmosphere, leading to the formation of a wide range of hydrocarbons and nitrile compounds. These include simple molecules such as ethane (C2H6), acetylene (C2H2), ethylene (C2H4), and hydrogen cyanide (HCN), as well as more complex organic species that may serve as precursors to prebiotic chemistry. The chemical diversity observed in Titan's atmosphere offers a natural laboratory for investigating processes analogous to those that may have occurred on early Earth.
The complexity of Titan's atmospheric chemistry is further enhanced by pronounced seasonal variations. Due to Saturn's 29.5-year orbital period and Titan's axial tilt of 26.7°, the moon undergoes extended seasons that significantly impact temperature gradients and atmospheric circulation. These seasonal dynamics, coupled with differences in solar insolation between hemispheres, lead to observable changes in the abundances and spatial distributions of minor species. Notably, several trace compounds exhibit pronounced hemispheric asymmetries that evolve over time as Titan progresses through its seasonal cycle [e.g., 2-4].
Over the past two decades, substantial progress has been made in understanding Titan's atmospheric chemistry through a combination of spacecraft and ground-based observations. The Cassini spacecraft's Composite Infrared Spectrometer (CIRS) provided an unprecedented wealth of high-resolution infrared spectral data during its 13-year mission in the Saturnian system [5]. In parallel, Earth-based infrared and radio telescopes, including the Atacama Large Millimeter/submillimeter Array (ALMA), have offered complementary views of Titan’s atmosphere, with steadily improving spatial and spectral resolution, as well as measurement sensitivity [e.g., 6-8]. Although Cassini delivered continuous coverage for nearly half a Titan year, it was unable to capture a complete seasonal cycle. This limitation underscores the importance of ongoing ground-based monitoring to develop a more comprehensive understanding of Titan's atmospheric processes.
In this study, we obtained high-resolution near-infrared spectra of Titan around 3 microns using the NIRSPEC spectrometer mounted on the Keck II telescope. The observations were conducted on October 31 and November 1, 2024, when Titan's solar longitude was approximately 173°, corresponding to northern autumn on Titan. The spectral range from 2.8 to 3.8 microns was covered at a resolving power of ~37,000 using a 0.288-arcsecond-wide slit. Distinct spectral features of CH4, CH3D, C2H2, C2H6, and HCN were clearly detected, consistent with previous studies [9-12]. Radiative transfer calculations indicate that the broad absorption feature near 3.3 microns, attributed to strong CH4 bands, is highly sensitive to the optical thickness of Titan’s haze layer. In this presentation, we present the inferred distributions of haze and minor species in Titan’s atmosphere based on our spectral data, and discuss their seasonal variations by comparing them with results from previous Cassini and ground-based observations.
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How to cite: Sagawa, H., Kawakita, H., Kobayashi, H., Bonev, B. P., Dello Russo, N., Vervack, R. J., and DiSanti, M. A.: Probing seasonal variations of trace gases in Titan's atmosphere using Keck/NIRSPEC, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–12 Sep 2025, EPSC-DPS2025-1186, https://doi.org/10.5194/epsc-dps2025-1186, 2025.
