Titan’s Upper Atmospheric Structure from Cassini/UVIS Observations

Titan is the only solar system satellite that possesses a significant atmosphere, which is composed mainly of N₂ and CH₄.  Within Titan’s upper atmosphere, solar UV photolysis of N₂ and CH₄ initiates the production of hydrocarbons and nitriles, and the photochemical growth terminates with the formation of a thick organic haze (Waite et al., 2005).  These products descend to Titan’s surface and lakes, possibly engaging on prebiotic chemistry with materials from these environments (Raulin et al., 2012).  The distributions of N₂ and CH₄ in Titan’s upper atmosphere, where photochemistry and haze formation are initiated, are highly variable for reasons that are poorly understood.  In particular, the Cassini Ultraviolet Imaging Spectrograph (UVIS) and Ion Neutral Mass Spectrometer (INMS) probed densities and temperatures in Titan’s thermosphere (Esposito et al., 2004; Waite et al., 2004).  N₂ and CH₄ densities within Titan’s thermosphere were retrieved from UV solar occultations (Capalbo et al., 2013, 2015), UV stellar occultations (Koskinen et al., 2011; Kammer et al., 2013; Yelle et al., 2021), UV dayglow (Stevens et al., 2015), and INMS in-situ observations (Yelle et al., 2006; Cui et al., 2009; Snowden et al., 2013). During Cassini’s Titan observations, the N₂ and CH₄ densities both varied by about one order of magnitude.  The density profiles do not appear to exhibit strong correlations with geophysical variables such as latitude, longitude, solar zenith angle, and local solar time (Müller-Wodarg & Koskinen, 2025).  UVIS scans of Titan’s airglow between 2004 and 2017 provide a global view of the atmosphere, which can help to uncover the origin of the variations.  However, most of the 635 UVIS scans were previously unanalyzed.  Having analyzed multiple UVIS scans, we present results based on our investigation of variability in Titan’s upper atmosphere.

We implement a fast, simplified optimal estimator to retrieve N₂, CH₄, and H densities from UVIS scans of Titan’s far ultraviolet (FUV) dayglow (Lavvas & Koskinen, 2022).  Our approach’s validity depends on the assumption that solar-driven processes dominate Titan’s dayglow emissions, which we previously demonstrated (Hoover et al., 2024).  To characterize variations in temperature and vertical mixing across latitude and time, we construct a Titan atmospheric structure emulator.  We create a parametrized pressure-temperature (P-T) profile in the mesosphere and thermosphere. For the lower atmosphere, we incorporate the SVRS temperature profile (Lombardo & Lora, 2023).  The SVRS dataset is strongly anchored in Cassini Composite Infrared Spectrometer observations of Titan’s lower atmosphere and simulations of Titan’s stratospheric dynamics.  We set the CH₄ volume mixing ratio at the stratopause (0.0146063) and use a parametrized K_zz profile (Yelle et al., 2008) to model the CH₄ mixing ratio profile throughout the atmosphere.  Our fit parameters are the mean temperature in the upper atmosphere (P < 3.9 ∗ 10⁻¹⁰ bar), mesopause temperature, K_zz value in the upper atmosphere, and a scale factor needed to match the H density profile from a photochemical model with the observations.  We use emcee (Foreman-Mackey et al., 2013) to retrieve atmospheric structure parameters by fitting the emulator model to the retrieved density profiles.

Based on these algorithms, we present N₂, CH₄, and H density profiles, as well as atmospheric structure parameters, retrieved from our dataset of UVIS scans.  Near the equator, the mean upper atmospheric temperature varies significantly over timescales below one year.  For some observations, we perform retrievals at multiple latitudes; in most of these cases, Titan’s mean upper atmospheric temperatures exhibit little change over latitude.  Changes in mean upper atmospheric temperature do not appear to be correlated with solar activity, suggesting that other factors, such as activity from Titan’s lower atmosphere or Saturn’s magnetosphere, may influence variations in Titan’s upper atmosphere more significantly.  To test the hypothesis that variability in Titan’s upper atmosphere is driven by waves and circulation patterns from the stratosphere (the "intrinsic variability" hypothesis), we compare our retrieved densities and temperatures with output from a Titan Thermosphere General Circulation Model (Müller-Wodarg et al., 2008; Müller-Wodarg & Koskinen, 2025).  Upper atmospheric K_zz values derived from our CH₄ profiles are more than one order of magnitude higher than those derived from Ar profiles measured by INMS (Yelle et al., 2008), indicating that CH₄ is undergoing significant escape.

 

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