UVIS airglow observations analysis: application for Titan's haze properties retrieval at multiple latitudes and timescales.

Titan's organic haze is central to the moon's atmospheric energy balance and photochemistry, and are intimately tied to cloud formation and the broader chemical cycles operating in the stratosphere [1, 2, 3, 4]. Constraining haze properties is therefore essential to validate the theoretical models describing their formation and growth [5, 6].

Several Cassini instruments have been used to measure haze extinction, through stellar occultations with UVIS [7] and VIMS [8], thermal emission with CIRS [9], or direct imaging with ISS [10]. However, extinction alone cannot separate particle size from number density. Most studies have worked around this by fixing particle size to Huygens/DISR values [11], which are only representative of a single location and altitude range. ISS scattered-light observations have allowed both quantities to be retrieved simultaneously [12, 13] near the detached haze layer (around 500 km).

We present the latest results of our analysis of the Ultraviolet Imaging Spectrograph (UVIS) limb airglow observations, provided by the latest UVIS Python package (UPyP) [14]. We use multiple flyby data within the same terrestrial year to collect UV emission from scattering by haze particles in the 1600-1900 Å range, at different phase angles, to fit the phase function of aerosols at different altitudes that is sensitive to particle size. We limit our analysis to phase angles below 150° as observations at higher angles appear contaminated by instrumental effects.

The diversity of observations allows us to retrieve the haze particle size and number density at multiple altitudes, latitudes and times. The inversion procedure considers a spherical layer structure of the atmosphere, including the haze single-scattering, N₂ rayleigh scattering, extinction by haze and gases, and airglow emission [15] depending on each observation geometry. At each altitude layer, the haze is represented by a log-normal size distribution with a mean radius set as a free parameter, and a number density set as a second free parameter. The two parameters are inverted using the maximum a-priori likelihood (MAP) [16]. The data is binned by latitude and local time to retrieve information at different latitudes and derive diurnal variations. The entire Cassini mission dataset is used to perform this analysis allowing us to retrieve the spatial and temporal evolution of the haze properties through out the mission.

 

References

[1] Courtin. "Aerosols on the Giant Planets and Titan". Space Science Reviews, 2005.

[2] Lavvas et al. "Condensation in Titan's Atmosphere at the Huygens Landing Site". Icarus, 2011.

[3] Larson et al. "Simulating Titan's Aerosols in a Three Dimensional General Circulation Model". Icarus, 2014.

[4] De Batz De Trenquelléon et al. "The New Titan Planetary Climate Model. II. Titan's Haze and Cloud Cycles". The Planetary Science Journal, 2025.

[5] Lavvas et al. "Surface Chemistry and Particle Shape: Processes for the Evolution of Aerosols in Titan's Atmosphere". The Astrophysical Journal, 2011.

[6] Lavvas et al. "Aerosol Growth in Titan's Ionosphere". Proceedings of the National Academy of Sciences, 2013.

[7] Koskinen et al. "The Mesosphere and Lower Thermosphere of Titan Revealed by Cassini/UVIS Stellar Occultations". Icarus, 2011.

[8] Bellucci et al. "Titan Solar Occultation Observed by Cassini/VIMS: Gas Absorption and Constraints on Aerosol Composition". Icarus, 2009.

[9] Vinatier et al. "Analysis of Cassini/CIRS Limb Spectra of Titan Acquired during the Nominal Mission II: Aerosol Extinction Profiles in the 600–1420 Cm^-1 Spectral Range". Icarus, 2010.

[10] Seignovert et al. "Haze Seasonal Variations of Titan's Upper Atmosphere during the Cassini Mission". The Astrophysical Journal, 2021.

[11] Tomasko et al. "A Model of Titan's Aerosols Based on Measurements Made inside the Atmosphere". Planetary and Space Science, 2008.

[12] Seignovert et al. "Aerosols Optical Properties in Titan's Detached Haze Layer before the Equinox". Icarus, 2017.

[13] West et al. "The Seasonal Cycle of Titan's Detached Haze". Nature Astronomy, 2018.

[14] Le Guennic et al. "Methods for analysing low signal-to-noise emission observations: the Cassini Ultraviolet Imaging Spectrograph pipeline and application to Titan airglow observations". The Planetary Science Journal, 2026 (accepted).

[15] Lavvas et al. "N2 State Population in Titan's Atmosphere". Icarus, 2015.

[16] Rodgers. "Inverse Methods for Atmospheric Sounding: Theory and Practice". World Scientific, 2000.