Seasonal variations of CH4, HCN, and C2H2 in Titan’s upper atmosphere from 13 years of Cassini/VIMS observations

The Cassini–Huygens mission (2004–2017), covering nearly half a Titan year, provided an unprecedented dataset to investigate the seasonal variability of Titan’s upper atmosphere. The Visual and Infrared Mapping Spectrometer (VIMS) onboard Cassini enabled long-term monitoring of the poorly characterized upper atmosphere of Saturn’s largest moon, detecting strong infrared non-LTE emissions from key species, including CH₄, HCN, and C₂H₂, which serve as tracers of Titan’s atmospheric chemistry and dynamics.

This study builds on previous analysis of VIMS daytime infrared spectra acquired from 2004–2012 (Dinelli et al., 2019), which revealed clear seasonal variability in the abundance and distribution of these molecules during northern winter (2004-2009) and early spring (2010-1012) and suggested an extended meridional circulation reaching well above 500 km, possibly up to 750–800 km or higher.

Here, we extend the dataset to 2013–2017 to complete seasonal coverage through northern late spring and southern late autumn. The analysis focuses on the 2.9–3.4 µm range, probing altitudes from 500 to 1100 km, spanning the upper mesosphere and lower thermosphere. The results show large enhancements of HCN and C₂H₂ near 800 km at the south pole during southern late autumn, consistent with an extension of the middle atmosphere circulation above 500 km. At similar latitudes, enhanced CH₄ concentrations are observed above 700 km, supporting the hypothesis that the meridional circulation may reach the lower thermosphere, although reduced photodissociation under weak autumn illumination might also contribute.

These results support a vertically extended meridional circulation, with subsidence over the winter pole linked to a north polar vortex and a subsequent reversal after its breakdown following the northern spring equinox.

 

References:

Dinelli, B.M., Puertas, M.L., Fabiano, F., Adriani, A., Moriconi, M.L., Funke, B., García-Comas, M., Oliva, F., D’Aversa, E., Filacchione, G., 2019. Climatology of CH₄, HCN and C₂H₂ in Titan’s upper atmosphere from Cassini/VIMS observations. Icarus 331, 83–97. doi: 10.1016/j.icarus.2019.04.026