Seasonal Variability of Stratospheric H2O on Titan

Seasonal Variability of Stratospheric H₂O on Titan

Titan is Saturn’s largest moon and one of the most complex Earth-like bodies in our Solar system. It hosts a thick, complex atmosphere with weather systems [1], rich C-N-H photochemistry [2], and unique surface features such as lakes of methane [3]. The presence of organic hazes and oxygen-bearing molecules in the atmosphere make Titan astrobiologically important and provides an analogous natural laboratory to study pre-biotic Earth [4] and exoplanets with similar climates. Understanding Titan’s atmosphere is also pertinent to inform NASA's Dragonfly mission set to arrive in 2034 [5].

Water vapour is an important, yet poorly understood presence in Titan’s atmosphere. It plays a vital role in distributing oxygen molecules, which are otherwise scarce, throughout the planet to form species such as CO, CO₂ and H₂CO [2]. Water vapour was first detected in Titan’s atmosphere in 1998 by the Infrared space Observatory [6]. Since then, only a handful of studies from Herschel [7], CIRS [8] and the INMS [9] instruments have provided observations. Due to modelling difficulty and its low abundances, there is limited information on seasonal, global and vertical abundances of Titan’s H₂O with research focusing on averages and single measurements.

146 far-IR observations acquired by CIRS on-board the Cassini spacecraft were analysed to form the first-reported global picture of H₂O abundances in Titan’s stratosphere across its 13-year mission, improving on previous studies. Using the most recent photochemical model [2] as an a priori in the NEMESIS radiative transfer modelling tool [10] and a new method of applying parameterised gaussian cross-sections [11] to fit the poorly understood hazes, we present results showing the seasonal variability of water vapour at pressures of ~ 0.1-10 mbar. We discuss our results and its implications, and compare our findings to previous work. 

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