Latitudinal and Temporal Variability of Water Vapour in Titans Stratosphere

Since its discovery around 20 years ago, water vapour (H2O) has been measured in Titans stratosphere from different space instruments. The estimations of its abundance obtained so far are complicated to reconcile without considering possible spatial and temporal variations, partly driven by the meridional circulation of Titans atmosphere. However, these variations cannot be fully confirmed given the very few H2O abundance measurements currently available. This still prevents photochemical models to fully constrain the external oxygen flux at Titan and, as a result, to determine the main source of water in Titans atmosphere. In this work, we try to confirm the existence of latitudinal/temporal variations of the H2O abundance in Titans stratosphere. For this, we exploit far-infrared nadir observations taken by the Focal Plane 1 (FP1, 10-640 cm-1) of the Cassini Composite Infrared Spectrometer (CIRS) during almost the entire Cassini mission (2005-2017). More particularly, we focus on spectra recorded at a spectral resolution of 0.5 cm-1, which is required to detect the weak H2O lines. To investigate the latitudinal and temporal variations of H2O, this 12.5-year CIRS FP1 dataset is split into 5 Titan months (2.5 Earth years) to allow sufficient temporal resolution around the seasonal changes. For each Titan month, average spectra are built for 6 latitudinal bins of 30 each, leading to a total of 30 spectra to analyze over the entire temporal range. The analysis, including the temperature, the aerosols and the H2O abundance retrievals, is performed using the NEMESIS software and will be presented. The retrieved H2O abundances will be shown and their latitudinal/temporal variations will be discussed in terms of their possible causes. Overall, the results of this work will help photochemical models to determine the nature of the external source of H2O in Titans atmosphere.