Presentation #402.02 in the session “Titan Atmosphere”.
Although water has been detected and quantified in Titan’s atmosphere by previous modeling efforts, its temporal and spatial variability has yet to be investigated over the entirety of Cassini’s operational lifetime. We have focused our search for latitudinal and temporal variation of water vapor in the 120-260 cm-1 range, using Cassini’s Composite Infrared Spectrometer (CIRS) instrument. CIRS Focal Plane 1 (FP1, 10-600 cm-1) has high spectral resolution (0.5 cm-1), which is required for the detection of weak water lines. Nadir spectra were chosen to cover all latitudes over Titan’s disk during the operational lifetime of Cassini. We split the 12.5-year data set (2005-2017) into five Titan months (2.5 Earth years) to allow for high temporal resolution of spectra around seasonal changes. Each Titan month was split again into six latitude bins corresponding to 30° latitude each, for a total of 30 bins over the entire temporal range. We modeled the data set using the Non-linear Optimal Estimator for MultivariatE Spectral AnalySIS (NEMESIS) planetary atmosphere radiative transfer and retrieval tool. We began by retrieving temperature profiles by modeling a series of methane lines between 125 and 155 cm-1. Aerosol, trace gas, and water vapor scaling factors were also retrieved from a set of modeled a priori estimates. The models targeted the entire 120-260 cm-1 range, which allowed for the robust detection and modeling of rotational water lines. In these models, contribution functions, which denote the sensitivity of our water retrievals, peaked in the lower stratosphere (90-180km). We subsequently derived stratospheric mixing ratios for water in each of the modeled bins. Here, we are able to provide new stratospheric mixing ratios that record Titan’s spatial and temporal dispersion of water vapor. These results will better constrain photochemical models and help determine whether the external source of water comes from interplanetary dust particles, Saturn’s rings, or from Enceladus. References:  Coustenis et al. Icarus 207 (2010): 461-476.  Jennings et al. Applied Optics 56 (2017): 5897-5897.  Irwin et al. Journal of Quantitative Spectroscopy & Radiative Transfer 109 (2008): 1136–1150.  Bauduin et al. Icarus 311 (2018): 288–305)