Observations of the temporal evolution of Saturn’s stratosphere following the Great Storm of 2010–2011

Context. Water vapour is delivered to Saturn’s stratosphere by Enceladus’ plumes and subsequent diffusion in the planet system. It is expected to condense into a haze in the middle stratosphere. The hot stratospheric vortex (the ‘beacon’) that formed as an aftermath of Saturn’s Great Storm of 2010 significantly altered the temperature, composition, and circulation in Saturn’s northern stratosphere. Previous photochemical models suggested haze sublimation and vertical winds as processes likely to increase the water vapour column density in the beacon. Aims. We aim to quantify the temporal evolution of stratospheric water vapour in the beacon during the storm. Methods. We mapped Saturn at 66.44 and 67.09 μm on seven occasions from July 2011 to February 2013 with the PACS instrument of the Herschel Space Observatory (Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA). The observations probe the millibar levels, at which the water condensation region was altered by the warmer temperatures in the beacon. Using radiative transfer modelling, we tested several empirical and physically based models to constrain the cause of the enhanced water emission found in the beacon. Results. The observations show an increased emission in the beacon that cannot be reproduced only accounting for the warmer temperatures reported in the beacon. An additional source of water vapour is thus needed. We find a factor (7.5±1.6) increase in the water column in the beacon compared to pre-storm conditions using empirical models. Combining our results with a cloud formation model for July 2011, we evaluate the sublimation contribution to 45–85% of the extra column derived from the water emission increase in the beacon. Conclusions. The observations confirm that the storm conditions enhanced the water abundance at the millibar levels because of haze sublimation and vertical winds in the beacon. Future work on the haze temporal evolution during the storm will help to better constrain the sublimation contribution over time.