Modeling the complete set of Cassini’s UVIS occultation observations of Enceladus’ plume

Abstract

The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed a plume of water vapor spewing out from the south polar regions of Enceladus via occultations 7 times during the Cassini mission. Five of them yielded spatially resolved data that allowed fits to a set of individually modeled jets. We created a direct simulation Monte Carlo (DSMC) model to simulate individual water vapor jets with the aim of fitting them to water vapor abundances along the UVIS line of sight during occultation observations. Accurate location and attitude of spacecraft together with positions of Enceladus and Saturn at each observation determine the relationship between the three-dimensional water vapor number density in the plume and the two-dimensional profiles of water vapor abundances along the line of sight recorded by UVIS. By individually fitting observed and modeled jets, every occultation observation of UVIS presented a unique perspective to the physical properties and distribution of the jets. The minimum velocity of water vapor in the jets is determined from the narrowest observed individual jet profile: it ranges from 800 m/s to 1.8 km/s for the UVIS occultation observations. 41 individual jets were required to fit the highest resolution UVIS dataset taken during the Solar occultation however, an alternative larger set of linearly-dependent jets cannot be excluded without invoking additional unrelated data from other instruments. A smaller number of jets is required to fit the stellar occultation data because of their spatial resolution and geometry. We identify a set of 37 jets that were repeatedly present in best fits to several UVIS occultation observations. These jets were probably active through the whole Cassini mission.