Science Seminars

2/16/2017 – Ganna Portyankina; Enceladus jets: deciphering CASSINI’s occultation observations with models

Speaker: Ganna Portyankina (LASP)
Date: Sunday, Feb 16, 2020
Time: 4:00 PM
Location: SPSC W120

Seminar Abstract:

One of the most spectacular discoveries of the Cassini mission is jets emitting from the southern pole of Saturn’s moon Enceladus. The composition of the jets is water vapor and salty ice grains with traces of organic compounds. Jets, merging into a wide plume at a distance, are observed by multiple instruments on Cassini. The mass fraction of icy grains to vapor in the jets is currently largely unconstrained varying from < 0.1 to 0.7. This large variation mostly stems from the lack of simultaneous measurements of abundances of icy grains and vapor which is needed to account for the temporal and spatial variations in both dust and gas outputs. The Cassini Ultraviolet Imaging Spectrograph (UVIS) has observed occultations of several stars and the Sun by the water vapor plume of Enceladus. On one occasion, UVIS was joined by Visual and Infrared Mapping Spectrometer (VIMS) and both instruments followed the Sun being occulted by the Enceladus jets. UVIS is best suited to derive water vapor abundances, while VIMS is sensitive to icy grains. We use a 3D direct simulation Monte Carlo (DSMC) model for Enceladus’ jets to model both vapor and ice components of the water jets. The Monte Carlo model tracks test particles from their source at the surface into space. The overarching result of the simulation run is a test particle number density along line-of-sight for each time point during the occultation observation for each of the jets separately. The geometry of the occultation observation is such that the line of sight at each point of time crosses multiple jets from different tiger stripes. Thus the relative strength of the jets must be determined to fit the observed UVIS and to a degree VIMS curves. Peaks in column density along UVIS line of sight can be explained with isolated highly collimated jets that have different production rates and vertical molecule velocities of 1.8 km/s. At least 43 individual jets are required to fit the observed UVIS data, while a combination of a larger set of linearly-dependent jets cannot be excluded. The modelled distribution of jets needed to reproduce the VIMS and UVIS results are different, indicating different mass fraction icy grains across the active south polar region. This means that the bulk plume mass fraction icy grains estimates do not provide a representative measure of this parameter. The variable composition also suggests variable influence of the subsurface processes in plume generation at different source fissures.