Constraining the Enceladus plume using numerical simulation and Cassini data

Abstract Since its discovery, the Enceladus plume has been subjected to intense study due to the major effects that it has on the Saturnian system and the window that it provides into the interior of Enceladus. However, several questions remain and we attempt to answer some of them in this work. In particular, we aim to constrain the H2O production rate from the plume, evaluate the relative importance of the jets and the distributed sources along the Tiger Stripes, and make inferences about the source of the plume by accurately modeling the plume and constraining the model using the Cassini INMS and UVIS data. This is an extension of a previous work (Yeoh, S.K., et al. [2015] Icarus, 253, 205–222) in which we only modeled the collisional part of the Enceladus plume and studied its important physical processes. In this work, we propagate the plume farther into space where the flow has become free-molecular and the Cassini INMS and UVIS data were sampled. Then, we fit this part of the plume to the INMS H2O density distributions sampled along the E3, E5 and E7 trajectories and also compare some of the fit results with the UVIS measurements of the plume optical depth collected during the solar occultation observation on 18 May 2010. We consider several vent conditions and source configurations for the plume. By constraining our model using the INMS and UVIS data, we estimate H2O production rates of several hundred kgs-1: 400–500 kg/s during the E3 and E7 flybys and ∼900 kg/s during the E5 flyby. These values agree with other estimates and are consistent with the observed temporal variability of the plume over the orbital period of Enceladus (Hedman, M.M., et al. [2013] Nature, 500, 182–184). In addition, we determine that one of the Tiger Stripes, Cairo, exhibits a local temporal variability consistent with the observed overall temporal variability of the plume. We also find that the distributed sources along the Tiger Stripes are likely dominant while the jets provide a lesser contribution. Moreover, our best-fit solutions for the plume are sensitive to the vent conditions chosen. The spreading angle of the jet produced is the main difference among the vent conditions and thus it appears to be an important parameter in fitting to these INMS data sets. In general, we find that narrow jets produce better fits, suggesting high Mach numbers (> 5) at the vents. This is supported by certain narrow features believed to be jets in both the INMS and UVIS data sets. This tends to rule out sublimation from the surface but points to a deep underground source for the plume. However, the underground source can be either sublimation from an icy reservoir or evaporation from a liquid reservoir. A high Mach number at the vent also suggests subsurface channels with large variations in width and not fairly straight channels so that the gas can undergo sufficient expansion. Additionally, the broad spreading angles inferred for the µm-sized grains (Ingersoll, A.P. and Ewald, S.P. [2011] Icarus, 216, 492–506; Postberg, F., et al. [2011] Nature, 474, 620–622) cannot be due to spreading by the gas above the surface alone. Some other mechanism(s) must also be responsible, perhaps occurring below the surface, which further points to an underground source for the plume.