CONDUIT FLOW FEEDING PLUMES ON ENCELADUS

There has been considerable debate about the origin and supply the Enceladus plume, with liquid, vapor and hybrid supply models, constrained by observed characteristics of the Enceladus plume and the E-ring. The plume exhibits a solid:vapor ratio of ~3:7, which supports neither a pure liquid nor a vapor source clearly. However, it has become apparent from Cassini UVIS analysis (Hansen et al., 2011) that the solid fractions of the jets are different from those of the overall plume, according to Porco et al. (2014) leading to a 6:1 solid:vapor ratio in the jets themselves, that more clearly supports a liquid source from a deep subsurface ocean or regional sea.

To test this model, we perform equilibrium thermodynamic and fluid mechanical analysis of multi-phase flow. We consider conduit and jet flow ascending from a deep liquid source and decompressing to below the triple point pressure. The model’s simplified chemistry includes only water driven by exsolving CO₂ (as per Matson et al., 2012), as a proxy for all driving volatiles. We assume adiabatic ascent from an initial 273 K source, with cooling buffered by phase changes. Phase changes down to the triple point pressure (~611 Pa) are small, resulting in ~2-3 wt% boiling due to decompression. Decompression to below the triple point occurs within a vent or jet, resulting in rapid simultaneously boiling and freezing at the ratio of the latent heats of evaporation and fusion. The result of the process is ~18 wt% net vapor content of the escaping materials. After this, decompression continues, no longer buffered by the latent heat of fusion. We assume rapid decompression and cooling, and depending on assumptions this can result in a few wt% sublimation (if adiabatic) or condensation (if isentropic) either way, with the latter being easier to justify. In both scenarios, the final solid:vapor ratio is somewhere in the 3.5:1 – 6.5:1 range, very consistent with the 6:1 inferred by Porco et al. (2014).

Our modeling supports the hypothesis that the Enceladus jets are drawing from a subsurface body of liquids through a conduit, in a manner akin to terrestrial explosive volcanic eruptions.

Hansen, C. et al. (2011) Geophys. Res. Lett. 38, L11202, doi:10.1029/2011GL047415.

Matson, D. et al. (2012) Icarus 221, 53-62.

Porco, C. et al. (2014) Astronom. J. 148:45, doi:10.1088/0004-6256/148/3/45.