Subsurface Gas Flow and Ice Grain Acceleration within Enceladus and Europa Fissures: 2D DSMC Models

Abstract:

The ejection of material from geysers is a ubiquitous occurrence on outer solar system bodies. Water vapor plumes have been observed emanating from the southern hemispheres of Enceladus and Europa (Hansen et al. 2011, Roth et al. 2014), and N₂plumes carrying ice and ark particles on Triton (Soderblom et al. 2009).

The gas and ice grain distributions in the Enceladus plume depend on the subsurface gas properties and the geometry of the fissures e.g., (Schmidt et al. 2008, Ingersoll et al. 2010). Of course the fissures can have complex geometries due to tidal stresses, melting, freezing etc., but directly sampled and inferred gas and grain properties for the plume (source rate, bulk velocity, terminal grain velocity) can be used to provide a basis to constrain characteristic dimensions of vent width and depth. We used a 2-dimensional Direct Simulation Monte Carlo (DSMC) technique to model venting from both axi-symmetric canyons with widths ~2 km and narrow jets with widths ~15-40 m. For all of our vent geometries, considered the water vapor source rates (10²⁷­ – 10²⁸ s^-1) and bulk gas velocities (~330 – 670 m/s) obtained at the surface were consistent with inferred values obtained by fits of the data for the plume densities (10²⁶ – 10²⁸ s^-1, 250 – 1000 m/s) respectively. However, when using the resulting DSMC gas distribution for the canyon geometries to integrate the trajectories of ice grains we found it insufficient to accelerate submicron ice grains to Enceladus’ escape speed. On the other hand, the gas distributions in the jet like vents accelerated grains > 10 μm significantly above Enceladus’ escape speed. It has been suggested that micron-sized grains are ejected from the vents with speeds comparable to the Enceladus escape speed.

Here we report on these results including comparisons to results obtained from 1D models as well as discuss the implications of our plume model results. We also show preliminary results for similar considerations applied to Europa.

References: Hansen, 2011. Geophys. Res. Lett. 38, L11202; Ingersoll, 2010. Icarus 206, 594 – 607; Schmidt, 2008. Nature 451, 685 – 688; Soderblom, 2009. Science 250, 412 – 415; Roth, 2013l. Science http://dx.doi.org/10.1126/science.1247051 2013