The two-phase water plumes arising from the Enceladus South pole of are a key signature of what lies below the surface. Multiple Cassini instruments measured the gas-particle plume over the warm Tiger Stripe region during several close flybys. Numerous observations also exist of the near-vent regions in the VIS and IR. The most likely source for these extensive geysers is a subsurface liquid reservoir of somewhat saline water and other volatiles boiling off through crevasse-like conduits into the vacuum of space.
We used a Direct Simulation Monte Carlo (DSMC) code (Yeoh et al., 2017) to simulate the plume, as it exits a vent, under axisymmetric conditions, in a vertical domain extending up to 10 km (Mahieux et al., 2019). We performed a parametric study of the following parameters: vent diameter, outgassed flow density, water vapor/ice mass ratio, gas and ice speed, ice grain diameter and vent exit angle. We constructed parametric expressions for the plume characteristics – number density, temperature, velocity components – at the 10 km upper boundary, where the flow has become free-molecular. We use these parametrizations to propagate the plumes to higher altitudes, assuming free-molecular conditions. This enables very rapid numerical computations – ~10 minutes – and tabulations of the density field in space.
We present a formal sensitivity analysis of twelve vent parameters – vent diameter, outgassed flow density, water gas/ice mass flow ratio, gas and ice speed, ice grain diameter, vent exit angle, latitude, longitude, azimuth and zenith angles of the venting direction – conditioned on the number density field measured by the INMS instrument, considering the 98 vent geometry reported in Porco et al. (2014).