Ingersoll, Andrew P., Porco, C. C., Helfenstein, P., West, R. A., et al. (2006). "Models of the Enceladus Plumes." American Astronomical Society, DPS meeting #38, #15.02.


Abstract
The gases in the plumes include H2O, CO2, N2, CH4, and possibly other hydrocarbons, according to the INMS team. The solid particles in the plumes are probably water ice, but the identification is less certain than for the gaseous components. The plumes emanate from warm cracks in the surface near the south pole. The gas has a scale height of 80 km, according to the UVIS team, and the particles have a scale height of 30 km, according to the ISS team. By integrating across the plumes in their images, the ISS team was able to infer the upward flux of particles vs. altitude. Close to the surface, the falloff of density with altitude is much steeper than that of an escaping atmosphere in which both the particles and the gas are moving upward with the thermal velocity of the gas. Thus some of the particles are falling back to the surface and some are escaping. The larger scale height of the gas implies that the escaping fraction is greater for the gas. I will present models that attempt to explain these plume data. The density of the gas and the size of the particles determine the degree of dynamical coupling between gas and particles. There are three models - a sublimating gas that picks up particles as it leaves the surface, sublimating gas that forms particles in flight as the pressure decreases, a boiling liquid that freezes by evaporative cooling as the pressure decreases. Each model has its own range of mass flux, density, particle size, and scale height for both gas and liquid. I will discuss the implications of the observations and models regarding the possibility of liquid water near the surface.