This project will develop a comprehensive numerical model of the formation and dynamical evolution of the Enceladus water vapor and ice particle plumes (EPs). Since the nature of the reservoir that produces EPs is hotly debated, including eruptions from chambers of liquid water, sublimation and venting of hydrate clathrate, and vapor diffusion through ice pores, the model proposed here will simulate different origins and allow comparison to available observations. The model describes the dynamics of compressible gas and of boiling liquids expanding through narrow channels, particle growth in the expanding vapor, condensation and sublimation on the walls, heat transfer and vapor diffusion in the surrounding ice, and feedback between these different elements. This work will advance our knowledge of the subsurface structure of Enceladus and of processes occurring on icy moons elsewhere in the solar system, producing the first complete model of the EP dynamics and thermodynamics.
The search for liquid water has broad societal impact by capturing the imagination of the public and of young people who might be considering scientific careers. Enceladus is a potentially important island of habitability: this project tests the hypothesis that liquid water exists below the surface and at what depth it might exist, and whether there is a source of chemical energy to support life.
Enceladus is a small moon of Saturn whose surface is water ice at a temperature of minus 200 C (minus 328 F). Yet there is evidence of liquid water only a few kilometers below the surface. The evidence is indirect and derives from the plumes of water vapor and icy particles that emanate from cracks near the south pole (Figs. 1 and 2). The goal of this research is to use observations of the plumes to constrain what the conditions are like below ground and ultimately to decide if liquid water is present. Liquid water is important because it is one of the ingredients for life. The other ingredients, broadly speaking, are nutrients and energy, and Enceladus seems to have both. The main chemical elements of life are hydrogen, oxygen, carbon, and nitrogen. The ice and water vapor have hydrogen and oxygen, and the edges of the cracks have hydrocarbons, which show up in the spectrum of sunlight reflected from their surfaces. Specifically, the chemical bond between carbon and hydrogen vibrates at a certain frequency, so light at that frequency is absorbed on its way to the detectors on the spacecraft. Nitrogen is present as ammonia, which is one of the gases in the plumes in addition to water vapor. Other nutrients are present in the ice particles, which have dissolved salts of various chemical compositions. This mixture of diverse elements has chemical energy, which could be enough to sustain life. If liquid water is present, it could signal a source of thermal energy that also could sustain life. Liquid water is thus the key to habitability on Enceladus and is therefore the principal focus of this research. If the plumes arise from controlled boiling of a liquid water reservoir, the bubbles would carry a large amount of liquid with them as the frothy mixture rises in the vents and escapes to space. Exposed to vacuum, the liquid cools by evaporation. The surface freezes first, but the expansion due to freezing of the interior causes the surface to shatter. This process continues until the mixture is reduced to a cloud of small ice particles imbedded in a cloud of water vapor. However if the plumes arise from sublimation of warm ice, the particles would have to condense from the vapor as it rises in the vents. A key quantity is the ice/vapor ratio – the mass of ice in the plumes compared to the mass of vapor. Theoretical work, which is part of this investigation, has shown that the ratio is small – less than a few percent – if the ice condenses from the vapor and the plumes arise from sublimation of warm ice. Observational work, which is also part of this investigation, has shown that the ratio is large – in the range 0.35 to 0.70. This strongly favors the liquid water source. This investigation also reveals that the bulk of the particles are moving at slow speeds relative to the gas. This is incompatible with the sublimation source, since the particles that condense from the vapor tend to move with it. Further work is needed on the behavior of liquids exposed to vacuum. Boiling need not be explosive: Friction with the walls raises the pressure at the bottom of the crack until it is nearly equal to the pressure of the saturated liquid underneath. This leads to controlled boiling, which is compatible with the steady appearance of the plumes. Further work is needed as well on the terrain surrounding the cracks where the plumes originate. Whether the bubbly liquid reaches the surface is still an open question. The habitable zone was once defined as the space between Venus and Mars where water can exist as a liquid. We now know that it is an archipelago that includes the icy moons in the outer solar system. Because of its active plumes, Enceladus is perhaps the best example of habitability beneath an icy crust. This investigation has expanded the range of places in our solar system where life could possibly exist.
