Saturday, April 30, 2016

The Contribution of Water Filled Cracks to Enceladus' Plumes

Controlled boiling on Enceladus. 2. Model of the liquid-filled cracks


Ingersoll et al


Controlled boiling will occur on Enceladus whenever a long, narrow conduit connects liquid water to the vacuum of space. In a companion paper we focus on the upward flow of the vapor and show how it controls the evaporation rate through backpressure, which arises from friction on the walls. In this paper we focus on the liquid and show how it flows through the conduit up to its level of neutral buoyancy. For an ice shell 20 km thick, the liquid water interface could be 2 km below the surface. We find that the evaporating surface can be narrow. There is no need for a large vapor chamber that acts as a plume source. Freezing on the icy walls and the evaporating surface is avoided if the crack width averaged over the length of the tiger stripes is greater than 1 m and the salinity of the liquid is greater than 20 g kg−1. Controlled boiling plays a crucial role in our model, which makes it different from earlier published models. The liquids on Enceladus are boiling because there is no overburden pressure—the saturation vapor pressure is equal to the total pressure. Salinity plays a crucial role in preventing freezing, and we argue that the subsurface oceans of icy satellites can have water vapor plumes only if their salinities are greater than about 20 g kg−1.

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