Evaporation from a porous cometary nucleus

In a porous cometary nucleus, ice sublimates from the volume of a surface layer rather than just from the upper boundary. Given a model for the porous medium, the equations of mass and heat transfer can be solved for any desired orbit. The temperature profile and the vapor flux as a function of depth in the upper layer of the nucleus may thus be obtained. We carry out such calculations for a spherically symmetric icy nucleus in the orbit of comet P/Halley, assuming different values of porosity and different models for the ice structure. We find that the upper layer may be divided in two zones: in the uppermost zone, whose thickness ranges from 100 μm to ∼1 mm, the vapor flux is directed outward, whereas in the lower zone, which is 1000 times thicker, the vapor flows in the opposite direction. The sublimation rate as a function of heliocentric distance depends strongly on the porosity of the nucleus and is little affected by other parameters related to the structure of the ice. This allows the determination of the porosity coefficient of a comet from observation of its water production rates at large heliocentric distances.