On Pore Size and Fracture in Gas-Laden Comet Nuclei

Abstract

We have followed the evolution of a porous comet nucleus made of gas-laden amorphous ice in the orbit of P/Halley. The gas is released from the ice as the ice crystallizes and flows through the porous medium, driven by pressure and temperature gradients. When the pressure gradient is sufficiently high to violate mechanical equilibrium—in view of the low tensile strength of cometary ice—the pores are allowed to widen, so as to release the pressure and regain stability. The criterion for pore widening is taken to be that of fracture of brittle material under tensile stress. The parameters of the problem are: the tensile strength of the ice (104-106 dyn/cm2), the gas content (5-10%), and the resistance of the ice to progressive pore widening (cracking) as a function of pore size. We find that, depending on these parameters, pores (tortuous capillaries) of several millimeters to a few centimeters open throughout the crystalline ice layer. Under extreme conditions, no stable configuration is obtained; namely, tensile failure occurs. Such a situation is interpreted as an explosion; it is found to occur at a depth of 120 m and at a heliocentric distance of ∼12 AU. Under milder conditions, very wide channels open up—over 1 m in diameter—which may serve as a vent for gas drainage that may appear as a jet.