THE FORMATION OF COMETARY SURFACE CRUSTS

Models describing the buildup of mantles on cometary nuclei are reviewed and their predictions are compared with observations. Mantles restrained by gravity forces only are unstable under almost all conditions. A new model that emphasizes the importance of cohesive forces within the refractory material of cometary nuclei is introduced to explain the prevailing inactivity of nuclear surfaces. Numerical calculations describe the growth of depleted layers of refractory matrix material (crusts), their activity levels, and the vapor pressure under such crusts. It is found that over a wide range of parameters cohesion of the matrix material is stronger than the vapor pressure building up underneath the crust. These conditions lead to stable crusts but per se do not explain the activity of cometary nuclei. Structural inhomogeneities of cometary nuclei with varying dust-to-ice ratios and physical parameters are inferred to explain the restricted and in many cases persistent activity of comets. (C) 1994 Academic Press, Inc.