THE FORMATION OF COMETARY SURFACE CRUSTS

Models describing the buildup of mantles on cometary nuclei are review ed and their predictions are compared with observations. Mantles restr ained 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 th e prevailing inactivity of nuclear surfaces. Numerical calculations de scribe the growth of depleted layers of refractory matrix material (cr usts), their activity levels, and the vapor pressure under such crusts . It is found that over a wide range of parameters cohesion of the mat rix material is stronger than the vapor pressure building up underneat h 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 paramete rs are inferred to explain the restricted and in many cases persistent activity of comets. (C) 1994 Academic Press, Inc.