A model of water ice sublimation from a porous cometary nucleus. (a) The cometary nucleus is initially assumed to consist mainly of water ice particles with a small amount of rocky debris uniformly contained within. (b) The water ice sublimates from the outer layer and the primitive region shrinks. (c) The remaining rocky debris accumulates on the surface to form a dust mantle. Since the dust mantle is highly porous and therefore permeable, the water vapor generated inside leaks out through the dust mantle. (d) Finally, the cometary nucleus transforms to a rocky asteroid after almost complete sublimation of water ice.