The formation of a rubble mantle
The cartoon above depicts a cross-section through a piece of a cometary nucleus and shows the formation of a rubble mantle.
At initial time T0, the comet nucleus consists of a mixture of ices (yellow) and rocks (red). At later time T1, sunlight from above heats the nucleus surface and sublimates the ice. Dust particles and the smaller rocks are entrained in the gas flow (arrows) and are ejected from the nucleus. Large rocks are too heavy to be lifted. By time T3, about half the surface is covered by large rocks left behind as a lag deposit. In the final time step T4, the surface is almost completely sealed by the rubble mantle. The time difference T4 - T0 is uncertain but probably very short. Rubble mantles could form within a single orbit.
The black, inactive part of the nucleus of comet Halley is probably a rubble mantle.
The rubble mantle shown here is distinct from the irradiation mantle caused by cosmic ray bombardment.