I have had a think about what a space probe would find at the surface of a comet (nucleus). I am thinking it would be made up of a biosynthetic petroleum gel. Long chain chiral - similar in chemical consistency to tar. The function of this skin of at least several metres would be to absorb the sun's heat (being black) and of absorbing meteoric impact. The functional interior organs of the comet need to be protected, and an energy source is required.