Comet Borrelly and the two kinds of cometary sub-nuclei

If we accept the hypothesis, that cometary nuclei may essentially be aggregates of sub-nuclei [Weissman, P.R., Are cometary nuclei primordial rubble piles? Nature 320, 242–244, 1986.], several independent kinds of observational facts point to the possibility, that there are at least two populations of sub-nuclei, and they can be mixed in the nucleus of a comet [Illés-Almár, E., On two different populations of cometary sub-nuclei, Antarctic Meteorites XX, June 6–8, Tokyo. Abstracts, pp. 93–94, 1995]. One kind of the sub-nuclei could be loose aggregates from fluffy material, while the other one may be more compact and cohesive.