3.5. From Interstellar Dust to Comets
A major advance in our understanding of comets in the 20th century was made by the space probes Vega 1 and 2 and Giotto (see Nature, comet Halley issue, vol. 321, 1986). Until that time no one had ever seen a comet nucleus. The critical new discoveries were: (1) the low albedo ( 0.04) of comets; (2) the size distribution of the comet dust extending down to interstellar dust sizes (10-15 - 10-18 g), (3) the organic fraction of comet dust (see Greenberg & Li 1999b and references therein). The current ground based observations of the volatile composition of comets implies a close connection with the ices of interstellar dust (see Crovisier 1999 and Irvine & Bergin 2000 for recent reviews).
Most of the current models of comet nuclei presume that to a major extent they are basically aggregates of the interstellar dust in its final evolved state in the collapsing molecular cloud which becomes the protosolar nebula. In addition to the chemical consequences of such a model, there is the prediction of a morphological structure in which the aggregate material consists of tenth micron basic units each of which contains (on average) a silicate core, a layer of complex organic material, and an outer layer of ices in which are embedded all the very small carbonaceous particles characterizing the interstellar UV hump and the far UV extinction. All these components have been observed in the comet comae in one way or another. (27) The implication is that space probes which can examine in detail the composition of comet nuclei will be able to provide us with hands-on data on most of the components of interstellar dust and will tell us what is the end product of chemical evolution in a collapsing protosolar molecular cloud. At this time many laboratories are preparing materials as a data base for comparison with what will be analyzed during the space missions.
27 PAH molecules, a significant constituent of interstellar dust (see Section 3.2.2), would also be present in comets if they indeed contain unprocessed interstellar matter. The presence of PAHs in comets has been suggested by the 3.28 µm emission feature detected in some comets (Bockelée-Morvan, Brooke, & Crovisier 1995). More specifically, a 3-ring PAH molecule - phenanthrene (C14H10) - has been proposed as the carrier for the 342-375 nm fluorescence bands seen in comet 1P/Halley (Moreels et al. 1994). Li & Draine (2002d) are currently working on this topic. Back.