2.6. Comets
The origin of comets is closely linked to the solar system and they played an important role in cosmogony. Cometary nuclei have been created far away from the early Sun (10) and have been mostly kept intact since their formation. Although it is generally accepted that comets contain the most pristine material in the solar system, it is still a matter of considerable debate whether they are made of unmodified protosolar nebula interstellar dust or this material has been (completely or partially) evaporated before becoming a part of comets (see Mumma, Stern, & Weissman 1993; Crovisier 1999; Irvine & Bergin 2000 for reviews).
The internal structure and chemical composition of cometary nuclei have been a topic receiving much attention. Before 1950, the prevailing view was that the comet nucleus is composed of a coherent swarm of meteoroidal-type particles which are independent of each other (termed the "flying sand-bank" model). It was shown by Russell, Dugan, & Stewart (1926) that the "swarm" concept of the cometary nucleus was inadequate since the solar heating would vaporize icy bodies up to 30 cm in diameter.
Since 1950 a number of models for the comet nucleus have been proposed. For the most part, the icy-conglomerate model (also known as the "dirty snowball" model) of Whipple (1950) has been the standard which others have followed. According to Whipple (1950), the comet nucleus is a solid body consisting of a conglomerate of refractory dust grains and frozen gases (mostly of H2O ice). More recently, the computer simulation of dust aggregates formed by random accumulation (Daniels & Hughes 1981) led Donn, Daniels, & Hughes (1985) to postulate the fractal model in which the comet nucleus is considered as a heterogeneous aggregate of ice and dust grains with substantial voids. Weissman (1986) proposed a primordial rubble-pile model as a modification of the basic icy-conglomerate model in which the cometary nucleus is envisaged as a loosely bound agglomeration of smaller fragments, weakly bonded by local melting at contact interfaces. Gombosi & Houpis (1986) suggested an icy-glue model. According to them, the comet nucleus is composed of rather large porous refractory boulders (tens of centimeters to hundreds of meters) "cemented" together with the icy-conglomerate type ice-dust grain mix ("Whipple glue").
Alternatively,
Greenberg (1982b)
proposed the interstellar
dust model of comets in which the basic idea is that comets
have formed directly through coagulation of interstellar dust
(Greenberg 1982b;
Greenberg 1998;
Greenberg & Li 1999b).
The morphological structure of comet nuclei is thus modelled
as an aggregate of presolar interstellar dust grains
whose mean size is of the order of one tenth micron.
The representative individual presolar grain consists of
a core of silicates mantled first by an organic refractory material
and then by a mixture of water dominated ices in which are embedded
thousands of very small carbonaceous particles/large molecules
(Greenberg 1998;
Greenberg & Li 1999b).
Greenberg and his co-workers have further shown how the IR
emission for several distinctly different types of comets
bear a general resemblance to each other by reproducing
the IR emission of various comets (Halley - a periodic comet
[Greenberg & Hage 1990;
Greenberg et al. 1996];
Borrelly - a Jupiter family short period comet
[Li & Greenberg 1998a];
Hale-Bopp - a long period comet
[Li & Greenberg 1998b];
and extra-solar comets in the
Pictoris disk
[Li & Greenberg 1998c])
within the framework that
all comets are made of aggregated interstellar dust.
10 The current view is that Jupiter family comets (with small inclinations and orbital periods P < 20 yrs) formed in the trans-Neptune region now known as the "Kuiper Belt"; Halley-type comets (with relatively longer periods 20 < P < 200 yrs and larger inclinations) as well as long-period comets (with all possible inclinations and orbital periods 200 < P < 107 yrs) formed somewhere beyond the orbits of Jupiter and Saturn. See Li & Greenberg (1998a) and references therein. Back.