Halley, Hyakutake

The recent ai roach of large comets such as IP/Halley, C71996 B2 Hyakutake, and C/1995 Ol Hale-Bopp to the Earth provided a good opportunity to investigate the detailed composition of cometary ices by various methods such as mass spectrometry, infrared spectroscopy, and radio emission. The composition of interstellar ices is compared with that of the cometary ices in Table 9.3. It is striking that cometary and interstellar ices have quite comparable relative molecular abundances. 

Fig. 4. Isotopic variations of hydrogen in the Solar System (adapted from Robert et al. 2000). The deuterium/ hydrogen ratio of different components is normalized to the D/H ratio of the Sun (as it was before deuterium burning), which is thought to represent H2 in the protosolar nebula. Numbers along the y-axis represent the numbers of cases. Terrestrial hydrogen is enriched in deuterium by a factor of about six relative to solar. Among Solar System objects analysed so far, carbonaceous chondrites, Antarctic micrometeorites (Engrand Maurette 1998) and chondruies from LL3 chondrites present a distribution of D/H values that centre around the terrestrial D/H ratio. Notably, comets analysed so far (Halley, Hale Bopp and Hyakutake, references given by Dauphas et al. (2000)) present D/H values about two times higher than the terrestrial value.

Table I. The main interstellar ice components (relative to H2O) compared to that inferred for several comets. Cometary references Halley (24) Hyakutake and Hale-Bopp (25). See Table 2 in Reference 6 for details.

Figure 4. Polarimetric color vs. phase angle for comet (+) IP/Halley, (o) C/1996 B2 (Hyakutake), and ( ) C/1995 01 (Hale-Bopp). The data are from [22, 28-31].

In late 1985, radar observations of comet Halley, which was much more active than IRAS-Araki-Alcock, yielded echoes with a substantial broadband component presumed to be from a large-particle swarm, but no narrowband component, a negative result consistent with the hypothesis that the surface of the nucleus has an extremely low bulk density. In 1996, Goldstone obtained 3.5-cm echoes from the nucleus and coma of comet Hyakutake (C/I996 B2). The coma-to-nucleus ratio of radar cross section is about 12 for Hyakutake versus about 0.3 for lAA. The radar signatures of these three comets strengthen impressions... 

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