O

rTTTrm

period comets [2]

Fig. 48. Differences between

Jupiter family and long-"

period comets [2]

observations can be made at radio and IR wavelengths. A study of HCN, HNC, CH3CN, CH3OH, H2CO, CO, CS, and H2S of comets in the radio range (4 Jupiter -family, 2 Halley-family, 13 long-period) has shown no significant differences among Jupiter family and long-period comets [27]. This result is not in contradiction with the finding of C-depleted short-period comets because the C2 and C3 bearing parent molecules (C2H2, C2H6, and C3H4) were not included in the radio observations. Methanol has been determined at radio wavelengths in more than 17 comets and has therefore one of the largest data base of parent species. Variations of CH3OH/H2O of 26% among comets and 0.5-5% among Oort-cloud comets seem to reflect real compositional differences [27, 60]. Comets rich in methanol also seem rich in formaldehyde [27].

Cometary parent molecules can also be observed at IR wavelengths, although only in bright objects. A study of six long-period comets [163] included CO, CH4, C2H2, C2H6, HCN, and CH3OH. Out of the six comets in this study, five comets and comet P/Halley show relatively similar abundances. Nevertheless, their nucleus source of CO still varied by a factor of 10 from comet to comet. The other species showed variations less than a factor of two. One comet however, C/1999 S4, is very different compared with the other long-period comets. It is depleted in almost all volatiles observed. Possibly C/1999 S4 is indicating different formation conditions with reduced abundances of minor species relative to H2 O and therefore variations among Oort-cloud comets. However, we have to wait for a larger statistical sample for a definite conclusion. In view of the importance of the possible existence of classes of comets with different chemical composition, further observations of large samples need to be made.

In summary, among the parent molecules nucleus CO seems to show large variations among comets up to a factor of ten. Such large differences among Oort cloud comets and the existence of a truly different comet (C/1999 S4) indicate compositional variations among long-period comets. Comparing long-period to short period comets, a class of Jupiter family comets depleted in C2 and C3 exists. This difference between short- and long-period comets could not yet be confirmed for parent molecules because of the insufficient detection limit for these parents for weak comets. We have to await further technical improvements and a larger statistical data base for a more detailed study of the compositional differences among comets.

6.4 Isotopic Ratios

Isotopic ratios provide severe constraints on the origin of comets. Table 5 provides a list of the abundances determined in comets in comparison to the values in the solar system and the interstellar medium (see also [206]). The excellent correlation of isotopic ratios of cometary parent molecules with the solar system values is consistent with formation of cometary nuclei in the solar nebula.

Table 5. Isotopic ratios

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