interferometric spectroscopy at ~3 pm from the KAO, illustrating the potential 

 of this technique in the future for the detection of other parent molecular 

 species. Surprisingly, neither the in situ measurements nor the remote ultraviolet 

 observations have reported the presence of any gaseous chemical species pre- 

 viously unknown in comets. However, there has been a report, from low- 

 resolution infrared spectroscopy, of the detection of spectral features due to 

 organic compounds in the cometary grains, but this interpretation remains 

 highly controversial. Unfortunately, the in situ dust analyses cannot resolve this 

 question since they could measure only the atomic constituents of the dust 

 particles; molecular bonds were broken upon impact of the dust on the instru- 

 ment because of the large velocity of the spacecraft relative to the comet. None- 

 theless, a significant fraction of the small particles consisted almost entirely of 

 the biogenic elements, C, H, O, and N, in various combinations. These results are 

 very preliminary, and much detailed information about the gas and dust in the 

 coma is expected after further analysis of the wealth of comet Halley data. How- 

 ever, it seems likely that new information will emerge to encourage further 

 remote study of biogenic elements in future comets, particularly "new" comets 

 which are of most interest in this regard. 



The infrared range is the best spectral region for analyzing micrometer-sized 

 particles, typical of cometary dust, wherein may be found the majority of the 

 biogenic elements and compounds that could be incorporated into the early 

 Earth by cometary impact. Several observations can be considered: 



1. Mapping of the dust between 1 and 18 ixm at moderate spectral resolution 

 (X/AX ~ 100 - 1000). The instruments to be used are infrared cameras with 

 medium-resolution filters. The scientific objectives in mapping the dust are 



a. Searches for the silicate signature at 1 and 1 8 jum. 



b. Searches for the signatures of O-H, C-H, and C-C bonds in the 1- to 

 7-jum region in order to recognize organic and carbonaceous dust grains as well as 

 the hydrated phases. Data from the Giotto mission to Halley point strongly to 

 the existence of carbon-rich or entirely carbonaceous grains. 



c. Search for an icy halo at 3 jj.vc\ (in the very center of the coma). 



d. Constraints upon the size and the composition of the dust, by analyzing 

 the general shape of the thermal flux between 4 and 18 jitm. 



2. Observations of cometary dust at longer infrared wavelengths 

 (20-200 /urn). The shape of the spectrum beyond 20 /Jim is very sensitive to the 

 size of dust particles. An accurate measurement of the slope of the spectrum 

 between 20 and 200 [dm would provide new constraints, especially for the large 

 particles. 



3. Observations of the coma with the high spatial resolution from LDR. 

 Eventually, LDR and/or FIRST will provide the possibility of directly observing 

 the primary constituents of the coma after they have been sublimated from the 

 nucleus. H 2 could be easily detected and mapped at 557 GHz. 



40 



