The composition of interstellar grains is a most important question for exo- 

 biology. Infrared telescopes in Earth orbit can make an important contribution 

 to answering this question. It is important to obtain spectra covering the full 

 wavelength scale from 2 to about 20 jum to identify as many characteristic 

 groups as possible. Because the 5- to 8-fim region of the spectrum, containing, 

 for example, the important C-C stretches and C-H bending modes, is blocked 

 from the ground, infrared observations from Earth orbit (ISO and SI RTF) are 

 required. These satellites have the added advantage of the reduced thermal 

 background of a cooled telescope. Thus, even in the 10-£tm atmospheric window, 

 they can make an important contribution because much fainter sources can be 

 observed than from the ground. 



Among the questions that have to be addressed are, "What is the composition 

 of molecular grain mantles accreted inside molecular clouds?" and "Are photo- 

 lyzed grain mantles an important component of the interstellar dust?" Carbon 

 dioxide is one of many molecules that cannot be observed from the ground. 

 Theoretical calculations have suggested thatC0 2 may be the dominant constitu- 

 ent of accreted grain mantles under certain physical conditions. Absorption by 

 atmospheric C0 2 makes detection of interstellar C0 2 impossible, even from air- 

 plane altitudes. Ultraviolet photolysis will produce complex molecules that 

 cannot be formed by grain-surface reactions. Detection of such molecules will 

 help unravel the photochemistry taking place. Specifically, the determination of 

 the relative abundance of aromatic to aliphatic species is very important. The 

 former may be related to dust formed in stars (e.g., small condensed carbona- 

 ceous grains), while the latter may be produced by photolysis. High-resolution 

 spectra in the 3.2- to 3.5-jum region could answer this question. The C-H stretch- 

 ing modes are weak compared to the continuum dust extinction. Because there 

 is significant atmospheric CH 4 absorption even at a good site (such as Mauna 

 Kea), cooled space infrared telescopes are needed. Faint sources with long path 

 lengths through the diffuse interstellar medium can then be studied. 



Infrared observation can also be used to study the formation sites of inter- 

 stellar grains. One particularly important question is, of course, "Do carbona- 

 ceous or other biogenic-element-bearing grains condense in the outflow from 

 novae and supernovae?" and if so, "What is their precise composition?" The 

 tentative identification of infrared fluorescence from PAHs around planetary 

 nebulae is particularly interesting. These species, which are presumably the 

 molecular precursors of carbon grains, have obviously been formed only recently 

 in these objects. 



High-resolution studies of the 7.7-jum band in a variety of sources could reveal 

 profile variations related to differences in the collection of PAHs. Correlation 

 studies among the different infrared emission bands are also important in this 

 respect and could produce insight into the actual synthetic route. As yet undis- 

 covered infrared emission features may be present in wavelength regions that are 

 not observable from airplane altitudes. Searches for the emission features in 



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