from Earth orbit will open the far-infrared and submillimeter regions where 

 these small molecules have detectable transitions. Very detailed spatial studies 

 are needed to determine the distributions of the various species over the Jovian 

 and Saturnian disks. Velocity can also serve as a useful discriminant against the 

 influence of local chemical and meteorological effects. Adequate determination 

 of H 2 and HD profiles around 7000 A requires a spectral resolving power of 

 >10 4 . Determination of isotopic and elemental ratios can also be made from 

 observations of NH 3 and phosphine, PH 3 , in the infrared region. High-altitude 

 or Earth-orbiting facilities are necessary to minimize the effects of atmospheric 

 water and carbon dioxide (see fig. 3.2). Typical resolving powers of 1000 or 

 better are required for adequate measurements of isotope ratios, e.g., 

 15 NH 3 / 14 NH 3 , although resolving powers of 10 s are necessary to observe indi- 

 vidual rotation lines. If sufficient spatial resolution (<1 arcsec) can be achieved, 

 then local effects such as atmospheric turbulence and chemistry may be studied 

 (see Appendix C). 



Questions about the evolution of atmospheres with time may be easier to 

 answer than questions about origins. We know from the detection of molecules 

 that are not predicted by simple thermodynamic condensation models of the pri- 

 mordial nebula that outer-planet atmospheres are not in chemical equilibrium. 

 We also know from the presence of these molecules, and from the existence of 

 unidentified colored species that cannot be formed by freezing any of the 

 known atmospheric constituents, that active chemistry is occurring, or has 

 occurred, in the outer solar system. We do not know the details of such chemis- 

 try or what drives it. For example, in the upper atmosphere of Jupiter, photo- 

 chemistry of methane is occurring, but are there other types of chemistry going 

 on? What are the identities and sources of the (colored) compounds observed in 

 the lower atmosphere where direct photochemistry of methane is unlikely? Are 

 any of these organic compounds, and is the photochemistry there in any way 

 related to the photochemistry likely to have been driven in the early atmosphere 

 of the Earth? Might there indeed be sufficient chemical gradients and energy 

 sources in the Jovian atmosphere to support the simple life forms once postu- 

 lated in a paper entitled "Floaters, Bobbers, and Sinkers"? 



Further information on the nature and distribution of compounds in plane- 

 tary atmospheres must be obtained. For this the infrared region of the spectrum 

 is potentially the most useful, and many observations (e.g., Voyager Infrared 

 Interferometric Spectrometer (IRIS)) have been made. However, increased spec- 

 tral and spatial resolution are required to determine how various chemical 

 species are distributed in the atmosphere and to sort out the region from 10 to 

 15 jum where many molecules have absorptions. Study of the nature and distri- 

 bution of compounds in the atmosphere will also yield information about the 

 effects of various energy sources (solar ultraviolet, energetic particles, lightning) 

 at different levels within the atmosphere and hence their relative importance to 

 atmospheric chemistry and evolution. 



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