Remote observations can also help determine the physical properties of 

 atmospheres, in addition to the chemical properties. Information regarding tem- 

 perature and pressure profiles and atmospheric dynamics can be obtained by 

 measurements in the infrared and near-infrared regions and to some extent in the 

 visible region. Both high spectral resolution and signal sensitivity are important, 

 as they are necessary for accurate determination of line profiles and continuum 

 absorption. Information in the far-infrared region of the spectrum will help 

 extend temperature profiles to regions deeper in the atmospheres than is cur- 

 rently possible. Observations over long periods of time will help clarify effects of 

 atmospheric dynamics on chemical abundances and, by inference, on local 

 production of chemical species. 



Another problem that may be clarified by observations from Earth-orbiting 

 facilities is the difference between the atmospheres of Uranus and Neptune and 

 those of Jupiter and Saturn. If all of these planets were formed by the same pro- 

 cesses from a nebula of homogeneous composition, then the atmospheres should 

 be similar, except for the effects of decreasing temperature with distance from 

 the Sun. Present data suggest that although the two pairs of atmospheres are 

 qualitatively similar, the elemental ratios (C/H, N/H, etc.) are quite different, 

 implying different sources or formation pathways. Because of their much greater 

 distance from us and the Sun, Uranus and Neptune appear much smaller in the 

 sky and are dimmer than Jupiter and Saturn. Useful data are much more diffi- 

 cult to obtain and instruments with greater sensitivity and less atmospheric 

 interference are needed for studies of Uranus and Neptune. 



One last field of study is the search for and characterization of tenuous atmo- 

 spheres in the outer solar system. Both Pluto and Triton (Neptune's satel- 

 lite) have atmospheres, but very little is known about them. More information 

 about such atmospheres would assist in understanding the overall distribution of 

 the biogenic elements within the solar system and the associated occurrences of 

 planetary atmospheres and hence their likelihood around planets elsewhere in 

 the universe. Observations using instruments with extremely high sensitivity are 

 necessary for these studies. 



The study of planetary atmospheres can best be accomplished by in situ mea- 

 surements from remote probes such as Galileo or Cassini. However, the number 

 of such missions planned for the next two decades is extremely limited and the 

 information obtainable relates only to a specific location and time, so that these 

 observations must be complemented and extended by global and synoptic obser- 

 vations from Earth or Earth-orbit. Of the orbital spacecraft listed in Appendix B, 

 those that will be most useful in making planetary observations are those capable 

 of detecting the ultraviolet, infrared, and submillimeter regions of the spectrum. 

 HST, ASTRO, IUE, FUSE, SOFIA, SIRTF, ISO, FIRST, and LDR satisfy these 

 requirements. For continuing efforts to determine the identities, abundances, 

 and distributions of molecular species, the greatest information to date has come 

 from the 5-/jm window and the 10- to 15-/um region in the infrared. From the 



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