Comets and asteroids present two challenges for study from Earth-orbiting 

 observatories. Studies of cometary comae are similar to the atmospheric obser- 

 vations of the giant planets described later, although the lack of pressure broad- 

 ening makes very high spectral resolution both extremely useful and desirable; 

 this can be obtained with LDR and/or a higher-resolution SI RTF spectrometer 

 than is currently planned. Determining the composition of the solid (ice or 

 mineral) surfaces can be performed by means of reflectance spectroscopy, at 

 reasonably high resolution, in the infrared. Some of this work can be done in 

 the near infrared when HST acquires this capability, but most likely an infrared 

 orbiting observatory such as SI RTF or ISO will be required. The WUPPE that is 

 part of the ASTRO Shuttle package will allow the size and composition of the 

 cometary dust to be inferred from the percentage of polarization in scattered 

 light from 1300 to 3300 A. 



The objectives for Titan, the only known planetary satellite with a significant 

 atmosphere, are similar to those below for the giant planets. Because of the very 

 small angular size of the satellite, the measurable flux is much smaller than that 

 for the planets, and initial observations may have to await the development of 

 more sensitive, second-generation HST instruments. 



For the giant planets the need is to increase knowledge of the inventory of 

 trace molecular species in the predominantly hydrogen atmospheres and to 

 determine, using line-shape measurements at high spectral resolution, the vertical 

 and meridional distributions of these species in the atmosphere. Isotope ratios 

 should also be determined for the most abundant molecules. When HST becomes 

 operational it will permit high-resolution studies of molecules detectable in the 

 ultraviolet and visible and, in addition, will allow these studies to be extended to 

 Uranus and Neptune. ASTRO will provide limited observations of solar system 

 targets, but should add significant information about their atmospheric composi- 

 tion. Further progress will require high-resolution infrared, submillimeter, and 

 millimeter observations from space using platforms such as SI RTF, ISO, or LDR. 

 Determination of abundances from such observations will need extensive labora- 

 tory data currently not available. 



6.1.4 Molecules in the Interstellar Medium 



The study of molecules in interstellar space is an important component of 

 exobiology. Three important questions to be answered in this field are the 

 extent of chemical evolution throughout the universe, the availability of bio- 

 genic elements, and the existence of molecules such as water and organic com- 

 pounds. The extent of chemical evolution can be determined by spectroscopic 

 studies of complex molecules in the gas phase of interstellar clouds, especially 

 in the infrared and the millimeter, and by studies of interstellar dust particles in 

 the infrared, visible, and ultraviolet. In the near future, most detailed observa- 

 tions of molecules will be undertaken via ground-based millimeter- and 



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