the Titan disk (Appendix C), which might be sufficient to detect limb darkening 

 or brightening, but not much in the way of detailed structure. In the far- 

 ultraviolet region, both cameras are hampered by the red sensitivity of the 

 detectors, particularly the charge-coupled devices (CCD) used by the WF/PC. 

 The ultraviolet filters have relatively poor transmission and appreciable out-of- 

 band transmission, particularly in the red. Moreover, the far-ultraviolet emissions 

 (e.g., of H, N, C, N 2 , and H 2 ) originate mainly in the exosphere of Titan, excited 

 by interaction with charged particles trapped on magnetic field lines of Saturn. 



Second is spectroscopy of Titan between 1000 and 3000 A. A study in the 

 ultraviolet range at medium and high resolution would be the best way to search 

 for the spectral signatures of the photodissociation products of organic mole- 

 cules that may be present in the atmosphere. The advantage of the HST, relative 

 to previous ultraviolet satellites such as the IUE, will be a higher spectral resolu- 

 tion and a higher sensitivity (Titan is too faint to be observed with IUE below 

 2000 A). The instruments to be used would be the faint-object spectrograph, for 

 a medium-resolution spectrum of the whole ultraviolet range, and the high- 

 resolution spectrograph, for investigating specific spectral signatures at high 

 spectral resolution using different modes (depending upon the wavelength and 

 the lines to be searched for). However, at this stage it is not clear whether either 

 of these two spectrographs has sufficient sensitivity to detect any species other 

 than the dominant atoms and diatomic molecules of the outer atmosphere 

 of Titan. 



Many complex organic molecules exhibit strong vibration-rotation lines in 

 the infrared and rotational transitions in the far-infrared, submillimeter, and 

 millimeter ranges. These lines cannot be observed from the ground. Moreover, in 

 the case of a weak source such as Titan, the background noise is very strong. A 

 cooled instrument in space, like SIRTF or ISO, is needed to perform the obser- 

 vations. An alternative solution is to observe Titan in the millimeter range with 

 a heterodyne system and long-baseline interferometer. This is certainly a ground- 

 based program to be developed in the near future. However, in the millimeter 

 range, rotational transitions, under the excitation conditions of planetary atmo- 

 spheres, are usually much weaker than transitions corresponding to higher J 

 values, i.e., in the submillimeter and far infrared. The search for complex mole- 

 cules on Titan with ISO, SIRTF, and LDR thus appears to be a promising 

 method, complementing ground-based millimeter programs. Molecules to be 

 searched for include the following nitrites: CH 3 CN (acetonitrile), C 2 H 5 CN (pro- 

 pionitrile), C 2 H 3 CN (acrylonitrile), and possibly adenine. The instruments to be 

 used would be high-resolution spectrometers with a spectral resolving power as 

 high as possible; however, at long wavelengths (>50 jum)the observations will 

 have to separate Titan from Saturn— i.e., will have to be performed with the best 

 possible angular resolution when Titan is near elongation. Initially, SOFIA may 

 offer the superior resolution needed and have the sensitivity to detect at least 

 the strongest features. 



36 



