Triton's Atmospheric Structure: What We Have Learned from Stellar Occultations and Prospects for the Future

J. L. Elliot (MIT and Lowell Observatory), C. B. Olkin (Lowell Observatory)

In 1993 we began a program for observing stellar occultations by Triton with the following objectives: (1) probe Triton's atmosphere in the microbar pressure region for comparison with models based on Voyager data (e.g. Strobel et al., Icarus 120, 266), (2) investigate the predicted seasonal changes in surface pressure (Spencer & Moore, Icarus 99, 261; Hansen & Paige, Icarus 99, 273) and (3) investigate spatial variability of the atmospheric structure. Observations have been successful for three stars, and the results are descibed by Olkin et al. (Icarus, in press) and Elliot et al. (Science, submitted). A large difference between the observations and models in the pressure and temperature at a radius of 1400 km (about 50 km altitude) may be due to seasonal change or inadequacy of the models. Triton's atmosphere has been found to be highly distorted from spherical symmetry, which has been interpreted as evidence for winds near the sonic velocity ( tex2html_wrap_inline26 140 m s tex2html_wrap_inline28 ). Based on current knowledge of Triton's atmosphere just described, our goals for future investigations of Triton's atmosphere are threefold: (i) map the central-flash with multiple chords in order to understand how Triton's atmosphere is distorted, (ii) obtain a light curve of greater S/N than we have at present in order to better establish Triton's temperature and pressure profiles so that present models based on Voyager data can be improved; and (iii) regularly probe Triton's atmosphere (annually if possible) in order to learn how its pressure changes with time. The prospects for observation of more high-quality Triton occultations are bright for the next three years (McDonald & Elliot, AJ 109, 1352), after which the Neptune system moves away from the galactic plane and the frequency of events diminishes. This work was supported, in part, by NASA Grants NAG5-3940 at MIT and NAG2-1078 at Lowell Observatory.