The thermal structure of Triton's atmosphere: Results from the 1993 and 1995 occultations.

C. B. Olkin, J. L. Elliot, H. B. Hammel, A. R. Cooray, S. W. McDonald, J. A. Foust, A. S. Bosh, M. W. Buie, R. L. Millis, L. H. Wasserman, E. W. Dunham, L. A. Young, R. R. Howell, W. B. Hubbard, R. Hill, R. L. Marcialis, J. S. McDonald, D. M. Rank, J. C. Holbrook, and H. J. Reitsema. Icarus 129, 178-201 (1997) .


This paper presents new results about Triton's atmospheric structure from the analysis of all ground-based stellar occultation data recorded to date, including one single-chord occultation recorded on 1993 July 10 and nine occultation lightcurves from the double-star event on 1995 August 14. These stellar occultation observations made both in the visible and in the infrared have good spatial coverage of Triton, including the first Triton central-flash observations, and are the first data to probe the altitude level 20-100 km on Triton. The small-planet lightcurve model of J. L. Elliot and L. A. Young (1992, Astron. J. 103, 991-1015) was generalized to include stellar flux refracted by the far limb, and then fitted to the data. Values of the pressure, derived from separate immersion and emersion chords, show no significant trends with latitude, indicating that Triton's atmosphere is spherically symmetric at ~50-km altitude to within the error of the measurements; however, asymmetry observed in the central flash indicates the atmosphere is not homogeneous at the lowest levels probed (~20-km altitude). From the average of the 1995 occultation data, the equivalent-isothermal temperature of the atmosphere is 47 +/- 1 K and the atmospheric pressure at 1400-km radius (~50-km altitude) is 1.4 +/- 0.1 bar. Both of these are not consistent with a model based on Voyager UVS and RSS observations in 1989 (D. F. Strobel, X. Zhu, M. E. Summers, and M. H. Stevens, 1996, Icarus 120, 266-289). The atmospheric temperature from the occultation is 5 K colder than that predicted by the model and the observed pressure is a factor of 1.8 greater than the model. In our opinion, the disagreement in temperature and pressure is probably due to modeling problems at the microbar level, since measurements at this level have not previously been made. Alternatively, the difference could be due to seasonal change in Triton's atmospheric structure.

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