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J. L. Elliot (MIT \& Lowell Obs.), D. F. Strobel, X. Zhu (JHU), L. H. Wasserman, O. G. Franz (Lowell Obs.)
Triton's occultation of a 10.6 magnitude star was observed with FGS #3 aboard the HST, which passed close enough to the center of Triton's shadow to record a central flash. These data have been used to infer a substantial increase in Triton's surface pressure since the Voyager encounter in 1989 and a previous stellar occultation in 1995 (Olkin {\it et al}., {\it Icarus} {\bf 129}, 178). Because the predominantly nitrogen atmosphere is in vapor-pressure equilibrium with surface frost, the surface-pressure increase implies a global warming of Triton's surface frost of 1-2 K over the 8-year period (Elliot {\it et al}., {\it Nature} {\bf 393}, 765).
Here we report results of inversions of these data to recover temperature, pressure, and number density profiles of Triton's atmosphere over an altitude range of 25-150 km. The upper part of the temperature profile shows a thermal gradient of ~0.3 K/km down to an altitude of ~50 km, where the profile becomes isothermal at a temperature of ~50-52 K. This result is somewhat warmer than the 48 K equivalent isothermal temperature reported by Tyler {\it et al}. ({\it Science} {\bf 246}, 1466) in the 0-50 km altitude range. Atmospheric models based on Voyager data (e.g. Strobel {\it et al}., {\it Icarus} {\bf 120}, 266; Krasnopolsky {\it et al}., {\it JGR} {\bf 98}, 3065; Lellouch {\it et al}., {\it Adv. Space Rev}., {/bf 12}, 113) do not have a steep enough temperature gradient in the 25-150 km region, nor do they have an isothermal region below 50 km. Below this isothermal region the temperature must decrease to a surface-frost temperature of 39-40 K.
This work was supported, in part, by STScI Grants GO-07489.01-96A at MIT and GO-07489.02-96A at Lowell Observatory.