Presentation #307.06 in the session “Centaur/KBO/Pluto Gathertown”. Cross-listed as presentation #114.06.
Pluto’s bulk atmosphere is supported by the vapor pressure of N2 ice, the most volatile of its surface constituents. The vapor pressure of N2 is a steep function of temperature: a 1.5 K increase leads to a doubling of Pluto's surface pressure and column abundance . As Pluto’s heliocentric distance has increased since its perihelion in 1988, a spectator sport has evolved — watching and waiting for Pluto’s bulk atmosphere to decrease due to expected drops in N2 surface temperatures. Despite a 20% decrease in received sunlight between 1988 and 2015, Pluto’s surface pressure increased over that period by roughly a factor of 2x per decade .
Recent models of Pluto's evolving surface pressure include scenarios in which Pluto’s maximum surface pressure occurs in this decade [2,3]. An occultation observed on 6-JUL-2019  suggests that Pluto’s surface pressure decreased by 20% from the 11.5 μbar value reported from the New Horizons flyby on 14-JUL-2015 . We observed an occultation of a V=13.0 star by Pluto on 15-AUG-2018 from about two dozen sites, including two located within 26 km of the central chord. The two lightcurves, observed at 5 and 10 Hz, yield a best-fit surface pressure of 11.13 ± 0.4 μbar. Central flash features constrain the geometry of the shadow path with incredibly small uncertainty (0.5 km rms), an improvement over the single-chord analysis of .
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