Presentation #102.04 in the session “Pluto System: Atmosphere”.
The New Horizons misson provided a snapshot of Pluto’s haze distribution and column abundance at the time of its flyby in July 2015. We report on results from a 15-AUG-2018 stellar occultation by Pluto that was observed from roughly 50 locations in the US and Mexico. This event was remarkable for three reasons. First, several sites were arranged in a picket fence along the cross-track direction, spaced by about 20-km intervals. About a dozen of these sites show central flash features in their lightcurves. Second, the central flash amplitudes change rapidly as a function of cross-track offsets, letting us determine the location of the shadow path on the Earth at the 500-m level (rms). The pre-event prediction - based on astrometry from the Gaia catalog and a post-New Horizons ephemeris for Pluto - was only off by 19.9 km in the down-track direction and 6.0 km in the cross-track direction. Third, the central flash amplitudes are very sensitive to haze opacity. We find a slight increase in haze opacity on 15-AUG-2018 compared to the New Horizons flyby opacities measured on 14-JUL-2015. This trend does not support the idea that Pluto’s haze abundance closely follows the solar cycle. In addition, we find that Pluto’s bulk column abundance has increased from 2015 to 2018, by an amount commensurate with the average rate of the past two decades (approximately doubling every ten years). Even though Pluto has been receding from the Sun since 1989, we do not find evidence that its atmosphere has begun to freeze out onto its surface.