Pluto's Atmosphere Persists

Pluto has a tenuous atmosphere with microbar-level surface pressure, composed primarily of nitrogen and containing a layered haze made of organic materials. An eccentric orbit combined with high obliquity leads to significant changes in solar insolation at different latitudes throughout the Plutonian year. Because the atmosphere is supported through vapor-pressure equilibrium with the surface ices, changes in surface conditions are intimately linked with the properties of the atmosphere. Thermophysical, volatile-transport models have been employed to anticipate Pluto's atmospheric evolution: predictions include atmospheric contraction or even collapse over the coming decades as well as an atmosphere that remains through the entire 248-yr revolution around the Sun (e.g. Young, 2013, ApJ Lett., 766, L22; Hansen et al. 2015, Icarus, 246, 183; Bertrand et al. 2018, Icarus, 309, 277; Johnson et al. 2021, Icarus, 356, id.114070). Previous work reported that Pluto's atmospheric pressure has been monotonically increasing since its definitive discovery in 1988 through 2016 (Meza et al. 2019, A&A, 625, id.A136) and then that the atmosphere had possibly begun freezing out in 2018-2019 (Arimatsu et al. 2020, A&A 638, L5; Young et al. 2021, AAS DPS Meeting #53, id.307.06). Observations of an occultation in 2020 did not show a pressure drop and were interpreted to be either a continued pressure increase (Poro et al. 2021, A&A, 652, L7) or a plateau phase (Sicardy et al. 2021, ApJ Lett, 923, L31). Here, we report results from nine successfully-observed stellar occultations by Pluto between 2017 August and 2022 August. The stellar magnitudes ranged from G=12.91 to 17.76 with geocentric relative velocities between 1.7 and 24.3 km/s. Four of these events had successful chords from multiple sites, while five events were from single sites. Our results indicate that Pluto's atmospheric pressure has been roughly holding steady since the New Horizons spacecraft flyby in 2015 and does not show any signs of collapse as of August 2022.