Pluto’s Atmosphere in Decline

Pluto is the only minor planet (excluding satellites) that is known to host a thin, global atmosphere. The atmosphere has microbar-level surface pressure, is composed primarily of nitrogen, and contains a layered haze made of organic materials (e.g. Gladstone et al., 2016, Science, 351, id. aad8866). Notably, Pluto’s atmosphere is intricately linked to its surface ices through vapor-pressure equilibrium (e.g. Elliot et al., 1989, Icarus, 77, 148). Because of the strong tie between the atmosphere and ices, the surface pressure is highly dependent on ice temperature, which is a function of orbital and seasonal timescales. On Pluto, with an eccentric orbit (e = 0.25) and high obliquity (~123 deg.), these changes are pronounced and the atmospheric properties can vary significantly on timescales of only a few decades. Thermophysical, volatile-transport models have been developed to study Pluto’s atmospheric evolution; predictions range from atmospheric contraction or collapse over the coming decades to an atmosphere that remains throughout Pluto’s entire 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).

Stellar occultation data are the most direct way to measure Pluto’s atmosphere from the Earth. Results from previous occultations reported that Pluto’s atmospheric pressure monotonically increased 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 ten successfully-observed stellar occultations by Pluto between 2017 August and 2023 July that have not yet been published. The stellar magnitudes ranged from G=12.91 to 18.4 with geocentric relative velocities between 1.7 and 24.5 km/s.  Four of these events had successful chords from multiple sites, while six events were from single sites. We carried out atmospheric fits assuming clear, isothermal atmospheres as well as atmospheres with a haze layer. Our results indicate that Pluto’s atmospheric pressure has begun decreasing in recent years.

This work is supported by NASA grant 80NSSC21K043.