Reconciling results of 2019 and 2020 stellar occultations on Pluto’s atmosphere

Abstract

A stellar occultation by Pluto on 5 September 2019 yielded positive detections at two separate stations. Using an approach consistent with comparable studies, we derived a surface pressure of 11.478 ± 0.55 µbar for Pluto’s atmosphere from the observations of this event. In addition, to avoid potential method inconsistencies when comparing with historical pressure measurements, we reanalyzed the data for the 15 August 2018 and 17 July 2019 events. All the new measurements provide a bridge between the two different perspectives on the pressure variation since 2015: a rapid pressure drop from previous studies of the 15 August 2018 and 17 July 2019 events and a plateau phase from that of the 6 June 2020 event. The pressure measurement from the 5 September 2019 event aligns with those from 2016, 2018, and 2020, supporting the latter perspective. While the measurements from the 4 June 2011 and 17 July 2019 events suggest probable V-shaped pressure variations that are unaccounted for by the volatile transport model (VTM), the VTM remains applicable on average. Furthermore, the validity of the V-shaped variations is debatable given the stellar faintness of the 4 June 2011 event and the grazing single-chord geometry of the 17 July 2019 event. To reveal and understand all of the significant pressure variations of Pluto’s atmosphere, it is essential to provide constraints on both the short-term and long-term evolution of the interacting atmosphere and surface by continuous pressure monitoring through occultation observations whenever possible, and to complement these with frequent spectroscopy and photometry of the surface.