Letter to the Editor

Evidence for a rapid decrease of Pluto’s atmospheric pressure revealed by a stellar occultation in 2019

¹ Astronomical Observatory, Graduate School of Science, Kyoto University Kitashirakawa-oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
e-mail: arimatsu@kwasan.kyoto-u.ac.jp
² Department of Earth Science, Okayama University, 1-1-1 Kita-ku, Tsushimanaka, Okayama 700-8530, Japan
³ Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University, 6-3 Aramaki -aza-aoba, Aoba-ku, Sendai 980-8578, Japan
⁴ Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
⁵ Japan Spaceguard Association, Bisei Spaceguard Center, 1716-3 Okura, Bisei, Ibara, Okayama 714-1411, Japan

Received: 18 February 2020
Accepted: 15 May 2020

Abstract

We report observations of a stellar occultation by Pluto on 2019 July 17. A single-chord high-speed (time resolution = 2 s) photometry dataset was obtained with a CMOS camera mounted on the Tohoku University 60 cm telescope (Haleakala, Hawaii). The occultation light curve is satisfactorily fitted to an existing atmospheric model of Pluto. We find the lowest pressure value at a reference radius of r = 1215 km among those reported after 2012. These reports indicate a possible rapid (approximately 21₋₅⁺⁴% of the previous value) pressure drop between 2016, which is the latest reported estimate, and 2019. However, this drop is detected at a 2.4σ level only and still requires confirmation from future observations. If real, this trend is opposite from the monotonic increase of Pluto’s atmospheric pressure reported by previous studies. The observed decrease trend is possibly caused by ongoing N₂ condensation processes in the Sputnik Planitia glacier associated with an orbitally driven decline of solar insolation, as predicted by previous theoretical models. However, the observed amplitude of the pressure decrease is larger than the model predictions.