Letter to the Editor

Constraints on the evolution of the Triton atmosphere from occultations: 1989–2022

¹ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, 5 place Jules Janssen, 92190 Meudon, France
e-mail: Bruno.Sicardy@obspm.fr
² Indian Institute of Space Science and Technology, Thiruvananthapuram, 695547 Kerala, India
³ Federal University of Uberlândia (UFU), Physics Institute, Av. João Naves de Ávila 2121, Uberlândia, MG 38408-100, Brazil
⁴ Laboratório Interinstitucional de e-Astronomia – LIneA, Av. Pastor Martin Luther King Jr 126, Rio de Janeiro, RJ 20765-000, Brazil
⁵ American Association of Variable Star Observers (AAVSO), 185 Alewife Brook Parkway, Suite 410, Cambridge, MA 02138, USA
⁶ Physical Research Laboratory, Ahmedabad, 380009 Gujarat, India
⁷ Universidade do Rio de Janeiro – Observatório do Valongo, Ladeira do Pedro Antonio 43, Rio de Janeiro, RJ 20.080-090, Brazil
⁸ Institut Polytechnique des Sciences Avancées IPSA, 94200 Ivry-sur-Seine, France
⁹ IMCCE, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, Univ. Lille, 75014 Paris, France
¹⁰ Observatório Nacional/MCTIC, Rio de Janeiro, Brazil
¹¹ TÜBITAK National Observatory, Akdeniz University Campus, Antalya 07058, Turkey
¹² Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹³ Federal University of Technology-Paraná (UTFPR/PPGFA), Curitiba, PR, Brazil
¹⁴ Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400 005, India
¹⁵ Indian Institute of Astrophysics, II Block, Koramangala, Bangalore 560034, India
¹⁶ Department of Physics, Indian Institute of Technology Bombay, Powai 400 076, India
¹⁷ Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital 263002, India
¹⁸ Akashmitra Mandal, Kalyan, 421301 Maharashtra, India
¹⁹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Rabiańska 8, 87-100 Toruń, Poland
²⁰ Udaipur Solar Observatory, Physical Research Laboratory, PO Box 198 Badi Road, 313001 Udaipur, India
²¹ Harvard College Observatory, Harvard University, 60 Garden St., Cambridge, 02158 MA, USA
²² Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
²³ INAF, Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
²⁴ Key Laboratory for Research in Galaxies and Cosmology, Chinese Academy of Sciences, 80 Nandan Rd., Shanghai 200030, PR China
²⁵ School of Astronomy and Space Science, University of Chinese Academy of Sciences, 1 East Yanqi Lake Rd., Beijing 100049, PR China
²⁶ Key Lab of Space Astronomy and Technology, National Astronomical Observatories, Beijing, 100101 PR China
²⁷ Shandong Astronomical Society, 180 West Wenhua Rd., Weihai, Shandong 264209, PR China
²⁸ Vygonnaia street, house 2/1, flat 59, Ussuriysk, Primorsky Krai 692527, Russia
²⁹ Koshkina Street, house 19 k. 1, flat 210, Moscow 115409, Russia
³⁰ SETI Institute, 339 N Bernardo Ave Suite 200, Mountain View, CA 94043, USA
³¹ Unistellar, 5 allée Marcel Leclerc, bâtiment B, Marseille 13008, France
³² Unistellar Citizen Scientist, Tsuchiura, Japan
³³ Unistellar Citizen Scientist, Kagoshima, Japan
³⁴ UNESP-São Paulo State University, Grupo de Dinâmica Orbital e Planetologia, CEP 12516-410 Guaratinguetá, SP, Brazil

Received: 28 November 2023
Accepted: 2 February 2024

Abstract

Context. In about 2000, the south pole of Triton experienced an extreme summer solstice that occurs every ∼650 years, when the subsolar latitude reached about 50°S. Bracketing this epoch, a few occultations probed the Triton atmosphere in 1989, 1995, 1997, 2008, and 2017. A recent ground-based stellar occultation observed on 6 October 2022 provides a new measurement of the atmospheric pressure on Triton. This is presented here.

Aims. The goal is to constrain the volatile transport models (VTMs) of the Triton atmosphere. The atmosphere is basically in vapor pressure equilibrium with the nitrogen ice at its surface.

Methods. Fits to the occultation light curves yield the atmospheric pressure of Triton at the reference radius 1400 km, from which the surface pressure is deduced.

Results. The fits provide a pressure p₁₄₀₀ = 1.211 ± 0.039 μbar at radius 1400 km (47 km altitude), from which a surface pressure of p_surf = 14.54 ± 0.47 μbar is deduced (1σ error bars). To within the error bars, this is identical to the pressure derived from the previous occultation of 5 October 2017, p₁₄₀₀ = 1.18 ± 0.03 μbar and p_surf = 14.1 ± 0.4 μbar, respectively. Based on recent models of the volatile cycles of Triton, the overall evolution of the surface pressure over the last 30 years is consistent with N₂ condensation taking place in the northern hemisphere. However, models typically predict a steady decrease in the surface pressure for the period 2005-2060, which is not confirmed by this observation. Complex surface-atmosphere interactions, such as ice albedo runaway and formation of local N₂ frosts in the equatorial regions of Triton, could explain the relatively constant pressure between 2017 and 2022.