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

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. <BR /> 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. <BR /> 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. <BR /> Results: The fits provide a pressure p<SUB>1400</SUB> = 1.211 ± 0.039 μbar at radius 1400 km (47 km altitude), from which a surface pressure of p<SUB>surf</SUB> = 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, p<SUB>1400</SUB> = 1.18 ± 0.03 μbar and p<SUB>surf</SUB> = 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 N<SUB>2</SUB> 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 N<SUB>2</SUB> frosts in the equatorial regions of Triton, could explain the relatively constant pressure between 2017 and 2022.