After New Horizons, a new Pluto Climate Model for new challenges

1. Introduction

Nine years after Pluto's flyby by New Horizons, there are still fundamental unanswered questions about Pluto's atmosphere. Here we focus on two specific plutonian mysteries. First, New Horizons revealed thin wave-like structures at global scales in the atmosphere, consistent with density perturbations previously seen in stellar occultations. It remains unclear whether these waves are orographic gravity waves initiated from winds flowing over Pluto's extensive topography or thermal tides resulting from the "breathing" of N2 ice in response to solar forcing. In addition, the impact of these waves on their climate has not yet been explored. Second, Pluto's atmospheric thermal structure measured by New Horizons [1,2] is presenting puzzling features such as: (1) a strong negative gradient between the stratosphere at 110K and the upper atmosphere at 70K, (2) a strong thermal gradient in latitude, still unexplained, and (3) a 3km-deep cold layer, which has been only partially explained.

Recent studies [3] suggest that the cooling of the upper atmosphere (1) may be explained by the presence of organic hazes. However the bulk material of the haze (organic vs ice) remains unclear, and the radiative impact of the haze on the atmosphere remains to be explored in detail with 3D climate models including haze and hydrocarbon ice microphysics, and tested against available thermal emission measurements of the atmosphere.

 

2. The new Pluto PCM

To tackle these scientific investigations, we created a completely new-generation planetary climate model (PCM) able to simulate Pluto at high spatial resolution, with haze microphysics processes, and over the long annual timescales. This Pluto PCM is derived from the LMD Generic PCM and from the Legacy Pluto GCM [4,5,6] and therefore benefits from the recent developments from the Generic PCM such as its efficient parallelization.

At the conference we will present this new tool and preliminary results regarding the impact of radiatively active hazes on Pluto's climate as well as the propagation of gravity waves in the atmosphere.

 

Acknowledgements

This work was supported by funding from ANR "Programme de Recherche Collaborative" 2024-2028 (ANR-23-CE49-0006).

References

[1] Hinson, D. P., et al., Radio occultation measurements of Pluto’s neutral atmosphere with New Horizons, Icarus, 290, 96–111, 2017.

[2] A. Dias-Oliveira et al., Pluto's atmosphere from stellar occultations in 2012 and 2013, ApJ 811, 53, 2015.

[3] Zhang, X., et al, Haze heats Pluto's atmosphere yet explains its cold temperature, Nature, 2017.

[4] Forget, F., et al., A post-new horizons global climate model of Pluto including the N2, CH4 and CO cycles, Icarus, 287, 54–71, 2017.

[5] Bertrand, T., et al., Pluto’s Beating Heart Regulates the Atmospheric Circulation: Results From High-Resolution and Multiyear Numerical Climate Simulations. JGR: Planets, 125(2), 1–24, 2020.

[6] Bertrand, T. and Forget, F.: 3D modeling of organic haze in Pluto’s atmosphere, Icarus, 287:72, 2017.