Radiative-convective models of the atmospheres of Uranus and Neptune: Heating sources and seasonal effects

¹ Laboratoire de Météorologie Dynamique/Institut Pierre-Simon Laplace (LMD/IPSL), Sorbonne Université, CNRS, École Polytechnique, Institut Polytechnique de Paris, École Normale Supérieure (ENS), PSL Research University, 4 place Jussieu BC99, 75005 Paris, France
e-mail: gwenael.milcareck@lmd.ipsl.fr
² Laboratoire Atmosphères, Milieux, Observations spatiales (LATMOS), IPSL, Observatoire de Versailles St-Quentin-en-Yvelines, Université de Versailles St-Quentin-en-Yvelines, CNRS, 11 boulevard d’Alembert, 78280 Guyancourt, France
³ Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), Observatoire de Paris, CNRS, Sorbonne Université, Université Paris-Diderot, Meudon, France
⁴ Laboratoire d’Astrophysique de Bordeaux, Université de Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁵ School of Physics & Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK

Received: 17 December 2023
Accepted: 30 March 2024

Abstract

Context. The observations made during the Voyager 2 flyby have shown that the stratosphere of Uranus and that of Neptune are warmer than expected by previous models. In addition, no seasonal variability of the thermal structure has been observed on Uranus since Voyager 2 era and significant subseasonal variations have been revealed on Neptune.

Aims. In this paper, we evaluate different realistic heat sources that can induce sufficient heating to warm the atmosphere of these planets and we estimate the seasonal effects on the thermal structure.

Methods. The seasonal radiative-convective model developed by the Laboratoire de Météorologie Dynamique was used to reproduce the thermal structure of these planets. Three hypotheses for the heating sources were explored separately: aerosol layers, a higher methane mole fraction, and thermospheric conduction.

Results. Our modelling indicates that aerosols with plausible scattering properties can produce the requisite heating for Uranus, but not for Neptune. Alternatively, greater stratospheric methane abundances can provide the missing heating on both planets, but the large values needed are inconsistent with current observational constraints. In contrast, adding thermospheric conduction cannot warm the stratosphere of both planets alone. The combination of these heat sources is also investigated. In the upper troposphere of both planets, the meridional thermal structures produced by our model are found inconsistent with those retrieved from Voyager 2/IRIS data. Furthermore, our models predict seasonal variations should exist within the stratospheres of both planets while observations showed that Uranus seems to be invariant to meridional contrasts and only subseasonal temperature trends are visible on Neptune. However, a warm south pole is seen in our simulations of Neptune as observed since 2003.