Determine the condensate species in cold planetary atmospheres

Determine which chemical species actually condense in the cold atmospheres of Uranus and Neptune, despite thermodynamic phase-rule constraints on the number of stable condensed species.

Background

The paper explains that modeling the vertical atmospheric structure of Uranus and Neptune requires first calculating the equilibrium gas-phase composition and then selecting the condensates that are stable under the relevant pressure and temperature conditions. Because many chemical species may condense in cold planetary atmospheres, identifying the physically realized condensate set remains unresolved. This uncertainty directly affects predictions of cloud composition, atmospheric depletion, and the inferred vertical structure.

References

Although the amount of stable condensed species in equilibrium with the gas phase is restricted by Gibbs' phase rule, it remains unknown which species actually condense.

— The atmospheric vertical structure of Uranus and Neptune from thermochemical models: the impact of model assumptions  (2608.13157 - Douçot et al., 13 Aug 2026) in Section 1, Introduction

However, NH$_3$ condensation has not yet been conclusively identified in the atmospheres of Uranus and Neptune.

— The atmospheric vertical structure of Uranus and Neptune from thermochemical models: the impact of model assumptions  (2608.13157 - Douçot et al., 13 Aug 2026) in Section 2.3, 1-bar temperature and thermal profile