Temperature profiles and variability of mean molecular weight in ice giants. Left panel: Uranus and Neptune temperature profiles from the Voyager 2 spacecraft, retrieved from radio occultations by Lindal et al. (1987, 1990); Lindal (1992). Saturation vapor pressure and triple point are indicated for 6% of CH₄ (the maximum observed value), which corresponds to a specific concentration of 0.30 kg/kg. Theoretically, methane only exists in a solid or gas form, and can only sublimate and condense. A lower methane concentration would shift the gas/iquid/solid transition curves toward the bottom of the plot. Right panel: Mean molecular weight variability in ice giants. We consider a mix of [85% H₂ + 15% He] for the background atmosphere composition corresponding to a 2.3 g mol⁻¹ mean molecular weight. We assume that CH₄ specific concentration follows saturation and its specific concentration in the deep atmosphere is set to 0.20 kg/kg for Uranus (equivalent to a 3.6% volume mixing ratio) and 0.30 kg/kg for Neptune (6.2% volume mixing ratio).