Microphysical modeling of methane ice clouds in the atmospheres of the Ice Giants

Abstract

Voyager 2 radio occultation measurements of Uranus and Neptune revealed a layer approximately 2-4 km thick near 1.2 and 1.6 bars, respectively, wherein the atmospheric refractivity exhibited a slope variation (1, 2). These findings were interpreted as indicating a region where methane gas was undergoing condensation, forming an ice cloud centered around this pressure level. While the formation of this putative cloud would explain the observed decrease in methane abundance with height above 1.2 and 1.6 bars, or the banded structure of Uranus through latitudinal variations in the opacity of this cloud, several recent works and observations do not provide direct evidence in favor of this cloud (3): (i) radiative transfer models show an enhancement in the scattering opacity at pressures near 4-6 bars, more consistent with the presence of H2S ice (4, 5); (ii) observations from ground-based telescopes (or observations from telescopes in orbit around the Earth) of methane clouds indicate cloud tops near 0.4 bars in both planets (6), approximately a scale height above the base of the putative methane cloud.