Convective storms and atmospheric vertical structure in Uranus and Neptune

Abstract

The ice giants Uranus and Neptune have hydrogen-based atmospheres with several constituents that condense in their cold upper atmospheres. A small number of bright cloud systems observed in both planets are good candidates for moist convective storms, but their observed properties (size, temporal scales and cycles of activity) differ from moist convective storms in the gas giants. These clouds and storms are possibly due to methane condensation and observations also suggest deeper clouds of hydrogen sulfide (H 2 S) at depths of a few bars. Even deeper, thermochemical models predict clouds of ammonia hydrosulfide (NH 4 SH) and water at pressures of tens to hundreds of bars, forming extended deep weather layers. Because of hydrogen’s low molecular weight and the high abundance of volatiles, their condensation imposes a strongly stabilizing vertical gradient of molecular weight larger than the equivalent one in Jupiter and Saturn. The resulting inhibition of vertical motions should lead to a moist convective regime that differs significantly from the one occurring on nitrogen-based atmospheres like those of Earth or Titan. As a consequence, the thermal structure of the deep atmospheres of Uranus and Neptune is not well understood. Similar processes might occur at the deep water cloud of Jupiter in Saturn, but the ice giants offer the possibility to study these physical aspects in the upper methane cloud layer. A combination of orbital and in situ data will be required to understand convection and its role in atmospheric dynamics in the ice giants, and by extension, in hydrogen atmospheres including Jupiter, Saturn and giant exoplanets. This article is part of a discussion meeting issue ‘Future exploration of ice giant systems’.

Zonal Winds of Uranus and Neptune: Gravitational Harmonics, Dynamic Self-gravity, Shape, and Rotation

“…As can be seen from Figure 3, the density of the models at the 1 bar pressure level is higher than what is usually inferred for Uranus (∼0.42 kg m −3 ) and Neptune (∼0.45 kg m −3 ; Hueso et al 2020). This is especially true for three-layered models, as they allow for a larger variation in density in the outer layers.…”

Giant Planets from the Inside-Out

“…Understanding the occurrence of these storms remains a significant challenge, given the stabilising effects of the molecular weight of moist air (i.e., convective inhibition), balanced by the buoyant effects of latent heat release (Guillot 1995;Leconte et al 2017). Unfortunately, the depth of this water-driven convection is hard to access on Jupiter and Saturn, but methane-driven convection on the Ice Giants may provide a vital means for testing convection in hydrogen-rich planets at higher, more accessible altitudes (Hueso et al 2020).…”

Zonal winds of Uranus and Neptune: Gravitational harmonics, dynamic self-gravity, shape, and rotation

“…As can be seen from Figure 3, the density of the models at the 1-bar pressure level is higher than what is usually inferred for Uranus (∼0.42 kg m −3 ) and Neptune (∼0.45 kg m −3 ) (Hueso et al 2020). This is especially true for 3-layered models, as they allow for a larger variation in density in the outer layers.…”