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 (H2S) at depths of a few bars. Even deeper, thermochemical models predict clouds of ammonia hydrosulfide (NH4SH) 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’.

中文翻译：

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天王星和海王星的对流风暴和大气垂直结构

冰巨星天王星和海王星拥有以氢为基础的大气层，其中有几种成分会在它们寒冷的高层大气中凝结。在两颗行星上观测到的少量明亮云系统是湿对流风暴的良好候选者，但它们的观测特性（大小、时间尺度和活动周期）与气态巨行星中的湿对流风暴不同。这些云和风暴可能是由甲烷凝结造成的，观测还表明在几条柱的深处有更深的硫化氢 (H2S) 云。更深层次的热化学模型预测氨氢硫化物 (NH4SH) 和水在数十到数百巴的压力下形成的云，形成扩展的深层天气层。由于氢的分子量低且挥发物含量高，它们的缩合产生了一个强大的稳定垂直的分子量梯度，比木星和土星中的等价梯度更大。由此产生的对垂直运动的抑制会导致潮湿的对流状态，这与地球或泰坦等氮基大气中发生的情况大不相同。因此，人们对天王星和海王星深部大气的热结构尚不清楚。类似的过程可能发生在土星木星的深水云中，但冰巨星提供了研究上部甲烷云层中这些物理方面的可能性。需要结合轨道和原位数据来了解对流及其在冰巨星大气动力学中的作用，进而在包括木星在内的氢大气中，土星和巨大的系外行星。本文是讨论会问题“冰巨星系统的未来探索”的一部分。