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![Montage of thermal emission observations of Uranus (top) and Neptune (bottom) that help characterise ice giant circulation. On the left, centimetre-wave observations from the VLA (Butler et al. 2012; de Pater et al. 2018; Molter et al. 2020) and millimetre-wave observations from ALMA (Tollefson et al. 2019; Molter et al. 2020) sense opacity variations in the deep troposphere. Fainted banded structure is visible on both planets, although maps were constructed from many hours of data, smearing features in longitude. On the right, 17–18 μm observations sense upper tropospheric temperatures (Orton et al. 2007b, 2015), whereas 7.9 and 13.0 μm sense stratospheric temperatures via methane and acetylene emission, respectively (Roman et al. 2020; Sinclair et al. 2020). Two sets of Uranus data are shown, one near equinox (top row) when both poles were visible, and one in 2015–18 (second row) when the north pole was in view. Hubble/WFC3 images of Uranus and Neptune in 2018 are shown in the centre for context, courtesy of the OPAL programme (https://archive.stsci.edu/prepds/opal/). All images have been oriented so that the north pole is at the top. On both planets, the dominant features at centimetre/millimetre wavelengths are the very bright poles, interpreted as regions of dry, subsiding air parcels at pressures greater than ∼1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\sim 1$\end{document} bar. Note that Uranus’ large polar region extends to ∼±45∘\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\sim \pm 45^{\circ }$\end{document}, while Neptune’s extends only to ∼65∘S\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\sim 65^{\circ }\mbox{S}$\end{document}](publication/339462079/figure/fig6/AS:962173773828126@1606411512297/Montage-of-thermal-emission-observations-of-Uranus-top-and-Neptune-bottom-that-help.png)
Montage of thermal emission observations of Uranus (top) and Neptune (bottom) that help characterise ice giant circulation. On the left, centimetre-wave observations from the VLA (Butler et al. 2012; de Pater et al. 2018; Molter et al. 2020) and millimetre-wave observations from ALMA (Tollefson et al. 2019; Molter et al. 2020) sense opacity variations in the deep troposphere. Fainted banded structure is visible on both planets, although maps were constructed from many hours of data, smearing features in longitude. On the right, 17–18 μm observations sense upper tropospheric temperatures (Orton et al. 2007b, 2015), whereas 7.9 and 13.0 μm sense stratospheric temperatures via methane and acetylene emission, respectively (Roman et al. 2020; Sinclair et al. 2020). Two sets of Uranus data are shown, one near equinox (top row) when both poles were visible, and one in 2015–18 (second row) when the north pole was in view. Hubble/WFC3 images of Uranus and Neptune in 2018 are shown in the centre for context, courtesy of the OPAL programme (https://archive.stsci.edu/prepds/opal/). All images have been oriented so that the north pole is at the top. On both planets, the dominant features at centimetre/millimetre wavelengths are the very bright poles, interpreted as regions of dry, subsiding air parcels at pressures greater than ∼1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\sim 1$\end{document} bar. Note that Uranus’ large polar region extends to ∼±45∘\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\sim \pm 45^{\circ }$\end{document}, while Neptune’s extends only to ∼65∘S\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\sim 65^{\circ }\mbox{S}$\end{document}
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Atmospheric circulation patterns derived from multi-spectral remote sensing can serve as a guide for choosing a suitable entry location for a future in situ probe mission to the Ice Giants. Since the Voyager-2 flybys in the 1980s, three decades of observations from ground- and space-based observatories have generated a picture of Ice Giant circulat...
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We use observations from one of the SOUTHTRAC (Southern Hemisphere Transport, Dynamics, and Chemistry) Campaign flights in Patagonia and the Antarctic Peninsula during September 2019 to analyze possible sources of gravity waves (GW) in this hotspot during austral late winter and early spring. Data from two of the instruments onboard the German High...
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... Surprisingly, both ice giants present equatorial regions with about 4% of CH 4 while their polar regions appear to be depleted by a factor of ∼2 Tomasko 2009, 2011;Sromovsky et al. 2014). These meridional variabilities are possibly caused by tropospheric circulation (e.g., Fletcher et al. (2020) and references therein). This was recently confirmed by Irwin et al. (2021) for Neptune, though derived abundances were somewhat slightly higher, i.e., 4-6% in the equatorial region and 2-4% in the southern polar region. ...
The exploration of carbon-to-oxygen ratios has yielded intriguing insights into the composition of close-in giant exoplanets, giving rise to a distinct classification: carbon-rich planets, characterized by a carbon–to–oxygen ratio ≥ 1 in their atmospheres, as opposed to giant planets exhibiting carbon–to–oxygen ratios close to the protosolar value. In contrast, despite numerous space missions dispatched to the outer solar system and the proximity of Jupiter, Saturn, Uranus, and Neptune, our understanding of the carbon-to-oxygen ratio in these giants remains notably deficient. Determining this ratio is crucial as it serves as a marker linking a planet’s volatile composition directly to its formation region within the disk. This article provides an overview of the current understanding of the carbon-to-oxygen ratio in the four gas giants of our solar system and explores why there is yet no definitive dismissal of the possibility that Jupiter, Saturn, Uranus, or Neptune could be considered carbon-rich planets. Additionally, we delve into the three primary formation scenarios proposed in existing literature to account for a bulk carbon-to-oxygen ratio ≥ 1 in a giant planet. A significant challenge lies in accurately inferring the bulk carbon-to-oxygen ratio of our solar system’s gas giants. Retrieval methods involve integrating in situ measurements from entry probes equipped with mass spectrometers and remote sensing observations conducted at microwave wavelengths by orbiters. However, these methods fall short of fully discerning the deep carbon-to-oxygen abundance in the gas giants due to their limited probing depth, typically within the 10–100 bar range. To complement these direct measurements, indirect determinations rely on understanding the vertical distribution of atmospheric carbon monoxide in conjunction with thermochemical models. These models aid in evaluating the deep oxygen abundance in the gas giants, providing valuable insights into their overall composition.
... Strong science drivers remain for multiple atmospheric probes to the giant planets (particularly Uranus, as discussed by Fletcher et al. 2020), despite the changing level of explicit support from survey to survey over the past three decades. In this paper, we present the overarching science drivers for including multiple probes on the UOP mission (Sect. ...
... Similar behavior in CH 4 , C 2 H 6 , and C 2 H 2 suggests that temperature variation rather than composition drives the radiance enhancement, while lack of longitudinal variation in continuum and CH 3 D radiance may be due to sensitivity to levels deeper than the radiance anomaly. Adapted from Rowe-Gurney et al. Fletcher et al. (2020) time is longer for hydrocarbon hazes mixed down from the stratosphere. Widescale upwelling would sustain the stable layer and help to suspend haze particles, while widescale downwelling would suppress formation of the stable layer. ...
... This is consistent with a meridional circulation, with cold air rising at mid-latitudes and subsiding at both the poles and the equator (Fig. 4). The para-H 2 fraction is at its minimum in areas of upwelling observed in the mid-latitudes yet at a much higher value in the high-latitude areas of the northern hemisphere that exhibited cooler temperatures Fletcher et al. (2020). ...
A major motivation for multiple atmospheric probe measurements at Uranus is the understanding of dynamic processes that create and maintain spatial variation in thermal structure, composition, and horizontal winds. But origin questions—regarding the planet’s formation and evolution, and conditions in the protoplanetary disk—are also major science drivers for multiprobe exploration. Spatial variation in thermal structure reveals how the atmosphere transports heat from the interior, and measuring compositional variability in the atmosphere is key to ultimately gaining an understanding of the bulk abundances of several heavy elements. We review the current knowledge of spatial variability in Uranus’ atmosphere, and we outline how multiple probe exploration would advance our understanding of this variability. The other giant planets are discussed, both to connect multiprobe exploration of those atmospheres to open questions at Uranus, and to demonstrate how multiprobe exploration of Uranus itself is motivated by lessons learned about the spatial variation at Jupiter, Saturn, and Neptune. We outline the measurements of highest value from miniature secondary probes (which would complement more detailed investigation by a larger flagship probe), and present the path toward overcoming current challenges and uncertainties in areas including mission design, cost, trajectory, instrument maturity, power, and timeline.
... Deeper down, the significant vertical reduction of the mean molecular weight at the CH 4 condensation level at ∼2 bar seems to act as a barrier to vertical motion (e.g., IRW22) and below this the vertical circulation appears to switch with air rising at equatorial latitudes and falling nearer the pole, leading to higher equatorial abundances of CH 4 , seen here, and also high deep H 2 S abundances seen at microwave wavelengths (Tollefson et al., 2021). This simple "stacked circulation" view (e.g., Fletcher et al., 2020;Tollefson et al., 2019), however, is inconsistent with the banded structure seen here at methane continuum wavelengths longer than 650 nm and may also be inconsistent with retrieved H 2 S abundances (at 1.5 μm) at pressures less than 5 bar (Irwin et al., 2019b), suggesting that the circulation may actually be even more complicated than previously thought. An interesting analogy is the distribution of NH 3 seen in Jupiter's deep atmosphere by the Juno spacecraft (Li et al., 2017), which appears to rise in an equatorial plume, with much greater abundances than at mid-latitudes, although a more zonal NH 3 structure is seen at the 0.5-2-bar level by both Juno , and also in mid-infrared observations (e.g., Fletcher et al., 2016); this is believed to arise and be maintained by a similar double "stacked cell" system (e.g., de Pater et al., 2023;Duer et al., 2021;Fletcher et al., 2021;Ingersoll et al., 2000;Moeckel et al., 2023;Showman & de Pater, 2005). ...
... The banding seen in the narrow windows of very low methane absorption longer than 650 nm is suggestive of a finer latitudinal scale variation than has been noted before on Neptune at the level of the H 2 S/haze Aerosol-1 layer at ∼5 bar and indicates that the circulation of Neptune's atmosphere is even more complicated than that suggested by the "stacked circulation" models of Tollefson et al. (2019) and Fletcher et al. (2020). Further work is needed to explore such changes more fully and recent observations with the JWST should provide strong new constraints. ...
Spectral observations of Neptune made in 2019 with the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope (VLT) in Chile have been analyzed to determine the spatial variation of aerosol scattering properties and methane abundance in Neptune's atmosphere. The darkening of the South Polar Wave at ∼60°S, and dark spots such as the Voyager 2 Great Dark Spot is concluded to be due to a spectrally dependent darkening (λ < 650 nm) of particles in a deep aerosol layer at ∼5 bar and presumed to be composed of a mixture of photochemically generated haze and H2S ice. We also note a regular latitudinal variation of reflectivity at wavelengths of very low methane absorption longer than ∼650 nm, with bright zones latitudinally separated by ∼25°. This feature, which has similar spectral characteristics to a discrete deep bright spot DBS‐2019 found in our data, is found to be consistent with a brightening of the particles in the same ∼5‐bar aerosol layer at λ > 650 nm. We find the properties of an overlying methane/haze aerosol layer at ∼2 bar are, to first‐order, invariant with latitude, while variations in the opacity of an upper tropospheric haze layer reproduce the observed reflectivity at methane‐absorbing wavelengths, with higher abundances found at the equator and also in a narrow “zone” at 80°S. Finally, we find the mean abundance of methane below its condensation level to be 6%–7% at the equator reducing to ∼3% south of ∼25°S, although the absolute abundances are model dependent.
... This asymmetry, as well as analyses of Voyager gravity measurements, suggests that zonal wind speeds are vertically confined globally and should decay with altitude in the upper troposphere (e.g. see Fletcher et al. (2020) for a recent review). As new observations and analysis techniques push limitations of spatial resolution and sensitivity, new features have emerged in images of Uranus that complicate this broader picture. ...
We present observations of Uranus in northern spring with the VLA from 0.7 cm to 5 cm. These observations reveal details in thermal emission from Uranus' north pole at 10s of bars, including a dark collar near 80N and a bright spot at the polar center. The bright central spot resembles observations of polar emission on Saturn and Neptune at shallower pressures. We constrain the variations in temperature and NH3/H2S abundances which could explain these features. We find that the brightness temperature of the polar spot can be recreated through 5 K temperature gradients and/or 10x depletion of NH3 or H2S vapor between 10-20 bars, both consistent with the presence of a cyclonic polar vortex. The contrast of the polar spot may have increased since 2015, which would suggest seasonal evolution of Uranus' polar circulation at depth.
... The observed 3-D spatial distribution of condensable gases as compared with a uniformly layered composition (i.e., only affected by condensation at the dew-point of the various gases) can be used to infer the general circulation in a planet's atmosphere [27,93,[148][149][150][151][152][153]. In addition to the observations reviewed in this paper, valuable input for such models is provided by data at 5-µm and mid-infrared wavelengths, which provide temperature maps in the upper troposphere-stratosphere as well as data on disequilibrium species and the ortho/para H 2 distribution. ...
... While the above circulation pattern also agrees with the para-H 2 data of both Uranus and Neptune from Voyager/IRIS (e.g., [151]), there are some observations that cannot be reconciled. The apparent increase in Neptune's retrograde zonal wind speed with altitude at the equator and mid-latitudes is opposite to what this circulation pattern pre-dicts, and suggests that the equator is either colder or denser than the mid-latitudes at altitudes > 1 bar [16]. ...
... In order to explain the latitudinal distribution of aerosol layers on Uranus, Sromovsky et al. [149] suggested a 3-layered vertical cell structure instead of the single cell discussed above. To reconcile these seemingly opposite circulation patterns led to the 2-layer stacked-cell circulation pattern, as sketched in Figure 23 [151]. ...
Radio observations of the atmospheres of the giant planets Jupiter, Saturn, Uranus, and Neptune have provided invaluable constraints on atmospheric dynamics, physics/chemistry, and planet formation theories over the past 70 years. We provide a brief history of these observations, with a focus on recent and state-of-the-art studies. The global circulation patterns, as derived from these data, in combination with observations at UV/visible/near-IR wavelengths and in the thermal infrared, suggest a vertically-stacked pattern of circulation cells in the troposphere, with the top cell similar to the classical picture, overlying cells with the opposite circulation. Data on the planets’ bulk compositions are used to support or disfavor different planet formation scenarios. While heavy element enrichment in the planets favors the core accretion model, we discuss how the observed relative enrichments in volatile species constrain models of the outer proto-planetary disk and ice giant accretion. Radio observations of planets will remain invaluable in the next decades, and we close with some comments on the scientific gain promised by proposed and under-construction radio telescopes.
... For example, Jupiter has several convective outbreaks in the 4-8 year timescale [33], while Saturn produces a massive outbreak (dubbed the Great White Spot; see Section 2.2.2) every 30 years [49]. The ice giants do not seem to demonstrate a regular periodicity, although this may due to limited observations and in-situ data from these planets [136]. Li and Ingersoll [64] showed that the timescales for convective events in H-He atmosphere is driven by the molecular weight gradient and the radiative cooling in the upper atmosphere. ...
The outer planets of our Solar System display a myriad of interesting cloud features, of different colors and sizes. The differences between the types of observed clouds suggest a complex interplay between the dynamics and chemistry at play in these atmospheres. Particularly, the stark difference between the banded structures of Jupiter and Saturn vs. the sporadic clouds on the ice giants highlights the varieties in dynamic, chemical and thermal processes that shape these atmospheres. Since the early explorations of these planets by spacecrafts, such as Voyager and Voyager 2, there are many outstanding questions about the long-term stability of the observed features. One hypothesis is that the internal heat generated during the formation of these planets is transported to the upper atmosphere through latent heat release from convecting clouds (i.e., moist convection). In this review, we present evidence of moist convective activity in the gas giant atmospheres of our Solar System from remote sensing data, both from ground- and space-based observations. We detail the processes that drive moist convective activity, both in terms of the dynamics as well as the microphysical processes that shape the resulting clouds. Finally, we also discuss the effects of moist convection on shaping the large-scale dynamics (such as jet structures on these planets).
... During the development of missions incorporating entry probes, selecting a trajectory to ensure a safe approach and successful probe delivery is a critical element of the mission design. The trajectory has to avoid rings and target the most desirable regions in the atmosphere, while balancing other requirements such as providing an optimal communication geometry between the probe and the relay carrier spacecraft as well as meeting science objectives (Ball et al. 2007;Fletcher et al. 2020). Due to the complexity of the problem, mission concept studies usually focus on the investigation of only a limited number of specific trajectories and probe delivery opportunities, restricting the science to a very small, predefined latitude range, while leaving a huge trade space unexplored (Banfield et al. 2018;Mousis et al. 2014Mousis et al. , 2018Hofstadter et al. 2019). ...
Exploring planetary atmospheres uncovers important information as to how our solar system formed and evolved. While remote sensing is extensively used, some crucial observations require in situ measurements by an atmospheric probe. Given their scientific importance, probe missions to Saturn, Uranus, and Neptune are under consideration for the coming decades. In anticipation of future probe missions, the software tool Visualization of the Impact of PRobe Entry conditions on the science, mission and spacecraft design (VIPRE) was developed as proof-of-concept to facilitate selection of probe entry locations. Currently, there is no analytical way to identify which interplanetary trajectory from thousands of feasible launch opportunities is optimal for a considered mission concept. The search and decision process for that solution is complex and relies on the intuition of mission designers, who focus on a subset of trajectories to make the trade space manageable. The idea of VIPRE is (1) to generate a multidimensional data cube showing relevant engineering and science parameters simultaneously for thousands of trajectories, and (2) to visualize the data for all entry sites over the body’s envelope. VIPRE lays a foundation to make available the data for browsing in a 3D visualization to identify the best family of solutions for a given mission. This paper introduces the validated and verified core algorithms of VIPRE, published on GitHub Probst. VIPRE serves as a basic framework to be used and extended for different purposes. The paper further presents the motivation for the development and algorithms; it explains the computation and data visualization strategy; and gives a list of suggested functionalities to extend and further develop VIPRE to fully leverage its potential.
... Despite the wavelength differences between data sets, there is no substantial variation observed in the winds. This indicates little vertical shear is present, even though wind gradients are expected in the 100 to 800 mbar region based on the latitudinal temperature structure [134,135]. Measurements on images from 2012 to 2014 showed no substantial changes in the zonal wind field, though the increase in cloud features allowed the extraction of a more complete wind profile [24,105]. ...
Each of the giant planets, Jupiter, Saturn, Uranus, and Neptune, has been observed by at least one robotic spacecraft mission. However, these missions are infrequent; Uranus and Neptune have only had a single flyby by Voyager 2. The Hubble Space Telescope, particularly the Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS) instruments, and large ground-based telescopes with adaptive optics systems have enabled high-spatial-resolution imaging at a higher cadence, and over a longer time, than can be achieved with targeted missions to these worlds. These facilities offer a powerful combination of high spatial resolution, often <0.05”, and broad wavelength coverage, from the ultraviolet through the near infrared, resulting in compelling studies of the clouds, winds, and atmospheric vertical structure. This coverage allows comparisons of atmospheric properties between the planets, as well as in different regions across each planet. Temporal variations in winds, cloud structure, and color over timescales of days to years have been measured for all four planets. With several decades of data already obtained, we can now begin to investigate seasonal influences on dynamics and aerosol properties, despite orbital periods ranging from 12 to 165 years. Future facilities will enable even greater spatial resolution and, combined with our existing long record of data, will continue to advance our understanding of atmospheric evolution on the giant planets.
... Uranus's winds also exhibit a surprising hemispheric asymmetry near the poles (Sromovsky et al. 2014;Karkoschka 2015), which may be seasonally driven. Whereas the cloud bands of Jupiter and Saturn are loosely associated with the zonal jets due to eastward jet peaks acting as transport barriers, Uranian cloud bands are seemingly not tied to the smooth wind structure (Fletcher et al. 2020b), which may hint at unresolved peaks in the zonal wind structure. Temporal tracking of cloud features in high-resolution images would reveal any such peaks, as well as any seasonal changes since Voyager 2. ...
... Depletion of gases such as methane (observed in the near-infrared) and H 2 S (observed in the microwave) around the poles seems to suggest that Uranus has a single deep circulation cell in each hemisphere in which air rises from the deep atmosphere at low latitudes, clouds condense out, and dry air is transported to high latitudes, where it descends . However, such a circulation pattern is inconsistent with observed cloud and temperature distributions in the upper troposphere, implying that the meridional circulation must be more complex, perhaps involving multiple stacked cells (Figure 6(d); Fletcher et al. 2020b). High-resolution maps of temperature and key chemical tracers of vertical mixing are necessary to unravel the meridional circulation. ...
Current knowledge of the Uranian system is limited to observations from the flyby of Voyager 2 and limited remote observations. However, Uranus remains a highly compelling scientific target due to the unique properties of many aspects of the planet itself and its system. Future exploration of Uranus must focus on cross-disciplinary science that spans the range of research areas from the planet’s interior, atmosphere, and magnetosphere to the its rings and satellites, as well as the interactions between them. Detailed study of Uranus by an orbiter is crucial not only for valuable insights into the formation and evolution of our solar system but also for providing ground truths for the understanding of exoplanets. As such, exploration of Uranus will not only enhance our understanding of the ice giant planets themselves but also extend to planetary dynamics throughout our solar system and beyond. The timeliness of exploring Uranus is great, as the community hopes to return in time to image unseen portions of the satellites and magnetospheric configurations. This urgency motivates evaluation of what science can be achieved with a lower-cost, potentially faster-turnaround mission, such as a New Frontiers–class orbiter mission. This paper outlines the scientific case for and the technological and design considerations that must be addressed by future studies to enable a New Frontiers–class Uranus orbiter with balanced cross-disciplinary science objectives. In particular, studies that trade scientific scope and instrumentation and operational capabilities against simpler and cheaper options must be fundamental to the mission formulation.
... Modern explorations of possible mission scenarios to the Ice Giants Uranus and Neptune [1,2,3,4] have motivated a revision of what we know about these planets and their planetary systems. Recent reviews explore their atmospheric dynamics [5], mean circulation patterns [6], and vertical structure [5,7]. These atmospheric themes connect with their internal structure [8] and formation mechanisms [9]. ...
... Another possibility is that methane could leak out to the stratosphere through warm regions like the hot south polar region [35] without the need for strong convection. However, this scenario is in conflict with circulation patterns that could explain the thermal structure observed in the planet's stratosphere [70,36,6]. Here we review observations of Neptune that suggest vigorous methane moist convection on discrete cloud systems. ...
... Such observations using radio interferometers result in a banded structure of the planets down to 50-80 bar [78,79]. This deep structure is interpreted as a signature of the latitudinal distribution of condensables such as H 2 S, and is interpreted as caused by a deep global circulation [6]. We here describe the expected multi-cloud layer structure of Uranus and Neptune and show caveats in the classical description of the vertical cloud structure. ...
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 small 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.
