Planetary Banding

The zonal, meridional, and vertical motions of Uranus’s troposphere and stratosphere may be studied via observations of zonal winds, temperature, composition, and aerosols, and how these parameters change over time. Contrast-enhanced images in reflected sunlight, such as those in Figures 1 and 2 (Fry et al., 2012; Karkoschka, 2015; Sromovsky et al., 2015), reveal a banded atmosphere, with fine-scale 5º–20º latitudinal lanes of different reflectivity. These contrasts are probably due to variations in aerosol optical depth with latitude, although Karkoschka (2015) suggested that at least one of the bands could have been caused by variations in aerosol absorption, suggestive of different materials in some bands. Unlike Jupiter and Saturn, this banding appears to have no strong correlation with the zonal “Winds and Temperatures,” which means that canonical ice giant “belts” and “zones” cannot be defined in quite the same way (Fletcher, de Pater, et al., 2020). Furthermore, zonal medians of the reflectivity maps (Sromovsky et al., 2015) indicate that these zonal contrasts are not static, but change from observation to observation, potentially because of obscuration of the banded structure by discrete features. However, millimeter (1.3–3.1 mm) observations of Uranus from ALMA in 2017–2018 and centimeter (0.9–10 cm) observations from VLA in 2015 (Figure 2, Molter et al., 2020) suggest subtle thermal banding in the 1–50 bar range that is qualitatively similar to that observed in reflected sunlight in the 1–4 bar range. This suggests that Uranus’s albedo contrasts could be related to thermal and/or compositional contrasts in the troposphere, although the long-term stability and location of the zonal bands remain to be rigorously assessed.

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Figure 6. Comparison of Uranus’s near-IR reflectivity (A) to the zonal-mean tropospheric temperatures (B) and cloud-tracked winds (C). Panel (A) shows Voyager 2 imagery poleward of ${60}^{\circ }$S (Karkoschka, 2015) with Keck H‑band imagery in 2012 northward of 60ºS (Sromovsky et al., 2015). Panel (B) comes from inversion of Voyager/IRIS 25–50 $\mu$m spectroscopy (Fletcher et al., 2020), the gray shaded regions represent the declining information content at low pressures. Zonal winds in (C) are adapted from Figure 15 of Sromovsky et al. (2015), combining Voyager 2 observations in the gray shaded area (squares, Karkoschka, 2015) with Keck observations (circles) at other latitudes, and the dotted curve represents a profile if reflected at the equator.

Winds and Temperatures

Tropospheric zonal winds are constrained by tracking of discrete cloud features by Voyager 2 (Karkoschka, 2015; Smith et al., 1986), the Hubble Space Telescope (Hammel et al., 2001; Karkoschka, 1998), and ground-based facilities like Keck and Gemini (Hammel et al., 2005b; Sromovsky & Fry, 2005; Sromovsky et al., 2009, 2015). These sources were used to develop a “canonical wind profile” by fitting Legendre polynomials to the measured drift of features (Figure 6c, Sromovsky et al., 2015), although there remain some dispersion and potential temporal variations around these measurements. Most of the tracked cloud features on Uranus are near the 1.3-bar methane condensation level or in the deeper 2–4 bar main clouds of H₂S ice (Irwin et al., 2018), but some can reach the 250–600 mbar region in the upper troposphere (Roman et al., 2018; Sromovsky et al., 2007, 2012). Some of the smaller-scale clouds evolve quite rapidly (Irwin et al., 2017), making it difficult to track them over multiple rotations. Nevertheless, Figure 6 shows that the cloud tracking has revealed a single, broad retrograde jet at Uranus’s equator, and a single high-latitude ~260 m/s prograde jet in each hemisphere near 50º–60º, quite unlike the multi-jet circulations of Jupiter and Saturn (see the reviews by Sánchez-Lavega et al., 2019). Note that it is common practice to define Uranus’s westward winds as positive (prograde) and the eastward equatorial jet as negative (retrograde), unlike on the other giants. Furthermore, reconstruction of the gravity field measured by Voyager 2 (Hubbard et al., 1991) out to the fourth-order harmonic (Kaspi et al., 2013) suggests that these tropospheric winds are confined to a layer approximately 1,000 km deep (i.e., down to 2 kbar).

Geostrophic balance suggests a correlation between horizontal temperature gradients and the vertical shear on the zonal winds (Holton, 2004). The thermal banding observed in the microwave in Figure 2 (Molter et al., 2020) is too fine scale to be related to the broad structure of the zonal flow (Sromovsky et al., 2015), but thermal-IR observations by Voyager 2 (Conrath et al., 1998; Flasar et al., 1987) and ground-based facilities (Figure 2, Orton et al., 2015; Roman et al., 2020) reveal larger-scale structure, with cool midlatitudes in the 80–800 mbar range, contrasted with a warmer equator and poles. The tropospheric temperature patterns in Figure 4b appear to be unchanging in the time since Voyager 2 and imply maximum positive windshears near ±15º–30º latitude on the flanks of the equatorial retrograde jet, and maximum negative windshear near the high-latitude prograde jets at ±60º–70º (Fletcher, de Pater, et al., 2020). The windshear is minimal (i.e., close to barotropic, with no wind variability with height) in the ±30º–50º latitude range associated with the midlatitude temperature minima, which is also where the most notable storm activity occurs (see “Uranian Meterology”). Taken together, this suggests that the cloud-top prograde jets of Sromovsky et al. (2015) should decay with altitude, potentially because of frictional damping related to the breaking of vertically propagating waves in the upper troposphere (Flasar et al., 1987). The equatorial winds would be more complicated, with the most rapid decay of the retrograde flow with height occurring away from the equator. Furthermore, existing measurements of latitudinal temperature gradients and zonal winds have relatively poor spatial resolution, so it is reasonable to assume that finer-scale temperature or wind bands exist throughout the troposphere, as suggested by the reflectivity (Figure 6a) and microwave emission maps. Future missions with multi-spectral mapping capabilities are needed to resolve this.

Finally, there are hints in Figure 6 that both the zonal winds (Sromovsky et al., 2015) and the tropospheric temperatures (Conrath et al., 1998; Orton et al., 2015) exhibit a north–south asymmetry and that this asymmetry might be weakening with time. Southern summertime winds at 50ºS–90ºS (Karkoschka, 2015) appear to be very different to corresponding northern latitudes 60ºN–83ºN during spring (Sromovsky et al., 2015), where zonal drift rates adhere to solid-body rotation. Long-term tracking of the winds will be required to understand whether this asymmetry is seasonal or a permanent feature.

Composition Contrasts

Besides temperatures, winds, and aerosols, planetary banding is observed in the gaseous composition in Figure 7. Fine-scale banding in microwave emission in Figure 7c (Molter et al., 2020) has already been discussed and can be interpreted in terms of variations in H₂S abundance as a function of latitude. The collision-induced continuum measured by Voyager 2 also reveals the latitudinal contrasts in para-hydrogen, the even spin isomer of H₂ (Conrath et al., 1998; Flasar et al., 1987; Fletcher, de Pater, et al., 2020). The para-H₂ fraction is quenched at 25% in the warmer deep troposphere, and chemical equilibration is sufficiently slow that upward motion can bring this low-para-H₂ air upward into the cooler upper troposphere, where the equilibrium para-H₂ abundance should be much larger. Sub-equilibrium para-H₂ in Figure 7a is therefore indicative of upwelling, which is seen at midlatitudes in the 80–800 mbar region. Conversely, super-equilibrium para-H₂ is observed at the equator and poles, which is taken to be an indicator of subsidence from the cold tropopause. Caution is required, though, as poorly understood catalysis processes on aerosol surfaces could cause the equilibration time scale to vary with location (Fouchet et al., 2003; Massie & Hunten, 1982).

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Figure 7. Zonal-mean composition gradients in para-H₂ disequilibrium (A), tropospheric CH₄ in approximately the 1–10 bar range (B), and tropospheric microwave emission indicating H₂S variations (C). Panel (A) shows the extent of disequilibrium for para-H₂ (sub-equilibrium in dotted lines indicating upwelling, and super-equilibrium in solid lines indicating subsidence) derived from Voyager or IRIS spectra (Fletcher et al., 2020). Gray shading indicates the lack of information content for p < 70 mbar. The latitudinal distribution of methane in panel (B) is from Karkoschka and Tomasko (2009) and Sromovsky et al. (2014). The deep distribution of H₂S is represented by the normalized microwave brightness in panel (C), as measured by ALMA and the VLA (Molter et al., 2020), where higher emission indicated depleted H₂S abundances, and the dashed line is indicative of the poleward CH₄ depletion as given in panel (B).

Small-scale contrasts in Figure 7b–c are superimposed onto a significant equator-to-pole decline in the volatile abundances CH₄ and H₂S. Uranus’s south polar depletion of methane was first detected via Hubble 300–1,000 nm spectroscopy in 2002 (Karkoschka & Tomasko, 2009), when the north polar region was still largely hidden from view. After equinox in 2007, further Hubble observations in 2012 (Sromovsky et al., 2014) and 2015 (Sromovsky et al., 2019) revealed that the north polar region exhibited a similar CH₄ depletion and that the equator-to-pole gradient from ~4% near the equator to ~2% at high latitudes appeared to be relatively stable over time. Furthermore, this gradient exhibited a steplike structure, with a transition in abundance near the ±30º–40º latitude region, although degeneracies with the assumed aerosol distribution can modify the derived gradients. Uranus’s polar methane depletion was also observed independently from ground-based facilities in the near-infrared, including the IRTF (Tice et al., 2013), Keck (de Kleer et al., 2015), Palomar (Roman et al., 2018), and the VLT and Gemini (Irwin et al., 2018; Toledo et al., 2018). Sromovsky et al. (2019) reproduced the spectra with a methane distribution that was latitudinally uniform for p > 5 bar (in the 2.7%–3.5% range, depending on aerosol scattering properties), and only latitudinally variable at lower pressures in the upper troposphere, with a factor of three decrease in the upper tropospheric CH₄ from 30ºN to 70ºN.

However, the implied equator-to-pole circulation, with rising methane-rich air at low latitudes, moving poleward as CH₄ is removed by condensation, precipitation, and sedimentation, before methane-poor air sinks at high latitudes, may in fact extend much deeper. VLA centimeter observations of Uranus between 1982 and 1994 (de Pater, 1991; de Pater & Gulkis, 1988; Hofstadter & Butler, 2003), probing down to 50 bars, revealed Uranus’s microwave-bright south polar region due to a strong depletion of absorbers, with a strong brightness gradient near 45ºS (Figure 2). VLA observations since 2003 show that the north polar region was similarly bright (de Pater et al., 2018; Hofstadter et al., 2004; Molter et al., 2020), and this high brightness can be primarily explained by the absence of H₂S down to ~35 bars (Molter et al., 2020). Whether this polar depletion of deep H₂S is caused by the same circulation and subsidence responsible for the shallow CH₄ depletion is an open question (Fletcher, de Pater, et al., 2020; Sromovsky et al., 2014). Furthermore, neither the methane nor the H₂S appear to be tracing the upper tropospheric circulation revealed by the para-H₂ and temperature distribution, which would suggest midlatitude upwelling rather than equatorial upwelling. Indeed, polar subsidence would tend to inhibit convection and CH₄ cloud formation (Sromovsky et al., 2012), and yet discrete clouds were clearly visible at Uranus’s north pole during early northern spring, as seen in Figure 1 (Sromovsky et al., 2014).

Convective Storms

Discrete meteorological features, from small bright clouds to planet-encircling waves, giant storms, and dark vortices, are superimposed on and intricately connected to the large-scale “Atmospheric Circulation” described herein. Some of the small-scale bright clouds, presumably comprised of methane ice reaching pressures of 300–600 mbar, high above the main H₂S cloud deck at 2–4 bars, may be convective in origin, driven by a combination of latent heat release as CH₄ condenses (Hueso et al., 2020; Stoker & Toon, 1989) and potentially by the energy released during the interconversion between para- and ortho-H₂, which have different specific heats (Smith & Gierasch, 1995). However, Uranus shares the same problem as the other giant planets, in that air saturated with condensates will be heavier than the surrounding “dry” H₂–He mixture (Friedson & Gonzales, 2017; Guillot, 1995; Leconte et al., 2017). As described in “Atmospheric Circulation,” the strong vertical gradient in molecular weight prevents convection extending over tremendous heights, instead limiting it to vertically thin layers (Gierasch & Conrath, 1987). Strong initial perturbations are therefore required to counteract this static stability (effectively a negative buoyancy) to drive self-sustaining CH₄-rich updrafts, but this has yet to be verified observationally (Hueso et al., 2020; Hueso & Sánchez-Lavega, 2019). Indeed, Guillot (2019) pointed out that this moist CH₄-driven convection is occurring at shallow pressures (0.1–1.5 bars) and low optical depths compared to the deep, hidden H₂O clouds of Jupiter and Saturn, meaning that Uranus (and Neptune) provide ideal destinations to investigate how convection operates on hydrogen-rich worlds where it is inhibited by the weight of the condensables. Furthermore, Uranus’s moist convection might be rather different from Jupiter’s, as H₂O will play a very limited role in the observable upper troposphere, and H₂S condensation provides only a limited source of latent heat to drive convection.

Hueso et al. (2020) defined Uranus’s convective features to be storm features that show divergence above the clouds over short time scales, but they point out that the frequency and distribution of such storms is not well known, and much of the observed cloud activity might not be convective. Instead, some bright features may be orographic structures associated with pressure perturbations at deeper levels (e.g., those associated with dark, hidden vortices), as on Neptune (Stratman et al., 2001). A particularly long-lived feature at southern midlatitudes, some 5,000–10,000 km wide, was known as the “Berg” and had bright features rising to the 550–750 mbar level, but with the main parts of the structure near 1.7–3.5 bars (de Pater et al., 2011). The Berg was captured by HST and Keck between 1994 and 2009 (de Pater et al., 2011, 2015; Hammel et al., 2005b; Sromovsky et al., 2009, 2015), and may even have been observed by Voyager in 1986 (Sromovsky & Fry, 2005). It oscillated in latitude (around 35.2ºS) and longitude, migrating toward the equator after 2005, reaching 27ºS in 2007, and disintegrating when it reached 5ºS in late 2009 (de Pater et al., 2011; Sromovsky et al., 2009). The Berg exhibited intense brightenings in 2004 and 2007, suggestive of convective storms within the feature (de Pater et al., 2015), but there was no evidence that it was related to a deeper unobserved vortex (Hueso & Sánchez-Lavega, 2019).

Discussion of strong convection naturally raises questions about the potential for lightning. Methane is non-polar, but H₂S, NH₃, NH₄SH, and H₂O provide the mixed-phase materials that could produce lightning (Aplin et al., 2020). Microphysical modeling suggests that the radio emissions discovered by Voyager (Zarka & Pedersen, 1986) are more likely related to the NH₄SH cloud than the deep water layers (Aplin et al., 2020).

Discrete Cloud Activity

Uranus has exhibited a range of discrete cloud phenomena since the early Voyager and Hubble observations. Eight features were observed between 35ºS and 70ºS by Voyager (Smith et al., 1986) near northern winter solstice. As winter proceeded in the mid-1990s, notable bright cloud features were observed at mid-northern latitudes (Karkoschka, 1998) as they were emerging into sunlight. Since 2000, ground-based facilities like Keck were frequently detecting cloud features (de Pater et al., 2002; Hammel et al., 2001, 2005b), including some in 2004 that reached sufficiently high altitude to be detected in the strong CH₄ band near 2.12 μ‎m (Hammel et al., 2005a). Northern midlatitudes 28ºN–42ºN appear to be intrinsically active (Sromovsky & Fry, 2007), particularly in the years following northern spring equinox (L_s = 0º, 2007), with clouds reaching 300–500 mbar, and new outbreaks observed in 2004–2006 and 2011 (Sromovsky et al., 2007, 2009, 2012). A record-breaking bright storm occurred at 15ºN in 2014, as shown in Figure 2 (de Pater et al., 2015), thought to be formed from a complex of smaller storms. And another at 32ºN possessed a longitudinally elongated tail in the 1–2 bar range, qualitatively similar to storm tails observed on Jupiter and Saturn (de Pater et al., 2015; Irwin et al., 2016, 2017; Sromovsky et al., 2015).

It is natural to ask whether Uranian storms exhibit a temporal dependency given that some of the brightest activity appeared to occur in the hemisphere emerging from the darkness of winter. Alternatively, maybe Uranian storms occur episodically because of powerful outbursts followed by long periods of quiescence as CAPE (convective available potential energy) accumulates, as suggested for Saturn’s annual storms (Li & Ingersoll, 2015). However, storm statistics are currently insufficient, and the observations are biased to the sunlit hemisphere, so robust assessment of the destabilizing influence of increased insolation remains to be performed. The one exception is in the polar domain in Figure 1, where the south pole displayed no discrete activity during southern summer (Karkoschka, 2015; Smith et al., 1986), whereas clusters of bright spots with 600–800 km diameters were observed in the north polar region after it emerged into sunlight in northern spring (Sromovsky et al., 2012, 2015). This notable asymmetry suggests some form of convective inhibition (or obscuration by overlying hazes) developing as spring turns to summer, something that will be testable as Uranus approaches northern summer solstice (L_s = 90º) in 2030.

Vortices and Waves

The vast majority of discrete features tracked on Uranus have been bright cloud features, rising above the main cloud decks. However, Uranus also infrequently exhibits dark anticyclonic ovals, which can form and dissipate on the time scales of years. Hubble was the first to observe a dark vortex in 2006 at 28ºN (Hammel et al., 2009), and since that time several more have been observed (e.g., Sromovsky et al., 2015). These are typically smaller than those observed on Neptune, and whether they are accompanied by bright (and more readily visible) companion clouds due to air being forced up and over the vortex (orographic clouds, Stratman et al., 2001) remains a topic of ongoing exploration. It is not clear whether the dearth of anticyclonic ovals on Uranus with respect to Neptune is an observational bias (e.g., because of different overlying aerosols and gas absorption) or a real difference between the two worlds.

Finally, Uranus exhibits wave phenomena that manifest as longitudinal reflectivity contrasts in the troposphere. A chain of diffuse bright features just north of the equator, first suggested by Keck images in 2003 (Hammel et al., 2005b), were observed by Keck in 2012 (Sromovsky et al., 2015) to be spaced every 30º–40º longitude. A second wave was captured by the 2012–2014 Keck observations, using adaptive optics and derotation techniques (Figure 6a, Fry et al., 2012; Sromovsky et al., 2015), revealing a transverse “scalloped” equatorial wave pattern just south of the equator, with diffuse bright features every 19º–21º longitude (wavenumbers 17–19) and a latitudinal amplitude of 2.4º–2.9º. Sromovsky et al. (2015) suggested that Kelvin waves or mixed internal gravity-Rossby waves may be at work, but this cannot be properly characterized until the dispersion relation (phase speed versus wavelength) is determined.

Wave phenomena are unlikely to be restricted to Uranus’s troposphere and may also be present in the stratosphere, modulating emission from stratospheric hydrocarbons (Roman et al., 2020) and generating rotational variability in disc-averaged mid-IR spectra observed by Spitzer (Rowe-Gurney et al., 2021).

Methane Photochemistry

Methane photolysis products dominate the stratospheric composition of both ice giants (Atreya et al., 1991), but the strength of vertical mixing by eddy diffusion (weak on Uranus, strong on Neptune) has stark consequences for the distribution of species, with Uranian methane photochemistry operating in a higher-pressure regime than on any other giant planet (Moses et al., 2018). The lower CH₄ homopause on Uranus (Herbert et al., 1987) has the surprising consequence that seasonal contrasts are more muted than on Neptune, and that CH₄ chemistry is not as well coupled to exogenic oxygenated materials deposited at lower pressures (see “Coupling to External Oxygen”). Nevertheless, photochemistry on Uranus results in a complicated mix of hydrocarbons (Atreya & Ponthieu, 1983; Dobrijevic et al., 2010; Moses et al., 2005, 2018.) that can be investigated via mid-infrared remote sensing (Encrenaz et al., 1998; Feuchtgruber et al., 1997; Orton et al., 1987; Orton, Fletcher, et al., 2014; Roman et al., 2020; Rowe-Gurney et al., 2021) and UV occultations (Bishop et al., 1990; Herbert et al., 1987). Acetylene (C₂H₂) was first discovered by the Infrared Space Observatory (ISO) (Encrenaz et al., 1998); ethane (C₂H₆), methyl-acetylene (C₃H₄), and diacetylene (C₄H₂) were observed by Spitzer (Burgdorf et al., 2006; Orton, Moses, et al., 2014). Some products, such as ethylene, propane, benzene, and methyl, have not yet been detected (Moses et al., 2020). The effects of the high-pressure photochemistry are apparent in mid-IR observations: Uranus’s hydrocarbons are confined to altitudes below the ~0.1 mbar level in Figure 4, and the ratio of ethane to acetylene is very different on Uranus compared to all the other giants (Orton, Moses, et al., 2014).

The latitudinal distribution of these species is expected to vary due to stratospheric circulation and seasonal photochemistry (Figure 8). Although photochemistry occurs primarily in UV sunlight (145 nm), and thus photolytic rates will be largest at polar latitudes that receive a higher annual average solar insolation than the equator, small amounts of solar Lyman alpha scattered from hydrogen in the local interstellar medium can provide a secondary photolysis source during the darkness of winter. Chemical abundances respond quickly at low pressures to changes in the UV flux, but Figure 8 shows that contrasts should decrease at higher pressures, where diffusion and chemical time scales increase, producing subtle hemispheric asymmetries at p < 1 mbar, with phase lags in response to insolation changes (Moses et al., 2018). Thus short-lived species like acetylene, methyl-acetylene, and diacetylene are expected to have maximum abundances at the summer or autumn poles, whereas long-lived species like ethane are expected to peak near the equator because photolytic destruction effectively competes with production in the high-latitude summer. However, these predictions assume uniform distributions of stratospheric CH₄ as the source material, which may not be the case given the strong equator-to-pole CH₄ gradients in the troposphere (see “Radiative Balance”). Furthermore, there remains some disagreement about what the stratospheric CH₄ abundance actually is, with a factor of six in stratospheric abundance between Spitzer results in 2007 (Orton, Moses, et al., 2014) and Herschel results in 2011 (Lellouch et al., 2015). Both inferences were from disc-integrated spectra, so this could be a sign of changing viewing geometry, latitudinal temperature gradients, changes in efficiency of vertical mixing, temporal variability, or some combination of the above.

Of these species, only emission from C₂H₂ has been mapped across Uranus, showing stark discrepancies from the photochemical model predictions (Moses et al., 2018; Roman et al., 2020), and implying a key role for circulation: either strong midlatitude upwelling of CH₄-rich air from the troposphere; or midlatitude subsidence in the stratosphere (see “Atmospheric Circulation”). The ground-based VLT maps of stratospheric C₂H₂ emission in Figure 2 suggest a distinct equatorial minimum at latitudes <±25º (Roman et al., 2020), counter to photochemical expectations, and the complete opposite of what is observed in the tropospheric temperatures.

The products of CH₄ photochemistry, such as benzene, acetylene, ethane, and propane (in addition to exogenic H₂O and CO₂) can condense to form thin haze layers in the 0.1–30 mbar range (Figures 4 and 5), some of which are visible in high-phase imaging from Voyager (Moses & Poppe, 2017; Rages et al., 1991; Romani et al., 1993; Toledo et al., 2018), and may be modulated by vertically propagating waves. The condensed hydrocarbons could also sediment downward to coat tropospheric hazes or serve as cloud-condensation nuclei for CH₄ and H₂S condensation at higher pressures.

Coupling to External Oxygen

Species that contain oxygen, like CO, CO₂, and H₂O, are present in the upper stratospheres of each of the giant planets (Feuchtgruber et al., 1997; Moses & Poppe, 2017), originating from cometary impacts, satellite debris, and ablation of interplanetary dust and ring particles. What makes Uranian photochemistry particularly interesting is that this oxygen deposition is separated from the methane photochemistry because of the sluggish vertical mixing, making it an intriguing counterpoint to the other giants (Moses & Poppe, 2017). Uranus’s stratospheric water was detected by ISO (Feuchtgruber et al., 1997); CO from the fluorescent emission in the infrared (Encrenaz et al., 2004) and sub-millimeter emission (Cavalié et al., 2014); and CO₂ from Spitzer (Burgdorf et al., 2006; Orton, Fletcher, et al., 2014). Photolysis of CO and CO₂ can lead to secondary peaks of hydrocarbon production at high altitudes above the levels where the CH₄ abundance is dropping away (Moses et al., 2018), whereas H₂O will condense to form a 10 mbar ice haze. An interplanetary dust particle source is sufficient to explain the observed amount of CO, CO₂, and H₂O in Uranus’s stratosphere (Moses & Poppe, 2017), although cometary impacts could also contribute (Lara et al., 2019).

Radiative Balance

The vertical distribution of hydrocarbons, particularly CH₄, C₂H₂, and C₂H₆, determine the radiative heating and cooling in Uranus’s middle atmosphere. For this reason, the presence of thin hazes and secondary peaks of hydrocarbon production can have a significant influence. Uranus’s upper troposphere and stratosphere are warmed by shortwave absorption by CH₄ and aerosols, and cooled by longwave emission from C₂H₂ and C₂H₆ in the stratosphere and from the collision-induced H₂–He continuum in the troposphere (Conrath et al., 1998; Li et al., 2018). However, it has long been noted that absorption of weak sunlight by CH₄ alone cannot balance the efficient cooling from H₂ and, to a lesser extent, the hydrocarbons, so the middle atmosphere is warmer (Orton, Fletcher, et al., 2014) than expected. Additional sources of heat from vertically propagating gravity waves or the radiative contribution of hazes have been invoked to explain the discrepancy between observations and predictions (Li et al., 2018).

Uranus’s extreme obliquity, resulting in a larger irradiance at the poles than the equator, could also yield seasonal temperature asymmetries. However, Uranus’s radiative time constant in the troposphere is longer than the Uranian year (Conrath et al., 1990; Friedson & Ingersoll, 1987), such that tropospheric temperatures are not expected to change significantly with time, and to track the annual average equilibrium values (Lunine, 1993). This is consistent with the lack of observed tropospheric temperature variation between Voyager (Figure 6) and the present day (Orton et al., 2015; Roman et al., 2020). Stratospheric temperature asymmetries have never been measured because of the extreme cold and resulting low mid-IR radiance, but Uranus’s stratospheric radiative time constant is actually longer than that on Neptune, due to the low CH₄ abundance and cold temperatures, such that stratospheric temperature asymmetries might be weaker on Uranus and again simply track the annual average (i.e., warmer at the poles and cooler at the equator). Future missions capable of measuring temperature contrasts between the summer and winter hemisphere are needed to address this question.

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Figure 9. Hubble observations at 619 nm, a weak methane absorption band, showing the evolution of Uranus’s reflectivity over a quarter of a century, spanning mid-winter, spring equinox, and mid-spring in the northern hemisphere. Observations before 2009 were acquired by the WFPC2 instrument, observations after 2009 were acquired by the WFC3 instrument, largely by the Outer Planet Atmospheres Legacy (OPAL) program since 2014.

Connection to the Interior

Although this article is focused on Uranus’s atmosphere, it cannot be considered in isolation from the deeper interior, and the external charged particle environment. It is not immediately clear where an ice giant atmosphere (molecular envelope) ends and its interior or icy mantle begins. Differential rotation due to zonal winds appears to be restricted to the outermost 1,000 km of Uranus’s 25,559 km equatorial radius (i.e., the outer 4%, down to ~2 kbar, Kaspi et al., 2013). Deeper down at ~20 kbar and 1,200 K, it is possible that H₂O is insoluble in H₂ (Bali et al., 2013), leading to the formation of a liquid water ocean at tens of kilobars (Bailey & Stevenson, 2015). This immiscibility can lead to sharp interfaces, or an ocean surface, within the interior (Bailey & Stevenson, 2021). This ocean would transition from being non-conducting to ionic or superionic at hundreds of kilobars and could be responsible for removing NH₃ by dissolution to explain the low N/S ratio detected in the atmosphere (see “Observations of Uranus”). The generation of Uranus’s internal dynamo (Ness et al., 1986) may be associated with convection in the partially dissociated fluid water layers (Redmer et al., 2011; Soderlund et al., 2013), and appreciable conductivity can be achieved at 20%–30% of the radii below the surface (Soderlund & Stanley, 2020). Dynamo simulations predict large-scale circulation in this watery layer, with equatorial upwelling and, depending on the thickness of the convecting layer, polar circulation cells (Soderlund et al., 2013), although it is unclear how this might relate to the circulation in the molecular envelope in “Atmospheric Circulation.” At even greater depths, high-pressure laboratory experiments suggest that water could form a thick superionic “icy mantle,” creating a solid-phase viscous lattice of oxygen ions surrounded by a sea of free hydrogen (Millot et al., 2018, 2019; Wilson et al., 2013).

These substantial transitions between exotic phases of matter in Uranus’s interior could lead to inhibition of convection and retention of interior heat (e.g., Hubbard et al., 1995), significantly influencing the thermal evolution of Uranus (Fortney et al., 2011). Indeed, different rates of hydrogen–water demixing, due to H₂O immiscibility and the formation of a water ocean, could explain the very different heat flow on Uranus compared to Neptune (Bailey & Stevenson, 2021). Furthermore, the presence of these transitions could successfully decouple motions in the observable “dry” atmosphere from those in the deeper, water-rich interior oceans. However, Helled et al. (2020) provided a cautionary note that the bulk oxygen content of Uranus is still unknown, to the extent that it might instead be dominated by rocky materials rather than water, and implying that the oft-used term “ice giant” may be misleading.

Connection to the Exterior

The circulation and chemistry of the middle atmosphere (e.g., the stratosphere) will also be intricately connected to processes shaping the upper atmosphere (e.g., ionosphere or thermosphere), above the homopause where hydrogen dominates. Weak solar heating alone cannot explain the high temperatures of Uranus’s thermosphere (Herbert et al., 1987), a deficit known as the “energy crisis,” and contributions from auroral heating and potentially the breaking of vertically propagating waves have been invoked to close the gap between observations and expectations. Extreme UV radiation, or impact ionization from the aurora, produces the ${\mathrm{H}}_3^{+}$ ion, which was first detected on Uranus in 1992 (Trafton et al., 1993) and has been monitored ever since, revealing a long-term cooling of Uranus’s thermosphere over three decades (Melin et al., 2019). That ${\mathrm{H}}_3^{+}$ production appears to be efficient on Uranus, but not Neptune, may be related to the weak atmospheric mixing and low homopause height. The thermospheric cooling trend appears to be decoupled from Uranus’s geometric season (i.e., it did not change after the 2007 equinox) but might be explained by a “magnetic season:” differing amounts of Joule heating at different points in the planet’s orbit, changes to the homopause height with time, or a hemispherically asymmetric homopause that changes the ${\mathrm{H}}_3^{+}$ density on the hemisphere visible from Earth (see Melin, 2020, for a review). Whether this redistribution of energy in Uranus’s upper atmosphere has any implications for the energy balance, circulation, and chemistry of the stratosphere remains to be seen.

Auroral emission has been observed via excitation of H Lyman-α‎ and H₂ bands due to precipitating energetic particles (Lamy, 2020). These are located at magnetic poles tilted from the rotational axis by ~60º, as observed by Voyager UVS (Herbert et al., 1987) and later Hubble (Lamy et al., 2012, 2017). Auroral brightening has also been tentatively detected in the infrared via ${\mathrm{H}}_3^{+}$ emission (Thomas et al., 2020). On Jupiter ion-neutral chemistry related to the aurora can lead to a unique balance of stratospheric chemicals and aerosols in the polar domain, so future observations could test whether this is also true on Uranus, given the midlatitude location of the magnetic poles.