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As Figure 5, but for Neptune, showing a contour plot of vertical aerosol structure (opacity/bar) inferred from our “snippet” Neptune retrievals, where for each wavelength the aerosol structure is retrieved from the wavelengths in a bin of width 0.1 μm centred on that wavelength. The red line in the opacity/bar key is again the assumed a priori value.
Source publication
We present a reanalysis (using the Minnaert limb‐darkening approximation) of visible/near‐infrared (0.3–2.5 μm) observations of Uranus and Neptune made by several instruments. We find a common model of the vertical aerosol distribution i.e., consistent with the observed reflectivity spectra of both planets, consisting of: (a) a deep aerosol layer w...
Citations
... Observations suggest that the sulfur to nitrogen ratio of ice giant atmosphere is significantly larger than one and the solar value by directly detecting H 2 S but not NH 3 in their atmospheres (Irwin et al. 2018(Irwin et al. , 2019. Therefore, the deep cloud deck at about 5 bar is more likely composed of H 2 S cloud instead of NH 3 (Irwin et al. 2022). Observations also suggest that there are high-altitude CH 4 clouds near the tropopause at 0.1 bar on ice giants, but they are more commonly seen and active on Neptune than Uranus (e.g., Gibbard et al. 2003;Irwin et al. 2022;Chavez et al. 2023). ...
... Therefore, the deep cloud deck at about 5 bar is more likely composed of H 2 S cloud instead of NH 3 (Irwin et al. 2022). Observations also suggest that there are high-altitude CH 4 clouds near the tropopause at 0.1 bar on ice giants, but they are more commonly seen and active on Neptune than Uranus (e.g., Gibbard et al. 2003;Irwin et al. 2022;Chavez et al. 2023). Summaries of cloud activities and storm events can be found in recent review papers (e.g., Hueso & Sánchez-Lavega 2019; Hueso et al. 2020;Fletcher 2021;Palotai et al. 2022). ...
... Clouds have a significant contribution to the opacity and albedo of ice giant atmospheres. Visible to near-infrared observations can help constrain the cloud structure and evolution from cloud opacity and spectroscopy on Uranus and Neptune (e.g., Hueso & Sánchez-Lavega 2019; Irwin et al. 2022;Chavez et al. 2023) and, therefore, provide useful information to help theorists understand and constrain the atmospheric dynamics on ice giants. ...
Storms operated by moist convection and the condensation of CH 4 or H 2 S have been observed on Uranus and Neptune. However, the mechanism of cloud formation, thermal structure, and mixing efficiency of ice giant weather layers remains unclear. In this paper, we show that moist convection is limited by heat transport on giant planets, especially on ice giants where planetary heat flux is weak. Latent heat associated with condensation and evaporation can efficiently bring heat across the weather layer through precipitations. This effect was usually neglected in previous studies without a complete hydrological cycle. We first derive analytical theories and show that the upper limit of cloud density is determined by the planetary heat flux and microphysics of clouds but is independent of the atmospheric composition. The eddy diffusivity of moisture depends on the planetary heat fluxes, atmospheric composition, and surface gravity but is not directly related to cloud microphysics. We then conduct convection- and cloud-resolving simulations with SNAP to validate our analytical theory. The simulated cloud density and eddy diffusivity are smaller than the results acquired from the equilibrium cloud condensation model and mixing length theory by several orders of magnitude but consistent with our analytical solutions. Meanwhile, the mass-loading effect of CH 4 and H 2 S leads to superadiabatic and stable weather layers. Our simulations produced three cloud layers that are qualitatively similar to recent observations. This study has important implications for cloud formation and eddy mixing in giant planet atmospheres in general and observations for future space missions and ground-based telescopes.
... It has been interpreted as either the simple base of the cloud layer (Lindal et al. 1987), or possibly evidence of static stability due to the inhibition of convection (Guillot 1995). The latter interpretation is supported by Irwin et al. (2022), who find that aerosols in this layer are too absorbing to be methane ice itself, and may represent haze particles that remain suspended due to weaker mixing in the stable layer. ...
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.
... However, water clouds are situated deep in the troposphere, perhaps at 50-200 bar, depending on the global abundance of O . CH 4 clouds condense much higher, around 1.3 bar (Irwin et al. 2022), and can be easily sampled by an entry probe assuming the probe passes through these ephemeral and localized clouds. The purported globally distributed H 2 S cloud deck around 3-6-bar will also be accessible to the entry probe; most probe designs target at least the 10 bar pressure level (Mousis et al. , 2018Simon et al. 2020;Orton et al. 2021a). ...
The Ice Giants represent a unique and relatively poorly characterized class of planets that have been largely unexplored since the brief Voyager 2 flyby in the late 1980s. Uranus is particularly enigmatic, due to its extreme axial tilt, offset magnetic field, apparent low heat budget, mysteriously cool stratosphere and warm thermosphere, as well as a lack of well-defined, long-lived storm systems and distinct atmospheric features. All these characteristics make Uranus a scientifically intriguing target, particularly for missions able to complete in situ measurements. The 2023-2032 Decadal Strategy for Planetary Science and Astrobiology prioritized a flagship orbiter and probe to explore Uranus with the intent to “...transform our knowledge of Ice Giants in general and the Uranian system in particular” (National Academies of Sciences, Engineering, and Medicine in Origins, worlds, and life: a decadal strategy for planetary science and astrobiology 2023-2032, The National Academies Press, Washington, 2022). In support of this recommendation, we present community-supported science questions, key measurements, and a suggested instrument suite that focuses on the exploration and characterization of the Uranian atmosphere by an in situ probe. The scope of these science questions encompasses the origin, evolution, and current processes that shape the Uranian atmosphere, and in turn the Uranian system overall. Addressing these questions will inform vital new insights about Uranus, Ice Giants and Gas Giants in general, the large population of Neptune-sized exoplanets, and the Solar System as a whole.
... 6). Figure 3a shows the variation of the sky brightness at p = 0.73 bar as a function of the zenith and azimuth angles simulated using the NEMESIS correlated-k radiative-transfer and retrieval code (Irwin et al. 2008). For these simulations, we used the aerosol/gas model described in Irwin et al. (2022) (referred to as IRW22 thereafter), where the sun is at a zenith angle of 79.1°. By integrating the radiance field over the sensor's FOV and corresponding spectral band at each pressure level, we derive the irradiance expected during the experiment. ...
... Irwin et al. 2018 and 2019, reported the detection of gaseous H 2 S above the main cloud deck, and based on these results, they argued that the principal constituent of this cloud is likely to be H 2 S ice. Most recently, Irwin et al. 2022 found that the main cloud deck is actually composed of a mix of haze and CH 4 ice at 1-2 bar, and that below this there is a deeper cloud of haze and H 2 S ice at p > ∼ 5 bar. The existence of H 2 S at these levels implies that the abundance of H 2 S exceeds that of NH 3 above the water condensation level, which leads to the expectation that deeper than ∼10 bar there exists a cloud of NH 4 SH, followed at even deeper levels by a cloud of H 2 O (Carlson et al. 1988). ...
... The colored rectangles indicate the wavelength range of channels 17 (green) and 18 (light brown) different cloud compositions. In all the simulations, the cloud opacity and effective radius are fixed to 1 km −1 and 2 µm, respectively, and methane gas absorption was included using the mole fraction retrievals given in Irwin et al. (2022). Here we see that while the methane ice absorption feature is masked by the methane gas absorption, the presence of NH 4 SH and H 2 S ices in the atmosphere can be determined by using the intensity measured in channels 18 and 19. ...
The aerosols (clouds and hazes) on Uranus are one of the main elements for understanding the thermal structure and dynamics of its atmosphere. Aerosol particles absorb and scatter the solar radiation, directly affecting the energy balance that drives the atmospheric dynamics of the planet. In this sense, aerosol information such as the vertical distribution or optical properties is essential for characterizing the interactions between sunlight and aerosol particles at each altitude in the atmosphere and for understanding the energy balance of the
planet’s atmosphere. Moreover, the distribution of aerosols in the atmosphere provides key information on the global circulation of the planet (e.g., regions of upwelling or subsidence).
To address this challenge, we propose the Uranus Multi-experiment Radiometer (UMR), a lightweight instrument designed to characterize the aerosols in Uranus’ atmosphere as part of the upcoming Uranus Flagship mission’s descending probe payload. The scientific goals of UMR are: (1) to study the variation of the solar radiation in the ultra-violet (UV) with altitude and characterize the energy deposition in the atmosphere; (2) to study the vertical distribution of the hazes and clouds and characterize their scattering and optical properties; (3) to investigate the heating rates of the atmosphere by directly measuring the upward and downward fluxes; and (4) to study the cloud vertical distribution and composition at pressures where sunlight is practically negligible (p > 4-5 bars).
The instrument includes a set of photodetectors, field-of-view masks, a light infrared lamp, and interference filters. It draws on the heritage of previous instruments developed at the Instituto Nacional de Técnica Aeroespacial (INTA) that participated in the exploration of Mars, where similar technology has demonstrated its endurance in extreme environments while utilizing limited resources regarding power consumption, mass and volume footprints, and data budget. The radiometer’s design and characteristics make it a valuable complementary payload for studying Uranus’ atmosphere with a high scientific return.
... In addition to capturing the reflection spectra of NDS-2018 and DBS-2019, the 2019 VLT/MUSE data also show distinct latitudinal variations over a wide range of wavelengths (473-933 nm), in particular the South Polar Wave (SPW) at ∼60°S, first seen in Voyager 2 images in 1989 (Smith et al., 1989), which is dark at blue-green wavelengths, but invisible at longer wavelengths. The spectral features of the SPW are very similar to those of dark spots and Karkoschka (2011a) concluded that it is caused by a darkening of particles at pressures >3 bar, which was later confirmed by Irwin et al. (2022c). The SPW has been visible ever since the Voyager 2 flyby in HST imaging observations and these observations are reviewed in detail by Karkoschka (2011b). ...
... The analysis in this paper follows on from a combined analysis of HST/STIS, IRTF/SpeX, and Gemini/NIFS observations of both Uranus and Neptune by Irwin et al. (2022c), which we will refer to henceforth as "IRW22" or the "holistic" model. Given that we do not know the composition of the aerosols in Neptune's atmosphere, and do not have a definite expectation of the volume mixing ratio profile of the main visible/IR absorber, methane, the simultaneous retrieval of aerosol and methane abundance profiles is a degenerate problem, even when analyzing the 800-860 nm region that can differentiate between aerosols and methane abundance (Karkoschka & Tomasko, 2011). ...
... These retrievals show several very clear latitudinal . Imaginary refractive index (n imag ) spectra of Aerosol types 1, 2, and 3 derived from fitting the disc-averaged Very Large Telescope/Multi Unit Spectroscopic Explorer observations with our modified aerosol model, compared with the corresponding n imag spectra from the IRW22 analysis of Hubble Space Telescope/Space Telescope Imaging Spectrograph, IRTF/SpeX and Gemini/NIFS Neptune observations (Irwin et al., 2022c). dependencies of the fitted parameters, although the agreement between the observed and fitted reflectivities at 511 and 831 nm is not perfect (Figure 7). ...
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.
... μm region. Aerosols in panel (a) are plotted in opacity/km at a reference wavelength of 5 μm, calculated following the scheme in Appendix C of Irwin et al. (2022). A logarithmic color bar is used to show structure within the aerosol cross-section. ...
Saturn's northern summertime hemisphere was mapped by JWST/Mid‐Infrared Instrument (4.9–27.9 µm) in November 2022, tracing the seasonal evolution of temperatures, aerosols, and chemical species in the 5 years since the end of the Cassini mission. The spectral region between reflected sunlight and thermal emission (5.1–6.8 µm) is mapped for the first time, enabling retrievals of phosphine, ammonia, and water, alongside a system of two aerosol layers (an upper tropospheric haze p < 0.3 bars, and a deeper cloud layer at 1–2 bars). Ammonia displays substantial equatorial enrichment, suggesting similar dynamical processes to those found in Jupiter's equatorial zone. Saturn's North Polar Stratospheric Vortex has warmed since 2017, entrained by westward winds at p < 10 mbar, and exhibits localized enhancements in several hydrocarbons. The strongest latitudinal temperature gradients are co‐located with the peaks of the zonal winds, implying wind decay with altitude. Reflectivity contrasts at 5–6 µm compare favorably with albedo contrasts observed by Hubble, and several discrete vortices are observed. A warm equatorial stratospheric band in 2022 is not consistent with a 15‐year repeatability for the equatorial oscillation. A stacked system of windshear zones dominates Saturn's equatorial stratosphere, and implies a westward equatorial jet near 1–5 mbar at this epoch. Lower stratospheric temperatures, and local minima in the distributions of several hydrocarbons, imply low‐latitude upwelling and a reversal of Saturn's interhemispheric circulation since equinox. Latitudinal distributions of stratospheric ethylene, benzene, methyl, and carbon dioxide are presented for the first time, and we report the first detection of propane bands in the 8–11 µm region.
... In a recent comparative study, Irwin et al. ( 2022 ) modelled observations of both Uranus and Neptune from 2002 to 2010, made with (1) HST 's Space Telescope Imaging Spectrograph (STIS); (2) the SpeX instrument of the NASA's Infrared Telescope Facility (IRTF); and (3) Gemini-North's Near-Infrared Integral Field Spectrometer (NIFS). These data, co v ering a wide range of wavelengths from 0.3 to 2.4 μm, enabled Irwin et al. ( 2022 ) to develop an 'holistic', simple model for the haze and cloud structure of both planetary atmospheres, consisting of (1) a deep layer of aerosol (Aerosol-1) extending up to the ∼5 bar level (the limiting pressure of sensitivity), composed most likely of photochemical haze and H 2 S ice; (2) a middle layer of aerosol at ∼2 bar (Aerosol-2), composed most likely of photochemical haze and methane ice; and (3) and extended layer of photochemical haze (Aerosol-3) reaching up from the Aerosol-2 level into the lower stratosphere. ...
... In a recent comparative study, Irwin et al. ( 2022 ) modelled observations of both Uranus and Neptune from 2002 to 2010, made with (1) HST 's Space Telescope Imaging Spectrograph (STIS); (2) the SpeX instrument of the NASA's Infrared Telescope Facility (IRTF); and (3) Gemini-North's Near-Infrared Integral Field Spectrometer (NIFS). These data, co v ering a wide range of wavelengths from 0.3 to 2.4 μm, enabled Irwin et al. ( 2022 ) to develop an 'holistic', simple model for the haze and cloud structure of both planetary atmospheres, consisting of (1) a deep layer of aerosol (Aerosol-1) extending up to the ∼5 bar level (the limiting pressure of sensitivity), composed most likely of photochemical haze and H 2 S ice; (2) a middle layer of aerosol at ∼2 bar (Aerosol-2), composed most likely of photochemical haze and methane ice; and (3) and extended layer of photochemical haze (Aerosol-3) reaching up from the Aerosol-2 level into the lower stratosphere. An unexpected by-product of this new 'holistic' model was that it can explain why Neptune is bluer than Uranus since the opacity of Neptune's Aerosol-2 layer at ∼2 bar is found to be roughly half that of Uranus. ...
... This allows light to penetrate deeper into Neptune's atmosphere before being reflected, leading to more absorption by red-absorbing gaseous methane and so making the planet appear bluer. When Irwin et al. ( 2022 ) compared the predicted visual colours from the HST /STIS observations of Uranus and Neptune, it was apparent that although Uranus was slightly more green than Neptune, the difference in colour was understandably nothing like as extreme as that shown in the early Voyager 2 observations (Figs 2 a and b), where the Neptune images were contrast-enhanced as previously outlined. The reconstructed colours were more similar to later 'true' colour renderings (Figs 2 c and d), but not identical, and we wondered if it might be possible to determine quantitatively whether there has been any significant change in the colours of Uranus and Neptune since the late-1980s. ...
We present a quantitative analysis of the seasonal record of Uranus’s disc-averaged colour and photometric magnitude in Strömgren b and y filters (centred at 467 and 551 nm, respectively), recorded at the Lowell Observatory from 1950 to 2016, and supplemented with HST/WFC3 observations from 2016 to 2022. We find that the seasonal variations of magnitude can be explained by the lower abundance of methane at polar latitudes combined with a time-dependent increase of the reflectivity of the aerosol particles in layer near the methane condensation level at 1 – 2 bar. This increase in reflectivity is consistent with the addition of conservatively scattering particles to this layer, for which the modelled background haze particles are strongly absorbing at both blue and red wavelengths. We suggest that this additional component may come from a higher proportion of methane ice particles. We suggest that the increase in reflectivity of Uranus in both filters between the equinoxes in 1966 and 2007, noted by previous authors, might be related to Uranus’s distance from the Sun and the production rate of dark photochemical haze products. Finally, we find that although the visible colour of Uranus is less blue than Neptune, due to the increased aerosol thickness on Uranus, and this difference is greatest at Uranus’s solstices, it is much less significant than is commonly believed due to a long-standing misperception of Neptune’s ‘true’ colour. We describe how filter-imaging observations, such as those from Voyager-2/ISS and HST/WFC3, should be processed to yield accurate true colour representations.
... We modelled the spectral signatures of NDS-2018 and DBS-2019 using the NEMESIS 17-21 radiative transfer model, adapting the procedure used by IRW22 (ref. 12) for the analysis of observations of Uranus and Neptune from 0.3 to 2.5 μm from HST's Space Telescope Imaging Spectro graph (HST/STIS), the NASA InfraRed Telescope Facility's (IRTF) SpeX instrument, and Gemini North's Near-Infrared Integral Field Spectrometer (NIFS). Updating the procedure slightly from IRW22, we parameterized the lowest 'Aerosol-1' haze as a vertically thin layer, centred at 5 bar, but left the other model parameters unchanged (Methods). ...
... Possible candidates for the Neptune 'chromophore' are discussed by IRW22 (ref. 12). We conclude that the complete spectral signature of NDS-2018 captured by the VLT's MUSE allows us to rule out the cloud 'clearing' scenario for dark spots with high confidence and demonstrates that dark spots are mostly the result of changes in the properties of the Aerosol-1 particles that reduce their single-scattering albedo and make them less reflective at short wavelengths. ...
... https://doi.org/10.1038/s41550-023-02047-0 of IRW22 (ref. 12), we find that although the overlying Aerosol-3 and Aerosol-2 aerosols are predicted to absorb some of the UV flux, in the absence of a substantial opacity of Aerosol-1 particles considerable UV could scatter conservatively down to very deep levels and photolyse light-sensitive gases (Supplementary Figs. 7 and 8). ...
Previous observations of dark vortices in Neptune’s atmosphere, such as Voyager 2’s Great Dark Spot (1989), have been made in only a few broad-wavelength channels, hampering efforts to determine these vortices’ pressure levels and darkening processes. We analyse spectroscopic observations of a dark spot on Neptune identified by the Hubble Space Telescope as NDS-2018; the spectral observations were made in 2019 by the Multi Unit Spectroscopic Explorer (MUSE) of the Very Large Telescope (Chile). The MUSE medium-resolution 475–933 nm reflection spectra allow us to show that dark spots are caused by darkening at short wavelengths (<700 nm) of a deep ~5 bar aerosol layer, which we suggest is the H2S condensation layer. A deep bright spot, named DBS-2019, is also visible on the edge of NDS-2018, with a spectral signature consistent with a brightening of the same 5 bar layer at longer wavelengths (>700 nm). This bright feature is much deeper than previously studied dark-spot companion clouds and may be connected with the circulation that generates and sustains such spots.
... A recent reanalysis of multiple space-and ground-based observations from 0.3 -2.5 µm [12], hereafter 'IRW22', found that the spectra of both Uranus and Neptune can be modelled very accurately with a simple atmospheric aerosol structure comprised of three main layers: 1) a deep H 2 S/photochemicalhaze aerosol layer with a base pressure > 5-7 bar (Aerosol-1); 2) a layer of methane/photochemical-haze just above the methane condensation level at 1-2 bar (Aerosol-2); and 3) an extended layer of small photochemical haze particles extending into the stratosphere (Aerosol-3). IRW22 also analysed HST/WFC3 observations of NDS-2018 [9], and Voyager-2/ISS observations of the GDS [4] together with a dark band near 60 • S, dubbed the 'South Polar Wave' (SPW) [13]. ...
... Radiative Transfer Analysis of NDS-2018 and DBS-2019. We modelled the spectral signatures of NDS-2018 and DBS-2019 using the NEMESIS [17][18][19] radiative transfer model, adapting the procedure used by IRW22 [12] for the analysis of HST/STIS, IRTF/SpeX and Gemini/NIFS observations of Uranus and Neptune from 0.3 to 2.5 µm. Updating the procedure slightly from IRW22 we parameterised the lowest 'Aerosol-1' haze as a vertically thin layer, centred at 5 bar, but left the other model parameters unchanged (see Methods). ...
... For reference, Supplementary Fig. 5 compares the imaginary refractive index spectra shown in Fig. 2b with those deduced for the blue-absorbing chromophore in Jupiter's atmosphere [20,21], showing the Aerosol-1 particles to have a very different spectral shape to those seen in Jupiter's atmosphere. Possible candidates for the Neptune 'chomophore' are discussed by IRW22 [12]. We conclude that the complete spectral signature of NDS-2018 captured by VLT/MUSE allows us to rule out the cloud 'clearing' scenario for dark spots with high confidence and demonstrates that dark spots are mostly the result of changes in the properties of the Aerosol-1 particles that reduce their single-scattering albedo and make them less reflective at short wavelengths. ...
Previous observations of dark vortices in Neptune's atmosphere, such as Voyager-2's Great Dark Spot, have been made in only a few, broad-wavelength channels, which has hampered efforts to pinpoint their pressure level and what makes them dark. Here, we present Very Large Telescope (Chile) MUSE spectrometer observations of Hubble Space Telescope's NDS-2018 dark spot, made in 2019. These medium-resolution 475 - 933 nm reflection spectra allow us to show that dark spots are caused by a darkening at short wavelengths (< 700 nm) of a deep ~5-bar aerosol layer, which we suggest is the H$_2$S condensation layer. A deep bright spot, named DBS-2019, is also visible on the edge of NDS-2018, whose spectral signature is consistent with a brightening of the same 5-bar layer at longer wavelengths (> 700 nm). This bright feature is much deeper than previously studied dark spot companion clouds and may be connected with the circulation that generates and sustains such spots.
... If the concentration of the condensible is high enough, compositional gradients stabilize the atmosphere to convection (Guillot 1995;Li & Ingersoll 2015;Leconte et al. 2017) and double-diffusive instabilities (Leconte et al. 2017) even if the lapse rate is superadiabatic. Molecular gradient-induced convective inhibition has been invoked to explain periodic storms in Saturn's atmosphere (Li & Ingersoll 2015), stable wave ducts for gravity waves in Jupiter's atmosphere (Ingersoll & Kanamori 1995), and the step-like behavior of methane's abundance in Uranus's condensation layer (Irwin et al. 2022). Neglecting this effect results in an underestimation of the deep atmospheric temperature in gas giant planets (Leconte et al. 2017). ...
Hycean worlds are a proposed subset of sub-Neptune exoplanets with substantial water inventories, liquid surface oceans, and extended hydrogen-dominated atmospheres favorable for habitability. We aim to quantitatively define the inner edge of the Hycean habitable zone (HZ) using a 1D radiative-convective model. As a limiting case, we model a dry hydrogen–helium envelope above a surface ocean. For a 1 bar (10,100 bar) atmosphere, the hydrogen greenhouse effect alone sets the inner edge of the HZ at 0.216 au (0.58, 3.71 au) for a Sun-like G star and at 0.0364 au (0.110, 0.774 au) for an 3500 K M star. Introducing water vapor into the atmosphere, the runaway greenhouse instellation limit is greatly reduced due to the presence of superadiabatic layers where convection is inhibited. This moves the inner edge of the HZ from ≈1 au for a G star to 1.6 au (3.85 au) for a Hycean world with a H 2 –He inventory of 1 bar (10 bar). For an M star, the inner edge is equivalently moved from 0.17–0.28 au (0.54 au). Our results suggest that most of the current Hycean world observational targets are not likely to sustain a liquid water ocean. We present an analytical framework for interpreting our results, finding that the maximum possible outgoing longwave radiation scales approximately inversely with the dry mass inventory of the atmosphere. We discuss the possible limitations of our 1D modeling and recommend the use of 3D convection-resolving models to explore the robustness of superadiabatic layers.
