Quenching of zonal winds in Jupiter's interior

Gravity and magnetic field data obtained by the Juno mission show that Jupiter's strong zonal winds extend a few thousand kilometers into the interior, but are quenched above the level where the electrical conductivity becomes significant. Here, we extend a simple linearized model [Christensen et al., Astrophys. J. 890, 61 (2020)] that explains the braking of the jets by the combination of stable stratification and electromagnetic effects. We show that in the inviscid limit, the process is essentially governed by a single parameter, which we call the MAC-number (for the forces acting on the flow-Magnetic, Archimedian, and Coriolis). The predictions for the drop-off of the zonal winds agree well with results from 3D-convection models. We run calculations that take the full range of density and electrical conductivity variations in the top 5,600 km of Jupiter into account. In order to satisfy constraints on the power driving the jets and on their effect on Jupiter's magnetic field, the top of the stable layer and the region where the jet velocity drops sharply must be near 2,000 km depth. The dissipation associated with quenching of the jets increases toward the poles, which can partly explain why the jets near [Formula: see text]20[Formula: see text] are faster than those at higher latitude.

A warm Neptune's methane reveals core mass and vigorous atmospheric mixing

Observations of transiting gas giant exoplanets have revealed a pervasive depletion of methane1-4, which has only recently been identified atmospherically5,6. The depletion is thought to be maintained by disequilibrium processes such as photochemistry or mixing from a hotter interior7-9. However, the interiors are largely unconstrained along with the vertical mixing strength and only upper limits on the CH4 depletion have been available. The warm Neptune WASP-107b stands out among exoplanets with an unusually low density, reported low core mass10, and temperatures amenable to CH4, though previous observations have yet to find the molecule2,4. Here we present a JWST-NIRSpec transmission spectrum of WASP-107b that shows features from both SO2 and CH4 along with H2O, CO2, and CO. We detect methane with 4.2σ significance at an abundance of 1.0 ± 0.5 ppm, which is depleted by 3 orders of magnitude relative to equilibrium expectations. Our results are highly constraining for the atmosphere and interior, which indicate the envelope has a super-solar metallicity of 43 ± 8 × solar, a hot interior with an intrinsic temperature of Tint = 460 ± 40 K, and vigorous vertical mixing which depletes CH4 with a diffusion coefficient of Kzz = 1011.6±0.1 cm2 s-1. Photochemistry has a negligible effect on the CH4 abundance but is needed to account for the SO2. We infer a core mass of11.5 - 3.6 + 3.0 M ⊕ , which is much higher than previous upper limits10, releasing a tension with core-accretion models11.

Vanadium oxide and a sharp onset of cold-trapping on a giant exoplanet

The abundance of refractory elements in giant planets can provide key insights into their formation histories1. Owing to the low temperatures of the Solar System giants, refractory elements condense below the cloud deck, limiting sensing capabilities to only highly volatile elements2. Recently, ultra-hot giant exoplanets have allowed for some refractory elements to be measured, showing abundances broadly consistent with the solar nebula with titanium probably condensed out of the photosphere3,4. Here we report precise abundance constraints of 14 major refractory elements on the ultra-hot giant planet WASP-76b that show distinct deviations from proto-solar and a sharp onset in condensation temperature. In particular, we find nickel to be enriched, a possible sign of the accretion of the core of a differentiated object during the evolution of the planet. Elements with condensation temperatures below 1,550 K otherwise closely match those of the Sun5 before sharply transitioning to being strongly depleted above 1,550 K, which is well explained by nightside cold-trapping. We further unambiguously detect vanadium oxide on WASP-76b, a molecule long suggested to drive atmospheric thermal inversions6, and also observe a global east-west asymmetry7 in its absorption signals. Overall, our findings indicate that giant planets have a mostly stellar-like refractory elemental content and suggest that temperature sequences of hot Jupiter spectra can show abrupt transitions wherein a mineral species is either present or completely absent if a cold trap exists below its condensation temperature8.

Evidence of hydrogen-helium immiscibility at Jupiter-interior conditions

The phase behaviour of warm dense hydrogen-helium (H-He) mixtures affects our understanding of the evolution of Jupiter and Saturn and their interior structures^1,2. For example, precipitation of He from a H-He atmosphere at about 1-10 megabar and a few thousand kelvin has been invoked to explain both the excess luminosity of Saturn^1,3, and the depletion of He and neon (Ne) in Jupiter's atmosphere as observed by the Galileo probe^4,5. But despite its importance, H-He phase behaviour under relevant planetary conditions remains poorly constrained because it is challenging to determine computationally and because the extremes of temperature and pressure are difficult to reach experimentally. Here we report that appropriate temperatures and pressures can be reached through laser-driven shock compression of H₂-He samples that have been pre-compressed in diamond-anvil cells. This allows us to probe the properties of H-He mixtures under Jovian interior conditions, revealing a region of immiscibility along the Hugoniot. A clear discontinuous change in sample reflectivity indicates that this region ends above 150 gigapascals at 10,200 kelvin and that a more subtle reflectivity change occurs above 93 gigapascals at 4,700 kelvin. Considering pressure-temperature profiles for Jupiter, these experimental immiscibility constraints for a near-protosolar mixture suggest that H-He phase separation affects a large fraction-we estimate about 15 per cent of the radius-of Jupiter's interior. This finding provides microphysical support for Jupiter models that invoke a layered interior to explain Juno and Galileo spacecraft observations^1,4,6-8.

Multishock to Quasi-Isentropic Compression of Dense Gaseous Deuterium-Helium Mixtures up to 120 GPa: Probing the Sound Velocities Relevant to Planetary Interiors

Shock reverberation compression experiments on dense gaseous deuterium-helium mixtures are carried out to provide thermodynamic parameters relevant to the conditions in planetary interiors. The multishock pressures are determined up to 120 GPa and reshock temperatures to 7400 K. Furthermore, the unique compression path from shock-adiabatic to quasi-isentropic compressions enables a direct estimation of the high-pressure sound velocities in the unexplored range of 50-120 GPa. The equation of state and sound velocity provide particular dual perspectives to validate the theoretical models. Our experimental data are found to agree with several equation of state models widely used in astrophysics within the probed pressure range. The current data improve the experimental constraints on sound velocities in the Jovian insulating-to-metallic transition layer.

The interiors of Uranus and Neptune: current understanding and open questions

Uranus and Neptune form a distinct class of planets in our Solar System. Given this fact, and ubiquity of similar-mass planets in other planetary systems, it is essential to understand their interior structure and composition. However, there are more open questions regarding these planets than answers. In this review, we concentrate on the things we do not know about the interiors of Uranus and Neptune with a focus on why the planets may be different, rather than the same. We next summarize the knowledge about the planets' internal structure and evolution. Finally, we identify the topics that should be investigated further on the theoretical front as well as required observations from space missions. This article is part of a discussion meeting issue 'Future exploration of ice giant systems'.

Remote determination of the shape of Jupiter's vortices from laboratory experiments

Jupiter's dynamics shapes its cloud patterns but remains largely unknown below this natural observational barrier. Unraveling the underlying three-dimensional flows is thus a primary goal for NASA's ongoing Juno mission that was launched in 2011. Here, we address the dynamics of large Jovian vortices using laboratory experiments complemented by theoretical and numerical analyses. We determine the generic force balance responsible for their three-dimensional pancake-like shape. From this, we define scaling laws for their horizontal and vertical aspect ratios as a function of the ambient rotation, stratification and zonal wind velocity. For the Great Red Spot in particular, our predicted horizontal dimensions agree well with measurements at the cloud level since the Voyager mission in 1979. We additionally predict the Great Red Spot's thickness, inaccessible to direct observation: it has surprisingly remained constant despite the observed horizontal shrinking. Our results now await comparison with upcoming Juno observations.

Revealing giant planet interiors beneath the cloudy veil

Observations from the Juno and Cassini missions provide essential constraints on the internal structures and compositions of Jupiter and Saturn, resulting in profound revisions of our understanding of the interior and atmospheres of Gas Giant planets. The next step to understand planetary origins in our Solar System requires a mission to their Ice Giant siblings, Uranus and Neptune.

Stability of H3O at extreme conditions and implications for the magnetic fields of Uranus and Neptune

The anomalous nondipolar and nonaxisymmetric magnetic fields of Uranus and Neptune have long challenged conventional views of planetary dynamos. A thin-shell dynamo conjecture captures the observed phenomena but leaves unexplained the fundamental material basis and underlying mechanism. Here we report extensive quantum-mechanical calculations of polymorphism in the hydrogen-oxygen system at the pressures and temperatures of the deep interiors of these ice giant planets (to >600 GPa and 7,000 K). The results reveal the surprising stability of solid and fluid trihydrogen oxide (H₃O) at these extreme conditions. Fluid H₃O is metallic and calculated to be stable near the cores of Uranus and Neptune. As a convecting fluid, the material could give rise to the magnetic field consistent with the thin-shell dynamo model proposed for these planets. H₃O could also be a major component in both solid and superionic forms in other (e.g., nonconvecting) layers. The results thus provide a materials basis for understanding the enigmatic magnetic-field anomalies and other aspects of the interiors of Uranus and Neptune. These findings have direct implications for the internal structure, composition, and dynamos of related exoplanets.

The formation of Jupiter's diluted core by a giant impact

The Juno mission¹ has provided an accurate determination of Jupiter's gravitational field², which has been used to obtain information about the planet's composition and internal structure. Several models of Jupiter's structure that fit the probe's data suggest that the planet has a diluted core, with a total heavy-element mass ranging from ten to a few tens of Earth masses (about 5 to 15 per cent of the Jovian mass), and that  heavy elements (elements other than hydrogen and helium) are distributed within a region extending to nearly half of Jupiter's radius^3,4. Planet-formation models indicate that most heavy elements are accreted during the early stages of a planet's formation to create a relatively compact core^5-7 and that almost no solids are accreted during subsequent runaway gas accretion^8-10. Jupiter's diluted core, combined with its possible high heavy-element enrichment, thus challenges standard planet-formation theory. A possible explanation is erosion of the initially compact heavy-element core, but the efficiency of such erosion is uncertain and depends on both the immiscibility of heavy materials in metallic hydrogen and on convective mixing as the planet evolves^11,12. Another mechanism that can explain this structure is planetesimal enrichment and vaporization^13-15 during the formation process, although relevant models typically cannot produce an extended diluted core. Here we show that a sufficiently energetic head-on collision (giant impact) between a large planetary embryo and the proto-Jupiter could have shattered its primordial compact core and mixed the heavy elements with the inner envelope. Models of such a scenario lead to an internal structure that is consistent with a diluted core, persisting over billions of years. We suggest that collisions were common in the young Solar system and that a similar event may have also occurred for Saturn, contributing to the structural differences between Jupiter and Saturn^16-18.