JWST is peering through Jupiter’s 5-micron atmospheric window to map rising and sinking air deep in the weather layers of all four giant planets. Existing observations only capture the topmost cloud deck; the circulation patterns that drive the banded stripes on Jupiter, Saturn, Uranus, and Neptune remain largely unknown. The canonical model—borrowed from Earth’s Hadley and Ferrel cells—may be wrong for these worlds. This project uses JWST’s NIRSpec and MIRI spectrometers to measure temperature, wind shear, and gases such as ammonia, phosphine, and water across latitudes, testing whether the textbook picture holds. If those gases show no variation with the bands, the Ferrel-like circulation model will need substantial revision. This is fundamental planetary science with no immediate practical application. But understanding how giant-planet atmospheres circulate directly informs models of brown dwarfs and exoplanets, where banding is commonly observed but poorly explained. Past fundamental work on planetary atmospheres has underpinned everything from weather satellite design to climate modelling—similar unexpected payoffs could emerge from a correct theory of deep tropospheric circulation.
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There is no better illustration of weather on other worlds than Jupiter’s colourful stripes, swirling vortices, and bright convective storms. Planetary banding also exists on Saturn, Uranus, and Neptune, where belts and zones of different temperatures and clouds are bordered by powerful winds. And yet existing observations reveal only the tip of the iceberg, the topmost level of a circulation system that extends deep into the abyssal layers. What controls the banded circulation of Jupiter and Saturn? Do the Ice Giants display a similar banded circulation in their deep, unexplored tropospheres? Is our ‘textbook picture’ of upwelling zones and downwelling belts, adapted from terrestrial meteorological concepts of Hadley and Ferrel circulation patterns, no longer fit for purpose? JWST observations acquired by our team between 2022-2024 allow us to rigorously search for rising and sinking motions by mapping gradients in temperatures, clouds, and gaseous composition in the deep weather layers of all four giant worlds. The exquisite infrared sensitivity, spectral coverage, and resolution of JWST is providing unprecedented access to the cloud-forming “weather layers” of the four giants. The NIRSpec/IFU (0.6-5.3 µm) and MIRI/MRS (4.9-28.5 µm) instruments are mapping the 3-8 µm spectrum in a way that was previously impossible for Uranus and Neptune, and challenging even for Jupiter and Saturn (spacecraft instruments have never covered the 5.2-7.3 µm range; ground-based instruments are hampered by terrestrial water and CO2). A dearth of methane and hydrogen absorption near 5 µm reveals the atmosphere at depth, and the NIRSpec and MIRI spectral range fully opens this window for discovery. Modelling of JWST’s full spectral range provides 3D mapping of temperatures, windshears, and aerosols, but the 3-8 µm region gives access to numerous gases within the weather layer: condensing gases like methane, ammonia, and water; disequilibrium species like phosphine, arsine, and carbon monoxide. We will search for latitudinal gradients in each of these species to test the canonical theory of belt/zone overturning motions on Jupiter and Saturn; and to establish the relevance of such circulations for the cloud layers of Uranus and Neptune. This requires revised continuum models for the underlying hydrogen, helium, methane and aerosol properties. If these gases do not vary on the scale of the planetary bands, then the Ferrel-like circulation model of rising and falling air within giant planet tropospheres would need substantial revision, with consequences for our expectations of planetary-scale banding on giant exoplanets and brown dwarfs.
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