Maria Zamyatina

Close-in gas giant atmospheres are in chemical disequilibrium. Solving 3D kinetics to capture one source of this disequilibrium, transport-induced quenching, is computationally demanding, so simplifications are used. General circulation models (GCMs) relax abundances on the timescale of a single conversion pathway. 1D models place a quench level and hold abundances vertically uniform above it. We test both simplifications on CH4 against aerosol-free, solar-composition GCM simulations with thermochemical kinetics for HAT-P-11b, HD 189733b, HD 209458b and WASP-17b. We find ten CH4-to-CO conversion pathways, varying with depth, longitude and planet. A single pathway controls CH4 quenching on the three cooler planets; on WASP-17b two compete. The CH4 quench level is 3D: where the atmosphere is hotter, or a faster pathway takes over, the conversion timescale shortens, the quench level rises and the quench-level abundance falls. On WASP-17b the pathway switch lifts the quench level over the substellar tropics, deepening equatorial CH4 depletion. A 1D quench treatment with the Zahnle & Marley timescale typically locates the quench level within a factor of two in pressure for CH4 and about four for CO on every planet. It fails above the quench level, where every quench scheme holds abundances vertically uniform. The minor carbon carrier, CH4 or CO depending on the planet, is wrong by up to two orders of magnitude, on HD 189733b in opposite directions at different latitudes. Quenching sets an abundance at the quench level but does not fix it thereafter; even without photochemistry, transport and conversion keep that abundance spatially structured.

The spectra of exoplanet atmospheres are affected by aerosols (clouds and hazes) of uncertain origin. Proposed aerosol formation mechanisms include gas condensation or photochemical reactions. We measured the transmission spectrum of the tidally locked gas giant exoplanet WASP-94A b and identified asymmetry in its atmosphere. The morning limb is cooler and cloudy, whereas the evening limb is hotter and exhibits gaseous water absorption features. We interpret this difference as being due to the formation of cloud droplets near the morning limb, which evaporate during circulation to the evening limb. The dominant aerosols are clouds cycling between the day and night sides of the atmosphere, not photochemical hazes. The resulting asymmetry can severely bias chemical abundance measurements, unless limb-resolved spectroscopy is available.

With its expanded wavelength coverage and increased precision compared to previous space-based observatories, JWST provides the opportunity to revisit benchmark planets and view them in a new light. Here, we conduct an in-depth study of the atmosphere of the hot-Saturn WASP-96 b combining a new JWST NIRSpec/G395H transit with archival NIRISS/SOSS and Very Large Telescope/FORS2 transmission spectra. The combined spectrum shows clearly visible features from H ₂ O, CO ₂ , and Na. CO, though, remains unconstrained, precluding a firm metallicity derivation from free retrievals alone. However, self-consistent grids yield a broadly superstellar atmospheric metallicity of 2–6× stellar. When combined with a roughly stellar C/O ratio ( 0.4 1 − 0.09 + 0.10 from self-consistent grids), we find that WASP-96 b potentially formed via core-accretion beyond the H ₂ O snowline and subsequently accreted volatile-rich material. Free retrievals also find a moderate preference ( ln B = 2.69) for models with SO ₂ versus without. WASP-96 b falls directly on the proposed “SO ₂ shoreline” and the retrieved SO ₂ abundance is well-matched to predictions from photochemical models. Our combined spectrum displays an optical slope, which our models fit with opacity from scattering aerosols—either small-particle condensate clouds or photochemical hazes—though we cannot completely rule out the broad wings of Na or the effects of stellar contamination. Future observations are necessary to disentangle these effects. Finally, we explore the possibility for limb asymmetry in WASP-96 b’s transmission spectrum and provide several tests to identify asymmetries in our data. We encourage the community to prioritize the development of a robust pathway to quantify the presence of limb asymmetry—particularly for low signal-to-noise cases.

We present the first atmospheric observation and characterisation of the aligned, 1468 K hot Jupiter, NGTS-2b, with one JWST NIRSpec/G395H transit. These observations complete the GO 3838 observing campaign of the BOWIE-ALIGN program, which aims to investigate the link between hot Jupiter atmospheric composition and formation history through the atmospheric analysis of planets orbiting F stars that are aligned and misaligned with the host stellar spin axis. The 2.84–5.18 μm spectrum shows weak absorption features attributed to H₂O and CO₂ absorption, which our free chemistry retrievals fit with posteriors that converge on high mean molecular weight solutions attained through significant H₂O mixing ratios. By comparing our results to interior modelling, we show that some of these solutions exceed the 43.5 × solar upper limit we obtained from our interior structure models. Such solutions are likely due to cloud-metallicity degeneracies and insufficient wavelength coverage to resolve them. We show that, in the case of our observations, the likelihood distribution of H₂O abundances is flat and uninformative, such that our retrievals are biased by the prior. Additionally, our statistically favoured atmospheric solution contains absorption from SO. The chemical abundances retrieved with this model are likely not astrophysically feasible and we demonstrate that the presence of SO is driven by only two data points. Our equilibrium chemistry retrievals hint at a subsolar C/O ratio and supersolar metallicity; however, we find wide posterior distributions that extend to solar values.

Over the past two decades, increasingly precise observations have revealed that exoplanet atmospheres are chemically diverse and often far from equilibrium, with processes such as vertical mixing, photochemistry, and atmospheric circulation producing significant departures from thermochemical expectations. As large surveys across a wide range of planets begin to uncover population-level chemical trends, a coherent interpretation of these patterns remains elusive, particularly for gas giants and Neptune-like planets where disequilibrium processes likely dominate. The ESA Ariel mission will provide the first homogeneous, statistically significant atmospheric data set for nearly a thousand exoplanets, offering an unprecedented opportunity to investigate the origins of chemical diversity across planetary populations. This white paper highlights central scientific questions for understanding atmospheric chemistry and recognized as priorities for the community. These questions span the mechanisms driving disequilibrium chemistry, the role of sulfur- and phosphorus-bearing species, the influence of stellar activity, the formation of haze precursors, and the chemical evolution of atmospheres around stars of different ages and types. Because these topics connect chemistry, physics, and planetary evolution, they form the core focus of the Ariel Chemistry Working Group, which synthesizes current knowledge and identifies the diagnostics best addressed by Ariel’s spectral capabilities. For each theme, we outline observational strategies and representative targets already included in the Mission Candidate Sample, illustrating Ariel’s ability to address these questions. By linking large-scale observations to predictive atmospheric models, this work supports refinement of Ariel’s target selection and enables population-level studies that will transform our understanding of planetary atmospheres.