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Abstract:

Hubble Space Telescope Wide-Field Camera 3 (HST/WFC3) observations spanning 2015 to 2021 confirm a brightening of Uranus' north polar hood feature with time. The vertical aerosol model of Irwin et al. (2023, https://doi.org/10.1038/s41550-023-02047-0) (IRW23), consisting of a deep haze layer based at ∼5 bar, a 1–2 bar haze layer, and an extended haze rising up from the 1–2 bar layer, was applied to retrievals on HST Space Telescope Imaging Spectrograph (STIS) (HST/STIS) observations (Sromovsky et al., 2014, 2019, https://doi.org/10.1016/j.icarus.2014.05.016, https://doi.org/10.1016/j.icarus.2018.06.026) revealing a reduction in cloud-top CH4 volume mixing ratio (VMR) (i.e., above the deep ∼5 bar haze) by an average of 0.0019 ± 0.0003 between 40–80◦N (∼10% average reduction) from 2012 to 2015. A combination of latitudinal retrievals on the HST/WFC3 and HST/STIS data sets, again employing the IRW23 model, reveal a temporal thickening of the 1–2 bar haze layer to be the main cause of the polar hood brightening, finding an average increase in integrated opacity of 1.09 ± 0.08 (∼33% increase) at 0.8 µm north of ∼45°N, concurrent with a decrease in the imaginary refractive index spectrum of the 1–2 bar haze layer north of ∼40°N and longwards of ∼0.7 µm. Small contributions to the brightening were found from a thickening of the deep aerosol layer, with an average increase in integrated opacity of 0.6 ± 0.1 (58% increase) north of 45°N between 2012 and 2015, and from the aforementioned decrease in CH4 VMR. Our results are consistent with the slowing of a stratospheric meridional circulation, exhibiting subsidence at the poles.

Abstract:

The characterisation of exoplanetary atmospheres has entered a new era with the advent of the James Webb Space Telescope (JWST), which enables the detection of molecular species in the atmospheres of both gas giants and smaller, potentially rocky planets. This thesis focuses on the development and application of atmospheric retrieval methods to interpret transmission spectra from JWST observations, with particular attention to assumptions in atmospheric models and their impact on the resulting inferences. The first part of this thesis examines how specific modelling choices can influence atmospheric retrievals. I investigate the role of planetary rotation in modifying effective gravity and demonstrate that the inclusion of centrifugal forces can alter retrieved atmospheric properties, particularly for low-density, fast-rotating planets that are common targets for transmission spectroscopy. I also explore the implementation of Centered Log-Ratio (CLR) priors for atmospheric composition, a method designed to better represent the statistical priors of gas abundances in retrievals. This is particularly relevant for smaller exoplanets, where the dominant atmospheric constituents are often unknown and conventional priors can lead to biased results. The second part of the thesis presents retrievals on JWST Early Release Science observations for the hot Saturn WASP-39 b. I apply the NEMESISPY retrieval framework to transmission spectra obtained using multiple JWST instruments using both free retrieval and equilibrium chemistry approaches. These analyses highlight the strengths and limitations of each method and illustrate how the choice of parameterisation and integration of chemical equilibrium codes can impact inferences about the atmospheric conditions on a planet. The final chapters focus on a 1.5 R_earth exoplanet L 98-59 d. Using a JWST transmission spectrum obtained with the NIRSpec G395H instrument, I perform a range of retrievals to test what can be inferred about its atmosphere. My analysis suggests the potential presence of a high mean molecular weight atmosphere dominated by the sulfur-bearing gases H2S and SO2. While these initial results are tentative, I outline an observational strategy targeting additional transits using complementary wavelengths to constrain its composition more robustly. Taken together, the work presented in this thesis examines assumptions in the technique of atmospheric retrieval and demonstrates its application to a diverse set of exoplanets. It shows how careful treatment of model assumptions and priors can improve the robustness of interpretations and enhance the scientific return from JWST observations. As the focus of exoplanet science shifts toward smaller and cooler planets, these tools and approaches will be essential for addressing fundamental questions about atmospheric diversity and the potential for habitability beyond our Solar System

Abstract:

ESA's Jupiter Icy Moons Explorer (JUICE) will provide a detailed investigation of the Jovian system in the 2030s, combining a suite of state-of-the-art instruments with an orbital tour tailored to maximise observing opportunities. We review the Jupiter science enabled by the JUICE mission, building on the legacy of discoveries from the Galileo, Cassini, and Juno missions, alongside ground- and space-based observatories. We focus on remote sensing of the climate, meteorology, and chemistry of the atmosphere and auroras from the cloud-forming weather layer, through the upper troposphere, into the stratosphere and ionosphere. The Jupiter orbital tour provides a wealth of opportunities for atmospheric and auroral science: global perspectives with its near-equatorial and inclined phases, sampling all phase angles from dayside to nightside, and investigating phenomena evolving on timescales from minutes to months. The remote sensing payload spans far-UV spectroscopy (50-210 nm), visible imaging (340-1080 nm), visible/near-infrared spectroscopy (0.49-5.56 μm), and sub-millimetre sounding (near 530-625 GHz and 1067-1275 GHz). This is coupled to radio, stellar, and solar occultation opportunities to explore the atmosphere at high vertical resolution; and radio and plasma wave measurements of electric discharges in the Jovian atmosphere and auroras. Cross-disciplinary scientific investigations enable JUICE to explore coupling processes in giant planet atmospheres, to show how the atmosphere is connected to (i) the deep circulation and composition of the hydrogen-dominated interior; and (ii) to the currents and charged particle environments of the external magnetosphere. JUICE will provide a comprehensive characterisation of the atmosphere and auroras of this archetypal giant planet.