Planetary Climate Dynamics

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:

We investigate the characteristics of equatorial waves associated with the maintenance of superrotation in the stratosphere of a Titan general circulation model. A variety of equatorial waves are present in the model atmosphere, including equatorial Kelvin waves, equatorial Rossby waves, and mixed Rossby–gravity waves. In the upper stratosphere, acceleration of superrotation is strongest around solstice and is due to interaction between equatorial Kelvin waves and Rossby-type waves in winter hemisphere midlatitudes. The existence of this "Rossby–Kelvin"-type wave appears to depend on strong meridional shear of the background zonal wind that occurs in the upper stratosphere at times away from the equinoxes. In the lower stratosphere, acceleration of superrotation occurs throughout the year and is partially induced by equatorial Rossby waves, which we speculate are generated by quasigeostrophic barotropic instability. Acceleration of superrotation is generally due to waves with phase speeds close to the zonal velocity of the mean flow. Consequently, they have short vertical wavelengths that are close to the model's vertical grid scale and therefore likely to be not properly represented. We suggest that this may be a common issue among Titan general circulation models that should be addressed by future model development.