The atmospheric chemistry of Earth-like exoplanets: a 3D perspective

Exoplanets or planets around other stars than the Sun are ubiquitous in the galaxy, with great diversity in stellar and planetary environments. The hundreds of known terrestrial exoplanets with mass and radius similar to Earth are preferentially found around M-dwarf stars. The James Webb Space Telescope (JWST) is providing the first glimpses into the atmospheres of these terrestrial exoplanets which is most easily achieved for planets on close-in orbits around a small and cool host star. The close-in orbits induce strong star-planet gravitational interactions that synchronize the planet's orbital and rotational velocity in a process called tidal locking, producing spin-orbit resonances akin to the Moon's 1:1 and Mercury's 3:2 spin-orbit resonance. Understanding the effects of spin-orbit resonances on atmospheric physics and chemistry is crucial to determine habitability, interpret spectroscopic observations, and put potential biosignatures (signs of biological processes in an atmosphere) into the environmental context. Comprehensive 3D General Circulation Models (GCMs) that describe the physical processes in a planetary atmosphere have been employed to explore the habitability and observability of such exoplanets. The simulations show that heat transport from a permanent dayside to a permanent nightside prevents atmospheric collapse for 1:1 resonant exoplanets. The atmospheric circulation depends on the planet's rotation rate and orbital configuration and determines the distribution of clouds and hazes. A 3:2 resonant orbit with a theoretically predicted eccentricity of 0.3 changes the stellar irradiation pattern and generally warms the planetary climate. Besides heating and driving circulation, spatially asymmetric energetic radiation from the host star also photolyses molecules, driving the atmospheric composition out of thermochemical equilibrium and motivating the use of 3D coupled Climate-Chemistry Models (CCMs). For 1:1 resonant exoplanets of an Earth-like atmospheric composition, photochemistry driven by M-dwarf radiation can form a global ozone layer, with stellar flares affecting the chemistry and spatial distribution of ozone. The photochemistry is highly sensitive to the stellar flux distribution, in particular at ultraviolet (UV) wavelengths. Other drivers of disequilibrium chemistry such as lightning and circulation as well as the dependence of 3D atmospheric chemistry on the orbital configuration are, as of yet, underexplored. This thesis aims to build upon previous work and investigates the 3D coupled interactions between the physical and chemical processes that govern planetary atmospheres for tidally locked exoplanets around M-dwarfs. I employ and further develop a 3D CCM -- consisting of the Met Office Unified Model and the UK Chemistry and Aerosols framework -- to simulate the physics and chemistry of exoplanet atmospheres. I configure two tidally locked exoplanets -- nominally Proxima Centauri b and TRAPPIST-1 d -- that are known to exhibit distinct circulation regimes, assuming 1:1 and eccentric 3:2 spin-orbit resonances and Earth-like atmospheric composition (N₂-O₂-CO₂-H₂O). The photochemical simulations consider the Chapman mechanism of ozone formation and the hydrogen oxide (HOₓ) and nitrogen oxide (NOₓ) catalytic cycles of ozone destruction. The CCM was previously used to simulate a 1:1 resonant exoplanet. For this study, I implement the calculation of varying stellar radiation with eccentricity and improve the photochemistry scheme to incorporate the latest stellar spectra in the calculation of spatially and temporally varying photolysis rates. At the time of writing, this is one of only four models able to do these calculations for exoplanets. In Chapter 3, I demonstrate that using the latest stellar spectrum drives enhanced ozone formation on Proxima Centauri b in a 1:1 spin-orbit resonance with ozone column densities ten times higher than previously found. The global ozone layer shows significant longitudinal variations with an accumulation at the location of permanent nightside gyres. I then investigate the potential for lightning initiation on a tidally locked exoplanet, finding that vigorous convection on the dayside hemisphere results in lightning flash rates (LFR) of up to 0.16 flashes km⁻¹ yr⁻¹, aligning with previously reported cloud coverage. The spatially asymmetric distribution of lightning flashes enhances the dayside hemisphere in NOₓ, which is then advected to the nightside where it reacts to form more complex molecules in the absence of photochemistry. Lightning-induced chemistry is not sufficiently abundant and found too deep in the atmosphere to show detectable features in simulated transmission spectra. Since the photochemical production of ozone is limited to the dayside hemisphere for 1:1 resonant exoplanets, a connection between the dayside ozone production region and the accumulation of ozone on the nightside has to exist. In Chapter 4, I investigate such dayside-nightside connections and how these explain the longitudinally varying ozone column densities. Transforming from the commonly used geographic to a tidally locked coordinate system to identify inter-hemispheric connections, I find that a stratospheric dayside-to-nightside circulation mechanism transports ozone from the dayside production regions to the nightside, where ozone-rich air then subsides and accumulates at the locations of permanent gyres. With age-of-air tracer experiments, I demonstrate that this circulation mechanism also affects other tracers, as long as the dynamical timescales are shorter than chemical timescales and a stratospheric dayside production mechanism is present. Chapter 5 compares Proxima Centauri b and TRAPPIST-1 d for both 1:1 and 3:2 spin-orbit resonances, the latter including an eccentricity of 0.3. These planets are known to exhibit distinct circulation regimes for a 1:1 resonance. Regardless of the resonance, TRAPPIST-1 d reaches the runaway greenhouse limit for its true irradiance within 1060 days of simulation. I artificially reduce the irradiance by 22% to ensure a stable climate for the 1:1 resonance, to still investigate the 3D atmospheric chemistry for a planet in this circulation regime. I report stable climates for Proxima Centauri b in both a 1:1 and 3:2 resonance. The 22% reduction in irradiance for TRAPPIST-1 d prevents the runaway greenhouse climate in the 1:1 resonant case. The atmospheric temperature of TRAPPIST-1 d in an eccentric 3:2 resonance increases significantly due to the eccentric orbit and a weaker stabilising cloud feedback, again driving the planet into a runaway greenhouse state. The distributions of ozone for the 3:2 resonant Proxima Centauri b and the 1:1 resonant TRAPPIST-1 d show latitudinal variations in ozone column densities, driven by an equator-to-pole circulation mechanism akin to the Brewer-Dobson circulation that controls the ozone distribution on Earth. Ozone production is stronger on Proxima Centauri b due to a higher UV flux. Ozone destruction is dominated by HOₓ cycling on Proxima Centauri and by NOₓ cycling on TRAPPIST-1 d, resulting from a much higher LFR (up to 1000 flashes km⁻² yr⁻¹) for the latter. I also demonstrate variability due to the rotation and eccentricity of Proxima Centauri b in a 3:2 spin-orbit resonance in Chapter 5. Especially prominent are the daytime-nighttime and periastron-apoastron cycles in water vapour (H₂O (g)) column densities at 48% and 12%, respectively. Surface temperature and ozone column densities have less pronounced cycles, but all show a brief time lag after periastron and apoastron passages, corresponding to the atmospheric response time. I use the 3D CCM data to generate synthetic emission spectra focusing on the mid-infrared (MIR) range (covered by JWST and also by the concept for the Large Interferometer For Exoplanets or LIFE). The 3D spatial variations and the observed geometry result in spectral fluctuations of up to 36 ppm for the 1:1 resonant exoplanets, depending on the orbital phase angle that we observe. The more homogeneous atmosphere of Proxima Centauri b in a 3:2 spin-orbit resonance lacks spectral fluctuations, presenting a discriminant from the 1:1 case and an important consideration for probing seasonally varying biosignatures. The thesis highlights the complex 3D interplay in planetary atmospheres between stellar radiation, orbital configuration, atmospheric dynamics, hydrological cycles including cloud formation, lightning initiation, and (photo)chemistry. Chapters 3 and 5 illustrate how the magnitude of ozone production depends on the stellar UV irradiation, but that ozone destruction can be controlled by either the hydrological cycles (producing HOₓ) or lightning initiation (producing NOₓ) and the (photo)chemistry following these processes. Lightning initiation depends on the irradiation and thermodynamics, while orbital parameters like the planet's rotation rate and eccentricity determine the distribution of incoming stellar radiation, in turn also affecting the atmospheric circulation and chemistry. I also illustrate how age-of-air experiments and coordinate system transformations unveil the role of circulation-driven atmospheric chemistry in Chapters 4 and 5, paving the way for further discoveries in climate-dynamics-chemistry interactions as described in Chapter 6. Contrasting the simulations of TRAPPIST-1 d in Chapter 5 underscores the sensitivity of planetary climates to orbital configurations, with implications for habitability studies. Distinct 3D variations in ozone distributions affect potential biosignature interpretation on rocky exoplanets. The presence of spectral fluctuations for the 1:1 resonant cases and the constant spectra for the 3:2 resonant cases provide an important discriminant between both orbital configurations and needs to be considered when studying seasonally varying biosignatures. I suggest various avenues for further research including 1) climate-dynamics-chemistry interactions, 2) additional disequilibrium chemical processes, 3) habitability for different spin-orbit resonances, 4) intercomparisons for models and (lightning) parametrizations, and 5) prebiotic chemistry in 3D. The 3D variations and potential spectral fluctuations can only be predicted and interpreted using complex 3D CCMs, emphasizing an important role for CCM simulations in developing the science objectives for observatories such as LIFE

Evidence for chromium hydride in the atmosphere of hot Jupiter WASP-31b

Context. The characterisation of exoplanet atmospheres has shown a wide diversity of compositions. Hot Jupiters have the appropriate temperatures to host metallic compounds, which should be detectable through transmission spectroscopy. Aims. We aim to detect exotic species in the transmission spectra of hot Jupiters, specifically WASP-31b, by testing a variety of chemical species to explain the spectrum. Methods. We conduct a re-analysis of publicly available transmission data of WASP-31b using the Bayesian retrieval framework TAUREX II. We retrieve various combinations of the opacities of 25 atomic and molecular species to determine the minimum set that is needed to fit the observed spectrum. Results. We report evidence for the spectroscopic signatures of chromium hydride (CrH), H2O, and K in WASP-31b. Compared to a flat model without any signatures, a CrH-only model is preferred with a statistical significance of similar to 3.9σ. A model consisting of both CrH and H2O is found with similar to 2.6 and similar to 3σ confidence over a CrH-only model and an H2O-only model, respectively. Furthermore, weak evidence for the addition of K is found at similar to 2.2σ over the H2O+CrH model, although the fidelity of the data point associated with this signature was questioned in earlier studies. Finally, the inclusion of collision-induced absorption and a Rayleigh scattering slope (indicating the presence of aerosols) is found with similar to 3.5σ confidence over the flat model. This analysis presents the first evidence for signatures of CrH in a hot Jupiter atmosphere. At a retrieved temperature of >1481-355 +264 K, the atmosphere of WASP-31b is hot enough to host gaseous Cr-bearing species, and the retrieved abundances agree well with predictions from thermal equilibrium chemistry. Furthermore, the retrieved abundance of CrH agrees with the abundance in an L-type brown dwarf atmosphere. However, additional retrievals using VLT FORS2 data lead to a non-detection of CrH. Future observations with James Webb Space Telescope have the potential to confirm the detection and/or discover other CrH features.Publisher PDFPeer reviewe

Evidence for chromium hydride in the atmosphere of hot Jupiter WASP-31b

Context. The characterisation of exoplanet atmospheres has shown a wide diversity of compositions. Hot Jupiters have the appropriate temperatures to host metallic compounds, which should be detectable through transmission spectroscopy. Aims. We aim to detect exotic species in the transmission spectra of hot Jupiters, specifically WASP-31b, by testing a variety of chemical species to explain the spectrum. Methods. We conduct a re-analysis of publicly available transmission data of WASP-31b using the Bayesian retrieval framework TAUREX II. We retrieve various combinations of the opacities of 25 atomic and molecular species to determine the minimum set that is needed to fit the observed spectrum. Results. We report evidence for the spectroscopic signatures of chromium hydride (CrH), H2O, and K in WASP-31b. Compared to a flat model without any signatures, a CrH-only model is preferred with a statistical significance of similar to 3.9σ. A model consisting of both CrH and H2O is found with similar to 2.6 and similar to 3σ confidence over a CrH-only model and an H2O-only model, respectively. Furthermore, weak evidence for the addition of K is found at similar to 2.2σ over the H2O+CrH model, although the fidelity of the data point associated with this signature was questioned in earlier studies. Finally, the inclusion of collision-induced absorption and a Rayleigh scattering slope (indicating the presence of aerosols) is found with similar to 3.5σ confidence over the flat model. This analysis presents the first evidence for signatures of CrH in a hot Jupiter atmosphere. At a retrieved temperature of &gt;1481-355 +264 K, the atmosphere of WASP-31b is hot enough to host gaseous Cr-bearing species, and the retrieved abundances agree well with predictions from thermal equilibrium chemistry. Furthermore, the retrieved abundance of CrH agrees with the abundance in an L-type brown dwarf atmosphere. However, additional retrievals using VLT FORS2 data lead to a non-detection of CrH. Future observations with James Webb Space Telescope have the potential to confirm the detection and/or discover other CrH features.</p