Planetary Climate Dynamics

Abstract:

The runaway greenhouse represents the ultimate climate catastrophe for rocky, Earth-like worlds: when the incoming stellar flux cannot be balanced by radiation to space, the oceans evaporate and exacerbate heating, turning the planet into a hot wasteland with a steam atmosphere overlying a possibly molten magma surface. The equilibrium state beyond the runaway greenhouse instellation limit depends on the radiative properties of the atmosphere and its temperature structure. Here, we use 1-D radiative-convective models of steam atmospheres to explore the transition from the tropospheric radiation limit to the post-runaway climate state. To facilitate eventual simulations with 3-D global circulation models, a computationally efficient band-grey model is developed, which is capable of reproducing the key features of the more comprehensive calculations. We analyze two factors which determine the equilibrated surface temperature of post-runaway planets. The infrared cooling of the planet is strongly enhanced by the penetration of the dry adiabat into the optically thin upper regions of the atmosphere. In addition, thermal emission of both shortwave and near-IR fluxes from the hot lower atmospheric layers, which can radiate through window regions of the spectrum, is quantified. Astronomical surveys of rocky exoplanets in the runaway greenhouse state may discriminate these features using multi-wavelength observations.

Abstract:

Sub-Neptunes are a subset of exoplanets that lie between the Earth and Neptune in size, have no solar system analogue and yet are one of the most common types of exoplanet in the galaxy. Some sub-Neptunes receive a similar level of stellar flux as Earth, making their atmospheres potentially cool enough to contain liquid water. The aim of this thesis is to simulate the atmospheres of these temperate sub-Neptunes and develop theories describing their atmospheric dynamics and potential habitability. I use a general circulation model to simulate the atmospheres of a range of dry, temperate sub-Neptunes. I show that their atmospheres are governed by horizontal weak temperature gradients over a broad range of parameter space. Their circulation is dominated by high-latitude jets, but heat is transported from the dayside to the nightside by a residual overturning circulation. I derive a scaling theory to link the strength of this circulation to the instellation. Next, I calculate the inner edge of the habitable zone for sub-Neptunes with a water surface – “Hycean worlds”. Using a 1D radiative-convective model, I show that compositional gradients induced by the condensation of water inhibit convection in a hydrogendominated atmosphere. The resulting temperature structures heat the surface and lead to the inner edge of the habitable zone moving outwards compared to traditional calculations. Lastly, I develop a general circulation model for use in hydrogen-dominated atmospheres with a non-dilute water vapour component. I demonstrate the model’s ability to simulate a range of sub-Neptune atmospheres with different deep water contents reaching as high as 70% of the atmosphere by mass. Future work can build on this model to understand how latent heating and compositional gradients impact the observable features and habitability of sub-Neptune exoplanets.