Evolution of the volatile inventory of rocky exoplanets in a runaway greenhouse state with the General Circulation Model ExoFMS-SOCRATES

Thesis

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 and 3-D radiative-convective models of steam atmospheres to explore the transition from the tropospheric radiation limit to the post-runaway climate state. To facilitate 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 analyse two factors that 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. 3-D GCM simulations were performed, indicating that the runaway greenhouse effect necessarily ends with a thin condensing region located at the uppermost layers of the atmosphere, mostly on the nightside, while dayside condensation is prevented by heating coming from the absorption of incoming stellar radiation caused by the opacity of water vapour. Surface pressures higher than 10 bar feature non-convective regions in the deep layers, whereas the surface temperature of post-runaway atmospheres seem only weakly sensitive to instellation. Combining five secondary eclipse observations of TRAPPIST-1b with JWST/MIRI could yield a 4.5 sigma detection if it has an atmosphere made out of 1 bar of water vapour. The maximum confidence level in the cases explored for TRAPPIST-1d is 2 sigma, making it unsuitable for thermal emission observations with JWST. Astronomical surveys of rocky exoplanets in the runaway greenhouse state may discriminate some of these features using multi-wavelength observations. Further understanding of runaway greenhouse climates will come from more holistic models, more spectroscopic measurements outside Earth-like conditions, and future observatories.

Authors: Boukrouche, R

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: extrasolar rocky planets; exoplanet atmospheres; habitable planets; greenhouse gases
• Subjects: planetary science