Using models of the Earth's atmosphere to assess exoplanet habitability
Yates, Jack Samuel
Recent advances in telescope technology have allowed us to detect planets and bodies that have the potential to be habitable. Habitability can be defined in a number of ways, but most commonly it is defined by the availability of liquid water. There are a vast number of factors that determine whether or not liquid water is present in an atmosphere or on a surface, and due to the limited observational data, our understanding of the role of each of these factors is poor, especially as we move further through the parameter space away from the Earth. Until data from the next generation of telescopes are available, attempts to constrain atmospheric habitability have to utilise computer modelling. Modelling has a long history in habitability studies, particularly with regards to the inner and outer boundaries of the circumstellar habitable zone (CHZ). Early models were 1-dimensional (1D), but in the last decade the balance has shifted towards 3-dimensional (3D) global circulation models (GCMs) that describe the air flow in a planetary atmosphere in a much more sophisticated way. In part this was due to the recognition of the importance of 3D processes like clouds and convection in the global energy balance, and in part due to the increasing prioritisation of planets that are dissimilar to Earth, such as M-dwarf planets, which show features such as tidal-locking and atmospheric jets that result in less spatial uniformity through the atmosphere, limiting the applicability of 1D models. As of this writing our current best hopes for habitability are M-dwarf planets such as the TRAPPIST planets and Proxima Centauri b that orbit in the habitable zone, with rocky compositions. M-dwarf planets were previously overlooked as candidate habitable planets in favour of G-star planets like the Earth. However, some researchers now favour M-dwarfs in light of modern GCM results, observational biases and planetary population statistics, demonstrating that we must be careful not to define habitability in a way that is too Earth-centric. In this thesis we expand on knowledge of habitability through models that are informed by Earth science, but that do not necessarily describe Earth-like environments. In Chapter 2, we consider an environment that has not been studied through the lens of habitability before: ultra-cool Y dwarf atmospheres. In the atmospheres of these bodies it is thought that there may be liquid water clouds and temperatures and pressures similar to those on the Earth's surface. However, as there is no surface it is important that any potential organisms are able to remain above the hot lower atmosphere and the cold upper atmosphere; we compare with the Earth's atmosphere, where microbes are able to stay in the atmosphere for weeks, even metabolising in clouds. We study this environment through a simple radiative or convective atmosphere paired with a model informed by nutrient-phytoplankton-zooplankton models from the Earth's ocean. We find that organisms similar in size to microbes can remain aloft in this environment due to upward convective winds. In Chapter 3, contrasting with the simple approach in the previous chapter, we describe the development of a highly-sophisticated, fully online, 3D photochemical model of an exoplanet atmosphere. We apply this model to a tidally-locked M-dwarf aqua planet with an Earth-like atmosphere, nominally Proxima Centauri b, to evaluate the impacts of the differing stellar energy spectrum and dramatically different global circulation on an ozone layer described through the Chapman mechanism and the hydrogen oxide catalytic cycle. We find that the ozone layer is unlike that seen in the Earth's atmosphere. The lack of UV photons from our quiescent M-dwarf results in very long chemical lifetimes, which means that the atmospheric transport becomes the dominant factor in the structure of the ozone layer. We see an accumulation of ozone in the night-side cold traps (or gyres) at low altitudes where transport is slow and lifetimes are long, resulting in a dramatic day-night contrast in ozone columns. Total ozone column is much smaller on an M-dwarf planet compared with the Earth, by around a factor of 10, owing to top-of-atmosphere UV flux. In Chapter 4, we develop on the results of Chapter 3 by altering certain parameters in the model and examining the effect on the climate. We find that dramatic changes occur when switching off the chemistry scheme and reverting to a prescribed Earth ozone layer. Specifically we find that the temperatures on the night side of the planet change by more than 50 K, accompanied by dramatic changes in the pole temperatures. In addition the cold traps move towards the equator and eastwards. These changes are caused by the smaller ozone columns that result from the interactive chemistry, which severely reduce night side atmosphere opacity. This opacity controls the night side cooling rate which in turn controls the atmospheric circulation through the day-tonight temperature contrast. We find that similar effects occur when switching off the hydrogen oxide catalytic loss cycle, though to a lesser extent. Furthermore, we examine the effects of electromagnetic flares on the chemistry, which do not seem to impact ozone columns, in agreement with previous works. Finally we demonstrate the changes in atmospheric ozone and climate in a 3:2 resonant orbit and with an Earth-like orbit and top-of-atmosphere flux. In sum, our results with this model show that the climate is highly sensitive to the ozone columns, and demonstrate the importance of fully-coupled 3D photochemical models, which have been used very rarely in exoplanet atmosphere modelling.
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