Using models of the Earth's atmosphere to assess exoplanet habitability

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

Ancient bacteria show evidence of DNA repair

Recent claims of cultivable ancient bacteria within sealed environments highlight our limited understanding of the mechanisms behind long-term cell survival. It remains unclear how dormancy, a favored explanation for extended cellular persistence, can cope with spontaneous genomic decay over geological timescales. There has been no direct evidence in ancient microbes for the most likely mechanism, active DNA repair, or for the metabolic activity necessary to sustain it. In this paper, we couple PCR and enzymatic treatment of DNA with direct respiration measurements to investigate long-term survival of bacteria sealed in frozen conditions for up to one million years. Our results show evidence of bacterial survival in samples up to half a million years in age, making this the oldest independently authenticated DNA to date obtained from viable cells. Additionally, we find strong evidence that this long-term survival is closely tied to cellular metabolic activity and DNA repair that over time proves to be superior to dormancy as a mechanism in sustaining bacteria viabilit

Microbial communities and processes in Arctic permafrost environments

In polar regions, huge layers of frozen ground, termed permafrost, are formed. Permafrost covers more than 25 % of the land surface and significant parts of the coastal sea shelfs. Its habitats are controlled by extreme climate and terrain conditions. Particularly, the seasonal freezing and thawing in the upper active layer of permafrost leads to distinct gradients in temperature and geochemistry. Microorganisms in permafrost environments have to survive extremely cold temperatures, freeze-thaw cycles, desiccation and starvation under long-lasting background radiation over geological time scales. Although the biology of permafrost microorganisms remains relatively unexplored, recent findings show that microbial communities in this extreme environment are composed by members of all three domains of life (Archaea, Bacteria, Eukarya), with a total biomass comparable to temperate soil ecosystems. This chapter describes the environmental conditions of permafrost and reviews recent studies on microbial processes and diversity in permafrost-affected soils as well as the role and significance of microbial communities with respect to global biogeochemical cycles

Survival of Methanogenic Archaea from Siberian Permafrost under Simulated Martian Thermal Conditions

Since ESA mission Mars Express determined water on Mars, a fundamental requirement for life, as well as the presence of CH4 in the Martian atmosphere, which could only have originated from active volcanism or from biological sources, it is obviously that microbial life could still exist on Mars, for example in the form of subsurface lithoautotrophic ecosystems, which are also exist in permafrost regions on Earth. Present work deals with the resistance investigation of methanogenic archaea from Siberian permafrost complementary to the already well-studied methanogens from non-permafrost habitats under simulated Martian conditions. The methanogenic archaea in pure cultures as well as in permafrost samples represent higher survival potential (up to 90 percent) than the referent organisms (0.3-5.8 percent) after 22 days of exposure to thermo-physical Martian conditions at low- and mid-latitudes. It is suggested that methanogens from terrestrial permafrost seem to be more resistant against Martian conditions and could be used as a prime candidates for the search for extraterrestrial life