In order to survive, evolve and thrive, life requires a biologically useful supply of energy and nutrients. While there is evidence for both throughout the solar system and beyond, quantifying the energetic threshold at which a given environment can be described as habitable remains difficult. This thesis explores how power (energy per unit time) can be used as a habitability predictor in extraterrestrial environments. The behaviour of life is simplified into a series of chemical processes which use energy and nutrients to create and maintain complexity — order from disorder — all while obeying the fundamental laws of thermodynamics. Crucially, the underlying thermodynamics of biology is split into two clear habitability-defining terms: the available power supply and the power demand posed by the environment.
We developed a new computational model for assessing the energetic and nutrient availability of the weakly constrained environments that are typical of astrobiology, astronomy and planetary science. NutMEG [Nutrients, Maintenance, Energy and Growth] can be used to estimate how much biomass an environment could provide were it exposed to life and how a microbial community might affect the local chemistry. We used the model to characterise the behaviour of methanogens in optimal conditions, and examine how the predictions change in energy- or nutrient-limited settings. For this application, NutMEG was configured to replicate methanogen growth behaviour from laboratory data available in the literature. As temperature rises from 280 to 330 K, NutMEG predicts exponential drops in final biomass (109–106 cells/L) and total methane production from a growth cycle (62–3 μM) despite an increase in peak growth rates (0.007–0.14 /hr). This owes to the increasing cost of survival diverting energy away from growth processes. Restricting energy and nutrients exacerbates this trend. With minimal assumptions NutMEG can reliably replicate microbial growth behaviour, but better understanding of the synthesis and maintenance costs life must overcome in different extremes is required to improve its results further.
We used NutMEG to examine the habitability of Enceladus’ subsurface ocean. The oceanic composition is difficult to characterise with current data and estimates are highly dependent on model-based interpretations, informed by Cassini measurements, which are also not yet tightly constrained. In light of these ambiguities, we considered a wide selection of parameter spaces to quantify the available energy for putative methanogens on Enceladus. We estimated the spontaneous power supply their metabolism could provide and compared it to expected power demands in order to map the icy moon’s habitability. On the one hand, Enceladus’ parameter space contains pockets in which life could thrive. On the other, there are swathes of the parameter space which appear uninhabitable. Enceladean habitability appears to be a delicate balance between the ocean’s temperature, pH, salinity and concentrations of carbonates, nutrients and dissolved gases (particularly H2); many of which are co-dependent. Variation in any one of these can tip the balance into uninhabitable conditions. These results do not aim to be pessimistic, but reflect how astrobiologists should be cautiously pragmatic in their approach to calculating the theoretical habitability of bodies which are not yet well characterised.
Finally, we extend this to explore the energetic controls on possible biomass and biosignatures on Enceladus. Peak methanogenic growth rates and biomass estimates for the ocean’s parameter space are defined, ranging from completely devoid of life to bustling with biology. We then consider hydrothermal activity as a source of hydrogen and carbon dioxide and quantify how this could improve methanogens’ chances of survival in Enceladus’ ocean. Using measurements from the Cassini mission and predictions of hydrothermal productivity we constrain the levels of biomass which could be supported in the bulk ocean in a steady state and discuss whether associated biosignatures could be detectable with future instruments. Much of the ocean is inflexible to small changes in biological behaviour, implying that methanogens fitting neatly into such conditions is improbable. However, some pockets of the parameter space at pH 8.5–9 are flexible, and tantalisingly coincide with the current best estimate of bulk ocean pH. In such regions, methanogens could occupy habitable niches in an ocean which behaves as-observed with biomasses of up to ∼10^10 cells/L, but this requires such life to be near the H2 source. Whether biosignatures could be detectable via an amino acid chirality analysis depends on the temperature of the habitat and the flow of material through the ocean, neither of which are understood well enough to draw concrete conclusions yet. At hydrothermal temperatures >370 K these biosignatures decay within months, but in the cool bulk ocean they could be preserved for millennia
