Couplage de modèles d'intérieur, d'atmosphère et d'écosystèmes pour l'inférence de l'habitabilité et des biosignatures

Is Earth’s ability to harbor life, habitability, a rare or unique occurrence in the Universe? What are remotely detectable signs of a biosphere, or biosignatures? These profound questions, once philosophical debates, may enter the realm of empirical science as the exploration of the solar system goes on with increasing ambition and means, as thousands of exoplanets have been detected in the past twenty-five years, and as upcoming ground and space telescopes might enable the characterization of their atmospheres. In this thesis, we leverage recent progress in theoretical and quantitative ecosystem science to present not only a biologist’s take on the meaning of habitability, but also the means of quantitative inference of habitability and biosignatures, as well as a path towards the assessment of astrobiological hypothesis on the exoplanet population level. Part I lays out the physiological modeling basis for energy-limited organisms and estimates model parameter values using published data to explore adaptation of micro organisms to temperature under the model's hypotheses. Second, Part II uses the modeling developed in Part I to assess the habitability and biosignatures of Saturn's icy moon Enceladus. Last, Part III couples this model to atmospheric, climate and geochemical modeling to predict patterns in exoplanet atmospheric composition versus orbital radius under various scenarios, thus paving the way to conceptualize exoplanet population level signatures of habitability and biosignatures.La capacité de la Terre à abriter la vie, autrement dit son habitabilité, est-elle rare ou même unique ? Que pourraient être des manifestations observables à distance, ou biosignatures, associées à l'existence d'une biosphère? Ces questions fondamentales pourraient passer des débats philosophiques à une science basée sur la donnée, alors que l'exploration du système solaire se poursuis et s'amplifie; ainsi que des milliers d'exoplanètes sont découvertes et que de nouveaux télescopes plus puissants sont mis en service dans l'espace comme au sol. Dans cette thèse, nous mobilisons des outils quantitatifs de la théorie des écosystèmes afin de donner non seulement un point de vue écosystémique au concept de l'habitabilité, mais également de concevoir les bases d'un moyen d'inférence quantitative de l'habitabilité et des biosignatures. La Partie I décrit un modèle de croissance microbienne limitée par l'accès à l'énergie chimique, estime la valeur des paramètres du modèle en utilisant des données publiées et explore l'adaptation des organismes à la température dans le contexte de ce modèle. La Partie II applique le modèle développé en partie I pour inférer quantitativement l'habitabilité et les potentielles biosignatures d'Encelade, une lune de glace de Saturne. Enfin, la Partie III couple ce modèle à des modèles d'atmosphère, de climat et de géochimie afin de simuler les corrélations entre la composition atmosphérique d'exoplanètes et leur situation autour de leur étoile sous différents scénarios. Ces travaux permettent de considérer les signatures d'habitabilité et les biosignatures au niveau de populations d'exoplanètes

Couplage de modèles d'intérieur, d'atmosphère et d'écosystèmes pour l'inférence de l'habitabilité et des biosignatures

La capacité de la Terre à abriter la vie, autrement dit son habitabilité, est-elle rare ou même unique ? Que pourraient être des manifestations observables à distance, ou biosignatures, associées à l'existence d'une biosphère? Ces questions fondamentales pourraient passer des débats philosophiques à une science basée sur la donnée, alors que l'exploration du système solaire se poursuis et s'amplifie; ainsi que des milliers d'exoplanètes sont découvertes et que de nouveaux télescopes plus puissants sont mis en service dans l'espace comme au sol. Dans cette thèse, nous mobilisons des outils quantitatifs de la théorie des écosystèmes afin de donner non seulement un point de vue écosystémique au concept de l'habitabilité, mais également de concevoir les bases d'un moyen d'inférence quantitative de l'habitabilité et des biosignatures. La Partie I décrit un modèle de croissance microbienne limitée par l'accès à l'énergie chimique, estime la valeur des paramètres du modèle en utilisant des données publiées et explore l'adaptation des organismes à la température dans le contexte de ce modèle. La Partie II applique le modèle développé en partie I pour inférer quantitativement l'habitabilité et les potentielles biosignatures d'Encelade, une lune de glace de Saturne. Enfin, la Partie III couple ce modèle à des modèles d'atmosphère, de climat et de géochimie afin de simuler les corrélations entre la composition atmosphérique d'exoplanètes et leur situation autour de leur étoile sous différents scénarios. Ces travaux permettent de considérer les signatures d'habitabilité et les biosignatures au niveau de populations d'exoplanètes.Is Earth’s ability to harbor life, habitability, a rare or unique occurrence in the Universe? What are remotely detectable signs of a biosphere, or biosignatures? These profound questions, once philosophical debates, may enter the realm of empirical science as the exploration of the solar system goes on with increasing ambition and means, as thousands of exoplanets have been detected in the past twenty-five years, and as upcoming ground and space telescopes might enable the characterization of their atmospheres. In this thesis, we leverage recent progress in theoretical and quantitative ecosystem science to present not only a biologist’s take on the meaning of habitability, but also the means of quantitative inference of habitability and biosignatures, as well as a path towards the assessment of astrobiological hypothesis on the exoplanet population level. Part I lays out the physiological modeling basis for energy-limited organisms and estimates model parameter values using published data to explore adaptation of micro organisms to temperature under the model's hypotheses. Second, Part II uses the modeling developed in Part I to assess the habitability and biosignatures of Saturn's icy moon Enceladus. Last, Part III couples this model to atmospheric, climate and geochemical modeling to predict patterns in exoplanet atmospheric composition versus orbital radius under various scenarios, thus paving the way to conceptualize exoplanet population level signatures of habitability and biosignatures

Couplage de modèles d'intérieur, d'atmosphère et d'écosystèmes pour l'inférence de l'habitabilité et des biosignatures

Is Earth’s ability to harbor life, habitability, a rare or unique occurrence in the Universe? What are remotely detectable signs of a biosphere, or biosignatures? These profound questions, once philosophical debates, may enter the realm of empirical science as the exploration of the solar system goes on with increasing ambition and means, as thousands of exoplanets have been detected in the past twenty-five years, and as upcoming ground and space telescopes might enable the characterization of their atmospheres. In this thesis, we leverage recent progress in theoretical and quantitative ecosystem science to present not only a biologist’s take on the meaning of habitability, but also the means of quantitative inference of habitability and biosignatures, as well as a path towards the assessment of astrobiological hypothesis on the exoplanet population level. Part I lays out the physiological modeling basis for energy-limited organisms and estimates model parameter values using published data to explore adaptation of micro organisms to temperature under the model's hypotheses. Second, Part II uses the modeling developed in Part I to assess the habitability and biosignatures of Saturn's icy moon Enceladus. Last, Part III couples this model to atmospheric, climate and geochemical modeling to predict patterns in exoplanet atmospheric composition versus orbital radius under various scenarios, thus paving the way to conceptualize exoplanet population level signatures of habitability and biosignatures.La capacité de la Terre à abriter la vie, autrement dit son habitabilité, est-elle rare ou même unique ? Que pourraient être des manifestations observables à distance, ou biosignatures, associées à l'existence d'une biosphère? Ces questions fondamentales pourraient passer des débats philosophiques à une science basée sur la donnée, alors que l'exploration du système solaire se poursuis et s'amplifie; ainsi que des milliers d'exoplanètes sont découvertes et que de nouveaux télescopes plus puissants sont mis en service dans l'espace comme au sol. Dans cette thèse, nous mobilisons des outils quantitatifs de la théorie des écosystèmes afin de donner non seulement un point de vue écosystémique au concept de l'habitabilité, mais également de concevoir les bases d'un moyen d'inférence quantitative de l'habitabilité et des biosignatures. La Partie I décrit un modèle de croissance microbienne limitée par l'accès à l'énergie chimique, estime la valeur des paramètres du modèle en utilisant des données publiées et explore l'adaptation des organismes à la température dans le contexte de ce modèle. La Partie II applique le modèle développé en partie I pour inférer quantitativement l'habitabilité et les potentielles biosignatures d'Encelade, une lune de glace de Saturne. Enfin, la Partie III couple ce modèle à des modèles d'atmosphère, de climat et de géochimie afin de simuler les corrélations entre la composition atmosphérique d'exoplanètes et leur situation autour de leur étoile sous différents scénarios. Ces travaux permettent de considérer les signatures d'habitabilité et les biosignatures au niveau de populations d'exoplanètes

Putative Methanogenic Biosphere in Enceladus's Deep Ocean: Biomass, Productivity, and Implications for Detection

International audienceAbstract Saturn's moon Enceladus is a top candidate in the search for extraterrestrial life in our solar system. Ecological thermodynamic modeling of the plume composition data collected by NASA's Cassini mission led to the hypothesis that a hydrogenotrophic methanogenic ecosystem might exist in the putative hydrothermal vents at Enceladus's seafloor. Here we extend this approach to quantify the ecosystem's expected biomass stock and production and evaluate its detectability from the collection of plume material. We find that although a hypothetical biosphere in Enceladus's ocean could be small (0.1 mL of material needs to be collected. This would require material from more than 100 fly-bys through the plume or using a lander. We then consider amino acid abundance as an alternative signature and find that the absolute abundance of amino acids, such as glycine, could be very informative if a detection threshold of 1 × 10 −7 mol L −1 could be achieved. Altogether, our findings set relatively high bars on sample volume and amino acid detection thresholds, but these goals seem within the reach of near-future missions

Putative Methanogenic Biosphere in Enceladus's Deep Ocean: Biomass, Productivity, and Implications for Detection

International audienceAbstract Saturn's moon Enceladus is a top candidate in the search for extraterrestrial life in our solar system. Ecological thermodynamic modeling of the plume composition data collected by NASA's Cassini mission led to the hypothesis that a hydrogenotrophic methanogenic ecosystem might exist in the putative hydrothermal vents at Enceladus's seafloor. Here we extend this approach to quantify the ecosystem's expected biomass stock and production and evaluate its detectability from the collection of plume material. We find that although a hypothetical biosphere in Enceladus's ocean could be small (0.1 mL of material needs to be collected. This would require material from more than 100 fly-bys through the plume or using a lander. We then consider amino acid abundance as an alternative signature and find that the absolute abundance of amino acids, such as glycine, could be very informative if a detection threshold of 1 × 10 −7 mol L −1 could be achieved. Altogether, our findings set relatively high bars on sample volume and amino acid detection thresholds, but these goals seem within the reach of near-future missions

Bayesian analysis of Enceladus' plume data to assess methanogenesis

International audienceObservations from NASA’s Cassini spacecraft established that Saturn’s moon Enceladus has an internal liquid ocean. Analysis of a plume of ocean material ejected into space suggests that alkaline hydrothermal vents are present on Enceladus’s seafloor. On Earth, such deep-sea vents harbour microbial ecosystems rich in methanogenic archaea. Here we use a Bayesian statistical approach to quantify the probability that methanogenesis (biotic methane production) might explain the escape rates of molecular hydrogen and methane in Enceladus’s plume, as measured by Cassini instruments. We find that the observed escape rates (1) cannot be explained solely by the abiotic alteration of the rocky core by serpentinization; (2) are compatible with the hypothesis of habitable conditions for methanogens; and (3) score the highest likelihood under the hypothesis of methanogenesis, assuming that the probability of life emerging is high enough. If the probability of life emerging on Enceladus is low, the Cassini measurements are consistent with habitable yet uninhabited hydrothermal vents and point to unknown sources of methane (for example, primordial methane) awaiting discovery by future missions

Co-evolution of primitive methane-cycling ecosystems and early Earth's atmosphere and climate

International audienceThe history of the Earth has been marked by major ecological transitions, driven by metabolic innovation, that radically reshaped the composition of the oceans and atmosphere. The nature and magnitude of the earliest transitions, hundreds of million years before photosynthesis evolved, remain poorly understood. Using a novel ecosystem-planetary model, we find that pre-photosynthetic methane-cycling microbial ecosystems are much less productive than previously thought. In spite of their low productivity, the evolution of methanogenic metabolisms strongly modifies the atmospheric composition, leading to a warmer but less resilient climate. As the abiotic carbon cycle responds, further metabolic evolution (anaerobic methanotrophy) may feed back to the atmosphere and destabilize the climate, triggering a transient global glaciation. Although early metabolic evolution may cause strong climatic instability, a low CO:CH4 atmospheric ratio emerges as a robust signature of simple methane-cycling ecosystems on a globally reduced planet such as the late Hadean/early Archean Earth

Inferring linguistic transmission between generations at the scale of individuals

International audienceAbstract Historical linguistics strongly benefited from recent methodological advances inspired by phylogenetics. Nevertheless, no available method uses contemporaneous within-population linguistic diversity to reconstruct the history of human populations. Here, we developed an approach inspired from population genetics to perform historical linguistic inferences from linguistic data sampled at the individual scale, within a population. We built four within-population demographic models of linguistic transmission over generations, each differing by the number of teachers involved during the language acquisition and the relative roles of the teachers. We then compared the simulated data obtained with these models with real contemporaneous linguistic data sampled from Tajik speakers from Central Asia, an area known for its large within-population linguistic diversity, using approximate Bayesian computation methods. Under this statistical framework, we were able to select the models that best explained the data, and infer the best-fitting parameters under the selected models. The selected model assumes that the lexicon of individuals is the result of a vertical transmission by two teachers, with a specific lexicon for each teacher. This demonstrates the feasibility of using contemporaneous within-population linguistic diversity to infer historical features of human cultural evolution

Early Mars habitability and global cooling by H<SUB>2</SUB>-based methanogens

International audienceDuring the Noachian, Mars' crust may have provided a favourable environment for microbial life1,2. The porous brine-saturated regolith3-5 would have created a physical space sheltered from ultraviolet and cosmic radiation and provided a solvent, whereas the below-ground temperature2 and diffusion6,7 of a dense, reduced atmosphere8,9 may have supported simple microbial organisms that consumed H2 and CO2 as energy and carbon sources and produced methane as a waste. On Earth, hydrogenotrophic methanogenesis was among the earliest metabolisms10,11, but its viability on early Mars has never been quantitatively evaluated. Here we present a probabilistic assessment of Mars' Noachian habitability to H2-based methanogens and quantify their biological feedback on Mars' atmosphere and climate. We find that subsurface habitability was very likely, and limited mainly by the extent of surface ice coverage. Biomass productivity could have been as high as in the early Earth's ocean. However, the predicted atmospheric composition shift caused by methanogenesis would have triggered a global cooling event, ending potential early warm conditions, compromising surface habitability and forcing the biosphere deep into the Martian crust. Spatial projections of our predictions point to lowland sites at low-to-medium latitudes as good candidates to uncover traces of this early life at or near the surface