Atmospheric Species and Spectral Radiation in Terrestrial Exoplanets : Implications for Astrobiology

Abstract: The physical properties of the planets and their parent stars are fundamental in the composition of atmospheres and radiative environments, which fundamentally determine their surface temperature and habitability. The atmospheric composition and radiation play a vital role in the emergence of life. This doctoral thesis presents two main results: 1. A method that uses the physical properties of the planets and their parent stars to infer the potential atmospheric compositions of the known exoplanets. For that, fundamental physics concepts and the basics of the kinetic theory of gases are used. Additionally, a new list of potentially habitable exoplanets is presented based on the resulting atmospheric compositions and the criteria that Earth-like atmospheres that can host liquid water should be considered habitable. The presented method also provides a preliminary classification of exoplanets similar to Earth (in terms of atmosphere) and their potential habitability. 2. A study of the impact of the radiation environment on the development and evolution of the human visual system towards optimal use of the available radiation. Human vision's possible evolutionary directions are presented while overcoming the shortcomings in the existing studies. The human visual system is hypothesised to have evolved in conjugation with the prevailing spectral radiation environment for photopic (daytime) and scotopic (night-time). One of the main novelties of this study is the comparison of the human vision bandwidth with the Full Width at half-maximum (FWHM) of the radiation reaching the shallow depths of the ocean, which may suggest that this is optimized for the development of animal sight during the formation of the early proto-visual system. Moreover, the observed maximum absorption wavelength during photopic vision (555nm) correlates with the maximum total energy for a 300 nm vision bandwidth. Furthermore, the analysis of the radiation environment at different solar zenith angles (SZA) during dusk suggests that the scotopic vision evolved to optimize information retrieval during these hours. The work presented in this thesis contributes to perform screening of Earth-like exoplanets and the study of astrobiological or space exploration aspects such as potential habitability, human-like vision, photosynthesis efficiency and evolution of life systems on exoplanets. 

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