14 Septembre – Thesis defense - Adam Jaziri

14 h Room Univers - Allée Geoffroy Saint Hilaire (Pessac)

Toward a generic 3D model for atmospheric photochemistry: from early Earth to exoplanets.

The study of planetary atmospheres has made immense progress thanks to the generalization of 3D hydrodynamic models called GCM (Global Climate Models). The multiplication of these models specific to each planet of the solar system is a study process that reaches its limits face to the great diversity of exoplanets. It is necessary to develop generic models in which the different parameters associated with the planet, such as its atmosphere, its rotation, its orbit and its star, can be flexible. Especially since the synergy between these models and the observations is growing due to the entry in a new phase for the observations of exoplanetary atmospheres thanks to the next launch of the JWST followed by the launch of ARIEL and the construction of the ELT. The generic LMDZ is a GCM developed considering this purpose. The objective of this thesis was to contribute to the improvement of this GCM by the implementation of a flexible and generic module of (photo)chemistry which was missing until now. This module allows to follow the evolution of chemical species connected by a network of reactions and photodissociations. The coupling between chemistry, dynamics and radiation transfer is essential. For instance, it forms on Earth an ozone layer responsible for the stratospheric temperature inversion, which the generic LMDZ can reproduce thanks to the developments made.
This new functionality of the code allowed two new studies to be carried out within the framework of this thesis. A first study was to test the stability of the Earth's atmosphere in a different context, on a planet in synchronous rotation around a red dwarf star. In the specific case of the temperate exoplanet Trappist-1e, which is one of the next JWST targets, we discussed quantitatively the observability of such an atmosphere. Then we studied for the first time in 3D the role of photochemistry in the oxygen enrichment of the Earth's atmosphere, which dates back about 2.5 billion years. Photochemistry results in a bistability of atmospheric composition that can explain a rapid runaway oxygen content. The close link between photochemistry and surface temperature during this runaway is discussed in order to refine the global understanding of the phenomenon.

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