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Experimental study of Pluto's atmosphere and aerosols

Abstract : On July 14th, 2015, NASA’s New Horizons spacecraft flew by Pluto, revealing a complex atmosphere and surface seen nowhere else in the Solar System. Pluto’s surface ices are composed of molecular nitrogen N2, methane CH4, and carbon monoxide CO. During Pluto’s elliptical orbit, these ices undergo a sublimation/condensation cycle resulting in a tenuous atmosphere (~11 µbar at the surface). This atmosphere is mostly composed of N2 and CH4, with ~500 ppm of CO. Subjected to extreme ultraviolet radiation and Lyman-α photons, it is the place of photochemical aerosol production, aerosols being solid particles in suspension in the atmosphere. The exact processes of formation of these aerosols are however not well constrained yet. These solid particles, whose chemical composition and optical properties are unknown, are observed up to more than 350 km of altitude in the atmosphere of Pluto. Numerical models have shown that the presence of these aerosols in the atmosphere could have an impact on the atmospheric chemistry and climate of Pluto. Moreover, it has been suggested that these aerosols sediment and constitute a source of organic matter on the surface of Pluto.During my Ph.D., I used an experimental approach to study the aerosols of Pluto, from their formation in the upper atmosphere to their evolution on the surface, through their interactions with the atmosphere. The formation of Pluto’s aerosols by photochemistry and their chemical composition are the subjects of the first and second part of this Ph.D. thesis (Chapter III and Chapter IV). The interaction of Pluto’s aerosols with solar radiation and the contribution of photochemical aerosols as a coloring agent on the surface of Pluto are the subjects of the third and fourth part of this Ph.D. thesis (Chapter V and Chapter VI).The experimental setup PAMPRE (Production of Aerosols in Microgravity by REactive Plasma), located at LATMOS, has been used to simulate the atmospheric chemistry of Pluto and to synthesize analogues of photochemical aerosols, usually called "tholins". Experiments have also been performed at GANIL, using the IGLIAS (Irradiation of astrophysical ices) experimental setup. By irradiating tholins with heavy ions, the objective was to simulate the ageing of organic matter on the surface of Pluto due to the charged particles constituting the galactic cosmic rays.Thanks to the physicochemical analyses carried out to characterize the chemical composition of Pluto-simulated atmosphere as well as that of the synthesized aerosol analogues, I was able to conclude to the importance of N2 and CO reactivity in the atmospheric chemistry of Pluto. The nitrogen constituting the molecules produced in the gas phase and ultimately incorporated in the solid particles is included not only in the form of terminal functional groups (amine, nitrile, isocyanide), but also in the form of nitrogen heterocycles (triazine, pyrazole, pyrazine, pyrrole). Regarding oxygen, only terminal oxygenated chemical functions (alcohol, carboxylic acid, carbonyl) were detected. These nitrogenous and/or oxygenated organic molecules are responsible for a strong absorption in the ultraviolet spectral range by Pluto aerosol analogues and a more moderate absorption in the visible and near-infrared. These results are consistent with spectral observations of Pluto’s surface and atmosphere by instruments onboard New Horizons. Finally, thanks to the experiments of irradiation of Pluto aerosol analogues by heavy ions, I was able to conclude that the surface of Pluto is processed by galactic cosmic ray irradiation, probably explaining the characteristic featureless spectra of the Cthulhu region.
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Submitted on : Tuesday, November 30, 2021 - 4:56:31 PM
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  • HAL Id : tel-03457696, version 1

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Lora Jovanovic. Experimental study of Pluto's atmosphere and aerosols. Earth and Planetary Astrophysics [astro-ph.EP]. Université Paris-Saclay, 2021. English. ⟨NNT : 2021UPASJ011⟩. ⟨tel-03457696⟩

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