Chemical Aging of Titan's Tholins Exposed to Solar Radiation in Low Earth Orbit onboard the EXPOSE Facility
Résumé
Titan's organic aerosols are produced from the chemistry occurring in the atmosphere from gaseous N2 and CH4 submitted to UV solar radiation and energetic particles from Saturn magnetosphere. Our direct knowledge about their chemical composition is limited but we know they are among the most complex organic materials on Titan, composed of a solid core incorporating at least both carbon, hydrogen and nitrogen atoms. This chemistry is unique in the solar system and it could help understanding pathways of the prebiotic chemistry that resulted in the emergence of life. Titan's aerosols composition is indirectly studied through the production and analysis of analogues, called tholins, using laboratory experiments which mimic Titan's atmosphere chemistry conditions. But usually, these tholins are analyzed straight after their production, and it is only recently that the question of their aging was raised. In particular, the aging of the Titan's aerosols submitted to solar radiation during their long descent to the satellite surface was studied in the laboratory using specific UV wavelengths (1). With the same objective, Titan's tholins were exposed to the direct solar radiation in low Earth orbit using the ESA's EXPOSE facility mounted outside the ISS for months. Two varieties of tholins with different chemical properties were produced with the PAMPRE laboratory experiment (2), the first one using a (98:2) N2:CH4 gaseous mixture, and the other one, a (90:10) one. Increasing the amount of initial methane results in a more aliphatic material. These samples were exposed to the same solar radiation for the months the EXPOSE facility was mounted outside ISS. The chemical properties of these samples measured before the flight and after their return to the laboratory were compared. They clearly showed that both tholins structure have been modified in some extent by the solar radiation. Thanks to control samples placed in the EXPOSE facility in blind cells, and in a facility in the laboratory, this evolution can be definitely attributed to UV radiation. We present here the corresponding results and the consequences we can deduce for the aging of the Titan's aerosols Ref:[1]Carrasco N.(2018) Nat. Ast. [2] Szopa C. (2005) Planet. Space Sci.