Résumé
Solar UV radiation is a major source of energy for initiating chemical evolution towards complex organic structures, but it can also photo-dissociate even the most complex molecules. Thus, solar UV can erase the organic traces of past life at the surface of planets, such as Mars, destroy organic molecules present on meteorites and micrometeorites, influence the production of distributed sources in comets or initiate chemistry in Titan's atmosphere. In the interstellar medium, the UV radiation field emitted by stars in the galaxy is also responsible for the chemical evolution and the extraordinary diversity of organic molecules detected. PSS (Photochemistry on the Space Station) was a Low Earth Orbit (LEO) experiment, implemented from mid-2014 to early 2016 on the EXPOSE-R2 platform outside the International Space Station. Its goal was to improve our knowledge about the chemical nature and evolution of organic molecules with astrobiological implications in space environments. It was a new step in a series of experiments conducted outside the MIR space station, in the ESA BIOPAN and previous EXPOSE facilities. In PSS, both vented and sealed cells were used allowing exposure of both solid and gaseous samples. Five kinds of experiments was carried out exposing molecules related to different environmental factors of astrobiological significance: the interstellar medium, comets & meteorites, Titan, Mars, as well as a set of samples to test the stability of biochips in space. In this talk we will describe the PSS experiment and focus on some results related to the stability of some prebioticaly relevant compounds such as glycine, the simplest amino acid, and nucleobases such as uracil, guanine and adenine. These molecules were both exposed in Low Earth Orbit and studied in the laboratory in order to derive their photochemical lifetime if they are ejected from comets on dust particles and orbit around the Sun before reaching the Earth as micrometeorites. The results can lead to better understand the contribution of cometary particles in the establishment of an organic reservoir on primitive Earth.