Evolution of Titan s high-altitude aerosols under ultraviolet irradiation

The Cassini-Huygens space mission revealed that Titan s thick brownish haze is initiated high in the atmosphere at about 1000 km of altitude, before a slow transportation down to the surface. Close to the surface at altitudes below 130 km, the Huygens probe provided information on the chemical composition of the haze. So far we do not have insights on a possible photochemical evolution of the aerosols composing the haze during their descent. We address here this atmospheric aerosol aging process, simulating in the laboratory how solar vacuum-ultraviolet (VUV) irradiation affects the aerosol optical properties as probed by infrared spectroscopy. An important evolution is found, which could explain the apparent contradiction between the nitrogen-poor infrared spectroscopic signature observed by Cassini below 600 km of altitude in Titan s atmosphere, and a high nitrogen content as measured by the Aerosol Collector and Pyroliser of Huygens probe at the surface of Titan.