Evolution of Titan’s high-altitude aerosols under ultraviolet irradiation
The NASA/ESA Cassini-Huygens space mission was an amazing success in the history of planetary exploration. In our laboratory we leverage its scientific legacy and reveal the evolution of the brownish haze enfolding Titan under the solar UV exposure, offering new perspectives on its impact on the chemical and climate balance in Titan’s atmosphere.
The paper in Nature Astronomy is here: https://go.nature.com/2H4S2mZ
Each planetary science mission provides the opportunity to make a pioneer opening into the fascinating worlds surrounding our planet. The Cassini space orbiter is one of them. It majestically finished its life in September 2017, during the Grand Finale, by entering the atmosphere of Saturn after 13 years of cumulated observations of the ringed planet and its icy satellites. The scientific legacy is huge. But all these discoveries give rise to new scientific questions that the finished mission will never be able to answer. To continue this exploration beyond the space mission, we chose to address some of these new questions in our laboratory, by mimicking Saturn and its moons with experimental simulations.
The present article opens new perspectives on the impact of the organic haze surrounding Titan, the largest moon of Saturn, on atmospheric chemistry and climate. Indeed, from the Cassini mission, we have learned that it was produced very high in the atmosphere at about 1000 km. At this altitude, the grains composing the haze reach nanometer-scale dimensions and spend about one-Titan day (eleven days on earth) in this atmospheric compartment. We also know that they are composed of prebiotic nitrogen-rich organic molecules. But at this high altitude they are also submitted to energetic solar far-Ultra-Violet radiations. How will these organic grains evolve under the harsh effect of these energetic photons?
To investigate this question, we first had to produce analogues of Titan’s haze grains. The top of Titan’s atmosphere is partially ionized, forming a natural plasma environment in which the organic grains are produced. So we synthesized analogues of this solid organic material by simulating Titan’s plasma chemistry with a plasma experiment at LATMOS (see Figure). Then we needed to submit the so-produced organic material to an analogue of the solar far-UV radiations. We found this artificial sun at the French synchrotron facility named SOLEIL (“Soleil” means Sun in French).
Now that the principles of the irradiation experiment had been established, the real difficulties could begin. The experimental beamline at SOLEIL that we needed to achieve these experiments is of high demand in the scientific community. After several applications for beamtime allocation we finally obtained, over the past two years, two one-week slots (24/24hrs) of irradiation time. So many ideas to be tested and so few and hard to obtain hours. Drastic choices had to be made: we decided to study the effect of two important wavelengths for Titan high atmosphere and a few irradiation durations for each wavelength to get insight on the kinetics of the possible evolution process. Among the three experimentalists (a PhD, an engineer and one professor) we had to organize our work into 8hrs shifts. We had to adapt the conditions of the irradiation experiments at any time of the day and night to progress with our tight experimental schedule. At the end of the campaigns we were tired and happy at the same time: no photons were wasted and all experiments ran as planned.
The data treatment of the spectra recorded during these intense campaigns required some quiet time and sharp brains. We did it afterwards without knowing if the samples had evolved or not with irradiation. How excited we were when we discovered that the samples had actually been affected! It meant that Titan’s haze was not only produced in the upper part of Titan’s atmosphere as shown by the Cassini mission, but that it would evolve during its journey in Titan’s atmosphere towards a resistant nitrogen-rich material. This change in optical and chemical properties is of high importance for Titan, impacting its atmospheric chemistry and climate. We hope that a future mission will be launched to Titan to characterize in situ this evolution process.