Insights into the role of polycyclic aromatic hydrocarbon condensation in haze formation in Jupiter’s atmosphere

Context. The haze in the atmosphere of Jupiter has been proposed to originate mostly from the condensation of low mass PAHs. Under this hypothesis, the transition of gas phase molecular species to solid particles is achieved by the formation of a critical nucleus composed of two small PAH molecules held together by van der Waals forces. Aims. Laboratory experiments coupled with theoretical calculations explore the thermodynamics and kinetics of PAH dimerization with the objective to assess this pathway in the production of nuclei of haze particles. Methods. We have performed experiments to identify the temperature range over which small PAH clusters form in supersaturated uniform supersonic flows. The kinetics of this formation has also been investigated. The chemical species present in the reactor are probed by a time-of-flight mass spectrometer equipped with a VUV laser for photoionisation of the neutral reagents and products. The experimental data were combined with theoretical calculations that employ careful consideration of the intermolecular interaction energies and intermolecular dynamics to estimate the binding energy, equilibrium constant, and rate constant. Results. We found that low-mass PAHs such as anthracene (C14H10) and pyrene (C16H10) can not homogeneously nucleate in the upper stratosphere at the altitude at which haze formation has been proposed to commence. Other chemical or physical processes should be considered to account for aerosol nuclei formation.