Can laboratory tholins mimic the chemistry producing Titanʹs aerosols? A review in light of ACP experimental results

The first results obtained by the ACP experiment onboard Huygens probe revealed that the main products obtained after thermolysis of Titanʹs collected aerosols, were ammonia (NH3) and hydrogen cyanide (HCN). Titanʹs aerosols, and their laboratory analogues named tholins, have been the subject of experimental or theoretical studies during the last four decades. These studies have been mainly devoted to understanding their origin and formation mechanisms, their physical, chemical and optical properties, and their role in the radiative equilibrium of the satellite. Before the arrival of the Cassini–Huygens mission, the dense layer of aerosols hid many aspects of the satelliteʹs surface and precious information about its composition. anʹs aerosols have been in the eye and mind of planetary scientists during such a long time, it is not surprising that a literature survey displays a good quantity of papers on aerosol analogues. With aerosol analogues we mean any material produced in a terrestrial laboratory under conditions that try to represent those of Titanʹs atmosphere. We present here a study aimed to understand the particularities of aerosol analogues synthesized in different laboratories around the world in order to determine some of their most representative chemical fingerprints and in some cases, to perform a direct comparison of the volatiles produced after a thermal treatment done in conditions similar to the ones used by the ACP experiment. From the information collected, we propose a broad classification of aerosol analogues highlighting the materials that can be more representative of Titanʹs aerosols in terms of their content of organic volatiles. We identify the laboratory analogs that best suit the ACP results; such identification is of prime importance to correctly predict the optical properties of Titanʹs aerosol and to accurately estimate their contribution in radiative equilibrium models and/or to assess their role in chemical reactions of astrobiological importance at Titanʹs surface.