Nitrile aerosol spectroscopy for the titan atmosphere

Saturn's largest moon Titan possesses a rich chemical environment. With an extended atmosphere ten times the height of Earth's, UV photolysis and fast particle bombardment in the outer-reaches efficiently produce higher-order species from the dominant molecular nitrogen and its minor methane gas fraction. Secondary products include hydrocarbons (such as ethane and acetylene), nitriles (cyanide family of molecules including acetonitrile) and small amounts of water (from trapped oxygen ions in the ionosphere) that drift down to cold, lower altitudes. Here, these species may undergo further reactions to form complex organic molecules and/or condense; forming microscopic ice particles or aerosols. Importantly, it is thought that these aerosol droplets act as vehicles to transport complex organics to the Titan surface. With its liquid hydrocarbon lakes and volcanic heat sources, the Titan surface possesses a remarkably active geology that could promote prebiotic chemistry if the required chemical ingredients are continually replenished via aerosol rainfall.

We study these unusual aerosols and map their spectral properties using a specialised gas cell at the Australian Synchrotron to mimic the conditions of the Titan atmosphere. This apparatus allows us to focus on the rapid condensation of nitrile gases (such as acetonitrile and propionitrile) into micron-size particles once supersaturated in our simulated Titan air. The terahertz and far-infrared beamline can then extract essential data on nitrile aerosol size, growth rate and ice particle morphology (crystal structure), which can be compared with the raw data obtained from spacecraft missions such as NASA Cassini-Huygens.