Experimental simulation of Titan's atmosphere: Detection of ammonia and ethylene oxide

Abstract

For several years now, an experimental simulation of Titan's atmosphere has been on going at LISA. A cold plasma is established in a gas mixture representative of the atmosphere of the satellite. In these experiments, more than 70 organic compounds have been identified, including the first identification in this type of experimental simulation of C ₄N ₂ already detected in its solid form on Titan, which suggests that the setup correctly mimics the chemistry of Titan's atmosphere. We have carried out the first experimental simulation including O-containing compounds in order to study the influence of the presence of CO on the chemical behavior of Titan's atmosphere. With the help of gas chromatography-mass spectrometry (GC-MS) and infrared spectroscopy (IRS) we can thus determine which minor species still undetected in Titan's atmosphere are likely to be present and understand the complex chemistry of the atmosphere of this satellite. Surprisingly we have identified unpredicted O-containing gaseous compounds, mainly ethylene oxide (also named oxirane, C ₂H ₄O). This molecule has been observed in the interstellar medium by observation in the millimeter region (Astrophys. J. 489 (1997) 553; Astron. Astrophys. 337 (1998) 275). On the contrary, the predicted O-compounds (formaldehyde and methanol) have not been identified in this experiment. Furthermore, we have identified NH ₃ in the gaseous products with an initial mixture of N ₂ (98%) and CH ₄ (2%). The paper describes the experimental device used in this work, in particular the IRS and GC-MS techniques. We also comment the results related to the detection of the O-containing compounds and NH ₃ and their implications on our knowledge of the chemistry of Titan's atmosphere and on the retrieval of the future Titan data expected from Cassini-Huygens.