Production yields of organics of astrobiological interest from H2O-NH3 hydrolysis of Titan's tholins
Résumé
Titan's environment hosts a complex organic chemistry that can be investigated from the Earth-based laboratory experiments. One of the key astrobiological questions of Titan's environment is the fate of the organic aerosols produced in the atmosphere, after they are deposited on the surface. In the present study, we report the geological structures these aerosols may encounter in the light of the last observations of the Cassini-Huygens mission. We chose to quantify the production of some astrobiologically interesting molecules in putative ammonia-water bodies likely to be present at the surface or subsurface of Titan, as crater melt pools or cryolavas. We synthesized laboratory analogs of Titan's aerosols from a continuous flow of a N2:CH4 (98:2) gas mixture irradiated by a cold plasma discharge. Aerosol analogs were recovered, partitioned in several 10.0 mg samples and placed in pure water and in 25 wt% ammonia solutions during ten weeks at 253 and 279 K, respectively. A derivatization reaction was performed on the residues obtained after evaporation of the hydrolysis solution using MTBSTFA in DMF as a reagent. Then some resulting products were identified and quantified using GC-MS analysis. Tholins were found to be very reactive toward an oxygen source. Urea was identified as the main product of Titan's tholins hydrolysis in ammonia-water solutions, with a production yield in mass, ranging from 6% to 12% at 279 K after 10 weeks. Several amino acids--alanine, glycine and aspartic acid--and perhaps the uracil nucleobase were also produced with yields from 0.001% to 0.4%. The determination of production yields carried out by the present study is a major step into the characterization of potential aerosols evolution on Titan.