The predictivity of photochemical models of Titan's atmosphere depends strongly on the precision and accuracy of reaction rates. For many reactions, large uncertainty results from the extrapolation of rate laws to low temperatures. A few reactions have been measured directly at temperatures relevant to Titan's atmosphere. In the present study, we observed the consequences of the reduced uncertainty attributed to these reactions. The global predictivity of the model was improved, i.e., most species are predicted with lower uncertainty factors. Nevertheless, high uncertainty factors are still observed, and a new list of key reactions has been established.

The discovery of large ( > 100 u) molecules in Titan's upper atmosphere has heightened astrobiological interest in this unique satellite. In particular, complex organic aerosols produced in atmospheres containing C, N, O, and H, like that of Titan, could be a source of prebiotic molecules. In this work, aerosols produced in a Titan atmosphere simulation experiment with enhanced CO (N 2 /CH 4 /CO gas mixtures of 96.2%/2.0%/1.8% and 93.2%/5.0%/ 1.8%) were found to contain 18 molecules with molecular formulae that correspond to biological amino acids and nucleotide bases. Very high-resolution mass spectrometry of isotopically labeled samples confirmed that C 4 H 5 N 3 O, C 4 H 4 N 2 O 2 , C 5 H 6 N 2 O 2 , C 5 H 5 N 5 , and C 6 H 9 N 3 O 2 are produced by chemistry in the simulation chamber. Gas chromatography-mass spectrometry (GC-MS) analyses of the non-isotopic samples confirmed the presence of cytosine (C 4 H 5 N 3 O), uracil (C 5 H 4 N 2 O 2 ), thymine (C 5 H 6 N 2 O 2 ), guanine (C 5 H 5 N 5 O), glycine (C 2 H 5 NO 2 ), and alanine (C 3 H 7 NO 2 ). Adenine (C 5 H 5 N 5 ) was detected by GC-MS in isotopically labeled samples. The remaining prebiotic molecules were detected in unlabeled samples only and may have been affected by contamination in the chamber. These results demonstrate that prebiotic molecules can be formed by the highenergy chemistry similar to that which occurs in planetary upper atmospheres and therefore identifies a new source of prebiotic material, potentially increasing the range of planets where life could begin.

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In this work Titan's atmospheric chemistry is simulated using a capacitively coupled plasma radio frequency discharge in a N(2)-CH(4) stationnary flux. Samples of Titan's tholins are produced in gaseous mixtures containing either 2 or 10% methane before the plasma discharge, covering the methane concentration range measured in Titan's atmosphere. We study their solubility and associated morphology, their infrared spectroscopy signature and the mass distribution of the soluble fraction by mass spectrometry. An important result is to highlight that the previous Titan's tholin solubility studies are inappropriate to fully characterize such a heterogeneous organic matter and we develop a new protocol to evaluate quantitatively tholins solubility. We find that tholins contain up to 35% in mass of molecules soluble in methanol, attached to a hardly insoluble fraction. Methanol is then chosen as a discriminating solvent to characterize the differences between soluble and insoluble species constituting the bulk tholins. No significant morphological change of shape or surface feature is derived from scanning electron microscopy after the extraction of the soluble fraction. This observation suggests a solid structure despite an important porosity of the grains. Infrared spectroscopy is recorded for both fractions. The IR spectra of the bulk, soluble, and insoluble tholins fractions are found to be very similar and reveal identical chemical signatures of nitrogen bearing functions and aliphatic groups. This result confirms that the chemical information collected when analyzing only the soluble fraction provides a valuable insight representative of the bulk material. The soluble fraction is ionized with an atmospheric pressure photoionization source and analyzed by a hybrid mass spectrometer. The congested mass spectra with one peak at every mass unit between 50 and 800 u confirm that the soluble fraction contains a complex mixture of organic molecules. The broad distribution, however, exhibits a regular pattern of mass clusters. Tandem collision induced dissociation analysis is performed in the negative ion mode to retrieve structural information. It reveals that (i) the molecules are ended by methyl, amine and cyanide groups, (ii) a 27 u neutral moiety (most probably HCN) is often released in the fragmentation of tholin anions, and (iii) an ubiquitous ionic fragment at m/z 66 is found in all tandem spectra. A tentative structure is proposed for this negative ion.

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[1] Titan's upper atmosphere produces an ionosphere at high altitudes from photoionization and electron impact that exhibits complex chemical processes in which hydrocarbons and nitrogen-containing molecules are produced through ion-molecule reactions. The structure and composition of Titan's ionosphere has been extensively investigated by the Ion and Neutral Mass Spectrometer (INMS) onboard the Cassini spacecraft. We present a detailed study using linear correlation analysis, 1-D photochemical modeling, and empirical modeling of Titan's dayside ionosphere constrained by Cassini measurements. The 1-D photochemical model is found to reproduce the primary photoionization products of N 2 and CH 4 . The major ions, CH 5 + , C 2 H 5 + , and HCNH + are studied extensively to determine the primary processes controlling their production and loss. To further investigate the chemistry of Titan's ionosphere we present an empirical model of the ion densities that calculates the ion densities using the production and loss rates derived from the INMS data. We find that the chemistry included in our model sufficiently reproduces the hydrocarbon species as observed by the INMS. However, we find that the chemistry from previous models appears insufficient to accurately reproduce the nitrogen-containing organic compound abundances observed by the INMS. The major ion, HCNH + , is found to be overproduced in both the empirical and 1-D photochemical models. We analyze the processes producing and consuming HCNH + in order to determine the cause of this discrepancy. We find that a significant chemical loss process is needed. We suggest that the loss process must be with one of the major components, namely C 2 H 2 , C 2 H 4 , or H 2 .

The polymeric composition of Titan's tholins--laboratory analogues of Titan's aerosols--is elucidated using high-resolution mass spectrometry. This complex organic matter is produced by plasma discharge in a gaseous nitrogen-methane mixture and analyzed with a hybrid linear trap/orbitrap mass-spectrometer. The highly structured mass spectra are treated with tools developed for petroleomics (Kendrick and van Krevelen diagrams), with original adaptations for nitrogen-rich compounds. Our goal is to find the best chemical basis set to describe the compositional space that these polymers occupy, to shed light onto the chemical structure of tholins. We succeeded in assigning the molecules identified in the mass spectra of tholins to a small number of regularly distributed X-(CH(2))(m)(HCN)(n) families, where the balanced copolymer (m = n) is determined to play a central role. Within each family, the polymer lengths n and m present Poisson-type distributions. We also identify the smallest species of a subset of families as linear and cyclic amino nitrile compounds of great astrobiological interest: biguanide, guanidin, acetamidine, aminoacetonitrile, and methylimidazole.