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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