The photochemistry of simple molecules containing carbon, hydrogen, nitrogen, and oxygen atoms in the atmosphere of Titan has been investigated using updated chemical schemes and our own estimates of a number of key rate coefficients. Proper exospheric boundary conditions, vertical transport, and condensation processes at the tropopause have been incorporated into the model. It is argued that he composition, climatology, and evolution of Titan's atmosphere are controlled by five major processes: (a) photolysis and photosensitized dissociation of CH4; (b) conversion of H to H2 and escape of hydrogen; (c) synthesis of higher hydrocarbons; (d) coupling between nitrogen and hydrocarbons; (e) coupling between oxygen and hydrocarbons. Starting with N2, CH4, and H2O, and invoking interactions with ultraviolet sunlight, energetic electrons, and cosmic rays, the model satisfactorily accounts for the concentrations of minor species observed by the Voyager IRIS and UVS instruments. Photochemistry is responsible for converting the simpler atmospheric species into more complex organic compounds, which are subsequently condensed at the tropopause and deposited on the surface. Titan might have lost 5.6 x 10(4), 1.8 x 10(3), and 4.0 g cm-2, or the equivalent of 8, 0.25, and 5 x 10(-4) bars of CH4, N2, and CO, respectively, over geologic time. Implications of abiotic organic synthesis on Titan for the origin of life on Earth are briefly discussed.

中文翻译：

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泰坦大气的光化学：模型和观测值之间的比较。

泰坦大气中包含碳，氢，氮和氧原子的简单分子的光化学已经使用更新的化学方案以及我们自己对许多关键速率系数的估计进行了研究。适当的大气圈边界条件，垂直传输和对流层顶的凝结过程已纳入模型。有人认为，泰坦大气的组成，气候和演化受五个主要过程控制：（a）CH4的光解和光敏解离；（b）H转化为H2并逸出氢；（c）合成高级烃；（d）氮与碳氢化合物之间的耦合；（e）氧与碳氢化合物之间的耦合。从N2，CH4和H2O开始，并引发与紫外线，高能电子的相互作用，和宇宙射线，该模型令人满意地说明了Voyager IRIS和UVS仪器观测到的次要物种的浓度。光化学负责将较简单的大气物质转化为更复杂的有机化合物，这些化合物随后在对流层顶上冷凝并沉积在表面上。Titan可能损失了5.6 x 10（4），1.8 x 10（3）和4.0 g cm-2，或相当于8、0.25和5 x 10（-4）条CH4，N2和CO的损失，分别在地质时期。简要讨论了土卫六上非生物有机合成对地球生命起源的影响。随后在对流层顶冷凝并沉积在表面上。Titan可能损失了5.6 x 10（4），1.8 x 10（3）和4.0 g cm-2，或相当于8、0.25和5 x 10（-4）条CH4，N2和CO的损失，分别在地质时期。简要讨论了土卫六上非生物有机合成对地球生命起源的影响。随后在对流层顶冷凝并沉积在表面上。Titan可能损失了5.6 x 10（4），1.8 x 10（3）和4.0 g cm-2，或相当于8、0.25和5 x 10（-4）条CH4，N2和CO的损失，分别在地质时期。简要讨论了土卫六上非生物有机合成对地球生命起源的影响。