Titan’s icy shell is expected to contain predominantly methane clathrate hydrates, water ice I_h, and possibly ammonia hydrates, beneath a cover of diverse organics formed via atmospheric photochemistry. The dissociation of clathrate hydrates has long been inferred as a potential replenishment mechanism for atmospheric methane; however, pure methane clathrates would be stable all the way to the surface. The melting of ammonia hydrates and subsequent interaction with methane clathrates could favor the dissociation of clathrates at much lower temperatures, due to the strong antifreeze effect of ammonia. To better understand the phase behavior of clathrate hydrates in the presence of ammonia, we have developed phase diagrams for the ternary system water–ammonia–tetrahydrofuran at 1 bar and in the temperature range 77–280 K via differential scanning calorimetry and Raman spectroscopy. We have been able to determine how ammonia promotes the start of a partial dissociation of THF–clathrates at temperatures far colder than the liquidus. We have also established that this ternary system exhibits a complex chemistry, with multiple phases forming in thermodynamic equilibrium because of a phase separation between a THF-dominated liquid and a H₂O–NH₃ dominated liquid. In addition to the expected THF–clathrates, we report the formation of other mineral phases such as ammonia hydrates, a new THF–NH₃-rich phase, and potentially mixed THF–NH₃ clathrates. Partial dissociation of ∼10% of the clathrate reservoir would release to Titan’s atmosphere methane amounts sufficient to sustain the hydrocarbon cycle for 650 My, which is commensurate with the age of the present atmosphere.

中文翻译：

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低温下三元水-四氢呋喃-氨系统的相图。Titan的包合物水合物和除气的含义

泰坦的冰壳，预计主要含有甲烷气水包合物水合物，水冰我_{^ h}以及可能由大气水化学形成的多种有机物覆盖下的氨水合物。长期以来，人们一直认为笼形水合物的分解是大气中甲烷的一种潜在补充机制。但是，纯甲烷包合物一直到表面都是稳定的。由于氨具有很强的防冻作用，氨水合物的熔化以及随后与甲烷笼形物的相互作用可能有利于笼形物在较低的温度下解离。为了更好地理解氨存在下笼形水合物的相态行为，我们通过差示扫描量热法和拉曼光谱学开发了1 bar且温度范围为77–280 K的三元系统水－氨水－四氢呋喃的相图。我们已经能够确定在比液相线低得多的温度下，氨如何促进THF-clathrates的部分解离。我们还确定，该三元体系具有复杂的化学性质，由于THF为主的液体与H的相分离，在热力学平衡中形成了多个相。₂ O–NH ₃为主的液体。除了预期的四氢呋喃盐包合物，我们还报告了其他矿物相的形成，例如氨水合物，一种新的富四氢呋喃盐-NH ₃相以及可能混合的四氢呋喃盐-NH ₃包合物。约10％的笼状储层的部分解离将向Titan大气释放足以维持650 My碳氢化合物循环的甲烷量，这与当前大气层的年龄相称。