Large lakes of liquid methane nestle between mountain ranges of solid water ice in the polar regions of Jupiter’s moon Titan (1, 2). This strange world illustrates in a quite dramatic fashion that the isoelectronic CH₄ and H₂O molecules display profoundly different physical properties including a 182 °C difference in their melting points at ambient pressure. Unlike methane, water molecules form strong hydrogen bonds with up to 4 neighbors, which explains the high melting point of ice compared to that of solid methane. Rearrangements of those hydrogen bonds, which take place as temperature and pressure are varied, give rise to a large family of complex network structures beyond the “ordinary” hexagonal form of ice, ice Ih (3). However, the structural diversity of H₂O does not end with the pure phases of ice. Water molecules can form cages around hydrophobic species such as methane to form clathrate hydrates (4, 5). These important inclusion compounds have been suggested as model systems for studying hydrophobic interactions (4), and they are also relevant for a wide range of industrial, geological, atmospheric, and cosmological settings (6, 7). Methane clathrate hydrate (MH) is one of the most thoroughly studied materials in this context with 3 distinct structural forms identified so far experimentally at different pressures (5). Schaack et al. (8) now report in PNAS the existence of a fourth hydrate of methane (MH-IV) that forms above ∼40 GPa and remains stable up to at least 150 GPa. Intriguingly, the water network of MH-IV takes on a very familiar form, that of ice Ih, but it is densely packed with methane molecules at a 2:1 H₂O:CH₄ ratio.

中文翻译：

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水和甲烷在极高的压力下保持在一起。

液态甲烷雀巢的大湖泊木星的卫星泰坦（极区固体水冰山脉之间1，2）。这个奇异的世界以一种非常戏剧化的方式说明了，等电子CH ₄和H ₂ O分子显示出巨大的物理特性，包括在环境压力下其熔点相差182°C。与甲烷不同，水分子与多达4个邻域形成强大的氢键，这解释了冰与固体甲烷相比的高熔点。这些氢键，这发生温度和压力的重排是多种多样的，产生一个大家族复杂的网络结构的超出冰的“普通”的六边形形式，冰我ħ（3）。但是，H ₂ O的结构多样性并没有以冰的纯相结束。水分子可形成围绕疏水性物质如甲烷以形成笼形水合物（笼4，5）。这些重要的包合物已被建议作为模型系统用于研究疏水性相互作用（4），并且它们也相关于广泛的工业，地质，大气，和宇宙的设置（6，7）。甲烷笼形水合物（MH）是在此情况下研究最深入的材料之一，到目前为止，在不同压力下通过实验确定了3种不同的结构形式（5）。Schaack等。（8）现在在PNAS中报告了甲烷的第四种水合物（MH-IV）的存在，其形成量约为40 GPa以上，并在至少150 GPa的范围内保持稳定。有趣的是，MH-IV的水网络呈非常熟悉的形式，即冰I h，但它以2：1 H ₂ O：CH _4的比率密密麻麻地充满了甲烷分子。