标题： 次海王星比看起来更干燥：重新思考富水世界的起源 显示英文标题

标题： Sub-Neptunes Are Drier Than They Seem: Rethinking the Origins of Water-Rich Worlds

摘要： 最近关于亚海王星大气中的生物标志气体的声明重新引发了对拥有表面海洋的富水亚海王星的兴趣，通常被称为Hycean行星。这些行星被认为是在雪线之外形成的，在向内迁移之前积累了大量H$_2$O（超过10重量%）。然而，目前的内部模型常常忽略了原始大气与熔融内部之间的化学平衡。在这里，我们计算了具有岩浆海洋的亚海王星合成种群的全球化学平衡状态。尽管许多最初积聚了5-30重量%的水，但内部-大气相互作用破坏了大部分水，将最终的H$_2$O质量分数降低到1.5重量%以下。因此，没有一个符合Hycean行星的阈值。尽管如此，我们发现仅在积聚最少冰的行星上存在H$_2$O主导的大气。这些行星在雪线内形成，碳和氢含量贫乏，并发展出质量分数低于1%的小包层，主要由内生水组成。相比之下，在雪线之外形成的行星积聚了更多的挥发物，但它们的水大多转化为H$_2$气体或被隔离到内部，导致大气中H$_2$O质量分数较低。大多数H$_2$O丰富的包层也与H$_2$完全互溶，使得单独的水层不太可能。 我们的结果推翻了冰层积聚与富水大气之间的传统联系，而是表明，H$_2$O主导的包层是通过在雪线内形成的氢贫行星中的化学平衡产生的。 显示英文摘要

摘要： Recent claims of biosignature gases in sub-Neptune atmospheres have renewed interest in water-rich sub-Neptunes with surface oceans, often referred to as Hycean planets. These planets are hypothesized to form beyond the snow line, accreting large amounts of H$_2$O >10 wt%) before migrating inward. However, current interior models often neglect chemical equilibration between primordial atmospheres and molten interiors. Here, we compute global chemical equilibrium states for a synthetic population of sub-Neptunes with magma oceans. Although many initially accrete 5-30 wt% water, interior-atmosphere interactions destroy most of it, reducing final H$_2$O mass fractions to below 1.5 wt%. As a result, none meet the threshold for Hycean planets. Despite that, we find H$_2$O-dominated atmospheres exclusively on planets that accreted the least ice. These planets form inside the snow line, are depleted in carbon and hydrogen, and develop small envelopes with envelope mass fractions below 1%, dominated by endogenic water. In contrast, planets formed beyond the snow line accrete more volatiles, but their water is largely converted to H$_2$ gas or sequestered into the interior, resulting in low atmospheric H$_2$O mass fractions. Most H$_2$O-rich envelopes are also fully miscible with H$_2$, making a separate water layer unlikely. Our results topple the conventional link between ice accretion and water-rich atmospheres, showing instead that H$_2$O-dominated envelopes emerge through chemical equilibration in hydrogen-poor planets formed inside the snow line.