Interior models of Uranus and Neptune often assume discrete layers, but sharp interfaces are expected only if major constituents are immiscible. Diffuse interfaces could arise if accretion favored a central concentration of the least volatile constituents (also, incidentally, the most dense); compositional gradients arising in such a structure would likely inhibit convection. Currently, two lines of evidence suggest possible hydrogen–water immiscibility in ice giant interiors. The first arises from crude extrapolation of the experimental H₂–H₂O critical curve to ∼3 GPa. The data are obtained for an impure system containing silicates, though Uranus and Neptune could also be “dirty.” Current ab initio models disagree (Soubiran & Militzer 2015), though hydrogen and water are difficult to model from first-principles quantum mechanics with the necessary precision. The second argument for H₂–H₂O immiscibility in ice giants, outlined herein, invokes reasoning about the gravitational and magnetic fields. While consensus remains lacking, here we examine the immiscible case. Applying the resulting thermodynamic constraints, we find that Neptune models with envelopes containing a substantial water mole fraction, as much as relative to hydrogen, can satisfy observations. In contrast, Uranus models appear to require , potentially suggestive of fully demixed hydrogen and water. Enough gravitational potential energy would be available from gradual hydrogen–water demixing to supply Neptune’s present-day heat flow for roughly 10 solar system lifetimes. Hydrogen–water demixing could slow Neptune’s cooling rate by an order of magnitude; different hydrogen–water demixing states could account for the different heat flows of Uranus and Neptune.

天王星和海王星的内部模型通常假设离散层，但只有在主要成分不混溶的情况下，才会出现尖锐的界面。如果吸积有利于挥发性最低的成分（顺便说一下，也是最密集的）的中心集中，则可能会出现扩散界面；在这种结构中出现的成分梯度可能会抑制对流。目前，有两条证据表明冰巨星内部可能存在氢-水不混溶性。第一个来自实验 H ₂ –H _{2 的}粗略推断O 临界曲线至~3 GPa。这些数据是针对含有硅酸盐的不纯系统获得​​的，尽管天王星和海王星也可能是“脏的”。当前的 ab initio 模型不同意（Soubiran & Militzer 2015），尽管氢和水很难从第一性原理量子力学以必要的精度建模。此处概述的关于冰巨星中H ₂ -H ₂ O 不混溶性的第二个论点引发了对引力场和磁场的推理。虽然仍然缺乏共识，但我们在这里研究了不兼容的情况。应用由此产生的热力学约束，我们发现海王星模型的包络包含大量水摩尔分数，与氢的摩尔分数一样多，可以满足观察。相比之下，天王星模型似乎需要，可能暗示氢和水完全分层。氢-水逐渐分层可以提供足够的重力势能，为大约 10 个太阳系生命周期提供海王星目前的热流。氢水分层可以将海王星的冷却速度降低一个数量级；不同的氢水分层状态可以解释天王星和海王星的不同热流。