Reaction between Hydrogen and Ferrous/Ferric Oxides at High Pressures and High Temperatures—Implications for Sub-Neptunes and Super-Earths
Reaction between Hydrogen and Ferrous/Ferric Oxides at High Pressures and High Temperatures—Implications for Sub-Neptunes and Super-Earths
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DOI：
10.3847/psj/acab03
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发表时间：
2023-02
影响因子：
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通讯作者：
H. W. Horn;V. Prakapenka;S. Chariton;S. Speziale;S. Shim
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文献类型：
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作者：
H. W. Horn;V. Prakapenka;S. Chariton;S. Speziale;S. Shim
海王星以下系外行星可能有厚厚的氢包层，因此在氢和下面的硅酸盐/金属之间形成了一个高压界面。一些亚海王星可能会通过大量的气体损失而转变为超级地球。如果氢在高压和高温下与氧化物和金属发生化学反应(P−T)，它可能会影响亚海王星和超级地球的核心和大气的结构和组成。虽然氢气在低压下是一种强大的还原剂，但在次海王星内部的P−T上，氢的行为是未知的，那里的氢是一种稠密的超临界流体。本文报道了利用脉冲激光加热的金刚石压腔与同步辐射X射线衍射相结合的方法，在20-40 Gpa和1000-4000K的温度范围内，对亚铁/铁氧化物与氢反应的实验结果。在这些条件下，氢会自发地将铁从氧化物中剥离，形成Fe-H合金，并将氧气释放到氢介质中。在可能发生这种反应的行星环境中，Fe-H合金可能下沉到地核的金属部分，而释放的氧可能以水的形式稳定在硅酸盐层中，提供了一种机制，以氢的形式进入海王星以下的深部。氧化还原反应产生的水也可以分配到海王星以下的大气中，这对了解其大气的组成具有重要意义。此外，从海王星以下转化而来的超级地球内部可能含有大量的氢和水(至少有几个wt%H2O)。这与较小的岩石行星不同，它们的形成相对干燥(可能是几百分之一重量%的H2O)。
Sub-Neptune exoplanets may have thick hydrogen envelopes and therefore develop a high-pressure interface between hydrogen and the underlying silicates/metals. Some sub-Neptunes may convert to super-Earths via massive gas loss. If hydrogen chemically reacts with oxides and metals at high pressures and temperatures (P−T), it could impact the structure and composition of the cores and atmospheres of sub-Neptunes and super-Earths. While H2 gas is a strong reducing agent at low pressures, the behavior of hydrogen is unknown at the P−T expected for sub-Neptunes’ interiors, where hydrogen is a dense supercritical fluid. Here we report experimental results of reactions between ferrous/ferric oxides and hydrogen at 20–40 GPa and 1000–4000 K utilizing the pulsed laser-heated diamond-anvil cell combined with synchrotron X-ray diffraction. Under these conditions, hydrogen spontaneously strips iron off the oxides, forming Fe-H alloys and releasing oxygen to the hydrogen medium. In a planetary context where this reaction may occur, the Fe-H alloy may sink to the metallic part of the core, while released oxygen may stabilize as water in the silicate layer, providing a mechanism to ingas hydrogen to the deep interiors of sub-Neptunes. Water produced from the redox reaction can also partition to the atmosphere of sub-Neptunes, which has important implications for understanding the composition of their atmospheres. In addition, super-Earths converted from sub-Neptunes may contain a large amount of hydrogen and water in their interiors (at least a few wt% H2O). This is distinct from smaller rocky planets, which were formed relatively dry (likely a few hundredths wt% H2O).