Parent bodies of carbonaceous chondrites that initially contained metallic iron potentially exert strong reduction power during aqueous alteration to generate molecular hydrogen in excess of hydrogen solubility in water-rich fluids. The surplus hydrogen escapes from the system, which is subsequently supplied to overlying regions in planetesimals. Based on this concept, we conducted chemical equilibrium modeling of the aqueous alteration and simulated gaseous H migration within the icy planetesimal that has a melted mantle and an icy shell during the early stages of radiogenic heating. In the chemical equilibrium modeling, we simulated the aqueous alteration of chondritic rocks at 0–350 °C and a water/rock mass ratio of 0.2–10 with initial CO contents of 0–10 mol% in the fluid. The results showed that the mineral assemblage and solution composition change with the temperature, water/rock mass ratio, and initial fluid composition. The reproduced mineral paragenesis and abundance well explain those of carbonaceous chondrites. Furthermore, it was revealed that the initial H fugacity of the system influences not only the stability of minerals and solution compositions, but also the preservation potential of organic molecules. Indeed, within these parameter spaces, the modeling results account for the organic/inorganic carbon-rich alterations reported for the Tagish Lake meteorite, Ceres, and Ryugu. Simulations of gaseous H migration in a planetesimal revealed that gaseous H in the deep interior can be transported to the interface with an icy shell even if the permeability is low. Moreover, it is highly possible that an H-rich layer would have been widely formed just below the icy shell. Therefore, it is expected that H-rich regions beneath the ice layer in planetesimals have substantial potential for the synthesis and preservation of organic molecules. These results imply that the alteration of carbonaceous chondrite parent bodies and C-complex asteroids is characterized by not only the type of parent bodies (e.g., formation age and distance from the Sun) but also the locations within their parent bodies.

中文翻译：

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冰冷星子中的水相变化：气态氢向外传输的影响

最初含有金属铁的碳质球粒陨石母体在水蚀变过程中可能发挥强大的还原能力，产生超过富水流体中氢溶解度的分子氢。多余的氢从系统中逸出，随后被供应到星子的上覆区域。基于这个概念，我们对放射性加热早期阶段具有熔化地幔和冰壳的冰微星子内的水蚀变和模拟气态氢迁移进行了化学平衡建模。在化学平衡模型中，我们模拟了球粒岩在 0–350 °C 条件下的水蚀变，水/岩体比为 0.2–10，流体中初始 CO 含量为 0–10 mol%。结果表明，矿物组合和溶液成分随温度、水/岩体比和初始流体成分的变化而变化。再现的矿物共生和丰度很好地解释了碳质球粒陨石的共生和丰度。此外，系统的初始H逸度不仅影响矿物和溶液成分的稳定性，还影响有机分子的保存潜力。事实上，在这些参数空间内，建模结果解释了塔吉什湖陨石、谷神星和龙宫报告的富含有机/无机碳的变化。对星子中气态 H 迁移的模拟表明，即使渗透率较低，内部深处的气态 H 也可以输送到与冰壳的界面。此外，富氢层很可能在冰壳正下方广泛形成。因此，预计星子冰层下方的富氢区域对于有机分子的合成和保存具有巨大的潜力。这些结果意味着碳质球粒陨石母体和C复合小行星的变化不仅取决于母体的类型（例如形成年龄和距太阳的距离），而且还取决于其母体内的位置。