Militzer Burkhard
Department of Earth and Planetary Science, University of California, Berkeley, CA 94720.
Department of Astronomy, University of California, Berkeley, CA 94720.
Proc Natl Acad Sci U S A. 2024 Dec 3;121(49):e2403981121. doi: 10.1073/pnas.2403981121. Epub 2024 Nov 25.
The Voyager spacecraft discovered that the ice giants Uranus and Neptune have nondipolar magnetic fields, defying expectations that a thick interior layer of planetary ices would generate strong dipolar fields. Stanley and Bloxham showed that nondipolar fields emerge if the magnetic field is only generated in a thin outer layer. However, the origin and composition of this dynamo active layer has so far remained elusive. Here, we show with ab initio computer simulations that a mixture of HO, CH, and NH will phase separate under the pressure-temperature condition in the interiors of Uranus and Neptune, forming a HO-dominated fluid in the upper mantle and a CH-NH mixture below. We further demonstrate that with increasing pressure, the CH-NH mixture becomes increasingly hydrogen depleted as it assumes the state of a polymeric C-N-H fluid. Since the amount of hydrogen loss increases with pressure, we propose that the C-N-H fluid forms a stably stratified layer. The magnetic fields are primarily generated in an upper layer that is HO-rich, homogeneous, convective, and electrically conducting. Under these assumptions, we construct ensembles of models for the interiors of Uranus and Neptune with the Concentric MacLaurin Spheroid method. We demonstrate that the phase separation of the solar-type HO-CH-NH mixture leads to models that match the observed gravity field and to layer thicknesses that are compatible with magnetic field measurements.
“旅行者号”航天器发现，冰巨行星天王星和海王星具有非偶极磁场，这与行星冰层的厚内层会产生强偶极磁场的预期相悖。斯坦利和布洛克斯汉姆指出，如果磁场仅在外层薄区域产生，非偶极磁场就会出现。然而，这种发电机活动层的起源和组成迄今仍不明确。在此，我们通过从头算计算机模拟表明，在天王星和海王星内部的压力 - 温度条件下，HO、CH和NH的混合物会发生相分离，在上地幔形成以HO为主的流体，在其下方形成CH - NH混合物。我们进一步证明，随着压力增加，CH - NH混合物在呈现聚合C - N - H流体状态时，氢含量会越来越低。由于氢损失量随压力增加，我们提出C - N - H流体形成一个稳定分层的层。磁场主要在富含HO、均匀、对流且导电的上层产生。基于这些假设，我们用同心麦克劳林球体方法构建了天王星和海王星内部的模型系综。我们证明，太阳型HO - CH - NH混合物的相分离会产生与观测到的重力场相匹配的模型，以及与磁场测量结果相符的层厚度。
