2024-11-25 カリフォルニア大学バークレー校(UCB)
＜関連情報＞
- https://news.berkeley.edu/2024/11/25/a-clue-to-what-lies-beneath-the-bland-surfaces-of-uranus-and-neptune/
- https://www.pnas.org/doi/10.1073/pnas.2403981121
惑星氷の相分離が天王星と海王星の非極性磁場を説明する Phase separation of planetary ices explains nondipolar magnetic fields of Uranus and Neptune
Burkhard Militzer
Proceedings of the National Academy of Sciences Published:November 25, 2024
DOI:https://doi.org/10.1073/pnas.2403981121
Significance
The Voyager spacecraft measured that Uranus and Neptune have nondipolar magnetic fields while strong dipole fields had been expected. Stanley and Bloxham thus proposed that the magnetic fields be generated only in a thin outer layer. Here, we predict what the materials in the interior layers are and why the lower layer is dynamo inactive. We demonstrate with ab initio simulations that planetary ices phase separate at high pressure into an upper, water-rich and a lower, hydrocarbon-dominated layer. The upper layer is convective and dynamo active while the lower layer is stably stratified. A signature of the stratification can be detected in normal modes, which lends support to placing a Doppler imager on a future Uranus mission.
Abstract
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 H2O, CH4, and NH3 will phase separate under the pressure–temperature condition in the interiors of Uranus and Neptune, forming a H2O-dominated fluid in the upper mantle and a CH4-NH3 mixture below. We further demonstrate that with increasing pressure, the CH4-NH3 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 H2O-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 H2O-CH4-NH3 mixture leads to models that match the observed gravity field and to layer thicknesses that are compatible with magnetic field measurements.
