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arXiv 1005.3523astro-ph.EPastro-ph.SR
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岩石系外行星中磁发电机的热力学极限

Thermodynamic Limits on Magnetodynamos in Rocky Exoplanets

Eric Gaidos, Clinton P. Conrad, Michael Manga, John Hernlund

备注：
Accepted to The Astrophysical Journal
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摘要：

为了确定磁性发电机是否在比地球更大或更热的岩石系外行星中运行，我们开发了一个分化的岩石行星及其热演化的参数模型。我们的模型再现了目前地球内部和磁场的既定性质。当应用于金星时，假设行星缺乏板块构造，并且具有粘度升高的脱水地幔，该模型显示发电机关闭或从未运行。我们的模型预测，在一个固定的行星质量，发电机的历史是敏感的核心大小，但不长寿命的，发热的放射性核素的初始库存。它预测，大于2.5个地球质量的岩石行星将不会发展内核，因为铁固相线的温度-压力斜率变得比核心的温度-压力斜率更平坦。相反，铁“雪”将在这些核心附近或顶部凝结，向上的潜热净转移将抑制对流和发电机。更大质量的行星可能由于核心冷却而具有贫血的发电机，但前提是它们具有移动的盖子（板块构造）。这些发电机的寿命随着行星质量的增加而缩短，但随着地面温度的升高而延长。像金星一样的大质量行星，有着停滞的盖子和更粘稠的地幔，将完全缺乏发电机。我们确定了岩石行星上的磁场的两个替代来源：在非常短的周期轨道上的行星的热或熔融的上层引起的涡电流，和发电机的离子导电层的“海洋”行星与水（冰）的上地幔中的~10%的质量。

英文摘要：

To ascertain whether magnetic dynamos operate in rocky exoplanets more massive or hotter than the Earth, we developed a parametric model of a differentiated rocky planet and its thermal evolution. Our model reproduces the established properties of Earth's interior and magnetic field at the present time. When applied to Venus, assuming that planet lacks plate tectonics and has a dehydrated mantle with an elevated viscosity, the model shows that the dynamo shuts down or never operated. Our model predicts that at a fixed planet mass, dynamo history is sensitive to core size, but not to the initial inventory of long-lived, heat-producing radionuclides. It predicts that rocky planets larger than 2.5 Earth masses will not develop inner cores because the temperature-pressure slope of the iron solidus becomes flatter than that of the core adiabat. Instead, iron "snow" will condense near or at the top of these cores, and the net transfer of latent heat upwards will suppress convection and a dynamo. More massive planets can have anemic dynamos due to core cooling, but only if they have mobile lids (plate tectonics). The lifetime of these dynamos is shorter with increasing planet mass but longer with higher surface temperature. Massive Venus-like planets with stagnant lids and more viscous mantles will lack dynamos altogether. We identify two alternative sources of magnetic fields on rocky planets: eddy currents induced in the hot or molten upper layers of planets on very short period orbits, and dynamos in the ionic conducting layers of "ocean" planets with ~10% mass in an upper mantle of water (ice).

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