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arXiv 2201.02976astro-ph.EP
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论富冰体的结构和长期演化

On the structure and long-term evolution of ice-rich bodies

Stephan Loveless, Dina Prialnik, Morris Podolak

摘要:

由于柯伊伯带天体和系外行星的发现,人们对富含冰的行星体的结构,特别是冰和岩石之间的区别越来越感兴趣。因此,我们进行了一系列的行星质量$M$的参数研究,产生半径$50 \aplt R \aplt 3000$ ~km,和岩石/冰质量比在0.25和4之间,在寒冷的环境中发展到4.5~Gyr,以获得目前的结构。我们使用的热演化模型,允许在多孔介质中的液体和蒸汽流动,解决流体静力平衡下的质量和能量守恒方程的球体在轨道上围绕一个中央恒星。该模型包括压力对孔隙度和熔融温度的影响,长寿命放射性同位素加热,以及温度依赖的蛇纹化和脱水。我们在参数空间[大小,岩石含量]中得到了形成岩石核的分化天体和未分化天体之间的边界:小天体,岩石含量低的天体,以及所考虑的最大天体,它们产生了高的内部压力,几乎没有达到熔化温度。最后分化的结构包括岩石核,富含冰的地幔,以及表面以下的薄而致密的地壳。我们得到并讨论了体积密度与半径的关系。研究了极冷环境的影响,发现在$\sim$ 20 K的环境温度下,小天体将冰以无定形形式保存到现在。

英文摘要:

The interest in the structure of ice-rich planetary bodies, in particular the differentiation between ice and rock, has grown due to the discovery of Kuiper belt objects and exoplanets. We thus carry out a parameter study for a range of planetary masses $M$, yielding radii $50 \aplt R \aplt 3000$~km, and for rock/ice mass ratios between 0.25 and 4, evolving them for 4.5~Gyr in a cold environment, to obtain the present structure. We use a thermal evolution model that allows for liquid and vapor flow in a porous medium, solving mass and energy conservation equations under hydrostatic equilibrium for a spherical body in orbit around a central star. The model includes the effect of pressure on porosity and on the melting temperature, heating by long-lived radioactive isotopes, and temperature-dependent serpentinization and dehydration. We obtain the boundary in parameter space [size, rock-content] between bodies that differentiate, forming a rocky core, and those which remain undifferentiated: small bodies, bodies with a low rock content, and the largest bodies considered, which develop high internal pressures and barely attain the melting temperature. The final differentiated structure comprises a rocky core, an ice-rich mantle, and a thin dense crust below the surface. We obtain and discuss the bulk density-radius relationship. The effect of a very cold environment is investigated and we find that at an ambient temperature of $\sim$20~K, small bodies preserve the ice in amorphous form to the present.

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