行星内部地震波及其对卫星轨道的影响

Seismic wavefields in a planet and their influence on the satellite orbit

本文指出地震学在天文和行星学科里的重要作用.我们主要介绍最近提出的“潮汐—地震波共振”(tidal-seismic resonance)效应,并且讨论它对卫星轨道演化的作用.当在同步轨道以下周期运动的卫星引起的引潮力的频率和行星内部自由震荡频率吻合时,就会发生潮汐—地震波共振.此时,行星内部的地震波将被激发并引起行星表面的显著位移.升高和下降的地面会对卫星产生一个力矩从而使得卫星轨道下降.因为潮汐共振引起的动态地面位移可以比单纯引潮力引起的位移大两个数量级,所以潮汐共振会显著加速卫星下降速率.我们用我们开发的三维地震波场模拟程序AstroSeis数值计算了潮汐—地震波共振对轨道的影响,进而推测这一共振效应可能对行星早期吸积速度有显著影响.另外,因为行星内部的Q值和S波的波速对潮汐共振影响很大,未来研究微重力环境下的小行星或陨石内部地震波的速度和Q值对研究行星演化和太阳系的形成至关重要.

Seismology can play an important role in both astronomy and planetary science.We will mainly discuss the recently proposed Tidal-Seismic Resonance(TSR)effect and its role on the orbital evolution of the moon around a planet.For a planet-moon system,TSR is expected when a tidal force frequency matches a free-oscillation frequency of the planet.TSR can excite large-amplitude seismic waves that can change the shape of the planet,which in turn,exerts a negative torque on the moon to cause it to fall rapidly toward the planet.We have developed a 3-D seismic wavefield modeling package,AstroSeis,to numerically study the moon's orbit change.We further speculate that TSR might be an important mechanism to accelerate the planet accretion process in the early history of star formation.Because TSR significantly depends on the seismic Q value and S-wave velocity of the planet when its size was still small,future work studying the seismic wave velocity and Q of some asteroids in micro-gravity environments is important to reveal the evolution history of planets and the solar system.