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arXiv 1012.3706astro-ph.EPastro-ph.SR
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长期混沌理论与水星轨道

Theory of Secular Chaos and Mercury's Orbit

Yoram Lithwick, Yanqin Wu

摘要:

我们研究了行星系的混沌轨道演化,重点是长期(即,轨道平均)相互作用,因为这些通常在长时间尺度上占主导地位。我们首先关注一个被多个大质量行星强迫的测试粒子的演化。它的偏心率和倾角都是线性的,轨道以恒定的频率进动。但是非线性可以使频率与行星的本征频率或这些频率的线性组合产生长期共振。这些非线性长期共振的重叠驱动长期混沌在行星系。我们首次通过计算非线性长期共振的位置和宽度来量化所产生的动态。当解析计算和数值积分的结果一起显示在新开发的“平均动量图”(MMM)中时,一致性是极好的。这张图对于非共面的行星系特别有启发性,并以图形的方式展示了混沌是从重叠的长期共振中出现的。然后,我们将这种新发现的理解应用于水星。以前的数值模拟已经确定水星的轨道是混乱的,水星甚至可能与金星或太阳相撞。我们表明,水星的混乱主要是由四种模式的组合,水星占主导地位的偏心率模式,金星占主导地位的倾角模式和水星的自由偏心率和倾角的共振之间的重叠。对太阳系的数值积分证实,这些共振角交替地振动和循环。我们能够计算出水星变得混乱的阈值:木星和金星必须有几个百分点的偏心率和倾斜度。水星似乎栖息在混乱的门槛上。

英文摘要:

We study the chaotic orbital evolution of planetary systems, focusing on secular (i.e., orbit-averaged) interactions, because these often dominate on long timescales. We first focus on the evolution of a test particle that is forced by multiple massive planets. To linear order in eccentricity and inclination, its orbit precesses with constant frequencies. But nonlinearities can shift the frequencies into and out of secular resonance with the planets' eigenfrequencies, or with linear combinations of those frequencies. The overlap of these nonlinear secular resonances drive secular chaos in planetary systems. We quantify the resulting dynamics for the first time by calculating the locations and widths of nonlinear secular resonances. When results from both analytical calculations and numerical integrations are displayed together in a newly developed "map of the mean momenta" (MMM), the agreement is excellent. This map is particularly revealing for non-coplanar planetary systems and demonstrates graphically that chaos emerges from overlapping secular resonances. We then apply this newfound understanding to Mercury. Previous numerical simulations have established that Mercury's orbit is chaotic, and that Mercury might even collide with Venus or the Sun. We show that Mercury's chaos is primarily caused by the overlap between resonances that are combinations of four modes, the Jupiter-dominated eccentricity mode, the Venus-dominated inclination mode and Mercury's free eccentricity and inclination. Numerical integration of the Solar system confirms that a slew of these resonant angles alternately librate and circulate. We are able to calculate the threshold for Mercury to become chaotic: Jupiter and Venus must have eccentricity and inclination of a few percent. Mercury appears to be perched on the threshold for chaos.

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