A simple model of the chaotic eccentricity of Mercury

TL;DR

This paper demonstrates that a simple integrable model with one degree of freedom can replicate Mercury's chaotic eccentricity variations, highlighting the influence of secular resonances and Venus' orbit excitation.

Contribution

It introduces a simplified integrable model that captures Mercury's eccentricity chaos without complex multi-planet interactions.

Findings

The model reproduces the amplitude and timescale of Mercury's eccentricity variations.

Chaos arises near the g1-g5 resonance between Mercury and Jupiter.

Venus' orbit excitation at Jupiter's precession frequency significantly influences Mercury's eccentricity.

Abstract

Mercury's eccentricity is chaotic and can increase so much that collisions with Venus or the Sun become possible (Laskar, 1989, 1990, 1994, 2008, Batygin & Laughlin, 2008, Laskar & Gastineau, 2009). This chaotic behavior results from an intricate network of secular resonances, but in this paper, we show that a simple integrable model with only one degree of freedom is actually able to reproduce the large variations in Mercury's eccentricity, with the correct amplitude and timescale. We show that this behavior occurs in the vicinity of the separatrices of the resonance g1-g5 between the precession frequencies of Mercury and Jupiter. However, the main contribution does not come from the direct interaction between these two planets. It is due to the excitation of Venus' orbit at Jupiter's precession frequency g5. We use a multipolar model that is not expanded with respect to Mercury's eccentricity, but because of the proximity of Mercury and Venus, the Hamiltonian is expanded up to order 20 and more in the ratio of semimajor axis. When the effects of Venus' inclination are added, the system becomes nonintegrable and a chaotic zone appears in the vicinity of the separatrices. In that case, Mercury's eccentricity can chaotically switch between two regimes characterized by either low-amplitude circulations or high-amplitude librations.