Abstract
Abstract A giant-impact origin for the Moon is generally accepted, but many aspects of lunar formation remain poorly understood and debated. Ćuk et al. proposed that an impact that left the Earth–Moon system with high obliquity and angular momentum could explain the Moon’s orbital inclination and isotopic similarity to Earth. In this scenario, instability during the Laplace Plane transition, when the Moon’s orbit transitions from the gravitational influence of Earth’s figure to that of the Sun, would both lower the system’s angular momentum to its present-day value and generate the Moon’s orbital inclination. Recently, Tian & Wisdom discovered new dynamical constraints on the Laplace Plane transition and concluded that the Earth–Moon system could not have evolved from an initial state with high obliquity. Here we demonstrate that the Earth–Moon system with an initially high obliquity can evolve into the present state, and we identify a spin–orbit secular resonance as a key dynamical mechanism in the later stages of the Laplace Plane transition. Some of the simulations by Tian & Wisdom did not encounter this late secular resonance, as their model suppressed obliquity tides and the resulting inclination damping. Our results demonstrate that a giant impact that left Earth with high angular momentum and high obliquity (θ > 61°) is a promising scenario for explaining many properties of the Earth–Moon system, including its angular momentum and obliquity, the geochemistry of Earth and the Moon, and the lunar inclination.
Highlights
The Moon is widely thought to have formed in the aftermath of a giant impact between the proto-Earth and another planetary body close to the end of terrestrial planet formation (Hartmann & Davis 1975; Cameron & Ward 1976; Canup & Asphaug 2001; Asphaug 2014; Barr 2016; Lock et al 2020)
The sequence of secular resonances encountered in the simulation shown in Figure 3, ending with a low-obliquity Earth, is typical of most of our integrations that have explored starting with a compact Earth–Moon system that has the present-day ecliptic angular momentum (AM) and obliquities of 62° or 65°, with various tidal properties of Earth and the Moon
We confirm the finding of Tian & Wisdom (2020) that the ecliptic AM of the Earth–Moon system is approximately conserved during the LPT
Summary
The Moon is widely thought to have formed in the aftermath of a giant impact between the proto-Earth and another planetary body close to the end of terrestrial planet formation (Hartmann & Davis 1975; Cameron & Ward 1976; Canup & Asphaug 2001; Asphaug 2014; Barr 2016; Lock et al 2020). Ćuk et al (2016) proposed that the tidal evolution of the Moon from a high-obliquity, high-AM Earth could result in both a large early lunar inclination (about 30°) and the transfer of AM from the Earth–Moon system to the Earth–Sun system. These dynamical effects are due to the instability of the orbits. We correct and improve the numerical integrator developed by Ćuk et al (2016) and use it to explore the dynamics of an Earth–Moon system with a high initial AM and obliquity, and determine whether this evolution can result in today’s observed configuration
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