Tidal evolution of the Moon from a high-obliquity, high-angular-momentum Earth

Abstract

In the giant impact hypothesis for lunar origin, the Moon accreted from an equatorial circumterrestrial disk; however the current lunar orbital inclination of 5◦ requires a subsequent dynamical process that is still debated. In addition, the giant impact theory has been challenged by the Moon’s unexpectedly Earth-like isotopic composition. Here, we show that tidal dissipation due to lunar obliquity was an important effect during the Moon’s tidal evolution, and the past lunar inclination must have been very large, defying theoretical explanations. We present a new tidal evolution model starting with the Moon in an equatorial orbit around an initially fast-spinning, high-obliquity Earth, which is a probable outcome of giant impacts. Using numerical modeling, we show that the solar perturbations on the Moon’s orbit naturally induce a large lunar inclination and remove angular momentum from the Earth-Moon system. Our tidal evolution model supports recent high-angular momentum 1 giant impact scenarios to explain the Moon’s isotopic composition and provides a new pathway to reach Earth’s climatically favorable low obliquity.