Origin of the lunar inclination from tidal interaction of multiple-moon system

TL;DR

This paper proposes that tidal interactions in a multiple-moon system under solar perturbation can explain the Moon's current high orbital inclination, which is difficult to account for with previous models.

Contribution

It introduces a new theory that tidal interactions among multiple moons can lead to inclination changes, explaining the Moon's current orbital tilt.

Findings

Tidal interactions can cause moon mergers or ejections.

Surviving moons can have inclinations exceeding ten degrees.

This mechanism explains the Moon's initial large inclination.

Abstract

According to the giant impact theory, the Moon formed through accreting the debris disk produced by a collision between Theia and the proto-Earth, and the predicted lunar orbital inclination relative to the Earth's equatorial plane is about within one degree when Moon formed. However, the current lunar orbital inclination with five degrees relative to the Earth's orbital plane requires the Moon's orbital inclination relative to the Earth's equator to be about ten degrees when traced back to the time of lunar formation. Since two moons are also a natural outcome of simulations of lunar formation from a protolunar disk produced by a giant impact, here we show that, under solar perturbation, gravitational tidal interaction between Earth and its two moons with negligible orbital inclination relative to Earth's equatorial plane could lead to a merger of one moon with Earth, or a merger of the two moons or an ejection of one moon, resulting that the surviving moon's orbital inclination relative to Earth's equator could exceed ten degrees. The theory proposed here may provide a way of explaining the initial large lunar inclination relative to the Earth's equator.