Origin of the Lunar Inclination from Tidal Interaction of Multiple-Moon System

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 1° when the Moon formed. However, the current lunar orbital inclination with 5° relative to the Earth’s orbital plane requires the Moon’s orbital inclination relative to the Earth’s equator to be about 10° 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 in the surviving moon’s orbital inclination relative to Earth’s equator exceeding 10°. The theory proposed here may provide a way of explaining the initial large lunar inclination relative to the Earth’s equator.