Origin of the Moon's orbital inclination from resonant disk interactions

The Moon is generally believed to have formed from the debris disk created by a large body colliding with the early Earth. Recent models of this process predict that the orbit of the newly formed Moon should be in, or very near, the Earth's equatorial plane. This prediction, however, is at odds with the known history of the lunar orbit: the orbit is currently expanding, but can be traced back in time to reveal that, when the Moon formed, its orbital inclination relative to the Earth's equator was I approximately = 10 degrees. The cause of this initial inclination has been a mystery for over 30 years, as most dynamical processes (such as those that act to flatten Saturn's rings) will tend to decrease orbital inclinations. Here we show that the Moon's substantial orbital inclination is probably a natural result of its formation from an impact-generated disk. The mechanism involves a gravitational resonance between the Moon and accretion-disk material, which can increase orbital inclinations up to approximately 15 degrees.