One of the Moon’s long-lasting mysteries might have finally been solved. Usually when satellites form from a disk of debris around the planet, they tend to orbit very close to the equatorial plane. The Moon doesn't seem to have to got that memo, though, and it orbits with an inclination 10 times larger than what is expected according to theory.
The Moon's inclination is measured with respect to the Earth’s orbital plane, also called the ecliptic. The Earth’s axis is tilted at 23.4° with respect to the ecliptic. The Moon’s tilt should be close to the Earth’s value, but it is actually only 5° from the orbital plane.
A solution to this discrepancy is outlined in a new study published in Nature. The Moon did form close to the equatorial plane of the Earth and then interactions with other small bodies falling on Earth created a wobble in the Moon which pushed it to its current inclination, according to the study.
Credit: NASA's Sci-jinks
The Moon formed when an object the size of Mars hit the Earth about 4.4 billion years ago. A debris disk, similar to Saturn's rings, formed around the young Earth and very quickly it coalesced into the Moon. At the time, Earth’s orbit was still full of planetesimals, large chunks of rocks and planetary fragments each around a few thousandths of the mass of the Earth.
During the first tens of millions of years after the formation of the Moon, these planetesimals collided with the Earth during a phase called the terrestrial late accretion. Although these objects were small compared to the Moon, they were able to “excite” the lunar orbit by passing near it on their way to the Earth and created an oscillation that over million of years settled our satellite on its current orbit. The planetesimals were rich in heavy elements (such as iron, gold, and so on) and the late accretion explains the abundance of these elements in the Earth's crust. If the accretion had happened earlier, they would have sunk deeper into the planet.
"This discovery places the Moon-forming event into the context of Earth formation," Kaveh Pahlevan, lead author of the study, told IFLScience. "About one percent accretion onto the Earth is required to explain the chemistry of the Earth's mantle. We have shown that the same amount of accretion can reproduce the excitation of the lunar orbit. Hence, this discovery presents a coherent picture of the final stage of accretion of the Earth following the Moon-forming event."
The study also highlights how serendipitous it is that our Moon survived the chaotic times in the early Solar System. By forming relatively late during Earth's formation, the Moon's only interaction with planetesimals is the orbital tilt and some craters. If it had formed earlier it might have been pushed towards our planet and we would have never witnessed our silver companion.
