New twist on Moon formation helps explain its weird orbit

The surprisingly large orbital tilt of the Moon, coupled with it being so unusually far away from the Earth, makes our nearest neighbour one of the strangest planetary bodies in the Solar System. Although many scientists agree that the ‘giant impact’ theory accounts for how the Moon got to where it is now, a new twist on this theory suggests that calamitous changes to Earth's rotation and the tilt of its spin axis were also affected by the impact.

The giant impact theory basically states that late in the formation of the Solar System a Mars-sized object grazed an early Earth, throwing off a mass of material from which the Moon condensed. This impact set the angular momentum for the Earth-Moon system, and gave the evolving Earth a five-hour day. Since the event, the Moon has moved away from us and Earth’s rotation has slowed to our current 24-hour day.

Aspects of the Moon, such as its composition help substantiate this theory, as the Moon is made of almost the same stuff as Earth, minus some volatile compounds that evaporated long ago. But this one simple feature makes the Moon very distinct. "Every other body in the solar system has different chemistry," said Sarah Stewart, professor of earth and planetary sciences at the University of California, Davis and senior author on a paper recently submitted to Nature on this new research.

However, if the Moon condensed from a disk of material rotating around Earth's equator, then it should be in orbit over the equator but its not; the Moon's current orbit is tilted five degrees off the equator. "This large tilt is very unusual. Until now, there hasn't been a good explanation," said Douglas Hamilton, professor of astronomy at the University of Maryland and a co-author of the Nature paper. "But we can understand it if the Earth had a more dramatic early history than we previously suspected."

This new model proposed by Stewart and colleagues suggests that a similar impact occurred that left a mass of molten material from which the Earth and Moon formed, but also that the high energy collision left the Earth spinning extremely fast – nearly twice the rate predicted by other models, knocking the Earths tilt so that its axis pointed towards the Sun.

According to the researchers, these factors contributed to establishing the moon's current weird orbit and as the Moon receded from the Earth, the forces from the Earth on the Moon became less important than gravitational forces from the Sun.

"As the moon moved outward, the Earth's steep tilt made for a more chaotic transition as the sun became a bigger influence," said co-lead author Matija Cuk of the SETI institute. "Subsequently, and over billions of years, the moon's tilt slowly decayed down to the five degrees we see today. So today's five degree tilt is a relic and a signature of a much steeper tilt in the past."

The team acknowledges that the model doesn't fully explain the Moon's orbit around Earth, but that their new model offers a framework for answering new questions in the future. "There are many potential paths from the moon's formation to the Earth-moon system we see today. We've identified a few of them, but there are sure to be other possibilities," said Hamilton. "What we have now is a model that is more probable and works more cleanly than previous attempts. We think this is a significant improvement that gets us closer to what actually happened."

* Full explanation of main image: In the "giant impact" model of the moon's formation, the young moon began its orbit within Earth's equatorial plane. In the standard variant of this model (top panel), Earth's tilt began near today's value of 23.5 degrees. The moon would have moved outward smoothly along a path that slowly changed from the equatorial plane to the "ecliptic" plane, defined by Earth's orbit around the sun. If, however, Earth had a much larger tilt after the impact (~75 degrees, lower panel) then the transition between the equatorial and ecliptic planes would have been abrupt, resulting in large oscillations about the ecliptic. The second picture is consistent with the moon's current 5-degree orbital tilt away from the ecliptic.