How The Moon Got Its Tilt Explained By New Model

Artist's impression of the formation of the Moon. NASA/JPL-Caltech/T.Pyle

It is often said the Moon plays a crucial role for life on Earth, since the gravitational pull of our natural satellite helps to stabilize our planet. New research now suggests that this role started as soon as the Moon formed 4.5 billion years ago.

For the study, American researchers looked for an explanation for the more than five-degree tilt the Moon's orbit has with respect to Earth’s orbit around the Sun. This is very different compared to most of the satellites orbiting other planets, as they usually orbit close to the planet's equator and not the orbital plane.

Looking for an explanation, the scientists discovered that when the Moon formed, it might have caused our planet to almost orbit on its side. It was only later that the Moon was responsible for straightening it up.

“Evidence suggests a giant impact blasted off a huge amount of material that formed the Moon,” said co-author Douglas Hamilton, professor of astronomy at the University of Maryland, in a statement. “This material would have formed a ring of debris first, then the ring would have aggregated to form the Moon. But this scenario does not quite work if the Earth’s spin axis was tilted at the 23.5-degree angle we see today.”

The computer simulation suggests that the impact might have pushed the tilt of Earth to more than 70 degrees and made our planet spin really fast, with the disk of debris forming the Moon around its equator, 15 times closer to where it is today. The Moon was then tugged by both our planet and the Sun in almost perpendicular directions, and might have remained in this “stalemate” scenario for millions of years.

The tidal influences of such a close Moon led to a progressive slowing down of Earth’s spin, which allowed our satellite to break free. Moving away from our planet, the Moon helped reduce Earth’s tilt, but it wasn’t a smooth transition. The change was abrupt, with our satellite's orbital tilt changing wildly.

“As the Moon moved outward, the Earth’s steep tilt made for a more chaotic transition as the Sun became a bigger influence,” lead author Matija Ćuk said in the statement. “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.”

These results, discussed in a paper published in Nature, solve several unknowns about the Moon, but there’s still more left to discover. Nevertheless, this research underlines the crucial role that the Moon has had on Earth.

“This work shows that there are multiple ways a planet could get a small axial tilt, making moderate seasons possible,” Ćuk said in another statement. “We thought Earth was this way because of the direction of the giant impact 4.5 billion years ago, but it looks like Earth achieved this state later through a complex interaction with the Moon and the Sun.”

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