A new study that reconstructs the deep history of our planet’s relationship to the moon shows that 1.4 billion years ago, a day on Earth lasted just over 18 hours. This is at least in part because the moon was closer and changed the way Earth spun around its axis.
“As the moon moves away, the Earth is like a spinning figure skater who slows down as they stretch their arms out,” explains Stephen Meyers, professor of geoscience at the University of Wisconsin-Madison and co-author of the study published this week [June 4, 2018] in the Proceedings of the National Academy of Sciences.
The study describes a statistical tool that links astronomical theory with geological observation. This is called astrochronology (time of the stars). It look backs on Earth’s geologic past, and reconstructs the history of the solar system so it can understand climate change as captured in rocks.
“One of our ambitions was to use astrochronology to tell time in the most distant past, and develop ancient geological time scales,” Meyers says. “We want to be able to study rocks that are billions of years old in a way that is comparable to how we study modern geologic processes.”
Earth’s movement in space is influenced by the other astronomical bodies that exert force on it, like other planets and the moon. This helps determine variations in Earth’s rotation around and wobble on its axis, and in the orbit Earth traces around the sun. These variations are collectively known as Milankovitch cycles and they determine where sunlight is distributed on Earth. But they have only been able to do this in the million-year range and they want to go to the billion year range because to the Moon’s age but it’s complicated by lack of knowledge of the history of the moon, and by what is known as solar system chaos, a theory posed by Parisian astronomer Jacques Laskar in 1989.
Last year, Meyers and colleagues cracked the code on the chaotic solar system in a study of sediments from a 90 million-year-old rock formation that captured Earth’s climate cycles. The further back in the rock record he and others have tried to go, the less reliable their conclusions have been..For instance, the moon is currently moving away from Earth at a rate of 3.82 centimeters per year. Using this present day rate, scientists extrapolating back through time calculated that “beyond about 1.5 billion years ago, the moon would have been close enough that its gravitational interactions with the Earth would have ripped the moon apart,” Meyers explains. Yet, we know the moon is 4.5 billion years old.
Meyers sought a way to better account for just what our planetary neighbors were doing billions of years ago to understand the Milankovitch cycles. He discussed this on his sabbatical e at Columbia University’s Lamont-Doherty Earth Observatory while in 2016. Alberto Malinverno, the Lamont Research Professor at Columbia heard it and thought. ‘I think I know how to do it! Let’s get together!'”
Since then they have combined a statistical method that Meyers developed in 2015 to deal with uncertainty across time with astronomical theory, geologic data and Bayesian inversion that allows the researchers to get a better handle on the uncertainty of the system under study.
With the approach, they could reliably assess from layers of rock in the geologic record variations in the direction of the axis of rotation of Earth and the shape of its orbit both in more recent time and in deep time, while also addressing uncertainty. They were also able to determine the length of day and the distance between Earth and the moon.
The study complements two other recent studies that rely on the rock record and Milankovitch cycles to better understand Earth’s history and behavior.A research team at Lamont-Doherty used a rock formation in Arizona to confirm the remarkable regularity of Earth’s orbital fluctuations from nearly circular to more elliptical on a 405,000 year cycle.
The study was funded by the National Science Foundation (EAR-1151438).