The Moon has been receding from Earth since its creation, and attempts at reconstructing the slow trajectory of the lunar exodus have reached very different conclusions. As a result, for much of its history, the location of the Moon in relation to Earth has been a mystery.
Now, new research has calculated the distance of the Moon 2.46 billion years ago, nearly doubling the age of the previous estimate. Because the length of the day is tightly, tidally tied to the location of the Moon, the research has also calculated how long a day lasted at the time: 17 hours.
To determine the distance of the Moon, scientists studied rhythmic patterns in Earth’s orbit and axis called Milankovitch cycles, explained Margriet Lantink, a geologist at the University of Wisconsin–Madison and lead author of the new study in the Proceedings of the National Academy of Sciences of the United States of America.
The idea of looking at Milankovitch cycles to say something about the history of the Earth-Moon system is not new, said Lantink, who conducted the research as a doctoral candidate at Utrecht University in the Netherlands. “But until recently, people have only been applying them to the youngest part of the geological record, where changes are very small,” she said.
In determining the way solar radiation is distributed on Earth, Milankovitch cycles influence changes in climate over very long periods of time. These changes can be captured in the geologic record, which Lantink investigated in banded iron formations (BIFs) at Joffre Gorge in Western Australia’s Karijini National Park.
By examining alternating layers of iron and clay in the BIFs, Lantink was able to identify patterns recording two key elements of Milankovitch cycles: orbital eccentricity and axial precession. Earth’s circular-to-elliptical orbital eccentricity cycle lasts about 100,000 years. Earth’s precession cycle, which describes wobbly changes in the direction of Earth’s rotational axis, currently lasts almost 26,000 years.
The BIFs showed a smaller repeating pattern inside a larger repeating pattern. Uranium-lead dating helped Lantink determine that the larger pattern was likely guided by eccentricity, whereas the smaller pattern recorded precession.
Using the precession cycle recorded in Joffre Gorge’s BIFs, which was closer to 11,000 years, Lantink and her colleagues calculated that at the time of the earliest deposits (2.46 billion years ago) the Moon was 321,800 kilometers from Earth—only 84% of its current distance. The precession analysis also indicated that Earth was spinning much faster at this time, resulting in 17-hour days.
“It turns out that the rate at which precession happens…depends on the Earth’s rotation rate and the distance to the Moon,” said Alberto Malinverno, a geophysicist at Columbia University who was not involved in the study.
“We showed that in very old rocks…you can recognize these Milankovitch cycles, and they are of high enough quality to say something about the Earth-Moon system,” said Lantink. “There’s a lot more possible than we thought.”
—Emily Shepherd (@emilyshep1011), Science Writer
11 November 2022: This article has been updated to correct the timing of the precession cycle recorded in Joffre Gorge’s BIFs