Moon has been drifting away from Earth for 2.4 billion years, rocks reveal
Even our nearest satellite can't stand our gravity
An analysis of Australian sedimentary rock has helped to show the Moon was about 60,000km closer to the Earth 2.46 billion years ago than it is now.
The current distance from the Earth to the Moon is around 384,400km (238,855 miles), but scientists have known for some time that it is drifting away. One of the scientific contributions of NASA's 1969 Apollo mission was to leave mirrors on the natural rocky satellite. The subsequent Lunar Laser Ranging experiment showed the Moon inches away from its home planet by 3.8cm (1.5 inches) every year.
The finding has presented scientists with something of a puzzle.
Extrapolating backwards, it suggests the Moon was in touching distance of the Earth around 1.5 billion years ago, whereas we know the Earth is about 4.5 billion years old from a bunch of other evidence and there's no evidence of a collision between the Moon and the Earth.
But the rate at which the Moon is sliding off into the firmament is not constant. Scientists have been trying to figure out how the rate changes, and how close the Moon and Earth once were by analysing evidence from rocks on our home planet.
Joshua Davies, Sussex University research fellow, and a post-doctoral researcher in geoscience at Utrecht University, Margriet Lantink, developed the technique to try to uncover details of the solar system's distant past. The focus of their study was sedimentary layers in Karijini National Park in western Australia, some of which reveal the oldest visible sections of the Earth's crust.
Sharing these ideas, Lantink and Davies explained how banded iron formations, formed on the ancient ocean floor, help determine the cycles of the Earth’s precession, the wobble in the Earth's axis related to its interaction with the Moon.
Since 1972, scientists have been working on the idea that ancient rock layers relate variations in climate induced by the so-called Milankovitch cycles, the effects of the Earth’s movement on climate. These cycles are linked to extreme cold and hot climate, which can affect the size of oceans and lakes, the kind of flora that grow and influence the evolution and migration of flora. They can also be linked to patterns in sedimentary rock.
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One of the Milankovitch cycles — the climatic precession cycle, which has to do with how the orientation of the Earth's spin axis "wobbles" over time – is directly related to the distance between the Earth and the Moon, say the authors. The current cycle has a duration of ~21,000 years, but "this period would have been shorter in the past when the Moon was closer to Earth," write the boffins.
Looking at evidence from the layers in the Australian rock, and confirming with evidence from similar formations in South Africa, the researchers show a 11,000-year cycle observed in the rocks is likely related to the climatic precession cycle, having a much shorter period than the current cycle.
They then worked backwards to show the precession signal reflected a distance between the Earth and the Moon 2.46 billion years ago.
Their study, written up in Proceedings of the National Academy of Sciences of the United States of America, concludes the Moon was around 321,800km (± 6,500km) from the Earth 2.46 billion years ago. That's around 62,000km closer to the Earth than it is now (the current position is 384,300km away) — or 1.5 times the circumference of Earth. The finding also suggests the length of the day on Earth was 17 hours at the time. The researchers are now looking for more rocks which could help uncover the past of the Earth-Moon dynamics.
"We now need other reliable data and new modeling approaches to trace the evolution of the Moon through time. And our research team has already begun the hunt for the next suite of rocks that can help us uncover more clues about the history of the solar system," they write. ®