The moon is currently moving 3.8 cm away from Earth each year. Credit: Shutterstock

Looking at the moon in the night sky, you would never imagine that it is slowly moving away from the Earth. But we know something else. In 1969, NASA’s Apollo missions installed reflective panels on the Moon. They showed that the Moon is there is currently moving away from Earth by 3.8 cm every year.

If we take monthcurrent rate of recession and project it back in time, we end up with a the collision of the Earth and the Moon about 1.5 billion years ago. However, the Moon was formed about 4.5 billion years agowhich means that the current level of recession is a poor indicator of the past.

Together with our research colleagues from Utrecht University and University of Genevawe used a combination of techniques to try to get information about the distant past of our solar system.

We recently found the perfect place to uncover the long-term history of our departing Moon. And this is not from studying the Moon itself, but from reading signals in ancient rock layers on Earth. Our latest research appears in the Proceedings of the National Academy of Sciences.

Reading between layers

In beautiful Karijini National Park in western Australia, some gorges cut rhythmically layered sediments 2.5 billion years old. These deposits are banded iron formations consisting of distinctive layers of minerals rich in iron and silica was once widely postponed at the bottom of the ocean and now found in the oldest parts of the earth’s crust.

Rock outcrops of Joffre waterfall show how alternating layers of reddish-brown iron with a thickness of a little less than a meter, art regular intervalsdarker, thinner horizons.

Our Moon has been slowly moving away from Earth for the past 2.5 billion years

Jofra Gorge in Karijini National Park, Western Australia, showing regular alternations between reddish-brown harder rocks and softer, clay-rich rocks (indicated by arrows) at an average thickness of 85 cm. These changes are explained by past climate changes caused by changes in the eccentricity of Earth’s orbit. Author: Fritz Hilgen, author provided

Darker intervals consist of softer rock that is more susceptible to erosion. A closer examination of the outcrops reveals the presence of additional regular changes of a smaller scale. The rock surfaces, which have been polished by seasonal river water flowing through the gorge, reveal an alternating pattern of white, reddish and blue-gray layers.

In 1972, the Australian geologist A. F. Trendal raised the question of the origin of the various scales of cyclic, periodic patterns seen in these ancient rock layers. He suggested that the patterns could be linked to past climate changes caused by so-called “Milankovitch cycles”.

Cyclical climate changes

Cycles of Milankovich describe how small periodic changes in the shape of the Earth’s orbit and the orientation of its axis affect the distribution of sunlight received by the Earth for many years.

Currently, the dominant Milankovitch cycles alternate every 400,000 years, 100,000 years, 41,000 years, and 21,000 years. These variations exert a strong control over our climate over long periods of time.

The main examples of the influence of Milankovitch climate forcing in the past are the occurrence severe cold or warm periodsalso how more humid or dry regional climates.

These climate changes have significantly altered conditions on the Earth’s surface, e.g size of lakes. They are an explanation for periodic greening of the Sahara desert and low oxygen levels in the deep ocean. Milankovitch cycles also influenced the migration and evolution of flora and fauna including ours own species.

And the signatures under these changes can be read cyclic changes of sedimentary rocks.

Our Moon has been slowly moving away from Earth for the past 2.5 billion years

Rhythmically alternating layers of white, reddish and/or bluish-gray rock with an average thickness of about 10 cm (see arrows). The alternations, interpreted as a signal of the Earth’s precession cycle, help us estimate the distance between the Earth and the Moon 2.46 billion years ago. Author: Fritz Hilgen

Recorded oscillations

The distance between the Earth and the Moon directly depends on the frequency of one of the Milankovitch cycles—cycle of climatic precession. this cycle occurs as a result of precessional motion (wobble) or changes in the orientation of the Earth’s axis of rotation over time. This cycle is currently ~21,000 years long, but this period would have been shorter in the past when the Moon was closer to Earth.

This means that if we can first find Milankovitch cycles in older sediments, and then find the Earth’s oscillation signal and establish its period, we can estimate the distance between the Earth and the Moon at the time the sediments were laid down.

Our previous studies have shown that Milankovitch cycles may be preserved in ancient banded ironwork in South Africathereby confirming Trendall’s theory.

Tapes iron formations Australia probably had deposited in the same ocean like the South African rocks, about 2.5 billion years ago. However, cyclical changes in Australian rocks are better defined, allowing us to study changes at a much higher resolution.

Our analysis of the Australian Banded Iron Formation showed that the rocks contain several scales of cyclic variation that roughly repeat themselves at intervals of 10 and 85 cm. By combining these thicknesses with the rate of sedimentation, we found that these cyclic changes occurred approximately every 11,000 years and 100,000 years.

Therefore, our analysis suggested that the 11,000 cycle observed in the rocks is likely related to a climatic precession cycle that has a much shorter period than the current ∼21,000 years. We then used this precession signal to calculate the distance between the Earth and the Moon 2.46 billion years ago.

We discovered that the Moon was about 60,000 kilometers closer to Earth (a distance of about 1.5 times the circumference of Earth). This would make the length of the day much shorter than it is now, about 17 hours rather than the current 24 hours.

Our Moon has been slowly moving away from Earth for the past 2.5 billion years

We found that the Moon was about 60,000 kilometers closer to Earth 2.46 billion years ago. Credit: Shutterstock

Understanding the dynamics of the solar system

Research in astronomy has provided models for the formation of our solar systemand observations of modern conditions.

Our research and some research by others represents one of the few methods of obtaining real data about the evolution of our solar system and will be crucial for future models of the Earth-Moon system.

It is very surprising that the past dynamics of the solar system can be determined by small variations in antiquity sedimentary rocks. However, one important piece of information does not give us a complete understanding of the evolution of the Earth-Moon system.

We now need other reliable data and new modeling approaches to track the Moon’s evolution over time. And our research team has already begun searching for the next set of rocks that could help us find more clues about the history of the solar system.

Reliable archives record changes in the Earth’s orbit

Additional information:
Margrit L. Lantink et al. Milankovitch cycles in banded iron formations bounded the Earth-Moon system 2.46 billion years ago, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2117146119


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