Looking at the Moon in the night sky, you would never imagine it slowly moving away from Earth, but we know otherwise. In 1969, NASA’s Apollo missions installed reflective panels on the Moon. These showed that the moon is currently moving 3.8cm away from Earth each year.
If we take the current rate of recession of the Moon and project it back in time, we end up with a collision between the Earth and the Moon about 1.5 billion years ago. However, the Moon formed about 4.5 billion years ago, indicating that the current rate of recession is a poor guide to the past.
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Scientists Joshua Davies, professor of earth and atmospheric sciences at the University of Quebec and Montreal, Canada, and Margriet Lantink, postdoctoral research associate in the Department of Geosciences at the University of Wisconsin-Madison, states United States, along with researchers from the University of Utrecht and the University of Geneva, are using a combination of techniques to try to obtain information about the distant past of our solar system.
They recently discovered the perfect place to uncover the long-term history of our waning Moon. And that’s not by studying the Moon itself, but by reading signals in ancient rock layers on Earth. In Karijini National Park, Western Australia, some gorges rhythmically cut through sediment in layers that are 2.5 billion years old.
These sediments are banded iron formations comprising distinct layers of iron- and silica-rich minerals, once widely deposited on the ocean floor and now found in the oldest parts of the earth’s crust.
The cliffs of Joffre Falls show how layers of reddish-brown iron formation less than a meter thick are alternated, at regular intervals, by darker and thinner horizons.
The darker intervals are composed of a type of rock that is softer and more susceptible to erosion. Closer examination of the outcrops reveals the presence of an otherwise regular and smaller-scale variation.
The rock surfaces, which have been polished by seasonal river water flowing through the gorge, reveal a pattern of alternating layers of white, reddish and bluish-grey.
In 1972, the Australian geologist AF Trendall raised the question of the origin of the different scales of cyclic and recurring patterns visible in these ancient rock layers. He suggested that the patterns could be linked to past variations in climate induced by so-called Milankovitch cycles.
cyclical climate change
The Milankovitch cycles describe how small periodic changes in the shape of the Earth’s orbit and the orientation of its axis influence the distribution of sunlight received by the Earth over the years. At this time, the dominant Milankovitch cycles change every 400,000 years, 100,000 years, 41,000 years and 21,000 years. These variations exercise a strong control over our climate for long periods of time.
The main examples of the influence of the Milankovitch climate forcing in the past are the occurrence of extremely cold or warm periods, as well as wetter or drier regional climatic conditions.
These climate changes have dramatically altered conditions on the Earth’s surface, such as the size of lakes. They are the explanation for the periodic greening of the Sahara Desert and the low oxygen levels at the bottom of the ocean.
The Milankovitch cycles have also influenced the migration and evolution of flora and fauna, including our own species. The signatures of these changes can be read through the cyclic changes of sedimentary rocks.
recorded oscillations
The distance between the Earth and the Moon is directly related to the frequency of one of the Milankovitch cycles – the climatic precession cycle. This cycle results from the movement of precession (oscillation) or change of orientation of the axis of rotation of the Earth over time.
It currently lasts about 21,000 years, but this period would have been shorter in the past, when the Moon was closer to Earth. This indicates that if we can first find Milankovitch cycles in ancient sediments, then find a signal of the Earth’s oscillation and establish its period, we can estimate the distance between the Earth and the Moon at the time the sediments settled.
Previous research by the team has shown that Milankovitch cycles can be preserved in ancient banded iron formation in South Africa, supporting Trendall’s theory. The Australian iron formations were probably deposited in the same ocean as the South African rocks around 2.5 billion years ago. However, the cyclic variations of Australian rocks are better exposed, allowing experts to study the variations with much greater resolution.
Analysis of the Australian Iron Formation has shown that the rocks contain multiple scales of cyclic variations which repeat at approximately 10 and 85 cm intervals. Combining these thicknesses with the rate of sediment deposition, these cyclic variations were found to occur approximately every 11,000 years and 100,000 years.
Therefore, the researchers’ analysis suggested that the 11,000-year cycle observed in the rocks is likely related to the climatic precession cycle, having a much shorter period than the current one around 21,000 years ago.
Then, scientists use this precession signal to calculate the distance between the Earth and the Moon 2.46 billion years ago.
The team found that the Moon was about 60,000 kilometers closer to Earth (this distance is about 1.5 times the circumference of the Earth). This would make the length of a day much shorter than it is now, at around 5 p.m. instead of the current 24 hours.
Understand the dynamics of the solar system
Astronomical research has provided models for the formation of our solar system and observations of current conditions. This study and some research by other experts represents one of the only methods to obtain real data on the evolution of our solar system and will be crucial for future models of the Earth-Moon system.
“It is incredible that the past dynamics of the solar system can be determined from small variations in ancient sedimentary rocks,” the researchers said. “However, one important piece of data does not give us a complete understanding of the evolution of the Earth-Moon system. »
“Now we need more reliable data and new modeling approaches to follow the evolution of the moon over time,” said the scientists, who added: “Our research team has already started the search for the next set of rocks that could help us uncover more clues to the history of the solar system.
With information from Space.com
Image in vedette : Diczie Quiel Sarino/
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