This article was originally published on The conversation. (opens in a new tab) The publication contributed the article to Space.com’s Expert Voices: Editorials and Perspectives.
Joshua Davies (opens in a new tab)Professor of Earth and atmospheric sciences, University of Quebec in Montreal (UQAM)
Margaret Lantin (opens in a new tab)Postdoctoral Research Associate, Department of Geosciences, University of Wisconsin-Madison
Looking at the moon in the night sky, you would never imagine it slowly moving away from Earth. But we know the opposite. 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 every year (opens in a new tab).
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. (opens in a new tab). However, the moon formed about 4.5 billion years ago (opens in a new tab)meaning that the current rate of recession is a poor guide to the past.
With our research colleagues from the University of Utrecht (opens in a new tab) and the University of Geneva (opens in a new tab)we’ve used a combination of techniques to try to get information about our solar system’s distant past.
We recently discovered the perfect place to uncover the long-term history of our receding moon. And that’s not by studying the moon itself, but by reading signals in ancient layers of rock on Earth. (opens in a new tab).
Related: How did the moon form?
Read between the layers
In the beautiful Karijini National Park (opens in a new tab) in Western Australia, some gorges cut through rhythmically layered sediments 2.5 billion years old. These sediments are banded iron formations, comprising distinctive layers of iron-rich and silica-rich minerals (opens in a new tab) once widely deposited on the ocean floor and now found on the oldest parts of the earth’s crust.
Cliff displays at Joffre Falls (opens in a new tab) show how layers of reddish-brown iron formation a little less than a meter thick are alternated, at regular intervals, by darker and thinner horizons.
The darker intervals are composed of a softer rock type that is more susceptible to erosion. A closer examination of the outcrops reveals the presence of a more 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-gray.
In 1972, the Australian geologist AF Trendall raised the question of the origin of the different scales of cyclic and recurring patterns. (opens in a new tab) 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 changes
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 (opens in a new tab) over periods of years.
Currently, the prevailing Milankovitch cycles change 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.
Key examples of the influence of Milankovitch’s climate forcing in the past are the occurrence of extreme cold (opens in a new tab) or hot periods (opens in a new tab)as well as wetter (opens in a new tab) or drier regional climatic conditions.
These climate changes have dramatically altered conditions on the Earth’s surface, such as the size of lakes (opens in a new tab). They are the explanation for the periodic greening of the Saharan desert (opens in a new tab) and low oxygen levels in the deep ocean (opens in a new tab). The Milankovitch cycles also influenced the migration and evolution of flora and fauna (opens in a new tab) including our own species (opens in a new tab).
And the signatures of these changes can be read through the cyclic changes of sedimentary rocks (opens in a new tab).
The distance between the Earth and the Moon is directly related to the frequency of one of the Milankovitch cycles – the climatic precession cycle (opens in a new tab). This cycle arises from the motion of precession (oscillation) or the change in orientation of the axis of rotation of the Earth over time. This cycle currently lasts about 21,000 years, 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 old 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.
Our previous research has shown that Milankovitch cycles can be preserved in an ancient banded iron formation in South Africa (opens in a new tab)thus supporting Trendall’s theory.
The banded iron formations in Australia were probably deposited in the same ocean (opens in a new tab) like the South African rocks, about 2.5 billion years ago. However, the cyclic variations of Australian rocks are better exposed, allowing us to study the variations at a much higher resolution.
Our analysis of the Australian Banded Iron Formation showed that the rocks contained multiple scales of cyclic variations that repeat at approximately 4 and 33 inches (10 and 85 cm intervals). By combining these thicknesses with the rate of sediment deposition, we found that these cyclic variations occur 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 the climatic precession cycle, having 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 (opens in a new tab).
We found that the moon was then about 37,280 miles (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 (opens in a new tab)and observations of current conditions (opens in a new tab).
Our study and some research done by others (opens in a new tab) 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 (opens in a new tab).
It is quite amazing that the dynamics of the past solar system can be determined from small variations in ancient sedimentary rocks. However, one important data point does not give us a full understanding of the evolution of the Earth-Moon system.
We now need more reliable data and new modeling approaches to trace the moon’s evolution over time. And our research team has already started the hunt for the next suite of rocks that can help us uncover more clues to the history of the solar system.
This article is republished from The conversation (opens in a new tab) under Creative Commons license. Read it original article (opens in a new tab).
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