Secrets of Moon’s slow escape revealed in Earth’s crust: ScienceAlert

Looking at the Moon in the night sky, you would never guess that it is 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 Moon’s current rate of recession 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, meaning that the current rate of recession is a poor guide to the past.

Together with our fellow researchers from the University of Utrecht and the University of Geneva, we are using a combination of techniques to try to gain insight into the distant past of our Solar System.

We recently discovered the perfect place to uncover the long-term history of our waning Moon. And it’s not from studying the Moon itself, but from reading signals in ancient rock layers on Earth.

Reading between layers

In the beautiful Karijini National Park in Western Australia, some canyons cut through 2.5 billion-year-old, rhythmically layered sediments. These sediments are banded iron formations, comprising distinctive layers of iron- and silica-rich minerals that were once widely deposited on the ocean floor and are now found in the oldest parts of the Earth’s crust.

Rock exposures at Joffre Falls show how layers of reddish-brown iron formation just under a meter thick are alternated, at regular intervals, by darker, thinner horizons.

The darker intervals consist of a softer type of rock that is more susceptible to erosion. A closer look at the outcrops reveals the presence of an additional normal, smaller-scale variant. The rock surfaces, which have been polished by the seasonal water of the river running through the gorge, reveal a pattern of alternating white, reddish and blue-gray layers.

In 1972, Australian geologist AF Trendall raised the question of the origin of the different scales of circular, repeating patterns visible in these ancient rock layers. He suggested that the patterns may be related to earlier climate variations caused by so-called “Milankovitch cycles”.

Cyclical climate changes

Milankovitch cycles describe how small, periodic changes in the shape of Earth’s orbit and the orientation of its axis affect the distribution of sunlight that Earth receives over time.

Currently, the dominant 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 the Milankovitch climate influence in the past are the occurrence of extreme cold or warm periods, as well as wetter or drier regional climate conditions.

These climate changes have significantly altered conditions on Earth’s surface, such as the size of lakes. It is the explanation for the periodic greening of the Sahara desert and the low oxygen levels in the deep ocean. Milankovitch cycles also affected the migration and evolution of flora and fauna, including our species.

And the signatures of these changes can be read through cyclical changes in sedimentary rocks.

Recorded swings

The distance between the Earth and the Moon is directly related to the frequency of one of the Milankovitch cycles – the cycle of climate transition. This cycle results from the precessional motion (oscillation) or change in orientation of the Earth’s axis of rotation over time. This cycle currently has a duration of ~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 the Milankovitch cycles in old sediments, and then find a signal of the Earth’s wobble and determine its period, we can calculate the distance between the Earth and the Moon at the time the sediments were deposited.

Our previous research showed that Milankovitch circles may be preserved in an ancient banded iron formation in South Africa, thus supporting Trendall’s theory.

The banded iron formations in Australia were probably deposited in the same ocean as the South African rocks, about 2.5 billion years ago. However, the cyclic variations in 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 variation that repeat at approximately 10 and 85 cm intervals. By combining these thicknesses with the rate at which the sediments were deposited, we found that these cyclical fluctuations occurred approximately every 11,000 years and 100,000 years.

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

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

Understanding the dynamics of the Solar System

Research in astronomy has provided models for the formation of our Solar System and observations of current conditions.

Our study and some research by others represents one of the only methods to obtain real data about the evolution of our Solar System and will be crucial for future models of the Earth-Moon system.

It is amazing that the dynamics of the Solar System in the past 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 other reliable data and new modeling approaches to track the Moon’s evolution over time. And our research team has already begun the hunt for the next suite of rocks that may help us uncover more clues about the history of the Solar System.

Joshua Davies, Professor, Sciences de la Terre et de l’atmosphère, Université du Québec à Montréal (UQAM) and Margriet Lantink, Postdoctoral Research Associate, Department of Geosciences, University of Wisconsin-Madison

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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