The moon has been trying to escape Earth for eons, new research suggests

Looking at the moon in the night sky, you would never believe that it is slowly moving away from earth. But we know otherwise. In 1969, NASA’s Apollo missions installed reflective plates on the moon. These have shown that the moon is currently moving 3.8 cm away from earth each year.

If we take the moon’s current rate of recession and project it into the past, a collision between the earth and the moon occurs about 1.5 billion years ago. However, the moon formed about 4.5 billion years ago, meaning the current rate of recession is a poor guide to the past.

Together with our research colleagues from Utrecht University and the University of Geneva, we have used a combination of techniques to try to gain 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 not by studying the moon itself, but by reading signals in ancient rock strata on Earth.

Reading between shifts

In Western Australia’s beautiful Karijini National Park, a series of canyons cut through 2.5 billion-year-old rhythmically stratified sediments. These sediments are banded iron formations that include prominent layers of iron- and siliceous-rich minerals that were once widely deposited on the sea floor and are now found on the oldest parts of the earth’s crust.

Cliff exposures at Joffre Falls show layers of red-brown iron formations nearly a meter thick alternating with darker, thinner horizons at regular intervals.

The darker intervals consist of a softer rock type that is more prone to erosion. A closer look at the outcrops reveals the presence of an additional regular, smaller-scale variation. Rock surfaces polished by seasonal river water flowing through the gorge reveal a pattern of alternating layers of white, reddish and bluish-gray.

In 1972 Australian geologist AF Trendall questioned the origin of the various scales of cyclical, recurring patterns visible in these ancient rock layers. He suggested that the patterns could be related to past climate variability caused by the 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 affect the distribution of sunlight that Earth receives over periods of years.

Currently, the dominant Milankovitch cycles change every 400,000 years, 100,000 years, 41,000 years, and 21,000 years. These fluctuations exert a strong influence on our climate over long periods of time.

Important examples of the influence of the Milankovitch climate forcing in the past are the occurrence of extreme cold or warm spells and wetter or drier regional climate conditions.

These climate changes have significantly altered the conditions at the Earth’s surface, such as the size of lakes. They are the explanation for the periodic greening of the Sahara and the low oxygen content in the deep sea. Milankovitch cycles have also influenced the migration and evolution of flora and fauna, including our own species.

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

Recorded wobble

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 precessional motion (wobble), or changing 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.

That is, if we can first find Milankovitch cycles in ancient sediments, and then find a signal of Earth’s wobble and determine its period, we can estimate the distance between Earth and the Moon at the time the sediments were deposited.

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

The banded iron formations in Australia were probably deposited in the same ocean as the South African rocks around 2.5 billion years ago. However, the cyclic variations in the Australian rocks are more visible, allowing us to study the variations at a much higher resolution.

Our analysis of the Australian Banded Iron Formation showed that the rock contained multiple scales of cyclic variation, repeating at approximately 10 and 85 cm intervals. When we combined these thicknesses with the deposition rate of the sediments, we found that these cyclical fluctuations occurred about every 11,000 and 100,000 years.

Therefore, our analysis suggests that the 11,000 cycle observed in the rocks is likely related to the precession climatic cycle, which has a much shorter period than the current ~21,000 years. From this precession signal we then calculated the distance between the earth and the moon 2.46 billion years ago.

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

Understand 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 few 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 quite amazing that the dynamics of past solar systems 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 follow the Moon’s evolution over time. And our research team has already begun searching for the next group of rocks that can help us find more clues about the history of the solar system.

This article was originally published on The Conversation by Joshua Davies at the Université du Québec à Montréal (UQAM) and Margriet Lantink at the University of Wisconsin-Madison. Read the original article here.