Looking up at the moon in the night sky, you'd never guess it's gently drifting away from Earth. But we know better. NASA's Apollo missions erected reflecting panels on the moon in 1969. These have shown that the moon is now moving away from the Earth by 3.8 centimeters each year. If we project the moon's present pace of recession back in time, we get a collision between the Earth and the moon roughly 1.5 billion years ago. However, because the moon originated roughly 4.5 billion years ago, the present recession rate is a poor predictor of the past.
We've been using a variety of methodologies, along with colleagues from Utrecht University and the University of Geneva, to try to learn more about our solar system's distant past. We recently located the ideal location for studying the long-term history of our receding moon. And it comes from interpreting signals in old layers of rock on Earth, not from examining the moon. Our most recent research was published in the Proceedings of the National Academy of Sciences.
Some gorges cut through 2.5 billion-year-old, rhythmically piled sediments in Western Australia's spectacular Karijini National Park. These sediments are banded iron formations, which consist of separate layers of iron and silica-rich minerals that were formerly widely deposited on the ocean bottom and are now located on the Earth's oldest portions. Cliff exposures at Joffre Falls illustrate how strata of reddish-brown iron deposits just under a meter thick alternate with darker, narrower horizons at regular intervals. The darker intervals are made up of a softer form of rock that is more prone to erosion. A deeper examination of the outcrops indicates the presence of a second, smaller-scale variety. Rock surfaces polished by seasonal river water rushing through the canyon reveal an alternating pattern of white, reddish, and blueish-gray strata.
A.F. Trendall, an Australian geologist, addressed the subject of the origin of the various sizes of cyclical, repeating patterns evident in these ancient rock strata in 1972. He hypothesized that the patterns were caused by historical fluctuations in climate caused by the so-called Milankovitch cycles. These climatic changes have had a substantial impact on the circumstances at the Earth's surface, such as lake size. They account for the periodic greening of the Saharan desert and low oxygen levels in the deep ocean. Milankovitch cycles have also affected flora and animal migration and evolution, including our own species. And the signs of these changes may be read in sedimentary rocks via cyclical changes.
The frequency of one of the Milankovitch cycles, the climatic precession cycle, is closely proportional to the distance between the Earth and the moon. The precessional motion (wobble) or changing the orientation of the Earth's spin axis throughout time causes this cycle. This cycle is presently 21,000 years long, however, it would have been shorter in the past when the moon was closer to Earth. This implies that if we locate Milankovitch cycles in old sediments, then find a signal of the Earth's wobble and determine its period, we may estimate the distance between the Earth and the moon at the time the sediments were formed.
Previously, we discovered Milankovitch cycles in an old banded iron deposit in South Africa, lending credence to Trendall's idea. Around 2.5 billion years ago, the banded iron formations in Australia were most likely deposited in the same ocean as the rocks in South Africa. The cyclic fluctuations in Australian rocks, on the other hand, are better exposed, allowing us to investigate the variations at a much greater resolution. Our investigation of the Australian banded iron formation revealed numerous scales of cyclical changes that roughly reoccur at 10 and 85-cm intervals. When we combined these thicknesses with the pace at which the sediments were formed, we discovered that these cyclical fluctuations occurred every 11,000 to 100,000 years.
As a result of our investigation, the 11,000 cycle identified in the rocks is most likely connected to the climatic precession cycle, which has a significantly shorter time than the present 21,000 years. This precession signal was then used to compute the distance between the Earth and the moon 2.46 billion years ago. We discovered that the moon was around 60,000 kilometers closer to Earth than previously thought (that distance is about 1.5 times the circumference of Earth). This would reduce the length of a day to around 17 hours rather than the present 24 hours.
Astronomical research has offered models for the genesis of our solar system as well as measurements of present conditions. Our work, along with some other studies, is one of the only ways to gather true data on the evolution of our solar system, and it will be critical for future simulations of the Earth-moon system. It's incredible how previous solar system dynamics may be deduced from minor differences in old sedimentary rocks. However, one essential data point does not provide a complete picture of the Earth-moon system's development. Other credible data and new modeling methodologies are now required to trace the evolution of the moon over time. And our study team has already begun looking for the next set of rocks that can help us unearth more information about the solar system's past.
Journal Information: Margriet L. Lantink et al, Milankovitch cycles in banded iron formations constrain the Earth–Moon system 2.46 billion years ago, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2117146119