A recent study published in Nature Geoscience challenges the conventional belief that Earth’s day length was consistently shorter in the past. Surprisingly, the research suggests that for approximately one billion years, the planet experienced a stable day length of around 19 hours. This intriguing period aligns with two significant increases in atmospheric oxygen, hinting at a potential connection between Earth’s rotation and its atmospheric composition.
Life today is filled with endless tasks and limited time to accomplish them. However, if we were to journey back in time to Earth’s ancient history, our days would have been even shorter.
The reason for shorter day lengths in the past can be attributed to the Moon’s proximity. As Ross Mitchell, a geophysicist at the Institute of Geology and Geophysics of the Chinese Academy of Sciences and lead author of the study, explains, “Over time, the Moon has siphoned off Earth’s rotational energy, propelling it into a higher orbit and distancing it from our planet.”
Previously, models of Earth’s rotation suggested a consistent trend of decreasing day length throughout history. However, Mitchell and Uwe Kirscher, co-author of the study and a research fellow at Curtin University in Australia, discovered a different pattern.
To determine ancient day length, researchers traditionally relied on sedimentary rocks that preserved intricate layering caused by tidal mud flats. By counting the number of sedimentary layers formed per month due to tidal fluctuations, scientists could estimate the length of ancient days. However, such records are scarce and often disputed. Fortunately, another method exists.
The study employs cyclostratigraphy, a geological technique that examines rhythmic sedimentary layering to identify astronomical “Milankovitch” cycles. These cycles provide insights into how changes in Earth’s orbit and rotation influence climate.
“Precession and obliquity, two Milankovitch cycles, are linked to Earth’s rotation axis wobbling and tilting in space. The faster rotation of early Earth can be detected through shorter precession and obliquity cycles in the past,” clarifies Kirscher.
Mitchell and Kirscher took advantage of the recent surge in Milankovitch records, with more than half of the data for ancient times generated within the last seven years.
“We recognized that the time had come to test a somewhat unconventional but entirely reasonable idea about Earth’s ancient rotation,” stated Mitchell.
One unproven theory posits that Earth’s day length might have remained constant during its remote past. Besides the oceanic tides influenced by the Moon’s gravitational pull, Earth experiences atmospheric tides driven by daytime heating.
While solar atmospheric tides are weaker than lunar oceanic tides, this wasn’t always the case. When Earth rotated faster in the past, the Moon’s gravitational pull would have been considerably weaker. Unlike the Moon, the Sun’s tide exerts a pushing force on Earth. Therefore, as the Moon slows down Earth’s rotation, the Sun accelerates it.
“As a result, if in the past these opposing forces had equalized, a tidal resonance would have caused Earth’s day length to halt its changes and remain constant for a certain period,” explains Kirscher.
Surprisingly, this is precisely what the new data compilation reveals. Earth’s day length seemingly ceased its gradual increase, stabilizing at approximately 19 hours between one and two billion years ago—an era referred to as the “boring billion” by Mitchell.
Remarkably, this period coincides with the two most significant spikes in oxygen levels. Timothy Lyons, a researcher at the University of California, Riverside, who was not involved in the study, finds it captivating, stating, “It’s fascinating to consider how Earth’s rotation could have influenced the evolving composition of the atmosphere.”
Thus, this new study lends support to the notion that Earth’s journey towards contemporary oxygen levels relied on extended day lengths, enabling photosynthetic bacteria to produce greater amounts of oxygen each day.
Reference: “Mid-Proterozoic day length stalled by tidal resonance” by Ross N. Mitchell and Uwe Kirscher, 12 June 2023, Nature Geoscience. DOI: 10.1038/s41561-023-01202-6
Frequently Asked Questions (FAQs) about ancient day length
Q: How long were Earth’s days in its ancient past?
A: According to the study, Earth’s days in its ancient past stalled at around 19 hours for approximately one billion years, challenging the belief that day length consistently decreased over time.
Q: What caused the stability in day length during that period?
A: The research suggests that a tidal resonance between the Moon’s gravitational pull and the Sun’s atmospheric tides may have caused Earth’s day length to remain constant for a significant period of time.
Q: How did researchers measure ancient day length?
A: Traditional methods involved analyzing sedimentary rocks with fine-scale layering caused by tidal mud flats. However, the study also utilized cyclostratigraphy, a geologic technique that detects Milankovitch cycles related to changes in Earth’s orbit and rotation.
Q: What is the significance of the period with stable day length?
A: The stable day length period aligns with two major increases in atmospheric oxygen. This suggests a potential connection between Earth’s rotation, day length, and the composition of the atmosphere.
Q: How does the study relate to the evolution of oxygen levels on Earth?
A: The findings support the idea that longer day lengths during the stable period allowed photosynthetic bacteria to generate more oxygen each day, contributing to the rise of oxygen levels in the atmosphere.
More about ancient day length
- Study: “Mid-Proterozoic day length stalled by tidal resonance” (Nature Geoscience): Link
- Institute of Geology and Geophysics of the Chinese Academy of Sciences: Link
- Curtin University: Link
- Milankovitch cycles: Link
- University of California, Riverside: Link