Surprisingly, the length of the day may have been determined by the amount of oxygen in the Earth’s atmosphere, which in turn was determined by the abundance of photosynthesizing organisms.
Although we assume that our days last 24 hours, early in the history of life on our planet, days were even shorter. Day length was shorter because the moon was closer. “Over time, the moon stole the rotational energy of the Earth to bring it into an orbit higher and farther from the Earth”said Ross Mitchell, a geophysicist at the Institute of Geology and Geophysics of the Chinese Academy of Sciences and lead author of a new study published in Natural Geosciences. “Most models of the Earth’s rotation predict that the length of the day is shorter and shorter going back in time,” said Uwe Kirscher, co-author of the study and now a researcher at Curtin University in Australia. But a slow and steady change in day length going back in time is not what Mitchell and Kirscher found. How do researchers measure ancient day length? In recent decades, geologists have used the analysis of some sedimentary rocks that retain layers. Count the number of sedimentary layers per month caused by tidal fluctuations and know the number of hours in an ancient day. But such tidal records are rare, and preserved ones are often disputed. Fortunately, there is another way to estimate the length of the day. Cyclostratigraphy is a geological method that uses rhythmic sedimentary stratification to detect “Milankovitch” astronomical cycles that reflect how changes in the Earth’s orbit and rotation affect climate. “Two Milankovitch cycles, precession and obliquity, are related to the wobble and tilt of the Earth’s axis of rotation in space. The faster rotation of the early Earth can therefore be detected in cycles of precession and obliquity shorter than the past.”Kirscher explained. Mitchell and Kirscher took advantage of a recent proliferation of Milankovitch records, with more than half of the data for ancient times generated in the past seven years. “We realized that it was finally time to test some sort of fringe, but entirely reasonable, alternative idea about paleorotation of the Earth.” Mitchell said. One unproven theory is that the length of the day may have stopped at a constant value in Earth’s distant past. In addition to the tides in the ocean related to the pull of the moon, the Earth also has solar tides related to the warming of the atmosphere during the day. Solar atmospheric tides aren’t as strong as lunar ocean tides, but that wasn’t always going to be the case. When the Earth rotated faster in the past, the moon’s pull would have been much weaker. Unlike the pull of the moon, the tide of the sun instead pushes the Earth. So while the moon slows down the Earth’s rotation, the sun speeds it up. “Because of this, if in the past these two opposing forces had become equal to each other, such a tidal resonance would have caused the Earth’s day length to stop changing and remain constant for some time.” Kirscher said. And that’s exactly what the new data collection showed. Earth’s day length appears to have stopped its long-term increase and flattened out to about 19 hours between roughly two and a billion years ago—”the billion years,” Mitchell noted, “commonly referred to as the ‘boring’ billion”. The timing of the stall intriguingly sits between the two largest increases in oxygen. Timothy Lyons of the University of California, Riverside, who was not involved in the study, said: “It is fascinating to think that the evolution of the Earth’s rotation may have influenced the evolution of the composition of the atmosphere.” The new study therefore supports the idea that Earth’s rise to modern oxygen levels had to wait longer days for photosynthetic bacteria to generate more oxygen each day.