2023-06-16 09:02:18
The gradual receding of the Moon has caused the length of the day to lengthen, reaching 24 hours today. But a new study shows that this duration would have stagnated at 19 hours for nearly a billion years, putting the entire Earth system on “pause”!
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Sometimes 24 hours seems too short to perform all the tasks that drive our daily lives. We can be reassured by saying that about 1 billion years ago, the impression would certainly have been worse! With days lasting just 19 hours, time must have raced by for the primitive organisms that populated the oceans at that time.
We have known for several decades that the length of the day was indeed much shorter in the past. As proof, the sedimentary records of the different tidal episodes. The tides represent a periodic phenomenon intrinsically linked to the rotation of the Earth and the presence of the Moon. By depositing each time a thin layer of sediments, the tides have thus left in certain sedimentary series the information making it possible to measure the length of the day during geological times. Scientists have thus discovered that the further back in time one goes, the shorter the duration of an Earth day. This phenomenon has been interpreted with regard to the distance of the Moon from the Earth.
The effect of the Moon, but not only!
Originally, the Moon was indeed much closer to us. It then moved away slowly under the effect of the differential which exists between the speed of rotation of the Earth and its speed of displacement in its orbit. The Earth indeed turns faster on itself than the Moon does the turn of the Earth, and that since the origin of the two stars. Thus, the bulging of the oceanic mass (the tides) that the Moon creates ends up exceeding it. This offset in turn affects the rotation of the Moon. There is thus a transfer of angular momentum from the Earth to the Moon which will expel the Moon into a gradually more distant orbit, in fact causing the rotation of the Earth to slow down. This is how the length of the earth’s day gradually lengthened until it reached the current value of 24 hours.
But it is this notion of “gradually” that has just been called into question by two scientists. It would indeed appear that this decrease in day length would not be linear over geological time. And for good reason, the Moon would not be the only star to have influenced the rotation of the Earth. In the equation, the Sun would have been forgotten.
Because if the mass of water on Earth is gravitationally excited by the attraction of the Moon, the light emitted by the Sun thermally excites our atmosphere, causing “atmospheric tides”. In an article published in the journal Nature Geoscience, the researchers show that this effect to some extent counterbalances that of the Moon. While the Moon tends to decelerate the Earth, the atmospheric tides produced by the Sun tend to accelerate it. Today, if the effect of the Moon is the most important, there would have been a period in the history of the Earth where these two forces would have canceled each other by entering into resonance, allowing the stabilization of the duration of the day. This phenomenon would have occurred during the Precambrian period.
An astonishing coincidence with the boring Billion
The authors of the study demonstrate the existence of this period thanks to the observation of cycles in the sediments, in connection with the orbital parameters of Milankovitch. They demonstrate that about 2 billion years ago, a period began during which day length stabilized at 19 hours. This phase lasted for 1 billion years. Surprisingly, it corresponds to a period commonly referred to as the “Boring Billion” or the “Great Discordance”.
The geological archives show that, between 1.8 and 0.8 billion years ago, nothing extraordinary happened: this period is marked by tectonic stability (no major paleogeographic evolution) and climatic (no glaciations), but also by a very slow biological evolution. As if the Earth had entered a state of stasis, in every way. So, would there be a link between this episode of the boring Billion and this temporary stabilization of the length of the day?
It’s possible. The explanation should be sought in the composition of the atmosphere. The high proportion of ozone present in the atmosphere 2 billion years ago, following the episode of the Great Oxygenation, would indeed have reinforced the effect of atmospheric tides, making it possible to counter that of lunar tides. and stabilizing day length at 19 hours. Climate change could have ended this period. Biological evolution would have started with the end of this episode, the progressive lengthening of days allowing photosynthetic organisms to produce more oxygen to support the development of a more complex life.
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