Billions of years that a day on Earth lasted 19 hours

For most of its history, Earth has been a strange place for humans. Features of our planet that we take for granted were very different hundreds or billions of years ago, such as the length of the day. We’ve known for a long time that our planet’s period of rotation has increased over time, but what we haven’t been clear about is the rate of braking. The logical thing would be to assume this was a constant, but in fact it has had periods where it slowed down more quickly than others. Indeed, A.J A recent study It indicates that a day on Earth lasted about 19 hours for nearly a billion years.

During a snowball episode about 600 million years ago, the Earth’s rotation was already 20 hours (Wikipedia / Oleg Kuznetsov – 3depix).

How is this possible? First, we must remember that the length of the day on Earth is constantly increasing due to the effects of tidal forces with the Moon and the Sun. The gravitational interaction between the three bodies causes the water in Earth’s oceans to go out of shape. Globular, in which two opposite bulges appear to be attracted to the Moon, slowing its rotation in the process. Likewise, the same tidal forces cause the moon to move further and further away from the earth. However, the Earth also has an atmosphere, and although the intensity of this effect is much less than in the case of the oceans, the important thing is that they are Reverse. That is, it speeds up rotation. The reason is that the swelling of the “atmospheric ocean” does not arise from the lunar gravitational field, but from the increase in temperature resulting from the absorption of sunlight by ozone and water vapor, and moreover, it is the sun that “pulls” this “and hence its effect on The rotation is different.

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Opposite effects of tidal forces on the oceans (increasing circulation period) and forces on atmospheric tides (period decreasing) (Mitchell et al.).

Both forces can cancel if the right conditions are met, that is, when the distance from the Earth to the Moon is less than it is now, as it really was in the past. And when did this coincidence happen? Well, this one is more difficult to figure out. Much more. The hypothesis that the moon’s gravitational effects on the atmosphere and oceans can cancel each other out is an old one, but only now have researchers Ross Mitchell and Uwe Kircher searching the geological record for evidence of a “stuck” rotation period. Unfortunately, these natural witnesses throughout the day are not in abundance. The primary method for determining cycle duration was to analyze regularly deposited ancient sediments, but the key to Mitchell and Kircher’s study is that in addition to the traditional analysis of counting layers in ancient sediments, they used cyclostratigraphy. This technique consists of investigating how the Milankovitch cycles affect sedimentary records. Two of the Milankovitch cycles, precession and the tilt of the Earth’s axis (tilt), depend on the length of the day. Shorter days indicate shorter slanting and spacing cycles. Determining the patterns of these cycles in sediments is a very complex task, but in the past seven years several studies have appeared that allow us to determine the length of Earth’s day.

For about a billion years, the Earth’s day lasted 19 hours (Mitchell et al.).

Mitchell and Kircher’s conclusion is that Earth’s day lasted 19 hours for about a billion years, from two billion to one billion years ago. At that time, it is believed that the distance from the Moon to the Earth was between 350,000 and 320,000 km (today it is 385,000 km). The results support the hypothesis that ocean tidal forces are canceled out by atmospheric tides. The pause in the increase in the length of the day coincides with the end of the Great Oxidation Event, when the Earth’s atmosphere experienced a significant increase in the proportion of oxygen of biological origin. At the same time, the end of the 19-hour day also coincides with a further increase in the amount of oxygen. Since the warming of the atmosphere depends on the presence of ozone, which in turn depends on the concentration of oxygen, Mitchell and Kircher suggest that the variability of this gas in the atmosphere may have been key to maintaining Earth’s 19-hour rotation.

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A model of change in the length of Earth’s day from 2022 (Farhat et al.).
Variation in the distance to the moon (Farhat et al.).

Previous models of Mitchell and Kircher’s article suggested that the rate of Earth’s rotation slowed down between approximately three billion and one billion years ago, but no slowdown like the one suggested by their study was detected, so more cyclostratigraphic analyses will be required. necessary to confirm this result. The first Earth’s day was about 6 hours long, but about 500 million years after its formation, it had already slowed down to 12 hours. The length of the day, in turn, affected the rate at which the moon was leaving. Shorter days mean less intensity of tidal forces on the oceans, i.e. a slower rate of departure than the current rate. Other factors, such as the extension of the oceans – during the ice ages – or the formation of supercontinents, also affected the variation in the length of the Earth’s day. In short, the length of the day and the distance to the Moon are crucial components for understanding our planet’s past.



Lovell Loxley

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