A day on Earth used to last only 19 hours. Now we know why. : ScienceAlert

There was a time in Earth’s turbulent history when everything almost came to a standstill. Tectonic activity subsided, geochemical processes emerged, and the evolution of life in its simplest forms didn’t do much.

Now, new research suggests that this period of nothingness — affectionately dubbed the “boring billion” — coincided with a time when planet Earth was floating in a steady state of perpetual rotation, a ball pirouetting in space, and a day ( one revolution) which lasted only 19 hrs.

This 19-hour “standstill” in Earth’s day length lasted about a billion years and was the result of a delicate balance of opposing forces in our planet’s distant past, according to a study by two geophysicists.

The graph shows how the Earth's day length increases, then decreases, and then increases again.
Earth’s day length “stagnated” at 19 hours for 1 billion years. (Mitchell & Kirscher, Natural Earth Sciences2023)

At this point, the Moon was closer to Earth and hovering at a constant distance, not escaping its gravitational embrace as before and since.

“Over time, the moon has stolen the rotational energy of the earth to propel it into a higher orbit farther from the earth,” explain Ross Mitchell of the Chinese Academy of Sciences and Uwe Kirscher of Curtin University in Australia in their published article .

As a result of the Moon’s outward movement, the Earth’s rotation is slowing and our sunlit days are lengthening a bit – not that we notice the smidgen lengthening of our 24-hour daily cycles.

Much previous research has examined how our planet’s days are slowly getting longer by more than 0.000015 seconds per year, according to current estimates.

Most models of the Earth’s rotation in studies like this predict that day length on our home planet has steadily increased over the past 3 to 4 billion years. For example, a 2018 study found that 1.4 billion years ago, a day on Earth lasted 18 hours.

However, there is another group of researchers who have been speculating since 1987 that the length of days on Earth may have remained the same for an extended period of time before slowly and steadily increasing again, up to the 24 hours we are today name a day.

But geological evidence pointing to a change in the Earth’s rotation is hard to find. The length of the day can be inferred from the growth of sun-tilted stromatolites and from tidal rhythmites, patterns of mud sediments formed by tides and preserved in the rock. However, these have rarely survived into the distant past.

In this new study, Mitchell and Kirscher used a flood of new geological data that has surfaced in recent years. Cyclostratigraphic data are records of rhythmic changes in Earth’s climate caused by astronomical forces, including Earth’s wobble and axial tilt.

“We realized it was finally time to test some kind of marginal but perfectly reasonable alternative idea to Earth’s paleorotation,” says Mitchell.

Their statistical analysis points to a flattening of Earth’s day length between 2 and 1 billion years ago, in the middle of the Proterozoic Epoch, culminating in the “Snowball Earth” and preceding the Cambrian explosion of life.

Mitchell and Kirscher wondered what might have propelled early Earth into a period of relative stability, and looked to other important events in our planet’s rocky history.

If their timing is correct, the daylength plateau followed significant fluctuations in early Earth’s atmospheric conditions: notably the Great Oxidation Event, in which oxygen levels rose and formed an ozone layer before falling again.

Mitchell and Kirscher say this added ozone may have absorbed more sunlight than water vapor, which could stimulate Earth’s lesser-known solar tides, which pulse in the atmosphere as it warms during the day.

The sun’s atmospheric tides are not as strong as the ocean tides, which are controlled by the moon’s gravitational pull. But when planet Earth spun faster in the past, the moon’s gravitational pull would have been weaker – a quarter of its current strength.

And if atmospheric tides accelerated from an injection of ozone and sunlight, as Mitchell and Kirscher suggest, that might have been enough to balance the opposing forces and lull the Earth into a very long, stable period of 19-hour days.

“At the resonance point, oceanic and atmospheric tidal torques would balance and stabilize the Earth’s spin rate at a constant day length,” Mitchell and Kirscher explain.

Of course, much more research is needed to “further test and more accurately determine the resonance period,” Mitchell and Kirscher note.

However, the duo concludes that their work is consistent with the notion that sudden climate change delayed the rise in oxygen levels and complex life on Earth “until the resonance was disrupted.”

“Longer days could [then] provide photosynthetic bacteria with sufficient sunlight to increase oxygen levels high enough to support large metazoans,” they write.

But judging by our growing understanding of the “boring billion,” expect these estimates of the early daily Earth’s rotation to change as more evidence comes to light.

Some researchers have argued in recent years that the boring billion was far more dynamic than its name suggests, and that its fertile soil provided a “slingshot” for complex life that we still marvel at today.

The study was published in Natural Earth Sciences.

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