How life could help atmospheric tides slow a planet’s rotation

Layers of Earth’s atmosphere

Some fairly advanced theorising here, but the possibilities look interesting. For example, could ‘resonant trapping’ exist?

Resonating oscillations of a planet’s atmosphere caused by gravitational tides and heating from its star could prevent a planet’s rotation from steadily slowing over time, according to new research by Caleb Scharf, who is the Director of Astrobiology at Columbia University.

His findings suggest that the effect is enhanced for a planet with an atmosphere that has been oxygenated by life, and the resulting ‘atmospheric tides’ could even act as a biosignature, reports Phys.org.

Tides can distort a planet’s mass, which in turn affects its rotation. We’re most familiar with gravitational tides, which on Earth we feel from the gravity of the Moon and the Sun. These gravitational tides create bulges as Earth spins, and the Moon and Sun tug on those bulges, slowing the spin.

By contrast, atmospheric tides, sometimes called thermal or solar tides, occur when sunlight heats the surface and air on Earth’s daytime side. That heating shifts the atmosphere’s mass from the hottest point to cooler points on the planet. As with gravitational tides, atmospheric tides cause bulges that are vulnerable to gravitational pulls. Those bulges subtly change the shape of Earth’s atmosphere, stretching it from a sphere to something slightly less symmetrical and more elliptical. Scharf suggests imagining a ‘handle’ on Earth, and that forces pulling on the atmospheric handle can then help speed up or slow down the planet’s rotation.

Resonant frequencies

Usually, the effects produced by these thermal tidal forces are relatively small, but the effects can be increased in certain circumstances, such as in resonances. These are natural vibrational frequencies that describe the undulating movement of bridges in the wind, or being pushed higher and higher on a swing. Atmospheric redistribution becomes amplified when the planet’s rotation rate matches the natural frequency of the atmosphere’s oscillation.

Scharf uses another metaphor to explain resonance: “It’s like playing a violin,” he tells Astrobiology Magazine. “The atmosphere is a violin string wrapped around the planet. If you draw the bow at the proper speed across the string, you get the right note and the loudest sound.”

Scientists believe resonance occurred with Earth when its days were about 21 hours long. That day length would have created a peak in the atmospheric movement, which means it would have felt the strongest tidal pulls from the Sun and the Moon, creating a particularly big ‘handle’ and maximum torque. At that resonance, the influence of a star over a planet’s atmosphere is at its largest, as are the effects on the planet’s rotation.

A phenomenon called ‘resonant trapping’ can occur when the opposing forces exerted on the atmospheric handle, and by the planet’s usual gravitational tides, reach equilibrium, locking in the planet’s rotation rate.

Breaking out of the trap

According to Scharf, research suggests that Earth may have been resonantly trapped at the 21-hour day length for “hundreds of millions of years,” perhaps in the Precambrian Era over 500 million years ago. The effects of resonant trapping are difficult to measure by themselves, but in general Scharf notes that planets with faster rotations have hotter equators and cooler poles. Being resonantly trapped may have affected Earth’s climate, but more important is the role of resonant trapping in climate evolution.

Resonance can be (and in the case of Earth would by necessity have been) broken by temperature fluctuations, such as a rapid warming after a deep freeze, which would re-initiate the increasing of the day length over millions of years as a planet’s rotation resumes slowing down.

For instance, it’s possible that 3 to 4 billion years ago, Earth had a 12-hour day, and that over time it lengthened to 24 hours. At some point in the distant future, an Earth day could be longer than 24 hours.

Continued here.

via Tallbloke’s Talkshop

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November 6, 2018 at 05:03AM

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