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Does life require Earth’s rate of rotation to lie in a certain range? What are those limits and what changes to life’s existence would there be as those limits were approached?

Mike Follows
Sutton Coldfield, West Midlands, UK

If Earth stopped spinning, gravity would remain essentially the same, but the Coriolis effect would cease, causing air to travel in straight lines rather than forming cyclonic systems like hurricanes. The planet would also lose its magnetic field, which protects us from solar wind and coronal mass ejections. As a result, it is likely that we would lose our atmosphere.

If Earth were tidally locked to the sun, one side would be in perpetual daylight and the other in constant darkness, creating extreme temperature differences. The effect of this on life has sparked speculation among science fiction writers – life could potentially exist along the terminator, the boundary between the two sides, but weather patterns and climate would change drastically. The tropical climate zone would be likely to expand, though the planet would be able to support less life.

If Earth's spin speed rose significantly, hurricanes and cyclonic winds would intensify, with more destruction

If the planet’s spin speed rose significantly, water would be pulled towards the equator, flooding land masses. Hurricanes and cyclonic winds would intensify, with more destruction.

If Earth’s spin were 17 times faster (with a period of about 85 minutes), objects at the equator would become weightless and rain would no longer fall there. At this speed, the equator would be moving at about 8 kilometres per second.

If the spin increased further, reaching the escape velocity (around 11 km per second), Earth would complete one full rotation every hour and objects at the equator would be flung into space. At higher latitudes, the speed would need to rise further for objects to become weightless. By then, however, it is likely that the atmosphere would already have escaped into space at the equator, making it irrelevant whether you remained attached to Earth.

When our planet first formed, it completed a revolution once every 6 hours or so. If its rotation continues to slow at the current rate of 1.7 milliseconds per century, its spin period will have doubled to two days by the time it is eventually swallowed as the sun expands to become a red giant in about 5 billion years. So, for truly scary scenarios, you will have to turn to science fiction.

Ron Dippold
San Diego, California, US

First, let us specify that the rate of rotation is relative to our sun. At zero rotation, one side of Earth would always face the sun and have daylight and the backside would be in eternal night. Planets like this exist in other systems, and our moon is tidally locked to Earth, so has zero rotation relative to us.

If Earth’s rotation stopped suddenly, the resulting 1670-km-per-hour air and water surges would kill almost everything. But if the change happened slowly enough or if the planet had never rotated, the situation would be surprisingly survivable for life in general.

The average global temperature would stay at around 15°C (59°F), because the solar inputs and outputs would be the same. However, the closest point to the sun would be much hotter and the furthest point much colder. This would drastically change ocean and wind patterns. Winds might be high near the terminator.

On the dark side, you would have a catastrophic loss of plants and then most other things would also die due to a lack of sunlight and food. But we know that some animals, such as deep cave dwellers, survive just fine without sunlight, and there is no reason why plants and many other organisms wouldn’t thrive with 24/7 sunlight on the day side. So I would say there is no lower rotation limit on some life existing, then evolving to fill this environment.

With the higher limit, though, there are bigger issues. Currently, any point at the equator is moving at about 1670 km/h. If this were boosted to 28,440 km/h – giving a rotation time of 85 minutes – the spin would counteract gravity. The escape velocity of Earth is 40,270 km/h, so organisms wouldn’t be flung off into space. Rather, they would be lifted into the air (very fun at first), then pulled back by gravity and smacked into some ground or water that is now moving, in relative terms, lethally fast. But gravity would have a greater effect the further you got from the equator, as the spin speed would fall as you moved closer to the poles. And I think lice could get used to 85-minute days.

Other things, however, would set an upper limit. Our oceans and atmosphere are only dragged along by friction from the rotation of the planet’s surface. At some point, the air and water drag won’t be able to keep up with Earth’s increased spin and the surface would be scoured by unending, killing-force winds and tsunamis. This would certainly happen with an 85-minute day, when water at the equator would start to slosh free, but it is also likely to occur even at less extreme spins. Most surface life would die. Yet there would be weaker winds and tsunamis towards the poles, and perhaps bacteria could survive deep inside the planet.

As Earth spins faster, the equator would bulge out further and the pole-to-pole distance would shrink. At some point, the planet would lose its structural integrity. At well below the 85-minute day, gravity would no longer hold down the water or land at the equator. And once the equator goes, the rest crumbles at the edge until you have a much smaller sphere.

There is life deep in the outer crust and perhaps extremophile bacteria exist in the outer mantle just under that, but, as far as we know, there is too much heat and pressure for life lower down.

In Surface Tension, a human ship crash-lands on an Earth-like planet where puddles of water are inhabited by microscopic life

So I can’t tell you exactly where the upper spin limit is, but there is one whenever the rate exceeds Earth’s structural integrity.

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