25 February 2009

A Tidally Locked Earth

Posted by Ryan Anderson

A while ago, I posted about an interesting abstract and poster at the Lunar and Planetary Science Conference discussion the possibility that tidally locked exoplanets might still be habitable.

Well, apparently the new Discovery series entitled “The World Without…” is doing an episode about what would happen if the Earth stopped rotating. One of their associate producers contacted me after reading my blog post about tidally locked exoplanets and asked me some more about what the earth would be like if it were tidally locked! I wrote a long response that I thought you might find interesting, so here it is!

The first thing that I should make clear is that a tidally locked planet still rotates once per orbit. If it didn’t, then the same side would not always face the star! I don’t know if that rules out a tidally-locked earth for your episode, but I will assume that it doesn’t.

If Earth were tidally locked, there would be no seasons. The only change in the amount of sunlight would come from the slight variation in distance from the sun due to Earth’s orbit being slightly out of round. Instead of seasons I suspect there would be zones of different climates depending how far away you are from the center of the side that always faces the sun. Right on the equator of the sun-facing side, I would expect very high temperatures. In the center of land masses you would probably have scorching hot deserts, and near the coasts there would be huge thunderstorms due to the rapid evaporation of the water. As you go farther away from the sub-solar point, the sun would get lower in the sky and you would have gradually cooler climates in rings. I think that the intense heating on the sunlit side and the cooler climates surrounding that area would set up circulation in the atmosphere similar to the Hadley cells that transport heat away from our equator (http://en.wikipedia.org/wiki/Hadley_cell) but the winds caused by this would not be affected by the coriolis force since the planet is rotating so slowly. So in general you would expect surface winds to blow cooler air toward the sub-solar point, where it would be heated, rise and then circulate back toward cooler climates.

The far side of the planet would be frigid, since it would never see the sun. Its only source of warmth would be ocean circulation and wilds from the warm half of the planet. Even on the sunlit side, much of the planet would never see the sun rise very high, and would be quite cold.

The slow rotation of the Earth would have an effect on the moon too. Due to the moon’s gravity, the Earth bulges a little bit toward the moon. Right now, the earth rotated much faster than the moon orbits, so the tidal bulge is always a little bit ahead of where the moon’s gravity “wants” it to be. This means that the moon’s gravity is actually slowing the earth’s rotation down and the moon is gradually moving away from the earth. If the earth were rotating really slowly, then the exact opposite would happen (I’m assuming that the moon starts off in its current orbit). The moon would gradually try to make the Earth spin faster, and in doing so the moon would lose energy and come closer to the Earth.

I would imagine that the lack of days, and the sun being at the same place in the sky all the time would have some interesting effects on life too, but I know very little about biology so I’ll leave that for someone more qualified.

They asked a follow up question, requesting more detail about the evolution of the Moon’s orbit around a non-rotating planet, so I dug out my old “physics of the planets” notes and did my best to answer:

Assuming that you start with the earth not rotating but the moon in its current orbit, the moon will exert a torque on the earth until the earth is spinning as fast as the moon is orbiting. To spin-up the earth, the moon will give up some of its angular momentum, which will cause it to gradually drop to a lower orbit. If I’m doing things correctly, the moon will be about 1/4 as far away as it currently is by the time it has transferred half of its angular momentum to the earth.

When the moon stopped moving inward, the Earth and moon would be locked facing each other, much like Pluto and its moon Charon. There would be no changing tides on earth because the tidal bulge would always face the moon. All of this tidal evolution wouldn’t happen immediately, it would probably take hundreds of millions, if not billions of years. During that time, the tides would occur with a frequency equal to the moon’s orbital period minus whatever the earth’s slow rotational period is.

Through all of this I ignored the torque and tides from the sun. Its effect will be less than that of the moon, so it might slow things down but I think the moon’s orbit and earth’s spin would still evolve in the same way, generally.

Again, this sort of bends the rules of the episode because the earth is technically rotating. If you held the earth fixed, then I think the tidal evolution would just continue until the moon crashed into us, since the moon would continue to try to give the earth angular momentum, but that angular momentum would “disappear” due to whatever magic is holding the earth fixed. This is not physically realistic because angular momentum is supposed to be conserved.