What would a tidally locked Earth be like?

[Tammuz]

Plenty of exoplanets we find are tidally locked, and reportedly they may be more common than previously thought.

This led me to the thought, what if the Earth (or a planet of the same size with the same distance to its star as we have, and the same kind of star at that) was tidally locked? One side always facing the sun and always having daylight, the other side always facing away, always being dark. I assume the side facing the Earth would be mercilessly hot, the other frigid cold, with a temperate band between them. But then, could the Earth even maintain its atmosphere in such a scenario? The side always facing the sun, how hot would it be? I assume it would be too hot for lakes or seas to remain over time.

 

[Elixir]

The temperature gradient would probably produce winds in the thousands of mph.

 

[ideologyhunter]

The rich would own all the houses built along the light/dark dividing line, and they'd be called Linies. The super-rich would own mansions that actually straddled the line -- the 1% Linies. Everyone else would either be charred and poor (Burnt Vermin) or pale and poor (Ghost Vermin). Tropical tree crops would be grown in a farming zone just inside the light zone. Root vegetables would be grown just inside the dark zone. Popular foods would include dates, parsnips, pomegranates, beets, cactus leaves, and potatoes. I guess if you never had pizza, pasta, ice cream, and muffins, you wouldn't miss 'em. Edge of Night would be the longest running soap opera, instead of ending in the 80s, as it did in our setup. As the World Turns would never build an audience and would be cancelled after a few episodes. At some point the Linies would demand that walls be built a hundred miles out on either side of the line, and the Burnt and the Ghosts would have to pay for them.

 

[bilby]

Edge of Night would be the longest running soap opera

And the most popular movie franchise would be Terminator.

 

[Spacetime Inhabitant]

With the Earth one also has to take into account the axial tilt, as this affects whether majority of Antarctica facing the sun. Whatever the case the melting of large parts of the Arctic and Antarctic ice will mean high sea level rise, so that is an additional bigger problem (this is if it suddenly happened today).
Also Earth has had various different ecologies in its history, so when did this process start. I assume that if it wasn't magically quick it would be a very long process. This means human civilization unlikely to arise, so no super rich to worry about. With far less liveable terrain evolution of life as we know it would be probably slowed, and definitely different to what our Earth experienced.

 

[Elixir]

Could it be set up so the planet remains tidally locked, but rotates on an axis that always points toward the sun? Or would its gyroscopic stability automatically overwhelm the tidal lock?
Just asking; I’d like my temperate zone garden to get light from all directions, instead of looking at the same bright spot in the same exact place on the horizon all the time.

 

[lpetrich]

Lorenz Energy Cycle: Another Way to Understand the Atmospheric Circulation on Tidally Locked Terrestrial Planets - IOPscience - rather difficult for me to follow, but go to Figure 2 in it, and it will show the calculated circulation for a slowly-rotating tidally-locked planet. The day-side hemisphere will be heated by the Sun, of course, with the point in the middle of it heated the most, the subsolar point, the point where the Sun is at the zenith. Air will rise there and travel in the upper hemisphere to the night-side hemisphere, and it will sink in the center of that hemisphere, the antisolar or absolar or contrasolar point, the point where the Sun is at the nadir. It will then travel back to the day-side hemisphere. Need an Antonym for “Subsolar” : r/words

Atmospheric Circulation of Tide-Locked Exoplanets | Annual Reviews - uses a counterpart of "antisolar point". Also has atmosphere temperatures in Figure 4.

If this Earth has an ocean near the subsolar point, the Sun will evaporate the water there and it will then rain back down as the air travels to the antisolar point.

In the first paper's calculation, the air will be 30 C out to about 45d from the subsolar point, 20 C at 60d, -10 C at 75d, -40 C at 90d (the terminator) and -60 C at the antisolar point.

In the fecond paper's calculation, the air will be 100 C at the subsolar point out to about 45d, then down to 30 C at the terminator, and then 0 C to the antisolar point. Scaling down to more pleasant termperatures means subtracting 70, giving 30, -40, -70 C.

The returning air is cold because it is coming out of the night side.

The first paper also has wind speeds, and the wind is the fastest over about 30d to 60d from the subsolar point: 15 meters/second. That's about as fast as the fastest ocean winds on our planet.

 

[lpetrich]

There is a certain problem for evolution on a tidally-locked planet. Lack of selection pressure for nighttime survival. On our planet, the day-night cycle provides such selection pressure, but it will be absent on a tidally-locked planet. That is because if the day side has tolerable temperatures, the night side will be too cold, thus giving no reason to visit there.

That will mean that night vision and above-water echolocation will have no chance of evolving, and we wouldn't be able to see anything on the night side. That means that we would not be able to see the stars.

 

[Swammerdami]

Gravitational force varies as inverse distance squared, so the massive Sun's gravitational force on the Earth is much greater than the Moon's -- it is about 390 times as distant, but about 27 million times as massive as the Moon.

HOWEVER tidal force, like magnetism, has a large inverse distance cubed component, which makes that component about twice as large for the Moon as for the Sun.

(With BOTH Moon and Sun having significant tidal effect on the Earth, does that make it harder for the Earth to get "locked" to the Moon?)

 

[Jokodo]

With the Earth one also has to take into account the axial tilt, as this affects whether majority of Antarctica facing the sun. Whatever the case the melting of large parts of the Arctic and Antarctic ice will mean high sea level rise, so that is an additional bigger problem (this is if it suddenly happened today).

I would assume "tidally locked" also implies no significant axial tilt. Either way, I doubt a sea level rise would be a likely consequence, more likely the opposite. With high evaporation on the sunny side and deep subzero temperatures on the dark side, I'd expect most of the Earth's surface water to get locked up a ice within a relatively short time. Winds from the sunny side will tend to carry high humidity air, and the water will precipitate as snow as soon as it cools off behind the terminator. Winds from the dark side, on the other hand, will be extremely arid. I'd expect a slow drying up of the oceans and a build up of an ice ring just behind the terminator (or maybe of two ice ridges in the tropics, not sure what the prevalent sounds would be on such a world), with only glacier flow and plates tectonics making some of that water available again. That's assuming at least and if the snow falls on land, if almost all of it falls onto an ocean that retains a liquid underside, things will be different.

 

[bilby]

There is a certain problem for evolution on a tidally-locked planet. Lack of selection pressure for nighttime survival. On our planet, the day-night cycle provides such selection pressure, but it will be absent on a tidally-locked planet. That is because if the day side has tolerable temperatures, the night side will be too cold, thus giving no reason to visit there.

That will mean that night vision and above-water echolocation will have no chance of evolving, and we wouldn't be able to see anything on the night side. That means that we would not be able to see the stars.

This doesn't seem plausible.

A prey animal that can flee across the terminator into darkness, where its predators cannot see it, would have an advantage; Consequently both predators and their prey would have an evolutionary pressure to develop systems to navigate in the darkness.

 

[Loren Pechtel]

Could it be set up so the planet remains tidally locked, but rotates on an axis that always points toward the sun? Or would its gyroscopic stability automatically overwhelm the tidal lock?
Just asking; I’d like my temperate zone garden to get light from all directions, instead of looking at the same bright spot in the same exact place on the horizon all the time.

I don't believe that could happen. There's nothing to cause the rotation axis to keep pointing at the star. And nothing to tidally lock, either.

 

[Loren Pechtel]

There is a certain problem for evolution on a tidally-locked planet. Lack of selection pressure for nighttime survival. On our planet, the day-night cycle provides such selection pressure, but it will be absent on a tidally-locked planet. That is because if the day side has tolerable temperatures, the night side will be too cold, thus giving no reason to visit there.

That will mean that night vision and above-water echolocation will have no chance of evolving, and we wouldn't be able to see anything on the night side. That means that we would not be able to see the stars.

This doesn't seem plausible.

A prey animal that can flee across the terminator into darkness, where its predators cannot see it, would have an advantage; Consequently both predators and their prey would have an evolutionary pressure to develop systems to navigate in the darkness.

You beat me to it. Doesn't even have to be animals, I could see plants extending onto the night side to provide some protection from grazers. Think of weeds that will grow back if the whole top of the plant is lost--except it's putting it in the dark rather than underground.

 

[Elixir]

Could it be set up so the planet remains tidally locked, but rotates on an axis that always points toward the sun? Or would its gyroscopic stability automatically overwhelm the tidal lock?
Just asking; I’d like my temperate zone garden to get light from all directions, instead of looking at the same bright spot in the same exact place on the horizon all the time.

I don't believe that could happen. There's nothing to cause the rotation axis to keep pointing at the star. And nothing to tidally lock, either.

Well, Loren, I already almost gave up on it but then I had a thought : what if 90% of the mass of this planet was on the sunward hemisphere? Isn’t there any orbit possible that would let the sun keep that hemisphere pointed at it.

 

[Jokodo]

There is a certain problem for evolution on a tidally-locked planet. Lack of selection pressure for nighttime survival. On our planet, the day-night cycle provides such selection pressure, but it will be absent on a tidally-locked planet. That is because if the day side has tolerable temperatures, the night side will be too cold, thus giving no reason to visit there.

That will mean that night vision and above-water echolocation will have no chance of evolving, and we wouldn't be able to see anything on the night side. That means that we would not be able to see the stars.

This doesn't seem plausible.

A prey animal that can flee across the terminator into darkness, where its predators cannot see it, would have an advantage; Consequently both predators and their prey would have an evolutionary pressure to develop systems to navigate in the darkness.

You beat me to it. Doesn't even have to be animals, I could see plants extending onto the night side to provide some protection from grazers. Think of weeds that will grow back if the whole top of the plant is lost--except it's putting it in the dark rather than underground.

A rhizome several kilometers long? There's got to be an extended twilight zone where the light gradually diminishes...

 

[Loren Pechtel]

Could it be set up so the planet remains tidally locked, but rotates on an axis that always points toward the sun? Or would its gyroscopic stability automatically overwhelm the tidal lock?
Just asking; I’d like my temperate zone garden to get light from all directions, instead of looking at the same bright spot in the same exact place on the horizon all the time.

I don't believe that could happen. There's nothing to cause the rotation axis to keep pointing at the star. And nothing to tidally lock, either.

Well, Loren, I already almost gave up on it but then I had a thought : what if 90% of the mass of this planet was on the sunward hemisphere? Isn’t there any orbit possible that would let the sun keep that hemisphere pointed at it.

Planets are basically spheres. Period. It's part of the definition of "planet" and effectively a certainty for anything inhabitable because I do not believe it's possible to have an escape velocity high enough to hold an atmosphere while being small enough to allow anything to resist gravity on a large scale.

(It is at least in theory possible to have a megastructure with an atmosphere and low gravity but any such structure must be active. Consider, for example, a Dyson Sphere at Earth's orbit. Assuming it's non-rotating it has a very low surface gravity but will hold an atmosphere, but it's many zeroes beyond anything which could be built. You either need some form of non-material support (of which there are not even any theoretical suggestions) or an outward force generated by objects moving above orbital velocity. Think along the lines of oodles of spinning rings inside it. For the simple case of using spinning rings to support an orbital ring I've done enough of the math to show that it's possible because force goes to zero as size goes to zero--enough rings with enough supports, a solution must exist. I very much doubt my math is up to figuring out a sphere.)

 

[Loren Pechtel]

A prey animal that can flee across the terminator into darkness, where its predators cannot see it, would have an advantage; Consequently both predators and their prey would have an evolutionary pressure to develop systems to navigate in the darkness.

You beat me to it. Doesn't even have to be animals, I could see plants extending onto the night side to provide some protection from grazers. Think of weeds that will grow back if the whole top of the plant is lost--except it's putting it in the dark rather than underground.

A rhizome several kilometers long? There's got to be an extended twilight zone where the light gradually diminishes...

Depends on how much atmosphere the world has.

However, I wasn't picturing that, but rather putting the safe part behind uneven terrain in permanent shadow.

 

[lpetrich]

A problem with that discussion. Day and night won't be all or nothing. It isn't all-or-nothing on our planet, and it won't be on an exoplanet with a thick-enough atmosphere.

• Sun at the zenith (+90d geometric elevation): 100,000 lux
• The sky with the Sun at the zenith: 20,000 lux
• Sunrise / sunset (0d): 410 - 585 lux
• Civil twilight: artificial illumination unnecessary
• Civil dawn / dusk (-6d): 2 - 3.5 lux
• Nautical twilight: ocean horizon visible
• Nautical dawn / dusk (-12d): 0.008 lux
• Astronomical twilight: most stars visible
• Astronomical dawn / dusk (-18d): 0.0006 lux

 Daylight and  Twilight and  Lux and The Different Types of Twilight, Dawn and Dusk and civil twilight - Glossary of Meteorology and nautical twilight - Glossary of Meteorology and astronomical twilight - Glossary of Meteorology

 

[lpetrich]

Plants can grow near the terminator line, but they will shade each other unless they grow far apart in the direction to the Sun. But that will likely cause selection pressure for growing in low light.

Light Requirements for Plants: Find Your Plant's PPFD and DLI
Natural Sunlight Intensity
PPFD = Photosynthetic Photon Flux Density = mcmol/m^2/s
Zenith sunlight = 2000 mcmol/m^2/s = 108,000 lux.

From that table, some plants need as little as 20 mcmol/m^2/s = 1000 lux.

What is the lowest intensity threshold of light for photosynthesis in any species? - Biology Stack Exchange
noted
An obligately photosynthetic bacterial anaerobe from a deep-sea hydrothermal vent | PNAS - estimating need only 1 mcmol/m^2/s = 50 lux. That's well into civil twilight.

So organisms could live in the twilight zone if they grow slow enough.

But from atmospheric-circulation calculations, if the temperatures are tolerable for us near the subsolar point, they are likely to be below freezing for the twilight zone.

 

[Loren Pechtel]

A problem with that discussion. Day and night won't be all or nothing. It isn't all-or-nothing on our planet, and it won't be on an exoplanet with a thick-enough atmosphere.

• Sun at the zenith (+90d geometric elevation): 100,000 lux
• The sky with the Sun at the zenith: 20,000 lux
• Sunrise / sunset (0d): 410 - 585 lux
• Civil twilight: artificial illumination unnecessary
• Civil dawn / dusk (-6d): 2 - 3.5 lux
• Nautical twilight: ocean horizon visible
• Nautical dawn / dusk (-12d): 0.008 lux
• Astronomical twilight: most stars visible
• Astronomical dawn / dusk (-18d): 0.0006 lux

 Daylight and  Twilight and  Lux and The Different Types of Twilight, Dawn and Dusk and civil twilight - Glossary of Meteorology and nautical twilight - Glossary of Meteorology and astronomical twilight - Glossary of Meteorology

This is a function of the atmosphere. Night is very abrupt on an airless world.