The major problem on a Tidal Locked Planet is that water ice accumulates on the dark side, until the illuminated side might become a huge desert. Temperatures might fall so much on the dark side, that even the atmosphere would start to sublimate.
Celestial mechanics Edit
Here is why some celestial bodies are tidal locked and others are not:
Solar System Edit
However, a clear look at the planets Mercury and Venus reveal that they are not tidal locked. Mercury has an elliptical orbit. At apogee solar gravity is weaker then at perigee. This explains why Mercury is a Low - spinning planet and not tidal locked. Venus, on the other hand, has a circular orbit and should be tidal locked. However, winds on Venus are very strong and have a circular movement, that is also transmitted to the planet. Venus is in fact in an equilibrium state between solar gravity that tries to slow it down and its air currents that try to speed it up.
A solar day on Venus lasts 116.75 Earth days . This is the time needed for the Sun to return to the same place on the sky. However, Venus has an atmosphere over 50 times denser then the Earth. Assuming an atmosphere as dense as Earth's, its effect on rotation would be 50 times lower. With other words, the day would last about 18 Earth years.
K - type stars Edit
If we think about K - type stars, their habitable zone is close to where Mercury is around the Sun. There, a planet would experience stronger tidal forces then Earth and Venus do. However, if the planet has a slightly elliptical orbit or if the planet has an atmosphere to sustain circular currents, there is a chance that it will not be tidal locked. Also, if the planet is young enough, it did not have enough time to slow down completely.
If we make an analogy to Mercury and Venus, a planet orbiting K - type stars should be a Low - spinning planet.
M - type stars Edit
Around M - type stars, the hosted planet needs to be very close to be in the habitable zone. At such close distance, it will be exposed to strong tidal forces. If the orbit is elliptical, the planet will become a tortured tidal world, like Io or even worse, given the large mass of the star.
However, air currents could maintain a centrifugal force, like on Venus. Stellar gravity would be 20 to 90 times stronger then Sun's at Earth's orbit. If we take Venus around Barnard's Star and put it in the habitable zone (0.067 AU), it will be exposed to 40 times stronger. The difference between revolution period and rotation period will be far smaller. One solar day will last 40 times longer (about 13 Earth years).
Now, if we give an atmosphere similar to Earth's, the effect will be interesting. The difference between rotation and revolution periods will be even smaller, of 720 Earth years.
Moons in the Solar System Edit
However, there is a problem. Earth's Moon follows a slightly elliptical orbit. Why is it tidal locked? Also, Saturn's moon Titan has an atmosphere and winds. Why is it tidal locked?
When a ball is moving, it is gradually slowing down, until at some point it stops. And usually, at that point, it moves a bit backwards. This is because the ball reaches something higher (for example a little rock or a stick). Something similar happens for planets and moons. As they rotate, they are in a sort of equilibrium. They are not quite spheres, but ellipsoids, slightly tilted and with the center of mass located at the geometrical center.
When a planet or moon is tidal locked, it is not in the same equilibrium. it no longer has an ellipsoidal shape. in fact, it bulges towards the and behind the sun. Also, the center of mass moves a bit towards the sun (or hosting planet) and is no longer the same with the geometrical center. It is like a ball that entered a small hole or like a wheel that has one part heavier then the other. If you push it, it will return to the previous equilibrium position. If you push harder, you might turn it over, but it will try again to go to the previous equilibrium state.
Moons that are tidal locked are actually librating. The Moon is the best example. A point that is in the center of the Earth oriented hemisphere of the moon, might at some point be a few degrees left or right. Because of this, small parts of the cosmic hemisphere of the Moon can be seen from time to time on Earth. Many satellites are librating.
If the solar day is too long and tidal forces are too strong, the planet will change its shape and the center of mass will move towards the sun. When this happens, the forces that maintain the planet spinning at a different rate then rotation period are too weak to turn the planet over once more. That is the point where a planet will become tidal locked. First it will start librating with large movements, but then, it will come to an equilibrium state, with limited libration.
The stronger the gravitational forces from the star, the higher are chances a planet will be tidal locked. The weaker the forces that keep a planet spinning (elliptical orbit, circular winds), the higher will be chances for a tidal lock. It is not clear what is the lowest limit where a planet can avoid becoming tidal locked.
Human intervention Edit
However, it is possible to make a tidal locked planet to rotate. This can be made with the help of guided impacts or with the use of massive explosions on the surface or by altering gravity or the fabric of time. If the force is not enough, the planet will be librating and not spinning. From time to time, additional forces are needed to keep the planet spinning.
The planet Edit
A tidal locked planet will accumulate all its water as ice on the dark side. If temperatures drop too much, it is possible that even the atmosphere will condense. This is why tidal locked planets must be made to rotate somehow.
The idea is to make the planet rotate very slow. A perfect target would be to make the day last for over 100 Earth years. This way, settlers will not have to relocate often. They can live for many years in the same place.
Simulation for the equator Edit
Night. Suppose the planet has a day of 200 Earth years. During the 100 years of night, snow accumulates in thick layers and gets compacted. At the end of the night, there could be layers of between 40 and 200 meters of ice. It will behave like a short glaciation.
Early morning. When morning comes, temperatures rise and the ice starts to melt, only that the process lasts for many years. Winds will rotate in the same direction, so during the morning they will bring cold air from the dark side, preventing for a while ices from melting. The process will result in massive flooding and strong erosion.
Late morning. About halfway between sunrise and noon (25 years from sunrise), ice should have finish melting. Temperatures increase fast. Plants rise from seeds that have been frozen or from seeds brought by humans. Since winds will continuously blow in the same direction (from sunrise to sunset), there is no way seeds can arrive by wind. Now, we are in a period of 10 to 20 years when the land is habitable and perfect for agriculture.
Noon. At 50 years after sunrise, we are at noon. With the sun above your head, temperatures rise dangerously close to water evaporation point. The land becomes a desert, water is fast evaporated and the ocean is scourged by a never ending hurricane.
After noon. At 75 years after sunrise, we have a second habitable period, only that this time is longer. This period starts later, because heat from the noon is brought here by the winds.
Evening. Until the sun reaches horizon, temperatures don't drop below freezing, because hot air is brought from the heated areas. At 100 years from sunrise, we finally have a sunset.
Early night. Soon after sunset, there is no light, but still there is warm. For a few years, as temperature remains above zero, plants try to grow from existing seeds, but they are unable to survive without light. This affects the population of seeds that might survive during the night.
Simulation for the temperate region Edit
Around temperate regions, there will be less heat, but seasons will certain exist. Let's suppose again that day length is 200 Earth years.
Night lasts for 100 Earth years. Snow falls and is compacted in layers of 40 to 100 meters thick.
Morning comes with a sunrise over the icy land. It will take much more time to melt all the ice. There is less sunlight and winds are bringing cold air from the dark hemisphere.
Noon is the place with maximum solar luminosity and the point where ice should be melted. It will be hot, but not extremely hot. Plants and animals will conquer the land.
Evening will come with a gradual decrease in temperature. Land will be habitable all time until sunset.
Early night does not last long. First temperatures below freezing might occur very soon after sunset, preventing seeds from germinating.
Slower rotation patterns Edit
We can simulate models where a day will last 1000 Earth years. In this scenario, settlers can live all their lives in a certain climate area, but their children will have to relocate. However, at such a low spinning rate, tidal forces will overcome rotational forces and will make the planet tidal locked. Settlers will have to use artificial forces to keep their planet spinning.
Assuming a day of over 1000 Earth years, during the night, glaciers will have enough time to form. However, they will not survive deep into the illuminated hemisphere. Climate patterns will be similar to a tidal locked planet, only that each climate region will move slowly towards the sunrise.
As glaciers get form in night and melt in day, sea level can very from a century to another, depending on the amount of existing land surface. Also, as they will stay for 500 Earth years, they will create typical Geographical patterns: glacier lakes, valleys and other similar features. During day, these features will be eroded, but not completely.
Another model Edit
An analysis of Saturnian moon Enceladus shows that rough terrain like the Tiger Stripes appears in various locations, but also with different ages. The most accepted theory is that at some point an impact made the moon turn over, but not to start rotating.
It could be possible to force a tidal locked planet to move a bit, not to start spinning. I we do this, a limited area from the illuminated hemisphere will enter night and a limited area from the dark hemisphere will be illuminated. If we do this, we can recycle a limited amount of water stocked on the dark hemisphere. We can repeat the process anytime we want, if we have the resources to do this.
This way, we can have a climate pattern for as long as we have enough water. Changes will be fast, probably violent, but followed by long peacefully periods. However, if we want to make a planet roll, we need to do this for at least 30 degrees. Objects in tidal locked equilibrium have their center of mass turned towards the star they are orbiting. If we don't twist enough the planet, it will only librate and return back to previous position. Still, a large libration can bring some light to the dark hemisphere to melt some ice and feed the oceans.
Maintaining a nearly tidal locked planet still spinning can be a challenge for many scientists. The climate patterns will be interesting. The need for recycling water on a tidal locked planet may force many scientists to find ways to make their planets somehow to rotate, even if this means forcing all settlers to relocate.