Weather in a tidally locked planet's crepuscular zone

Question

I'm currently designing a world for a project I am working on. It's supposed to be a grounded and comparatively realistic setting. The setting is a tidally locked planet in its star's habitable zone, where liquid water is only possible on the crepuscular ring surrounding the planet perpendicular to the direction from which starlight hits. The planet is much larger than Earth, and has no iron core or magnetic field, and is subject to extreme weather conditions, but also negligible tectonic movement.

A single moon creates what the inhabitants know as day and night, circling above the habitable ring. The moon has a surface of ice, and a high albedo, reflecting the sunlight onto the planet's surface to create the day-night cycle for the crepuscular zone between the dark side and the sunward side.

I'm trying to figure out realistic wind and ocean currents. This planet's weather conditions would be quite extreme as far as I can tell, but the direction of the wind and potential weather cells elude me. I've tried researching some things myself too, but I've not found any answers that really fit the criteria of this world. The most I've found described the entire surface of a tidally locked planet and its wind cells, which helped only marginally.

Answer 1

I am definately not an astrometeorologist, so take the following with a grain of salt. I believe that on a completely tidally locked planet, even the crepuscular zone can't support life.

My reasoning is as follows

1. the dark side of the planet will be very cold, and the bright side very hot.
2. Any water originally on the bright side would have long-since evaporated and moved either to the crepuscular zone or to the dark size.
3. Water in the crepuscular zone will regularly cycle into the atmosphere due to the normal effects of evaporation/aerosolisation/transpiration etc.
4. Once in the atmosphere: some may blow into the bright zone - and will eventually return back to the crepuscular or dark zones (as it can't settle as rain): some may fall again in teh crepuscular zone (as rain/hail/snow); but some will blow into the dark zone and will then freeze out of the atmosphere as snow/ice.
5. Once water has frozen in the dark zone it is more or less stuck there - it cant evaporate as it is too cold so won't re-enter the atmosphere. A small amount may flow back into the crepuscular zone via glaciers, but that would only be for water deposited repatively close to the zone boundary - any water vapour that is deposited more than a few 100 km into the dark zone is there for keeps.
6. As there is no planetary rotation, there are none of the normal temporal variations in temperature and/or coreolis forces that drive the earths weather, so I would expect there to be very little 'weather' anywhere on the planet.
7. and as there is effectively a perpetual 'drain' of water from the crepuscular zones to the dark zone (extending over geological time periods) I would expect the crepuscular zone to be very very dry.

Is the following a realistic scenario for you? Rather than the planet being totally tidally locked it retains a very slow residual rotation - maybe as little as one rotation per 10,000 years. If that was the case then the crepuscular zone would slowly move around the planets surface. On one side of the planet the CZ would catch up on geologically historic ice deposits, so glaciers and melt ice would continuously replentish water supplies. On the other side of the planet the CZ would slowly enter the edge of the bright zone so would be pretty dry and dusty. With such a slow rotation rate, the zone only moves at around 4 km/year at the equator so depending on the CZ's width (maybe 1000 km?) it might only be necessary to rebuiuld/move once every 5-10 generations.

I think this is arguably realistic in terms of the physics and atmospheric dynamics, and could have potential plot bonuses - 5000 year old ruins reappearing across the horizon from some long-lost city that has passaged through either the bright or dark zone...