Tidally locked world

[DMScott]

Anyone ever ran a campaign in a tidally-locked world?

I'm thinking of an generally earthlike world that hasn't always been tidally locked but became that way in the recent past (within the last 50 years or so).

Hmm. That would require a close approach by (or to) another planetary body in the very recent past of your world, which is probably going to be pretty catastrophic. You might be better off assuming your world is something like an earthlike moon around a gas giant which has always been tidally locked. Creationism makes lousy science, but it does mean you don't need to have the setup be stable for very long (in planetary terms) in order to have interesting life show up .

The problem with having it be something that happens to an earthlike world is that the tidal locking might be the least of the effects you have to worry about - the close approach of an equal or larger sized planet is probably going to wreak havoc with your world's plate tectonics, climate, etc., and it might be hard to justify having any higher life left.

 

[Liminal Syzygy]

Thanks everyone for the feedback! Work has me fully occupied right now but will get back to worldbuilding in a few weeks.

 

[kipling]

Thanks everyone for the feedback! Work has me fully occupied right now but will get back to worldbuilding in a few weeks.

I'll have to check but I think that World-Building by Stephen Gillett addresses this briefly, indicating that the convection currents from the atmosphere would distribute the heat to some extent, but I suspect that cold and colder are reasonable choices.

Found this on a website(http://www.treitel.org/Richard/rass/tidelocked.html):

On sci.space.tech, Jason C. Goodman once wrote:

On Earth, the atmosphere and ocean carry about equal amounts of heat; Frank Crary's got the right idea when he suggests fluid motions would temper the climatic extremes. A nonrotating world will have much more efficient thermal recirculation; on Earth, the Coriolis force requires winds and currents to move nearly perpendicular to thermal gradients, rather than straight downgradient.

I'm a graduate student in climatology; I sat down and thought about this problem for a few hours, and cooked up a "box model" for a nonrotating Earthlike planet, in which the day-side and night-side boxes attempt to reach hot and cold equilibrium temperatures while thermal pressure-driven winds carry heat from warm to cold. I'm not sure I believe the result: the atmosphere can carry heat to the cold side so efficiently on a nonrotating planet that the temperature difference is only 5-15 degrees C! The winds in this model are 250 m/s!

If this is true, expect tropical weather with constant torrential rains in the "noontime", winds stronger than any Earthly tornado constantly during "morning" and "evening", and calm, clear, cool nights.

I'll check this result later; model details available on request.

There was some dispute, and Goodman agreed that box models may not be the best way to answer this kind of question. Later, when he tried to use a more sophisticated model to check his results, it "blew up" on him, so the question remains somewhat open.

And Brian Davis posted this Q&A (http://www.treitel.org/Richard/rass/tidelock01.txt):

Q: Can a tidally-locked world support an atmosphere? I heard it would all freeze out on the dark side, right?

A: It's a common idea that tidally-locked worlds can't have much of an atmosphere, because the dark side would get so cold that any gases would condense there, freezing the atmosphere onto the unlit side of the planet. But the atmosphere can transport heat, lots of it, as can ocean circulation if it's present. So, under what conditions can you have an atmosphere that doesn't "collapse" (freeze out)? A simple global circulation model was applied to the problem and generated some surprising results. First, at Earth-normal levels of sunlight, both a carbon dioxide atmosphere like Venus or a nitrogen dominated atmosphere like Earth's would NOT freeze out. For a Earth-like planet sporting one atmosphere of carbon dioxide, day-side temperatures would reach 330 K (or 135 F), while the dark-side would be just below 270 K (25 F, just below freezing) - at a slightly higher pressure, the dark-side temperatures would be above the freezing point of water and global oceans would even be possible. So while the dark-side can get cold, the atmosphere would not come close to freezing out.

Even more interesting is the winds: one simple assumption is that warm day-side air would rise and flow back to the dark-side, cooling and sinking there before flowing back across the terminator onto the day-side (ground level winds blow towards the center of the day-side). In fact the model showed that in the entire upper atmosphere superrotates, rotating around the planet even faster than the planet rotates, similar to Venus. Meanwhile the lower atmosphere has a very different circulation, with warm day-side air streaming at ground level across the equatorial terminators at mean speeds of 20 mph, cooling and returning to day-side across the poles (equatorial winds blow away from the day-side, while winds over the poles blow towards the day-side). So heat is rapidly distributed across the whole surface, warming the dark-side significantly and preventing atmospheric collapse.

The model was also used to study how the atmosphere was affected by the planet size, starspots (where the insolation of a M-type star can drop by a factor of two for several months), or different compositions. It was remarkably robust, and collapse was prevented even by some extremely thin atmospheres (like 10 mb of carbon dioxide, similar to Mars, under Earth-normal insolation). Liquid water oceans under a breathable (nitrogen/oxygen) atmosphere might even be possible: the dark-side must be warm enough not to trap all the water as ice, which might make the day-side so warm that UV-photolysis and hydrogen escape would lead to water loss - but M-type stars are so cool they have very little energy shortwards of 0.2 micrometers, thus UV-photolysis might not be significant.

In short, not only can such a tide-locked planet maintain an atmosphere, but it might even be habitable over much of its surface, with an active water cycle and maybe even a near-breathable surface.

Source: Joshi, Haberle, & Reynolds, "Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability", Icarus V129, pp450-465, 1997

 

[Nifft]

I think that an interesting world would be one tidally locked to a big, shiny (and perhaps even weakly radiating) gas giant. Thus, one side would be blocked from direct sun light by the planet, but would have indirect light for most of the year. The other side would have very bright direct light, and then darkness.

Seasons without days. Interesting.

-- N

 

[Liminal Syzygy]

Wow kipling, thanks! Very thought provoking... Looks like I may have to just take a theory (most likely whatever would be most interesting) and run with it.

 

[Goobermunch]

I had an interesting vision of what could have happened to your world to cause it to be tidally locked. It will depend on whether you use elemental planes or not. It has the nice benefit of mitigating some of the temperature variances inherent in the system. It also has the nice benefit of being perfectly explainable: it's magic!

My initial thought was something along the lines of magical catastrophe = comet hitting the planet. So I had a vision of an enormous mountain of magical ice jutting from the center of the "day side." Perhaps at its heart there is a seed crystal from the paraelemental plane of ice. The mountain never melts, but it cools the day side enough that it's, if not hospitable, at least habitable.

On the other side of the world, there's an enormous volcano (think Olympus Mons (or bigger)). The planet's core was, in fact, a gate to the paraelemental plane of magma. When the comet struck, the magma blew out the crust on the opposite side. Under normal circumstances, the planet's crust would have cracked, but due to divine intervention and the combined might of your world's greatest wizards, the world is intact (if barely), and the volcano merely leaks heat into the night side instead of spewing smoke, gas, and lava.

It's a temporary solution, but adequate for the moment.

--G