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PostPosted: Tue Jan 05, 2016 8:51 pm 
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I think that most RPGs tend to concentrate on one type of Tidal-Locked world, the "Mercury Model". BUT, especially around Red stars, you can have very cold worlds that are also tidally locked. The world could easily be Temperate or even Cold at the Sunpole and Frigid at the Nightpole. You don't have to have a baking desert at the Sunpole on every single world...

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PostPosted: Sat Jan 23, 2016 10:37 am 
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There is a paper about the winds, temperature patterns, and precipitation/evaporation on the surfaces of tidally-locked Earth-sized watery planets: Merlis, T.M. and Schneider, T. "Atmospheric dynamics of Earth-like tidally locked aquaplanets", in the Journal of Advances in Modelling Earth Systems, 2010. It considers in detail a planet rotating once per year and another rotating once per 24 hours (for the Coriolis effects), whereas it would have been better to have had something in between. Nevertheless it is indicative.

It looks as though the dark side will be uniformly cold 240–250 K, while the light side will be cool near the terminator, rising to over 300 K (but not over 310 K) in an extensive patch around the subsolar point. In the case of very slow rotation the warm patch is circular, wet in the middle and with a dry ring at its edges. In the case of rotation as fast as Earth it is a large ∑-shaped region with a >-shaped rainy region superimposed.

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Last edited by Agemegos on Wed Jan 27, 2016 12:53 am, edited 1 time in total.

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PostPosted: Tue Jan 26, 2016 9:36 pm 
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Thanks for sharing. I'll have to take a look at that one.

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PostPosted: Wed Apr 05, 2017 8:43 pm 
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Cyborg IM1 wrote:
I think that most RPGs tend to concentrate on one type of Tidal-Locked world, the "Mercury Model". BUT, especially around Red stars, you can have very cold worlds that are also tidally locked. The world could easily be Temperate or even Cold at the Sunpole and Frigid at the Nightpole. You don't have to have a baking desert at the Sunpole on every single world...

Yep. And the advantage of cooler-than-Earth tidelocked worlds is that they have temperate conditions on the sunny side, where there is light to drive photosynthesis, instead of near the terminator where it is dim.

On the other hand, Joshi et al. showed that even a slight atmosphere (100 millibars of CO2) will transport enough heat to the darkside to keep the temperature there above the freezing point of all atmosperic gasses, dispelling the suggestion of atmospheric collapse in the habitable zone. But Merlis and Schneider [2010] show that even with water acting as a working fluid to transport latent heat the dark side can get cold enough for water to freeze. They show the whole dark side as more uniform in temperature than I expected, and only about as cold as Antarctica in winter (which is warmer than I expected), but that's still room for an immense ice cap, and there is warm-moist circulation to transport the water there. The sunny sides of tide-locked worlds could be rather dry. And if they dry out the circulation of heat becomes less efective that Merlis and Schneider assume, whereupon the sunny side gets hotter and the dark side colder….

So a lot depends on return circulation of water as a liquid or ice, which as far as I know has not been studied.

First observation: a substantial fraction of the water is going to rain ou as a liquid as soon as high winds carry it to somewhere cooler than the crayfish-shaped warm zone, perhaps not even below freezing point given latent heat, it can flow back as a liquid.

Second observation: the backs of my envelopes suggest to me that even a hemispheric icecap as thick as that in the Antarctic would not contain all the water on Earth, and the Antarctic ice-cap maintains an equilibrium mass that is quite sensitive to global temperature, returning water to the oceans by glacial flow. Since the dark side of a tibe-locked Earthlike world looks like being no colder than Antarctica there ought to be a return of water to the sunny side by glaciers flowing to the terminator. Of course the glaciers will also flow to the interior of the dark side until an equilibrium (level ice plain?) is achieved, which will mean a lot of water locked up there. Habitability for a tide-locked world will depend on a modestly high initial endowment of volatiles and be favoured by a geothermal flux high enough to promote the lubrication of the beds of glaciers.

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PostPosted: Sat Apr 08, 2017 4:39 am 
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Cyborg IM1 wrote:
Wouldn't orbital eccentricity also cause tidal heating of the core? Possibly enough to allow an otherwise solid planet to have a small molten core?

Only if the tidal bulge moves or changes: tidal heating comes from the kneading effect of mantle material being forced to change shape. So if you have a tide-locked planet in a near-circular orbit there is no effect. Io is heated so much because its orbit is significantly eccentric, which means (1) that the tidal bulge rocks east and west a bit because the uniform rotation of the moon cannot match its non-uniform orbital motion dictated by Kepler's Third Law, and (b) that the tidal bulge rises and falls a bit because the tidal stress is greater at perijove than at apojove.

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PostPosted: Sat Apr 08, 2017 12:56 pm 
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I think I must have asked this on a different thread, but what are the consequences of a tide-locked world lacking a magnetic field? What kind of solar wind do you get from a red dwarf?


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PostPosted: Sun Apr 09, 2017 3:03 am 
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Matt Wilson wrote:
I think I must have asked this on a different thread, but what are the consequences of a tide-locked world lacking a magnetic field?

I don't know, and I think that the generation of planetary magnetic fields is not well understood.

Bear in mind that synchronously rotating planets are not non-rotating. A planet in the habitable zone of an M5V (such as Proxima) would have an orbital period of 16 hours, and therefore a rotational period of 16 hours. It would be actually rotating faster than Earth does.
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What kind of solar wind do you get from a red dwarf?

"M stars exhibit significant temporal variability as a consequence of phenomena occurring in the region from their photospheres to their coronae. As a result, they emit large amounts of ultraviolet (UV) radiation and X-rays during their active periods and strong stellar winds. This stellar activity can have several effects on a planet. In this paper we focus on the interaction of stellar winds with the atmosphere of planets around these stars. Stellar winds are mainly charged particles (protons and electrons) ejected from the outer stellar atmosphere driven by the pressure-expansion of the hot corona. It has been calculated that the M dwarf stellar winds are denser and faster than winds from solar-like stars (Wargelin and Drake, 2001), and these winds may be able to erode the atmospheres of planets around the stars."

Zendejas, J., Segura, A., & Raga, A. [2010] "Atmospheric mass loss by stellar wind from planets around main sequence M stars"

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