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Message: <blockquote data-quote="kipling" data-source="post: 1227055" data-attributes="member: 14955">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(<a href="http://www.treitel.org/Richard/rass/tidelocked.html" target="_blank">http://www.treitel.org/Richard/rass/tidelocked.html</a>):And Brian Davis posted this Q&A (<a href="http://www.treitel.org/Richard/rass/tidelock01.txt" target="_blank">http://www.treitel.org/Richard/rass/tidelock01.txt</a>):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</blockquote>

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