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before we can discuss Tidal locking effects, we need a set of commonly agreed upon points with which one can do analysis. Characteristics of Atmospheric Gases. kPa = 0.145 PSI, 1 ATM = 101.325 kPa STP is 101.325 kPa at 273k Elemental Helium : Boiling point 4.2 K , Lambda point 2.2 K 5.048 kPa, Critical point 5.19 K , 227 kPa Neon: Boiling point 27.1 K , Triple point 24.6 K, 43.37 kPa, Critical point 44.4918 K, 2769 Kpa Argon : Boiling point 7.302 K, Triple point 83.81 K, 68.9 kPa, Critical point 150.8 K, 4,870 kPa Molecular Hydrogen:Boiling point 20.271 K , Triple point 13.8033 K, 7.041 kPa Critical point 32.938 K, 1286 KPa Nitrogen: Boiling point 77.355 K, Triple point 63.151 K, 12.52 kPa, Critical point 126.2 K, 3.4 MPa Oxygen: 90.188 K, Triple point 54.361 K, 0.1463 kPa, Critical point 154.581 K, 5043 KPa Heteroatomic. Methane, Melting point 90.7 K Boiling point 111.66 K, Triple point 90.68 K 11.7 kPa, 190.8 K 4,640 kPa Carbon Dioxide, MP 216.6 K, Triple point 216.55 K 517 kPa, Critical point 304.19 K 7,380 kPa Water, MP 272.16, BP Triple point 273.16 K, 0.61kPa, Critical point 647.096 K, 22.06 MPa Bold pressures indicat the possibility for periodic liquid precipitation of the gas type as pressure falls, as pressures fall below the pressure limit, snowing of the gas. What is the point here. Bioneogenic planets start with atmospheres of CO2, Water, Methane, Nitrogen and other gases. If major surfaces of the planet are below the triple point temperature (216.5K, 273.16 K, 90.68, 273.16 K). If temperature falls below the temperature of one of the gases, say water, you have the formation of Ice, but the atmosphere can create a dynamic equilibrium. If two are below, for example carbon dioxide, the atmospheric pressure drops as CO2 accumulates in the cold spot and sublimates with circulation. If temperature falls below three, water will become locked by liquid carbon dioxide, which will be stabilized by a sea of methane. At still lower temperature methane freezes, the atmospheric pressure drops further still, if the temperature falls below the triple point for nitrogen there will first be a rain of nitrogen, but as pressure drops it will start snowing essentially locking these on the cold side of the planet. If there are enough of these, the planet might, over time rotate and boil them onto the other side creating a very slow rotation phenomena, but not so fast as to prevent sublimation on the slow moving side, even if life were to form it complex life would be routinely crushed my massive rolling glaciers of ice that melt and reform. However because the pressure on the exposed side is so low, stellar winds and flares will almost certainly kick the isolated gases out of the atmosphere, some will condense on the other side, others will end up speeding out to space. How cold to dark sides of tidally locked planets get. Mercury- closer than the habitable zone. On the dark side of mercury, whose lit side reaches 700K the temperature is average of 100K, if mercury had a primordial atmospheric it would Move 7/8ths of the Argon, Helium and Hydrogen to the dark side, prior to their expulsion No water on the sun facing side, all Water would be locked in ice on the dark side and at poles. No carbon dioxide on sun facing side, carbon dioxide would sublimate on the dark side. Freeze or rain methane in spots. Rain Nitrogen (Oxygen would never form because all CO2 is locked up in ice), which would transfer heat from the surface to space, and further cool down the atmosphere. Our moon, and imperfectly locked example of a Habitable zone planet. Lets go a little colder, lets look at the moon, which is heated up every 30 days on one side. At this distance from its stars, so called habitable zone the dark side has a basal temperature of 30K to 50K degrees. Lets argue that the moon was tidally locked with the sun, Water would boil and sublimate (snow) on dark side. Followed by carbon dioxide. https://upload.wikimedia.org/wikipedia/commons/thumb/1/13/Carbon_dioxide_pressure-temperature_phase_diagram.svg/330px-Carbon_dioxide_pressure-temperature_phase_diagram.svg.png First if that atmospheric pressure were high enough it would rain CO2, then as the pressure fell temperature would fall and snow of carbon dioxide would coat the ice, this would be followed by methane which would eventually freeze on top of the CO2, later Ntirogen and Oxygen would rain and then freeze on the dark side. Surprisingly Argon and Neon might join the club. The planet would conduct heat to the surface, and this would potentially result in subterrestrial lakes with hot vents, while these potentially melt the most volatile substances on top, the level of insulation spreads this out and limits it they will sublime after melting and vaporization on other areas and at the edges. This will drive eventually water from the edges to the center, the volatiles will flow to the edges and be blow off by solar winds, the pressure would drop keeping ice and carbon dioxide perpetually frozen or sublimating. Caveots, larger planets, better hold atmosphere have greater thermal heat retention and lower surface area to mass, higher potential for subglacial oceans.

http://www.compoundchem.com/2014/07/25/planetatmospheres/ Technically if hydrogen is metallic at high pressures its no longer a gas.

http://sciencenordic.com/scientists-discover-cause-behind-prehistoric-climate-change? This article discusses the global phenomena that occurred as the northern hemisphere warmed 14 to 15 thousand years ago. The oceans, a giant temperature modulator,mwould disequilibrate with atmospheric temperatures causing droughts, or floods and temperatures could shift wildly.