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I'm building a cold, water-based ocean-planet for my current setting, its temperature being below 0°c at all times and in all of its regions. Even despite the freezing temperatures, the planet's oceans are in a liquid state because of its thick, earth-like atmosphere, the large amounts of salt, ethanol and the biological anti-freeze proteins it contains due to local microbes. It's about 1.5 times as massive as earth and orbits its host-star just behind the system's frost-line.

My question would be: is this type of planet even stable to begin with? or would there be some chemical reactions taking place that would render it completely uninhabitable?

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  • $\begingroup$ Is your ocean primarily water mixed with these anti-freeze proteins, ethanol, and salt, or is it some different main liquid? $\endgroup$
    – Neil Iyer
    Sep 17, 2023 at 23:22
  • $\begingroup$ @Neil lyer it's primarily water! Only now realized that the answer didn't specify that, just edited it. $\endgroup$
    – NimRad
    Sep 17, 2023 at 23:25
  • $\begingroup$ Thanks for clarifying. $\endgroup$
    – Neil Iyer
    Sep 17, 2023 at 23:25
  • $\begingroup$ Do the local microbes actively spew out these anti-freeze proteins? Wikipedia says that "1 μM of Euplotes focardii consortium ice-binding protein (EfcIBP) is enough for the total inhibition of ice re-crystallization in –7.4 °C temperature" so do they use something like that, or is that not applicable here? $\endgroup$
    – Neil Iyer
    Sep 17, 2023 at 23:34
  • $\begingroup$ I figure the main purpose is to keep their bodies unfrozen, so it would probably be trace-amounts. As it says in the same wiki article, it mainly helps them tolerate the cold, not directly inhibiting ice-formation. $\endgroup$
    – NimRad
    Sep 17, 2023 at 23:40

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https://en.wikipedia.org/wiki/Supercooling It could lead to intersting pehenomena though.. were at the poles, supercooled ice structures could appear. Imagine a world, were the smallest turbulance can fast-grow ice mountains, surrounded by relative pure ethanol. Now if those structures regularly dissolve, it could give your planet a fascineating "landscape" ecology.. Islands are born from turmoil, drift towards dissolution

Example: You hit with a bullet or a spear on the water, the supercooled water starts to form a island.. which draws water crystalls from the water, while the ethanol remains, ever more concentrated as a later around the pillar island, which starts to turn in the water. Now we fire upon the ethanol. Heat is created. The Island formation continous were no infrared reaches and stops in the warm zone. As new structures emerge from the depths, with the island ever turning over.. mad havoc.

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    $\begingroup$ that sounds really interesting! i like that quite a bit actually. though i'm not following entirely in some aspects. could you elaborate on how those structures might form? $\endgroup$
    – NimRad
    Sep 18, 2023 at 11:14
  • $\begingroup$ Same as the ice forming in a super cooled water-bottle. All it takes is some local pressure drop or disturbance. $\endgroup$
    – Pica
    Sep 18, 2023 at 12:24
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    $\begingroup$ Supercooling is unstable, which is why a small disturbance can cause a large phase change. On a realistic planet with disturbances from day/night cycles, weather, currents, and dissolved impurities a supercooled state won't last very long. $\endgroup$
    – JanKanis
    Oct 12, 2023 at 12:55
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    $\begingroup$ This actually happens in Antarctica. They are called "Brinicles" or "Icy fingers of death". Supercooled water below the ice shelf starts freezing, forming fingers reaching down to the shallow sea floor. The ice may spread further on the surface and freeze slow-moving animals like clams and sea stars, or immobile ones like anemones. theguardian.com/news/2023/feb/16/… $\endgroup$ Oct 14, 2023 at 15:31
  • $\begingroup$ @ChristmasSnow thanks, so cool and i never knew.. $\endgroup$
    – Pica
    Nov 27, 2023 at 11:49
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Pure ethanol and pure water are miscible--ethanol and salt water are not. So you would not have an ocean of alcoholic salty water--you'd have an ocean of salt water with a separate layer of ethanol floating on top, with each layer having separate freezing points.

If you want the world to be habitable to humans, you've got another problem: ethanol is famously flammable. You cannot have an oxygen-rich atmosphere over this ocean. You'd be better off just making it hypersaline.

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    $\begingroup$ The 'salting-out' of an organic material from water by adding a salt to the solution is well known. See for example researchgate.net/publication/… $\endgroup$
    – Penguino
    Sep 18, 2023 at 3:53
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    $\begingroup$ Specifically referencing alcohol: scientificamerican.com/article/separate-liquids-with-salt $\endgroup$ Sep 18, 2023 at 5:54
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    $\begingroup$ @AlexP Basically when you have something that's more soluble and something that's less soluble, one tends to drive out the other. (Assuming they don't react to form something different anyway.) This is actually quite useful in a lot of chemical production processes for separating the things you want from the things you don't. $\endgroup$
    – Perkins
    Sep 18, 2023 at 23:06
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    $\begingroup$ The fact that you can salt out alcohols does not mean that no quantities of salt and alcohol can coexist in solution, it's a question of concentration (and in reality the two phases are a water-rich and alcohol-rich phase, not brine and pure alcohol). Further, sodium chloride appears to be ineffective for salting out ethanol...apparently potassium carbonate is used instead. $\endgroup$ Sep 22, 2023 at 16:55
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    $\begingroup$ Another issue in an oxygen-rich environment is that surely something would arise that metabolizes the alcohol for energy, preventing it from building up in the first place. The sheer quantity of carbon needed to cover an entire planet with oceans containing enough ethanol to strongly affect their freezing point is another problem. That ethanol is equivalent to a lot of biomass. $\endgroup$ Sep 22, 2023 at 17:02
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If this planet is mostly water without much of a rocky/metal core, it will actually have a solid ice core. Water solidifies at much higher temperatures if the pressure reaches ~1 GPa, which in Earth gravity would be 100 km deep, but in your world a bit shallower.

The temperature being below 0°C everywhere, without any ice forming seems a bit unlikely. If the temperature is just below 0°C at the equator, it will be a lot colder at the poles, and I would guess that even a highly saline ocean will develop an ice cap on the poles. Having a 0° axial tilt might help as then you have no seasons, but even so my intuition is that ice caps would form.

Another question regarding habitability would be the availability of minerals required for life, e.g. iron, phosphate, etc. I don't know enough about the cycles of all of them, but it is likely that some of them would sink into the solid core, and thus not be available for life at the surface.

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