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I plan on having an ocean-planet with a breathable atmosphere built into my current setting, orbiting its host-star (a K4v main-sequence star) a short distance beyond its frost-line, with oceans that are much deeper than those of earth, making up most of the planet's mass.

It doesn't have any natural satellites, is about 1.4 times as massive as earth (8.361*10^24 Kg) has a radius of 1.3 Earth-radii (8,282.3 km) a rotational period of 4.4 days, inclination of 5°, and a surface-gravity of 8.129m/s².

i'm not sure if those mass-radii ratios are entirely realistic because planet-radii seem behave weirdly when it comes to ocean-planets, but those were the proportions i ultimately went with.

My plan was for the planet to have surface-temperatures that constantly remain below 0°c, with minimum temperatures reaching up to -130°c.

Now i know that our own solar system has a purported ocean-moon, i.e. Europa, but unlike the planet i planned on implementing in this setting, it neither has a liquid surface, nor a breathable atmosphere.

I intentionally describe it as "liquid", because it doesn't necessarily have to have oceans made of water, it would be enough for them to be of a liquid which doesn't produce deadly vapors that could kill humans.

My question would be: Is it possible for a planet like this to exist, and under what circumstances could this planet retain liquid oceans under temperatures below 0°c while still having a breathable atmosphere?

Additionally, what could it look like given the question's parameters? (i.e. things like color of the oceans and atmosphere given the chemicals they're made of, or possibly the weather it might produce.)

*edit: You might realize that some of the responses look like as if they're referencing the question incorrectly. I edited the question heavily to make some more clarifications, hence the bounty as well.

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    $\begingroup$ Earth has "surface-temperatures reaching as far as -15°C". The year-round average in the Antarctic interior is apparently -57°C; plenty of other places will occasionally get as cold as -15°. So are you interested in a planet that has some places that are sometimes below freezing, or some places that are always below freezing, or some/all places with a year-round average below freezing, etc etc. All of these options are wildly different; and if you're using average temps then obviously there will be some times & places that are hugely warmer or colder than your single figure. $\endgroup$
    – Ben
    Sep 17, 2023 at 3:33
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    $\begingroup$ That seems an incredibly narrow temperature range though. Temperatures on Earth range from something like -90°C to +50°C, with a global average of something like +14°. If your global average temperature is only a little below freezing, then I'd be surprised if it never had places with maximum temps well above freezing. To get maximum temperatures (e.g. near the equator, if the planet has low axial tilt) a little below freezing, I think (though I don't have calculations) you'd need a global average a lot colder than -15° (and extremely cold at the poles in winter). $\endgroup$
    – Ben
    Sep 17, 2023 at 4:16

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Similar to the other answers, yes there are lots of liquids that melt below freezing and at a pressure of 1 atmosphere.

Liquid Possibilities

Butane and Pentane

You said that your planet had temperatures as low as -15ºC (5ºF). In that case, I would suggest using butane and/or pentane. These hydrocarbons have a low freezing point and could potentially work.

As for the toxicity problem, both butane and pentane are quite toxic, but if you redesign your lifeforms that would be inhabiting this planet to cope with these hydrocarbons, it could work.

Salt Water

If you really don't want a toxic liquid, you could use very very salty water (like much more salty than ocean water which has a 3.5% salinity rate) and that is totally non-toxic.

The way this works is that the water only freezes and the salt doesn't which decreases the freezing point of the water.

So if you used a concentration of 27% (270‰) salt in your salt water, the ocean could stay liquid till -21ºC and you could adjust your lifeforms' biology accordingly to cope with the very high salt concentrations.

Note: Also note that your salt will consist of mostly table salt ($NaCl$), but there could be $KCl$, $MgCl_2$, $CaCO_3$, $CaSO_4$ (and other sulfates - $SO_4^{2-}$), and some bromine (ex. $KBr$) and strontium (ex. $SrSO_4$).

Atmosphere

You edited your question to ask about the colors of the ocean, the weather, and the atmosphere.

If you had a hypersaline ocean (which I would recommend more), you could have an atmsophere quite similar to ours with nitrogen, oxygen, and argon (from the decay of $^{40}K$ to $^{40}Ar$). You could make your atmosphere locally rich in some other elements, like the noble gasses (most likely neon) and maybe some chlorinated and flourinated compounds.

If you choose hydrocarbons (butane and pentane), you can not have a oxygen rich atmosphere as the whole ocean would combust. Instead you could have nitrogen, argon, and some other gasses that you choose, though your gravity is too weak to retain any hydrogen or helium. You probably would have a little $CO_2$ produced from the absorbtion of any oxygen by the hydrocarbons.

The life on the hydrocarbon planet could always consist of obligate anaerobes (die in the presence of oxygen), aerotolerant organisms (don't die in the presence of oxygen but can't use it), or facultive organisms (can use oxygen and not use it and survive either way).

You would also have a probem, because without free $O_2$, you can't form $O_3$ (ozone) which is one of the ways to combat UV radiation. However, since your planet is more distant from its star and your star is more red (K4V), this shouldn't be as much as a problem.

Note: In all these scenarios, I am just trying to make the world's atmosphere similar to earth's. You can always add some other gasses to your atmosphere (noble gasses, flourinated and chlorinated compounds, sulfur compounds, etc.)

Weather

In the salt water scenario, I'm pretty sure that the weather would be similar to earth's unless you plan on adding some other gasses to the atmosphere.

The hydrocarbon scenario would have very different weather from earth (in ways I can't even predict) as there would be very little oxygen in the atmosphere and it would probably mostly be in the form of $CO_2$.

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  • $\begingroup$ 27% salt water is "non-toxic" in the since that you won't die just by breathing its fumes. But that much salt in your environment is still going to be really bad for human health. $\endgroup$
    – Nosajimiki
    Sep 18, 2023 at 14:07
  • $\begingroup$ Butane and Pentane sound like an interesting option honestly, but i'm a bit concerned with how flammable they both are. The planet's atmosphere would have to be made of something else other than oxygen to prevent the oceans from combusting, which i wanted to avoid because i aimed for something breathable. Unless the temperature somehow stops them from reacting. $\endgroup$
    – NimRad
    Sep 18, 2023 at 14:08
  • $\begingroup$ @Nosajimiki Yes I do realize that that would be bad for human health, but it isn't straight out toxic and also, there aren't many very good candidates here. Finally, the lifeforms on the world this is on could always adapt to the extremely high salt concentrations. $\endgroup$
    – Neil Iyer
    Sep 19, 2023 at 1:23
  • $\begingroup$ @NimRad I am pretty sure that low temperatures don't stop the reactions and probably only slow them down. If you would like you use those hydrocarbons (butane and pentane), your oceans wouldn't really exist and you would probably get CO₂ and water (H₂O), so no, I wouldn't suggest using it if you have a oxygen-rich atmosphere as you would end up with a huge greenhouse effect (from the CO₂), which would make your planet hot, negating the whole subzero temperatures thing. $\endgroup$
    – Neil Iyer
    Sep 19, 2023 at 2:32
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The lower the atmospheric pressure, the lower the temperature at which water boils and becomes water vapor. At a low enough atmospheric pressure liquid water is not possible since water will be either solid ice or gaseous water vapor, and ice will sublimate directly to water vapor.

A planet habitable for liquid water using lifeforms in general must have liquid surface water, and you want a habitable planet with liquid. Planets habitable for humans would be a subset of planets habitable for liquid water using life in general.

So your planet could have many other liquids on it, but it must have large oceans, or even a world-wide global ocean, made of reasonably pure water. Thus the amounts of other liquids present must not be enough to poison hypothetical native lifeforms featuring in the story, and must not be enough to poison any humans who drink it (possibly after purifying it) - unless such poisoning is a plot point.

And if the planet has a dense enough atmosphere water may be stay liquid at temperatures lower than the freezing point of water.

Here is a link to a phase diagram of water:

https://en.wikipedia.org/wiki/Phase_diagram#/media/File:Phase_diagram_of_water_simplified.svg

And apparently water freezes at about 0 degrees C at any atmospheric pressure similar to that of Earth. Water would remain liquid at minus 15 degrees C only at pressures about a thousand times higher than Earth's atmospheric pressure.

The atmospheric requirements for your human characters, or any plants and animals with similar requirements, are described in Stephen H. Dole, Habitable Planets for Man (1964). Clearly humans could survive in an atmosphere with the right composition a few times as dense as Earth's atmosphere, but nowhere near as dense as would be necessary for water to be liquid at - 15 degrees C.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf

Clearly the oceans can't be made of pure water, they have to include other ingredients which act as antifreeze, preventing the mixture from freezing, and those natural antifreezes have to be non poisonous for any lifeforms which drink the water, including possibly humans if the story requires it.

Neal lyer suggested using very, very salty salt water in the oceans of your world.

I note that blood is water mixed with many other chemicals and so should have a lower or higher freezing temperature than water. And of course it is perfectly safe to drink non infected blood.

On Earth, a lot of organisms which live in cold climates have natural forms of antifreeze in their bodies, often in their blood or sap. And since some organisms prey on those plants and animals, the natural antifreezes are not poisonous to Earth organisms, at least they are not poisonous to any Earth lifeforms which eat those planets and animals.

Antifreeze proteins are ice-binding or ice-structuring proteins that prevent water from freezing by adsorbing to the ice surface and stopping the growth of minute ice crystals to large crystals in a non-colligative manner. The antifreeze proteins are found in species like fish, arthropods, plants, algae, fungi, yeasts and bacteria. The diversity, distribution and classification of antifreeze proteins were highlighted in this review.

https://pubmed.ncbi.nlm.nih.gov/27739374/

So perhaps the other chemicals in the ocean of your world might include a combination of salt, as suggested by Neil lyer, and ethanol as suggested by L.Dutch, and various antifreeze proteins, which together help the water to say liquid at minus 15 degrees C, and which are in combination are not poisonous to native life and perhaps not even poisonous to humans if the plot requires it.

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  • $\begingroup$ That's a very interesting take actually! I've heard about the anti-freeze proteins before but i didn't make that connection somehow. Could be possible that the planet has some microbes or algae on it which can produce them naturally. $\endgroup$
    – NimRad
    Sep 17, 2023 at 3:53
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There are many substances which are liquid below 0°C (and 1 atm pressure):

  • methane (melts at −182 °C, boils at −161.5 °C),
  • methanol (melts at −97.6 °C) and
  • ethanol (melts at −114 °C)

are just three of them, which could be abundant enough to form an ocean on an hypothetical planet.

But making a planet habitable without water is nowhere possible, as long as you want humans in the picture. Though the ethanol ocean would give something to party while waiting for the end, humans cannot live without water.

Which is why there is such enthusiasm whenever ice is found on the Moon or on Mars: because it helps solving one of the major problems for a permanent base.

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You say it doesn't have to be an ocean of water, but... if you want an atmosphere with oxygen and an ocean that won't produce toxic-to-humans vapors, you really don't have a lot of options. It has to be something that won't spontaneously react with oxygen, which means no hydrocarbons or carbohydrates. Bromine won't burn, but it's ridiculously toxic, doesn't have a freezing point that low, and you wouldn't realistically be able to get an ocean of it. A mixture of carbon tetrachloride, trichloroethylene, and chloroform would give you a liquid ocean with a low enough boiling point, but... chloroform. I trust you can see the problem there with human habitability. And again, good luck getting enough of that together to form an ocean. Perfluorocarbons would be human-compatible and have sufficiently low freezing points, and even come with the bonus of dissolving enough oxygen into the liquid phase that humans could not drown, because the liquid would be breathable. (Although they could still suffocate from the effort of breathing dense liquid, or freeze to death if they fell in.) But again... good luck collecting an ocean's worth of perfluorocarbons. And any of those options still leave open the question of where the oxygen in the atmosphere coudld come from--if the ocean doesn't contain oxygen in its chemical makeup, how are biological or any other sort of process supposed to free up a bunch of it in gaseous form?

Your best bet is just making the oceans hypersaline. I am not certain just how much maximum freezing-point depression you can get by dissolving various types of salts in water, but Don Juan Pond in Antarctica provides an existence proof that hypersaline bodies of water can naturally exist at least down to -50C.

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    $\begingroup$ Wikipedia says that "high concentrations of carbon tetrachloride can affect the central nervous system and degenerate the liver and kidneys. Prolonged exposure can be fatal." So the CCl₄ is also problematic and trichloroethylene is also problematic (it produces anesthetic effects}. $\endgroup$
    – Neil Iyer
    Sep 19, 2023 at 2:36
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I know you asked for a liquid, but have you considered a ocean of sand?

Sand can be fluidized with air, as certain other particles can. You could have an ocean that contains no water, and cannot freeze. You could have 'vents' that have a constant flow of hot gases from underground pumping into the sand, fluidizing it.

If the air pressure is high enough on your planet (Likely preventing whatever you wanted to be there.) the water will remain liquid even below 0.

You said no moon, but if you had a belt around your planet, like Saturn, you could have enough tidal force to 'knead' the ocean, keeping it moving enough to stay water.

It is possible your water could have a alien form of algae that generates enough heat to keep the surface from freezing, but this likely stretches science.

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  • $\begingroup$ fluidized sand... i didn't even know that was possible, but i love the way it looks, just watched a video on it- I'm not entirely sure if this would work on a planetary scale, but if it could, that would be a very interesting option. $\endgroup$
    – NimRad
    Sep 20, 2023 at 21:45
  • $\begingroup$ @NimRad Brandon Sanderson did it in Tress and the Emerald Sea. Except that planet was fluidized magical spores. Probably not possible, but this is not hard science here, this is rule of cool! $\endgroup$ Sep 20, 2023 at 21:54
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Geothermal "Antarctica" Planet


Your planet is very Earth-like in composition with a few minor differences: it has more water, substantially more geothermal activity, but absorbs less solar radiation.

This combination of factors leads to atmospheric temperatures similar to those of Antarctica (with below-freezing temperatures for water-ice at the surface) but with geothermal activity keeping the oceans warm enough to circulate as liquid water.

Eventually things will freeze solid as those geological processes wane over geological timescales... but there are still hundreds of millions of years for life to evolve and flourish in the meantime.

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