Okay, so I have a sapient alien species that has a carbon-based biochemistry, and is extremophilic, able to survive having their internal temperature lowered to as cold as 253 K. They are able to do so by having a substance dissolved in their blood and other bodily fluids that lowers the freezing point of water by 20 K, as well as using proteins that can function at low temperatures. What substance (or substances) can lower the freezing point of water by that much, and is nontoxic enough for an organism to survive having it in their bodies 24/7? And, can be produced via biochemical reactions?

  • $\begingroup$ Antarctic fishes. $\endgroup$
    – AlexP
    Mar 31, 2021 at 18:39
  • $\begingroup$ "able to survive having their internal temperature lowered by as much as 253 K"... "lowers the freezing point of water by 20 K". You realize there's an order of magnitude difference there? Did you lose a digit? Given that first figure is getting near the point where many gasses start to liquefy, you may have some difficulties... $\endgroup$
    – Matthew
    Mar 31, 2021 at 18:42
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    $\begingroup$ “The North American wood frog (Rana sylvatica), for instance, can survive freezing temperatures for as long as seven months, relying on a natural antifreeze in its blood to protect its organs.” (Morell 2001) asknature.org/strategy/compounds-protect-from-freezing $\endgroup$ Mar 31, 2021 at 18:49
  • $\begingroup$ Jostling the water molecules with microwave radiation might potentially be an option. It would consume food, and probably not work in confined spaces, which is kind of the point of hibernation, but it would be an option. $\endgroup$ Mar 31, 2021 at 19:03
  • $\begingroup$ @TheMadmanandtheFool wouldn't it be easier to jostle them with metabolic heat? I mean, it works for all the species on Earth that have to deal with chilly temperatures. Plenty of species can operate at -20*C $\endgroup$ Mar 31, 2021 at 19:51

2 Answers 2


The easiest thing would be to use ethanol... it mixes nicely with water, and you can choose a sufficiently alcoholic mix to fit your cold-weather needs. 40% ethanol by volume is enough to keep a pure water/ethanol mix liquid at -20°C, and I'm sure it can work for your needs, too.

Ethanol is readily synthesisable from a whole range of carbohydrate feedstocks and is a useful solvent in its own right. A mixed ethanol-water biochemistry is inevitably going to be a fair bit different from our own, but not so wildly different that it won't inevitably involve carbohydrates, lipids and proteins. One thing you can be reasonably assured of is that they won't get drunk on the same things we do... the fact that high levels of ethanol are toxic for the biochemistry of most terrestrial organisms doesn't make it intrinsically bad to all possible forms of life, so it would be necessarily harmless to your aliens.

They may not like hot weather, due to alcohol's lower boiling point and higher vapour pressure than water at the same temperature. Alcoholic sweat will help cooling, of course.

Cremation is likely to be a risky end-of-life choice for this species. In fact, their general relationship with fire seems likely to be a much more cautious one than our own.

  • $\begingroup$ That sounds about right. In fact, you've solved the problem I posed. $\endgroup$ Mar 31, 2021 at 21:15
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    $\begingroup$ The question asks for 20K, or -253°C. The freezing point of pure ethanol is 150K or -123°C. It won't work at 120°C lower than that $\endgroup$
    – nzaman
    Apr 1, 2021 at 6:00
  • $\begingroup$ @nzaman the question clearly had a typo, and it didn't take much to recognise this and understand the OP's intent. Moreover, the question was fixed nine hours ago. $\endgroup$ Apr 1, 2021 at 6:40
  • $\begingroup$ "use ethanol" adapt this method for humans (through some sort of tech or gene engineering) & they're going to wake up drunk & have the mother of all hangovers, which is cool. $\endgroup$
    – Pelinore
    Apr 1, 2021 at 11:35
  • $\begingroup$ How many animals actually use this method? $\endgroup$
    – John
    Apr 1, 2021 at 15:13

Antifreeze proteins plus urea

Most real animals use antifreeze proteins. they have independently evolved multiple times.

by itself this is not enough however, the limit on known antifreeze proteins is -13 degrees C. However another common method is retaining Urea in the blood, many deep water fish do this. so try combining these methods. enter image description here

  • $\begingroup$ "many deep water fish do this" so do wood frogs. .. "this is not enough however, the limit on known antifreeze proteins is -13 degrees C" not enough? why not? anything below freezing gets the job done, below freezing all biological activity stops, there's no advantage to going colder once you're there. $\endgroup$
    – Pelinore
    Apr 2, 2021 at 0:57
  • $\begingroup$ "below freezing gets the job done" I'd best expand that a bit .. frozen is frozen, once you're there you're there, you won't get more expansion of the already frozen water, extra ice crystals won't form because you've gone even further below freezing [pauses to ponder // can't be bothered to Google] or have I got that wrong? $\endgroup$
    – Pelinore
    Apr 2, 2021 at 1:09
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    $\begingroup$ @Pelinore I don't think you understand how these animals work, some freeze many others keep functioning, cold water fish famously live in water that is below freezing thanks to its salt content. Supercooled water is a thing. Also -13 is not enough because the OP specifically asks for -20. $\endgroup$
    – John
    Apr 2, 2021 at 1:57
  • $\begingroup$ Hmm .. just checked back on the question & then checked the edits to see if it had changed .. somewhere I got the impression he was asking for sub light space travel for cryo pods to freeze them so they wouldn't age, so staying active wouldn't have been a desired option .. seems that was erroneous... now wondering where I got that impression .. never mind, just ignore me & carry on :) $\endgroup$
    – Pelinore
    Apr 2, 2021 at 2:21
  • $\begingroup$ The wood frog uses both urea & glucose & freezes sold by the way, just in case that's any use to your answer, I note you've not included them in your list. $\endgroup$
    – Pelinore
    Apr 2, 2021 at 2:32

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