For my setting, I want to create an ice planet composed primarily of water, containing a subsurface ocean. It would be roughly the mass of Earth. As cybernetic/genetic modification is prevalent in this world, the surface temperature doesn't matter to me as much, though I'd like for it to be somewhere near -13 to -20 degrees Celsius.

Would it be possible for a planet such as this to retain an atmosphere breathable by humans without its solid ice surface melting away?


3 Answers 3


Yes. To be breathable by humans, you just need to have enough oxygen, and not too much actively toxic stuff.

Not having toxic stuff is easy. Just start with not much atmosphere--water vapor and a bit of nitrogen, perhaps.

Then, you just need to accumulate oxygen. There are two broad classes of ways to manage that: biotic, and abiotic. An iceworld is ideal for abiotic accumulation of oxygen. The ice and underlying water isolates any reducing materials in the mantle or core from the atmosphere, so any molecular oxygen that is produced won't be immediately destroyed, and very slow processes can over geologic timescales produce large total quantities of oxygen. Specifically, a world in just the right mass range could lose hydrogen to space after photodissociation of water, while retaining the liberated oxygen. And you can make that go faster by putting your world around a hotter star, or in a high radiation environment like Jupiter's magnetosphere. In fact, this exact process happen to Jupiter's ice moons, though they are all small enough that they can't retain a whole lot of the oxygen that is produced on their surfaces.

Biotic production is a little trickier in this kind of environment. You basically need photosynthesis, which means the world has to be very fine-tuned to produce a thing enough ice crust for light to make it all the way into the underlying ocean, or you need to posit life forms that have evolved to live in the ice and survive near the surface. There are Earthling organisms that can live in snow or glaciers, but they rely on the fact that temperatures sometimes get high enough for liquid water to exist. You would need more exotic organisms that use some combination of natural antifreeze or having a low enough albedo that they can warm themselves to melt pockets of water around themselves just from absorbing sunlight, in addition to using sunlight for photosynthesis.

  • $\begingroup$ The lack of any CO2 would cause a bit of an issue for photosynthesis though. I sometimes wonder if this type of world would be harder to colonise in the long term than a CO2 rich but oxygenless world like Mars. If some oxygen leaks on a CO2 world then you can always make more, but if some CO2 leaks on an oxygen world then it's just gone, permanently reducing the amount of biomass your colony can support. On an ice world like this there are no rocks you can extract carbon from, so it seems like any colony is doomed to slowly dissipate into nothing. $\endgroup$
    – N. Virgo
    Feb 12, 2023 at 7:25
  • 2
    $\begingroup$ (Still, if needed, you could probably add some underwater volcanism that happens to supply just the right amount of CO2 for plants to grow - it's not really a problem as far as the question is concerned, I just thought the above was interesting to think about. This answer gets my +1.) $\endgroup$
    – N. Virgo
    Feb 12, 2023 at 7:33
  • $\begingroup$ a world in just the right mass range could lose hydrogen to space after photodissociation of water You're describing Jupiter's moon Europa, though this has an extremely thin atmosphere. Maybe a bigger Europa would hold an atmosphere better. It may help if it's also slighly warmer, both to promote loss of hydrogen and increase the vapour pressure of H2O in the atmosphere. en.wikipedia.org/wiki/Europa_(moon) $\endgroup$ Feb 12, 2023 at 21:30
  • $\begingroup$ @LevelRiverSt If you read a bit farther, you'll see that "this exact process happen to Jupiter's ice moons". Which, yes, includes Europa. $\endgroup$ Feb 12, 2023 at 21:33
  • $\begingroup$ According to Wikipedia, Callisto is mostly CO2, but Ganymede is indeed mostly O2 (and at higher pressure, though still tiny, despite surface gravity and escape velocities being similar.) I wasn't aware of Ganymede having O2, actually that makes Ganymede a better candidate for life than Europa as it has surface rock (which presumably has some chemically reducing substances in it) as well as subsurface water and O2. The fourth Galilean moon, Io, is of course too hot for liquid water due to tidal heating. $\endgroup$ Feb 12, 2023 at 21:51

Retaining an atmosphere is not a major problem. Your planet needs to be rather more massive than the likes of Europa or Pluto to hang onto an atmosphere while being close enough to a star to have a temperature of around -20 Celsius. Given its low density as compared to Earth, it may be quite large.

Having an atmosphere breathable by humans is more of a problem, because you need a lot of free oxygen. Oxygen is extremely reactive: Earth's early atmosphere didn't have any. All the oxygen in Earth's atmosphere has been liberated as a waste product by living creatures. The Great Oxidation Event, a little over two billion years ago, was the process of transformation of the atmosphere, causing a mass extinction. This could not happen until everything else in the atmosphere, oceans and land that could readily combine with oxygen had done so.

Does your ice planet have life? Does it have lots of life, enough to have chemically processed all of the atmosphere and ocean over time? If so, how is that life powered? On Earth, that's done by sunlight powering photosynthesis. But if your planet is entirely covered by a thick layer of ice, sunlight can't reach the liquid water where life can exist. Thick ice is not transparent. If the planet is almost entirely water, life can't be powered by radioactivity (which usually happens via heat).

Having enough life to maintain a breathable atmosphere seems to be your major problem.


Consider Antarctica

Antarctica's interior has an average temperature of -57℃ and yet has a perfectly breathable atmosphere. While that's a nasty year-round average, it's only a bit colder than the coldest permanently inhabited rural town on Earth, Oymyakon, which has average winter temperatures of -50℃ and enjoyed its lowest verified temperature of −67.7℃ (it has an unverified minimum of -71.2℃).

Antarctica obviously has its challenges, including:

  • It's honking cold and, therefore, somewhat uncomfortable.
  • Due to the cold, the relative humidity is about 0.03% (dry, dry, dry, dry, dry).

But, otherwise, it has a perfectly breathable atmosphere.

Considering that your planet's -20℃ is equivalent to only -4℉, it's actually not all that cold. I live in an area that regularly has weeks of -29℃ in the winter (and one year having a month of -40℃) and there's no problem breathing here.

And the ice doesn't melt — which it won't do other than through sunlight sublimation unless the temperature rises (kinda not surprisingly) to 0℃ and above.

The problem is what happens after a time of things consuming the oxygen

Sublimation will put some oxygen back into the atmosphere, and obviously your planet has oxygen in the atmosphere if you say so (because there's nothing there consuming it and no reason it can't be there, you have water, after all).

Until people show up and start breathing it.

Granted, unless you bring a LOT of people, it'll take forever to consume the oxygen. Once they leave, sublimation will eventually restore it (or, more specifically, will believably restore it), but sublimation is really slow.

Nevertheless, I don't see why you can't have oxygen to start with and a reasonable amount to last whatever time you need people to be there, unless you bring a LOT of people (I'm thinking millions...).


Your planet's pretty balmy for an ice planet. It will have a breathable atmosphere and the ice will only melt through sublimation.


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