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I'm working on a sci-fi setting which includes an aquatic space-faring species.

When a human space vessel is ruptured and depressurizes, the gas can escape rapidly and we immediately suffer from the effects of vacuum.

For a water-filled vessel and an aquatic species, how would the ship being ruptured affect the occupants? My first thought is that the water would mostly stay together. Water in vacuum begins to boil from lack of pressure, which cools the water and can result in ice forming.

In a violent emergency where the ship's hydrosphere is exposed, would the mass of water form an icy shell and protect the remainder of the water from boiling away? Would the mass of water get cold in whole, or just near the edges? Or would something else happen?

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If the water is already in microgravity and isn't mostly constrained by structures, the vapor pressure inside will tend to blow the mass apart into smaller masses, which will in turn blow apart more. At some point in this process, evaporative cooling will freeze the water, ending the cycle (ice has plenty of structural strength to contain water's vapor pressure at low temperatures and in small volumes). The result, however, would be closer to a gentle "snow explosion" than "boiling away". The process would take time, of course, likely much more time than explosive decompression of an air-filled volume the same size; if the aquatic space crew have good reactions/training and can move quickly (as many fish can, for a short time) they have a good chance to get into a sealed space before conditions become fatal.

Also, human skin, at least, can contain the vapor pressure of body temperature water for a while (not indefinitely, but pressure would be relieved by blowing internal contents out of existing orifices before the skin would rupture, unless it's already torn or punctured and can tear outward from the existing damage). The same may be true of your merstronauts.

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    $\begingroup$ "merstronauts" - Thank you for this ingenious word! :) $\endgroup$ – Martin Carney May 20 at 19:29
  • $\begingroup$ You're very welcome! :D $\endgroup$ – Zeiss Ikon May 20 at 19:29
  • $\begingroup$ There will also be sublimation of the frozen water directly to gas (think disappearing ice cubes), so even if a protective shell forms it won't last long... $\endgroup$ – BobT May 21 at 15:30
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    $\begingroup$ Compared to the boiling/freezing process, sublimation is slow, slow, slow (because it must add both latent heat of fusion and latent heat of vaporiation to the ice). We also aren't talking about a vapor pressure like that of carbon dioxide even just about its own sublimation temperature; we're talking about a vapor pressure at triple point conditions that's a soft vacuum on its own (below .01 atm, as I recall). $\endgroup$ – Zeiss Ikon May 21 at 15:47
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It takes a surprising amount of energy to form a gas bubble in water. As an example, consider a glass full with some soft drink at rest. Usually there will be bubbles of $CO_2$ going up continuously, but they will all originate from a certain number of points on the glass surface, not from within the liquid. These points are impurities in the glass' surface that ease the bubble forming process in a catalytic way.

As such, when you depressurize the ship, bubbles will form immediately on all surfaces. The bubbles on the surfaces will repressurize the water with their vapor pressure, preempting it from exploding in a vapor explosion. This repressurization will also, slowly, push the water out of the ship, where it will form big giant drops that are again stabilized by the vapor pressure on their surface as the water continues to evaporate. It's more of a tooth-paste-squirt effect than an explosion.

The really nasty part is, that the bubbles will also form on the skin of your species. This means, that any individual at rest will quickly not be swimming in water but be immersed within a bubble of low pressure vapor, powerless to move. And if the individual tries to swim before the bubble forms, it will create huge bubbles itself on the pulling sides of its fins.

This effect is even stronger if you consider that your water dwellers will be warmer than the water they swim in. This additional warmth means that the water bubbles that form on their skin have a higher gas pressure than the water bubbles that form on a cold wall. So the astronauts will be surrounded by bubbles before the ship's cold structure is covered in gas.

However, this temperature effect can also work for good: Assume that you have some machines in the spaceship that give off heat. If those machines are significantly warmer than the people on the ship, the boiling at the warm machines will keep pressure high enough for the people to survive until the machines are surrounded by gas or have cooled to the range of the body temperature of your species.

Ice won't form until enough water has evaporated from a surface to cool the remaining water down to 0°C. And when that happens, all your crew members will be drifting within their respective gas bubbles, extremely likely already dead.

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  • $\begingroup$ Well that's a nice and horrifying scenario... but the question is, is there enough gas in water diluted to form deadly gas bubbles around the personnel and controls? $\endgroup$ – Tschallacka May 21 at 7:53
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    $\begingroup$ @Tschallacka You misunderstood how the bubbles form: They are water vapor. Physically, there's really not that much of a difference between a boiling liquid and dissolved $CO_2$ gassing out. In both cases, you first need a bubble to form (takes energy to displace the liquid and overcome the surface tension), and then the molecules (water or dissolved gas) can leave the liquid whenever they get to the surface with enough energy (heat). Once the vapor bubbles form on the skin, they will quickly grow from the evaporating water. $\endgroup$ – cmaster May 21 at 17:19
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    $\begingroup$ It gets worse; the pressure drop would cause bubbles INSIDE the aquanauts, AKA 'the bends'. Just as bringing deep sea fish to the surface kills them through depressurisation, the ship occupants will die rapidly once depressurised. $\endgroup$ – Sir Adelaide May 24 at 6:25
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    $\begingroup$ @G0BLiN I've written this answer with what I know from my physics class at university. As such, it's basically integrated textbook info, for which I'm unable to find references: The facts and connections were stored in my head, and the location where I found it / which professor said it was filtered out by my brain as utterly irrelevant... Anyway, Zeiss Ikon's answer is half-good in that it correctly identifies some effects, but it ignores the surface effects and overestimates the speed of evaporative cooling (water has a large thermal capacity, but slow evaporation at room temperatures). $\endgroup$ – cmaster Oct 10 at 14:36
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    $\begingroup$ So, if you had balloon filled with water in space and pricked the balloon, you'd get pretty much what Zeiss Ikon's answer describes. But if you have a water filled spaceship that depressurizes due to local damage, you'd get what my answer describes. $\endgroup$ – cmaster Oct 10 at 14:39
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The water would all boil away. The main reason water can freeze in a vacuum chamber is because it is under gravity which applies pressure in lieu of an atmosphere. If your ship is really massive and really cold, it might be able to exert enough gravity to do this.

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    $\begingroup$ Hmm. The vacuum pump could still be running, extracting water vapour. I think what you're really seeing is gravity keeping the water in one place where it can freeze together, rather than as a finely dispersed cloud of snow. $\endgroup$ – Starfish Prime May 20 at 20:26
  • $\begingroup$ Opps, hit submit will while still trying to figure out conflicting sources. Thanks. $\endgroup$ – Nosajimiki May 20 at 22:01
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    $\begingroup$ @Nosajimiki You can edit an answer to improve it. $\endgroup$ – Martin Bonner supports Monica May 21 at 16:09

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