Hypothetical scenario: There is a dome on the moon containing an Olympic-size swimming pool. Our intrepid hero is receiving a scuba diving lesson at the bottom of the pool (3m) when a hull breach occurs and the pool is exposed to vacuum. How long does he have to live?

This question is similar to Boiling ponds and pools on Mars?. The biggest difference is that the gravity is much lower, which greatly reduces the available pressure to breathe with and might plausibly cause asphyxiation.

  • $\begingroup$ Can you explain the difference from the similar more clearly? Is it just the difference between Mars' surface atmosphere and vacuum? If there is a "hull breach" what does "otherwise protected from the elements" mean? What elements, and how? Are you sure there is no answer to this question there already? $\endgroup$
    – uhoh
    May 10, 2018 at 7:17
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    $\begingroup$ The pressure at 3 m depth under the gravity of 1,62 m/s² of the moon is too low for breathing. A scuba tank with pure oxygen would not help. The water would evaporate into the vacuum and the pressure at the bottom would decrease to zero. But the initial pressure at the bottom of the pool after the hull breach would be about 0.05 bar. The fictional hero would loose consciousness very soon. $\endgroup$
    – Uwe
    May 10, 2018 at 14:38
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    $\begingroup$ The good news is that you don't need scuba gear. Lungs can breathe up to about a 0.05atm pressure, about 0.5m on Earth and 3m on the Moon. The bad news is that @uwe's point about pressure being too low to breathe: Once there's a vacuum on top, the 0.05 atmosphere pressure at the bottom corresponds to over 20000m altitude on Earth; you simply can't breathe there. You'd need over 60m of water to even have a chance. $\endgroup$ May 10, 2018 at 17:13
  • $\begingroup$ Thanks you two. It sounds like the difference between vacuum and Mars ' atmosphere is significant. So does this mean that scuba diving in a pressurized dome even without the hull breach would be an iffy proposition? $\endgroup$ May 10, 2018 at 22:25
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    $\begingroup$ 0.05 bar is below the Armstrong limit of 0.063 bar. The lungs would be filled with pure water vapor. There is no partial pressure left for oxygen, at 0.063 bar the boiling point of water is 37 °C. But to use a 3 m long snorkel in the pool on the moon with closed hull is not recommendable, breathing would be very hard and exhausting. You have to avoid to inhale the previous exhaled carbon dioixde from the snorkel tube. So inhaling fresh air goes through the snorkel, but exhaling used air only into the water. $\endgroup$
    – Uwe
    May 11, 2018 at 12:15

1 Answer 1


Answer: Not long to live. Assuming he is not hypothermic, he will be unconscious in about a minute and brain dead in 4 minutes.

The hydrostatic pressure at 3m pool depth under Moon gravity is 35mmHg. The vapor pressure of water at body temperature is 47mmHg. Our hero's lungs are full of water vapor. Respiratory efforts would slosh this water vapor back and forth in the airways, providing no oxygen to the alveoli even if breathing pure O2.

enter image description here source

Interestingly, respiratory efforts are driven by blood CO2 levels, not O2 levels (unless O2 is exceptionally low). Respiratory efforts would continue to remove CO2 from the hero's blood even if efforts were not replenishing O2. So our hero would not be in particular respiratory distress, and may pass out before realizing he is in trouble.


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