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Why you need a pressure suit

Air and fluids · level 4, lesson 5 of 6

A question to start

Even with an oxygen mask, a person standing on Mars without a suit would be in deadly danger within seconds. Why is a mask not enough?

The idea

Water boils when the pressure around it falls low enough, even if it is not hot. The lower the pressure, the lower the boiling point.

Your body is at 37 °C. Water at 37 °C boils if the pressure around it falls below about 6.3 kPa (6,300 Pa). Below that pressure, the water in your mouth, eyes and lungs would start to boil, at body temperature.

Pressure around you Mars Earth
At the ground 0.64 kPa 101 kPa
Compared with 6.3 kPa 10 times too low 16 times above it

So on Mars you need a pressure suit: a sealed suit full of gas that pushes on your body from outside, the way Earth's air does. It also gives oxygen and keeps out the cold.

Mars 0.646.3 kPaEverest 33Earth 101 body water boilsbody water stays liquid

The scale is squeezed: each equal step to the right multiplies the pressure by the same amount.

Worked examples

1. How far below? \(6.3 \div 0.64 \approx 10\). Mars's air pressure is about 10 times too low.

2. Everest. At about 33 kPa, Everest is 5 times above the line. Climbers need oxygen, but their blood does not boil.

3. A suit. A suit must hold its gas at well above 6.3 kPa and give enough oxygen. If the gas is pure oxygen, it can work at a lower total pressure than air, because all of its push is oxygen.

Try it

  1. Why would water in your body boil on Mars, even though it is cold there?
  2. Below about what pressure does water at 37 °C boil?
  3. How many times is Earth's sea-level pressure above that line?
  4. Give two jobs a pressure suit does.
Answers
  1. The pressure is so low that water boils even at body temperature.
  2. About 6.3 kPa.
  3. \(101 \div 6.3 \approx 16\) times.
  4. Any two: it pushes on the body like air does; it gives oxygen; it keeps out the cold.

Common mistake

"Your blood would boil instantly and you would explode." Skin and blood vessels hold the body together, so you would not explode. But without a suit, a person would soon pass out for lack of oxygen, and body water would start to boil away.

Where it leads