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What Mars air is made of

Air and fluids · level 4, lesson 2 of 6

A question to start

Earth's air and Mars's air both contain carbon dioxide, nitrogen, argon and oxygen. But the mixes are almost opposite. How different are they?

The idea

Air is a mixture of gases. Scientists give the share of each as a percentage by volume: out of every 100 particles of air, how many are that gas.

Gas (share by volume) Mars Earth
Carbon dioxide 95.1% 0.04%
Nitrogen 2.59% 78.08%
Argon 1.94% 0.93%
Oxygen 0.16% 20.95%
Water vapour about 0.02% about 1%, but it changes a lot
MarsEarthcarbon dioxidenitrogenoxygen thin end slice on Mars: nitrogen and argon

So Mars air is nearly all carbon dioxide, while Earth air is mostly nitrogen with a fifth oxygen.

Mars air is so thin that, even though it is 95% carbon dioxide, a person could not get oxygen from it directly. But carbon dioxide contains oxygen, and it can be split. NASA's MOXIE experiment on the Perseverance rover did exactly that: it made 122 grams of oxygen from Mars air, at up to 12 grams an hour.

Worked examples

1. Out of 10,000 particles of Mars air, about 9,510 are carbon dioxide, 259 nitrogen, 194 argon and only 16 oxygen.

2. Out of 10,000 particles of Earth air, about 7,808 are nitrogen, 2,095 oxygen, 93 argon and 4 carbon dioxide.

3. Running MOXIE. At 12 grams an hour, 122 grams would take \(122 \div 12 \approx 10\) hours of its best running.

Try it

  1. What is most of Mars's air made of?
  2. What is most of Earth's air made of?
  3. Out of 10,000 particles of Mars air, about how many are oxygen?
  4. How much oxygen would MOXIE make in 5 hours at its best rate?
Answers
  1. Carbon dioxide.
  2. Nitrogen.
  3. About 16.
  4. \(5 \times 12 = 60\) grams.

Common mistake

"Mars has more carbon dioxide than Earth, so its air must be thick." It is the share that is high. The air itself is thin, so there is still not much of anything in each cubic metre.

Where it leads