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How thin is Mars air?

Air and fluids · level 4, lesson 1 of 6

A question to start

NASA describes Mars air as about 1% as dense as Earth's. What does "dense" mean, and where does that number come from?

The idea

The density of a stuff is its mass in each cubic metre:

\[\text{density} = \frac{\text{mass}}{\text{volume}}.\]

Air at the ground on Earth has a density of about 1.2 kg/m³. Mars air at the ground is about 0.016 kg/m³. That is about 75 times less dense, or about 1.3% of Earth's. NASA often rounds it to "about 1%".

Earth: 1.2 kg/m³Mars: 0.016 kg/m³
Air at the ground Mars Earth
Density about 0.016 kg/m³ 1.217 kg/m³
Pressure 636 Pa 101,400 Pa
Air above each square metre about 170 kg about 10,300 kg

Notice the density is 75 times less, but the pressure is 160 times less. They are not the same, for two reasons: Mars air is mostly carbon dioxide, whose particles are heavier than those in Earth's air, and Mars air is colder. Both make Mars air a bit denser than its low pressure alone would suggest.

Worked examples

1. How many times less dense? \(1.217 \div 0.016 \approx 76\). About 75 times.

2. As a percentage. \(0.016 \div 1.217 \approx 0.013\), which is 1.3%.

3. A big tent. A Mars tent holding 50 m³ of Mars air contains \(50 \times 0.016 = 0.8\) kg of it. On Earth, the same tent holds \(50 \times 1.217 \approx 61\) kg of air.

Try it

  1. What does density measure?
  2. How much is 20 m³ of Earth air, in kg?
  3. How much is 20 m³ of Mars air, in kg?
  4. Why is Mars air's density not 160 times less than Earth's, like its pressure?
Answers
  1. The mass in each cubic metre.
  2. \(20 \times 1.217 \approx 24\) kg.
  3. \(20 \times 0.016 = 0.32\) kg.
  4. Mars air is colder and made of heavier particles, so it is denser for its pressure.

Common mistake

"Mars air is thin because it is made of light gas." Its particles are actually heavier than those in Earth's air. It is thin because there is far less of it.

Where it leads