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A Mars night

Heat and matter · level 4, lesson 6 of 6

A question to start

At the Viking 1 site, the air warmed to −31 °C by day and fell to −89 °C by night: a swing of 58 °C, every sol. Why does Mars swing so much more than Earth?

The idea

Three things make a Mars night so cold.

1. Very little air. The air above each square metre of ground weighs as much as the pressure it makes: \(m = p/g\). Mars has only a sixtieth as much air over each square metre as Earth.

Air over each m² of ground Mars Earth
Surface pressure 636 Pa 101,400 Pa
Gravity 3.71 N/kg 9.81 N/kg
Mass of air, \(p/g\) about 171 kg about 10,300 kg

So the air holds little heat to give back at night, and lets the ground's infrared escape to space.

2. Dry air. Earth's air has water vapour and clouds, which hold heat in. Mars's has almost none.

3. Soil that keeps its heat near the top. Mars soil is loose and dusty, with thin gas in its gaps, so heat travels through it very slowly. The lander InSight measured how well it conducts heat: 0.039 W per metre per kelvin, several times less than dry sand on Earth. Only a thin top layer warms by day, and it loses that heat fast at night.

−89 °C−31 °Cdawnafternoonnight

Worked examples

1. The air's mass. Mars: \(636/3.71 = 171\) kg. Earth: \(101{,}400/9.81 = 10{,}340\) kg. About 60 times as much.

2. Heat in the air. To cool the whole air above 1 m² by 10 °C, it must lose \(mc\Delta T\): Mars, \(171 \times 840 \times 10 = 1.4\times10^6\) J; Earth, \(10{,}340 \times 1005 \times 10 = 1.0\times10^8\) J. Earth's air holds over 70 times as much.

3. The swing. \(-31 - (-89) = 58\) °C, every sol.

Try it

  1. Find the mass of air over 1 m² of a place on Mars where the pressure is 870 Pa.
  2. Why does a little air mean a big swing in temperature?
  3. Why do clouds keep Earth's nights warmer?
  4. What does it mean that Mars soil conducts heat poorly?
Answers
  1. \(870/3.71 = 234\) kg.
  2. There is little air to store heat by day and give it back at night, and little to block the ground's infrared.
  3. They send back some of the infrared the ground gives out.
  4. Heat moves into and out of it slowly, so only a thin top layer warms by day and it cools fast at night.

Common mistake

"Mars is cold only because it is far from the Sun." Distance makes Mars colder on average, but the huge swing between day and night comes from its thin, dry air.

Where it leads