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How dust storms grow

Air and fluids · level 8, lesson 6 of 6

A question to start

Every Mars year, dust storms cover areas the size of continents for weeks. About once every three Mars years, one grows until it wraps the whole planet. How can a local storm grow so big?

The idea

A dust storm feeds itself. It is a positive feedback loop:

  1. Wind lifts dust into the air.
  2. The dust absorbs sunlight and warms the air around it.
  3. The warmed air sets up bigger temperature and pressure differences.
  4. Those drive stronger winds, which lift more dust.

The loop is strong on Mars because its thin air has so little mass to warm. The same sunlight heats it far faster than Earth's thick air.

Heating the air above 1 m², if dust absorbs 290 W Mars Earth
Air above each m² 171 kg 10,340 kg
Heat to warm it 1 K (specific heat × mass) \(754 \times 171 = 129{,}000\) J \(1{,}000 \times 10{,}340 = 1.03\times10^7\) J
Warming rate, before any losses about 8 K an hour about 0.1 K an hour
wind lifts dustdust absorbs sunair warmsstronger winds

The biggest storms usually start in southern spring and summer, when Mars is nearest the Sun and sunlight is strongest. The 2018 global storm made the sky so dark over Opportunity, an optical depth of about 11, that the rover could not charge its batteries, and it never woke up again. In the end, storms die down when the dust shades the ground, the surface cools, and the winds near the ground weaken.

Worked examples

1. Mars. \(290/129{,}000 = 2.2\times10^{-3}\) K/s, or about 8 K an hour.

2. Earth. \(290/(1.03\times10^7) = 2.8\times10^{-5}\) K/s, about 0.1 K an hour: 80 times slower.

3. Three Mars years. \(3 \times 687 = 2{,}061\) Earth days, about 5.6 Earth years, matching NASA's "about 5½ Earth years" between global storms.

Try it

  1. Why is a dust storm a positive feedback?
  2. Why does sunlight heat Mars's air so much faster than Earth's?
  3. If the dust absorbed 150 W/m², about how fast would Mars's air warm?
  4. Give one thing that stops a dust storm growing for ever.
Answers
  1. Each step makes the next one stronger: more dust gives more heating, stronger winds and more dust.
  2. There is about 60 times less air above each square metre to warm.
  3. \(150/129{,}000 = 1.2\times10^{-3}\) K/s, about 4 K an hour.
  4. The dust shades the ground, so the surface cools and the winds that lift dust weaken.

Common mistake

"Dust storms are dangerous because of the wind." On Mars, the push of even the strongest storm winds is gentle. The real danger is the dust: it blocks the sunlight that solar-powered machines need.

Where it leads