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The sky of Mars

Waves, sound and light · level 6, lesson 5 of 6

A question to start

On 15 April 2015, the Curiosity rover photographed its first colour sunset on Mars. The sky around the Sun was blue, the opposite of an Earth sunset. Why?

The idea

Two things make Mars's sky different.

1. Very little air. There is about 60 times less air above each square metre of Mars than above Earth. So there are far fewer gas molecules to make a Rayleigh-blue sky.

2. Lots of fine dust. Mars's air is full of very fine dust, with grains about the size of a wavelength of light or a little bigger. They scatter light quite differently from tiny molecules. NASA explains that the fine dust lets blue light pass through more efficiently, keeping it close to the Sun's part of the sky, while yellow and red light is scattered widely across the sky.

So:

  • Daytime: the widely scattered yellow and red light fills the sky, which looks a dusty orange-brown.
  • Sunset: the light crosses a long path through the dust, and the blue glow around the Sun stands out.
The sky Mars Earth
Air above each square metre about 170 kg about 10,300 kg
What scatters most of the light fine dust gas molecules
Daytime sky dusty orange-brown blue
Sky around the setting Sun blue orange and red
dusty orange-brown skyblue near the Sun

Worked examples

1. Earth's sunset. Tiny molecules scatter the blue away, leaving red and orange.

2. Mars's sunset. Dust lets blue through towards the Sun's direction and scatters red and yellow widely, so blue stays near the Sun.

3. Little air. \(10{,}300 \div 170 \approx 60\): Mars has about 60 times less air above each square metre to scatter light the Rayleigh way.

Try it

  1. Why is there so little Rayleigh-blue light in Mars's daytime sky?
  2. What scatters most of the sunlight on Mars?
  3. Why is the sky around the setting Sun blue on Mars?
  4. Why is Mars's sunset effect strongest at sunset rather than noon?
Answers
  1. There is about 60 times less air to scatter it.
  2. Fine dust in the air.
  3. The dust lets blue light through towards the Sun's part of the sky while scattering red and yellow widely.
  4. At sunset the light crosses a much longer path through the dusty air.

Common mistake

"Mars's sky is blue too, just fainter." Mars's daytime sky is a dusty orange-brown. The dust, not the gas, decides its colour.

Where it leads