Why the Sky on Mars Is Pink
High Winds, Martian Dust Storms & Airborne Oxidized Iron Particles
Apr 29, 2009
When the first human explorers walk on the surface of Mars and look up into the sky, they will see a strange pinkish yellowish Martian sky. Why does the sky on Mars have such a different color from Earth's familiar blue sky?
Why Earth's Sky Is Blue
Earth's sky is blue because nitrogen and oxygen molecules in Earth's atmosphere reflect blue light more efficiently than red light. Scientists refer to this reflection as light scattering. Scattering of light off small particles is called Rayleigh scattering, which is much more efficient for shorter wavelength blue light than for longer wavelength red light.
Incoming sunlight strikes one of the nitrogen or oxygen molecules and reflects of in some random direction. The light photon then often strikes a second molecule and is scattered again. If the second or subsequent final scattering is in a downward direction, the light can be seen by an observer on the ground coming from a portion of the sky away from the incoming sunlight. With enough scattered blue light coming from all directions the entire sky will look blue.
Martian Dust Storms
Mars has its ruddy red color because there is a large amount of iron oxide (rust) in the soil. The Martian soil contains many small dust particles that are only about a millionth of a meter in size. Even though Mars has a very thin atmosphere, it has strong winds. The wind picks up these fine dust grains in small scale dust devils and in planet wide dust storms. These Martian dust storms dwarf the most severe dust storms on Earth's deserts.
These dust storms blow large amounts of fine iron oxide dust grains into the Martian upper atmosphere.
Sky on Mars
The iron oxide dust grains in the upper atmosphere of Mars affect the color of the Martian sky. These dust grains are the right size to scatter all colors of light from red to blue by Rayleigh scattering. If, as on Earth, Rayleigh scattering were the only effect, then the Martian sky would be white. However the iron oxide dust grains in the upper atmosphere of Mars do a very effective job of absorbing shorter wavelength blue light. That preferentially allows the longer wavelength red and yellow light to reach the Martian surface. So the sky on Mars has a light yellowish pinkish color.
The Martian sky will certainly be strange sight to future human explorers.
Further Reading
Freedman, R.A. and Kaufmann, W.J., Universe 8th ed., Freeman, 2008.
Chaisson, Eric and McMillan, Steve. Astronomy Today, 5th ed. Pearson Prentice Hall, 2005.
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