Why Does Mars Have Blue Sunsets? - Beyond The World
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Why Does Mars Have Blue Sunsets?

Earth and Mars are a bit like mirror worlds. Mars is the Red Planet. Earth is the pale blue dot. Mars is a frigid desert. Earth is full of water and life. But there’s another curious difference. The sky on Mars is red, while its sunsets are blue.

In the case of Mars, it’s more of a local phenomenon; dust particles and not atmospheric composition which dominate scattering in Mars.

When light is incident on an air molecule, known as Rayleigh scattering. Rayleigh scattering occurs when the object a photon scatters off (the air molecule) is much smaller than the wavelength of the photon. The closer the wavelength is to the size of the molecule, the more likely it is to scatter. This means that red wavelengths (which are the longest in visible spectrum) don’t scatter with air molecules much, while blue wavelengths (which are the shortest) tend to scatter a lot. This is why the sky appears blue, since so much of the blue light is scattered.

When the Sun is low in the sky, it’s light has to travel a long path through the atmosphere to reach you. As the light travels through the atmosphere some of the photons are scattered off the air molecules. When the photons scatter off air molecules, they scatter randomly in all directions, so usually when a photon scatters, most of it scatters away from your line of sight. Since blue photons scatter much more often than red ones (blue wavelength is shorter and more comparable to the size of atmospheric gas molecules on earth), much of the blue light is scattered away. This leaves red photons to reach your eye. Hence the Sun looks red when low in the sky. When the Sun is overhead, the path it takes to reach you is much shorter, so only a bit of the blue light is scattered. So the Sun looks yellow.

Now, Mars has a much thinner atmosphere (1% of Earth’s), so the amount of Rayleigh scattering is much less. But Mars also has a dry, dusty surface, and a weaker surface gravity, so the atmosphere of Mars is often filled with fine dust particles (if you watched the movie Martian, you can recall it). These particles are larger in size than the main atmospheric gases of Mars’s atmosphere, also more comparable in size to the larger wavelengths of visible light, so most of the light is scattered by Mie scattering.

On the contrary, Rayleigh scattering due to atmospheric composition dominates in the case of earth. While, for Mars, it is the larger dust particles which play the major role and push the scattering characteristics to the Mie region. One of the main differences between Rayleigh and Mie scattering is that Rayleigh scattering tends to occur in all directions, but Mie scattering varies with scattering angle. Also, good to know that the scattering cross-section in Mie scattering region has oscillatory characteristics. Effectively, what this means is that longer wavelengths (reds) tend to scatter more uniformly, while shorter wavelengths (blues) tend to scatter at slight angles. This further means that the blue light tends to be deflected less than red light which implies Mars can have a dusty red daytime sky, and a blue sunset.

Mie scattering does occur on Earth as well, but since Mie scattering is less efficient than Rayleigh scattering it’s never strong enough to give us a blue sunset. It can (rarely) produce a blue moon. It happened more than 100 years ago though; due to the volcanic eruption of Krakatoa in 1883, which sent so much ash into the atmosphere it produced brilliantly red sunsets and visibly blue moons all across the globe for nearly two years.

As a result, the phrase “once in a blue moon” came to mean a rare occurrence.

The indefatigable robotic rovers we’ve sent to Mars, Curiosity, Spirit, and Opportunity, have witnessed and recorded the strange phenomenon. Surprisingly, Earth and Mars are the only two planets in the Solar System where we can see sunsets.

Because Mercury lacks an atmosphere, we would see the Sun disappear while the temperature fluctuated between 427°C (801°F) and -173°C (-279°F) as we moved from day to night. It also has an extremely long day, rotating on itself every 58 and a half days. Going to Venus, on the other hand, would be even worse. The thick cloud cover and extremely dense atmosphere would stop the rays of the Sun from reaching us. And the high temperature and acid rain would easily melt our suits – and eventually our faces – off.

Within its dense atmosphere, Titan might be able to provide a rare sunset on occasion. But, for the time being, our Earthly sunsets and videos of Martian sunsets will have to suffice.

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