When most people think about sunsets, they imagine warm, romantic colors filling the sky as the sun sinks low on the horizon. Brilliant shades of pink, orange, and red illuminate the hemisphere overhead. On Mars, those spectacular evening light shows are a whole lot cooler, at least when it comes to the color palette.

What Makes A Sunset?

Why do we have sunsets? Aside from the obvious fact that the Earth turns and therefore, the sun sets, the reason behind our daily light show is pretty cool. It has to do with something called Rayleigh scattering. When sunlight passes through Earth’s atmosphere, its light is refracted as photons hit particles in the air and break into component colors based on wavelength. The air molecules in the atmosphere scatter mostly blue light when hit straight on, which is why we have a blue sky during the day.

In the evening, when the sun’s light passes through our atmosphere at an oblique angle, the scattering of the light changes. More particles get in the way of the photons as the angle of the sun changes, scattering more of the blue light to the point where it is no longer the dominant color. With the majority of the shorter wavelengths on the blue end of the spectrum have been bounced away in other directions, we’re left seeing the longer wavelengths of red, orange, and yellow.

Why Mars Looks Blue

Rayleigh scattering isn’t the only type of particle-based light refraction that can happen in an atmosphere. While Rayleigh scattering is caused when photons interact with particles that are smaller than them, something called Mie scattering occurs when the particles are larger than the photons. Rayleigh is dominant on Earth, where our atmosphere is thick, but on Mars, Mie is the winner.

What happens on Mars is a near opposite of what happens here on Earth. The larger particles scatter the longer wavelengths at more direct angles, which is why Mars’s daytime sky appears reddish-orange. When the sun sinks low at the end of a Martian day, its light has to pass through more of the dust flying around in the atmosphere, leaving a bluish hue in the wake of the dispersed red wavelengths.

Once In A Blue Moon

Although Rayleigh scattering, which is far more intense than Mie scattering, is what we typically see here on Earth, it’s not unheard of for Mie scattering to take the stage. When the massive volcano, Krakatoa, erupted in Indonesia in 1883, it sent out massive amounts of dust and volcanic debris. The explosion was so powerful that these particles traveled to the upper layers of the atmosphere, where they were then carried across the globe by the jet stream.

With the larger particles spread throughout the atmosphere, Mie scattering had a chance to amaze (and probably alarm) many people on Earth with a rare real-life blue moon. The phenomenon lasted for about two years, after which point the dust dispersed and settled enough for Rayleigh scattering to resume its place.

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