Negative Sunset

Earth has blue skies and red sunsets. Interestingly enough, Mars has red skies and blue sunsets. While at first glance, it might seem curious that the two planets seem to have the opposite skies to each other, it starts to make more sense when you look at what causes the colour of a planet’s sky in the first place.

In the universe, two things cause colour — chemistry and physics. And this time, it’s physics that takes centre stage. Earth’s sky is blue because of an effect called Rayleigh scattering. Sunlight, as any schoolkid who’s ever played with a prism will tell you, is made up of all the colours of light. A whole spectrum. But not all photons are created equally. Blue photons have more energy than red, which means that in Earth’s atmosphere the blue photons are more likely to bounce off the molecules in the air, and scatter. These photons ricochet away in all directions, so Earth’s sky looks blue, both from below and from above. At sunset, the sun appears low to the horizon, so the light you see is passing through a much greater amount of atmosphere than it would be normally. By the time that light reaches you, most of the blue photons have already been scattered away, leaving you with the characteristic warm golden glow of a sunset. The technical term for this in astronomy is “air mass”, and it means you can’t really observe any stars or other objects too low to the horizon. The sky eats too many photons, so your data wouldn’t be much good.

But on Mars… Well, Mars doesn’t have nearly as much atmosphere as Earth. While it wouldn’t exactly be like in Total Recall, you wouldn’t be able to breathe on Mars. The atmosphere’s thin enough that you’d suffocate if you didn’t get terminal frostbite first. The sky of Mars isn’t exactly red either. Nor is the rest of it really. It’s a kind of pinkish butterscotch colour. Martian sky is more or less the same colour as martian soil, and the reason is that they’re coloured by the same thing. Mars is an arid, dusty little planet, and while there aren’t enough molecules in the atmosphere to scatter very much light, there is plenty of dust!

Martian dust reflects the same pinkish colour when it’s in the air as it does on the ground, so the Martian sky appears dusty pink. Because in the Martian atmosphere, it’s the redder photons being scattered, when the Sun sets on Mars, the increased air mass scatters away most of the red photons. The result — Martian sunsets are blue!

I guess Earth and Mars really do have opposite sunsets.


About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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