Science Fun: Extra-terrestrial Sunsets

15 Sept

I loudly and proudly admit that my science fiction is as ‘soft’ as butter in places. The Rule of Cool covers everything I write, but that doesn’t mean I don’t value logical consistency and every so often, there’s a science question that sticks in my head.

Nonalith was born in a world I built on the run, and when thinking of how I could show that my protagonist was no longer on Earth, I decided (completely spur of the moment) to make the sky pink. And then I kept writing. And I kept thinking. And I found myself wondering:

“If the sky isn’t blue, what colour should the sunset be?”

What makes the sky blue?

Before answering this question, we need to explore why Earth’s sky is blue in the first place.

It all starts with electromagnetic radiation coming at us from the sun. Once it hits Earth’s atmosphere, the different wavelengths of light are scattered by molecules and tiny particles. Some wavelengths (i.e. colours) reach us, and some do not.

On Earth, we have a lot of oxygen and nitrogen, which are good at scattering blue and purple light. That’s why our sky appears blue during the daytime.

At sunset, light has longer to travel through the atmosphere because of the angle of the sun. As a result, blue wavelengths are scattered out of our line of sight before it can reach us, leaving us with the stunning reds and oranges we associate with sunsets. Red has the longest wavelength we can see, which is why it’s the last colour to disappear.

What about other planets?

So what about other planets? Well, different atmospheres mean different colours.

Uranus, for example, has blue daytime skies that change to brilliant turquoise at sunset. This is because its atmosphere is rich in hydrogen, helium and methane. These gases are good at scattering blue and green light, but will mostly absorb red.

It’s not just about what gases are present, either. Mars has a lower atmospheric density than Earth and a lot of dust. Light scatters off the dust particles, which gives the planet its iconic rosy-pink sky and a blue sunset.

It’s almost impossible to predict what a sunset might look like on another planet without a detailed understanding of the atmosphere. Every planet is different, and (probably) beautiful in its own way. But lucky us, NASA has given us a glimpse into the possibilities by modelling what some of these sunsets might look like…

NASA Goddard: NASA Scientist Simulates Kaleidoscope of Sunsets on Other Worlds (Youtube, 2020)

What else could this mean?

This brings up many other interesting questions, the biggest one in my mind being: “Could humans breathe on a planet without a blue sky?”

Not every blue sky is built the same: Jupiter, Saturn, Uranus and Neptune all have blue skies because their atmospheres contain a lot of hydrogen. And we certainly couldn’t safely breathe that in!

And if the sky isn’t blue, it’s likely that the atmosphere is either full of gases that would kill us or is missing something important. So to answer our question, I’m going to say…almost certainly not. But it’s not impossible because, hey, nothing is impossible within the realms of speculative fiction!

Another cool thing about Earth’s sky is that although there is a lot of violet light scattered in the atmosphere (and the sky should look more purple), the human eye is not very good at picking it up. This isn’t the case with other animals. Bees, for example, can’t see red light but can see in the ultraviolet spectrum. Imagine how a bee would see a sunset — how different must it look?

How can this stuff be incorporated into speculative fiction?

If you’re writing HARD science fiction, I recommend thinking twice before having your humans take off their spacesuits on a planet with a funky-looking sky. And if you’ve created an alien species that evolved on a non-Earthlike planet, consider what technological support they would need before visiting us down here.

On the other hand, if you’re writing soft science fiction or fantasy, don’t worry — Rule of Cool will always apply!

And if you’re wondering whether my protagonist in Nonalith survived under a pretty pink sky…well…you won’t find any spoilers here!