Mars is quite possibly the quintessential alien planet. It’s about as far removed from what we earthlings know to be reality. The planet itself is a waterless, red desert, incapable of housing life (at the moment, at least); Mars’ days are a whole 37 minutes longer than Earth’s days; and maybe the most alien of all, the day-time sky on Mars is red and the sunsets are a beautiful blue. Weird, ay?
But why are the sunsets on Mars blue? Why is there day-time sky red? It’s actually pretty simple.
It’s pretty much the same reason that we earthlings are used to blue skies in the day and red skies at sunset: light from the sun refracts different colours, depending on what is in the atmosphere. For example, earth’s atmosphere is mostly made up of air particles. Air particles refract blue light, creating the blue-coloured sky you see every day (unless you live in Britain). As the day goes on and the sun sets, the light has to travel further, making it bounce off even more particles, this is why there is a wider range of colours towards the end of the day. The red we see at sunset is mostly the result of volcano ash, which scatter more red light.
On Mars, the atmosphere is mostly made up of carbon dioxide and very fine red dust. This mix refracts the red light that makes up the standard Martian sky colour. When the sun sets on Mars, the light is bounced around from more particles and causes the eerie blue sunsets we see in the videos sent back from the Mars rovers.
So, if humans ever decide to colonise Mars, they’ll have to get used to this reverse colour scheme for the sky, which, granted, will probably be one of the simpler adjustments.