Sunsets on Mars are blue: Why is it so?

If you ever have the chance to look at the images of a sunset in Mars, you’re in for a surprise! Sunsets on Mars are not red or orange, they are blue! Yes, the pictures from various robotic trips to the red planet show the fascinating blue sunset on the planet.   

So how would you describe Earth’s sky? Predominantly blue during the day which makes its way to orange-red at dawn and dusk, correct? It’s the other way round for Mars. It has an orange-brown daylight sky that converts into a bluish palette at sunset. But the question is if the same sun is shining on both planets, why is there is colour difference in its light? The secret lies in the Martian atmosphere.  

Varying colours of sunset: The science 
The sun emits a spectrum of rays ranging from high-frequency gamma rays to low-frequency radio waves. A part of this radiation called the visible spectrum can be detected by our eyes. This visible spectrum, which you generally call ‘white light,’ is further composed of seven different wavelengths that we all know by the name VIBGYOR (or the rainbow colours).  

Now, these rays emitted by sun reach different planets with different intensities. This is because, other than the sunlight itself, the colours that our eyes see also depend on the medium (particles) through which it passes. You would have surely studied this in your physics class.  

So, when sunlight passes through different particles the light can get absorbed, reflected, or scattered, depending on the nature of the particle. Some particles tend to scatter more of the longer red wavelengths, whereas others go for the shorter blue waves. And so the resultant colour that we see varies accordingly. 

Why are Mars sunsets blue?  
Now let’s apply this to sunsets on Mars and Earth. The Martian atmosphere is dominated by large-sized dust particles. These particles cause something called ‘Mie Scattering,’ which filters out the red light from the sun’s rays and only lets the blue reach our eyes. 

But then why do sunsets on earth look red? Because the size of particles is very small in Earth’s atmosphere, as compared to the wavelength of light. The tiny particles present in Earth’s atmosphere scatter the blue wavelengths through a process called ‘Rayleigh Scattering’, so by the time the light reaches our eyes, only the red remains. 

Simple put, think of the atmosphere as a filter for sunlight. Earth’s atmosphere is good at filtering out blue light and letting in red light, whereas the Martian atmosphere is better at filtering out red light and letting in the blue.