When I feel down about the state of the world, about the indifference of the Universe, I always think of rainbows. They are a trick of the light when it encounters rain. The colours that make up what we consider white light are made visible as water droplets don’t scatter all the colours in the same way. And suddenly, beautifully, you have a rainbow. Sometimes, you get a double rainbow. If you are high enough, you can see it go all the way around in a circle, with no ends for leprechauns to place gold. But there is another thing that makes rainbows special. Nobody else has seen the same rainbows as you. It doesn’t matter if they are standing right next to you. Light doesn’t work that way. It moves in straight lines from the rain droplets to your eyes, and your eyes alone. The rainbows that you see are just for you. No one else in the whole universe ever has or ever will see that exact rainbow.

But a rainbow, even in its uniqueness tells us something about the way we understand the Universe. It shows us that our senses are limited. We see sunlight as colourless. Where have these colours been hiding? Are there even more that we do not see? And the answer is a resounding yes.  

A few years back, relaying more fun facts about these colourful events, I was asked if rainbows exist on other worlds. Obviously, we could speculate on planets around distant stars, but we are confident that they exist on a world that is literally next door in cosmic terms. There’s a moon in Saturn called Titan. Titan is the biggest satellite of Saturn, bigger than Mercury. It is also the only other place in the solar system with rivers, lakes, seas, and most importantly for our discussion, rain. The only problem is that is not the rain we are all familiar with. It is far too cold on Titan for that. The liquid there is methane (and other hydrocarbons) and it rains occasionally, although not as often as on Earth. If you were in a spaceship around Saturn and you looked at Titan, you would see a hazy orange world. Nothing too exciting about it. You wouldn’t see the lakes and rivers. But if you could see the world in infrared, the light that is emitted by warm bodies, suddenly the clouds would become transparent, and you would catch glimpses of sunlight reflecting on those lakes. Spacecrafts that we have sent there have seen them. 

But let me take you a step further. Imagine yourself on the cold surface of Titan. Still, with your infrared equipment, picture a more sophisticated version of night-vision goggles. It is raining and the Sun is shining through some clouds. There in the sky, you would see a rainbow. Very similar to Earth’s own but only visible to you thanks to infrared tech. It’s an invisible rainbow to humans, but it is there. As unique and incredible as the one that we can experience here.