Blue Skies, Smiling at Me
Do you look around the natural world and wonder? Wonder why things work the way they do, why things are the way they are? If you do, then there’s a bit of physicist inside of you. Physics is all around us.
Let’s wonder together about the sky. Why is the sky blue? Does the sky have a color on other planets? And if so, is the sky on other planets blue as it is on Earth? Visible light is a mixture of every color in the rainbow. When light strikes the surface of any object, some colors get absorbed and some get reflected. Whatever colors are reflected determine how we see the object. For example, the shirt I’m wearing right now looks red because it reflects mostly red light and absorbs other colors.
When light passes through a planet’s atmosphere, the same thing happens – some colors of light are absorbed while the rest go straight through. In Earth’s case, light in the blue color range gets absorbed by small particles (“aerosols”) in the upper atmosphere, which causes them to wiggle back and forth. A particle can’t continue to hold the extra energy from the light for too long, however, so it eventually stops wiggling by re-emitting the blue light it absorbed. This emitted bit of blue light comes out in a random direction, eventually being absorbed by another particle in the atmosphere and then being re-emitted in another random direction. This process is repeated over and over, spreading blue light across the sky. Our sky is colored because Earth has an atmosphere; it is blue because aerosol particles absorb and re-emit mostly blue light.
Any celestial object that has an atmosphere can have a colored sky. Other objects in our solar system – planets and moons – have atmospheres. The sky as seen from one of these objects has a color. Not necessarily blue, however. The sky color depends on what color or colors of light are absorbed as it passes through the atmosphere, which depends on the particles in it.
On Mars, the sky is a yellow-brown during the day