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 and purple-red at sunrise and sunset.
The atmosphere of Uranus contains methane, which causes the sky to be greenish-blue. And on Titan (one of Saturn’s moons) traces of hydrocarbons (think smog!) in the thick, mostly nitrogen atmosphere gives the sky a light tangerine color (pictured below).
We take our blue sky for granted. How would our perception of the world be different if our sky were green, say, instead of blue?
The color of light absorbed by a particle in an atmosphere depends on its size. Naturally occurring aerosols in Earth’s atmosphere tend to be about the same size; particles of that size absorb mostly blue light. Particulates resulting from a volcanic eruption or a large forest fire also tend to be uniform size, although typically larger than the aerosols that make our sky blue. However, man-made particulates, like greenhouse gases and hydrocarbons, tend to be distributed over a wide range of sizes.
Although sky color is not one of the reasons that Santa Clara is committed to sustainability – part of one of the five priorities in our Strategic Plan – sky color can, perhaps, tell you something about how a region values environmental stewardship.