Originally posted by ArMaP
It's not oxygen that makes the sky blue, it's the fact that the atmosphere is dense enough for scattering the light.
According to this, if a planet has a thinner atmosphere it should have less scattering, but it will still have some, making the sky a lighter blue.
All of this if I am not wrong, this is the idea I have about it, but I may be slightly wrong.

Why is the sky blue? Parents and teachers have traditionally made up lots of reasonable explanations. Adults are supposed to know the answers to such simple questions, right? Unfortunately, all of the explanations that I have ever heard are at least partly incorrect.

The correct answer is: Only because of a remarkable coincidence.

The Earth's atmosphere is primarily made of Oxygen and Nitrogen. It happens that both these are made up of atoms of about the same size. It also turns out to be true that EVERYTHING has lots of empty space between and around the atoms that it is made of.

Our Sun produces light which is actually a mixture of light of a lot of colors (called a spectrum, another subject). You can prove this with a prism, which separates the colors of what we normally call white light or sunlight. These various colors are actually just different sizes (wavelengths or frequencies) of waves of radiation (light). Our eyes only recognize the total of all this light, and so it appears to us to be a bright yellow, almost white color.

It turns out that RED light in the sunlight is the lowest frequency and therefore is the biggest wave (yet another subject!). These big waves happen to be around twice as big as the size of the waves of blue and violet light also in the sunlight. When these BIG waves pass through the earth's atmosphere (or any other collection of oxygen and nitrogen molecules) they are not affected very much by the relatively SMALL atoms of Oxygen and Nitrogen; so most RED light continues straight on through. (They are able to go around the atoms more than bumping into them.)

The BLUE (and violet) light in the sunlight is the highest frequency (of what we can see) and therefore has the littlest waves. These smaller waves happen to be around half the size of the red waves just discussed. When these SMALL waves of blue light pass through the earth's atmosphere, many of them eventually "crash into" the relatively LARGE atoms of Oxygen and Nitrogen. They tend to "bounce off" (are reflected or SCATTERED) and then they go all directions including sideways and down toward us.

A more scientific way of saying this is to say that the Oxygen and Nitrogen atoms are of a size that has a "natural vibration rate" (called frequency) that is closer to the rates of vibration of BLUE light. So the blue light can cause those atoms to start vibrating. The vibrating atoms then give off blue light when they stop vibrating, and that new blue light can go in any direction. (It's a little more complicated than that, but the general idea is correct.)

The REALLY complete explanation of this is called the Rayleigh Theory of Scattering!

As sunlight is passing through the atmosphere horizontally above you, the RED light tends to go straight through, while the BLUE light tends to get bounced around (what is called "scattered") as described above. When you look up at open sky, you see this scattered light which is mostly BLUE.

Please notice that I keep saying things like "usually", "mostly", "tends",and not "always". Remember that there is a LOT of empty space between the atoms in the air. Some blue light CAN make it through a good distance of air. But, the farther that the light has to pass through air keeps making it more and more likely that the blue light will be scattered while a good amount of the red light will continue on through.

Originally posted by Dan Tanna
I love this picture. Martian landscape looks alot like earth.
I will have to have a good hunt for the origional.

Originally posted by vze2xjjk
John Lear mentions Zorgon in one of his youTube videos. This is # 1 of 4