#scifi So far we have ignored one of the most important controls on the apparent color of a strange sun: the planetary atmosphere.  On the surface of a planet, we have to consider both absorption and scattering by molecules and particles in the atmosphere.  These processes strongly influence the apparent color of both sun and sky.  Since sky light falls on everything, while sun light falls only on non-shadowed areas. shadows will tend toward the sky color.  As a general rule, this is obvious only when the sun and sky colors are obviously different, as is true near sunrise and sunset on Earth, and is clearest on a light surface such as snow.  Shadows will not differ in color from illuminated areas unless the color of the sky differs from that of the sun.  So the color of shadows can be determined by sky color.

It is no coincidence that most common gasses are transparent in the visual wavelengths.  After all, our vision evolved to see through an atmosphere composed primarily of nitrogen, oxygen, argon, water vapor and carbon dioxide—eyes wouldn’t be much use in a band which was absorbed by the atmosphere.  There are a few colored gasses—Ozone is blue, for instance, while chlorine is green. But these gasses cannot explain sky colors which differ from those of the sun.  Consider ozone, which is frequently blamed for the blue of the sky.  This gas absorbs red wavelengths, allowing the blue through.  From the surface of a planet, it will absorb some of the red light from any light source above the ozone layer. But the light sources above the ozone layer are basically the sun and stars, both of which will appear bluer as a result.  If there is light scattered above the ozone, it will appear bluer also – but the effect of a colored gas is to color sky and sun by the same amount.  Given how readily our eyes see ambient light as white, we probably would not even notice the effect.

Furthermore, most colored gasses are extremely reactive, which makes them unlikely atmospheric constituents unless there is a constant source.  Ozone, for instance, is a part of the Earth’s atmosphere first because photosynthesis keeps producing free oxygen (O2), and second because ultraviolet light acts on that diatomic oxygen to produce a layer of ozone.

Sky color is the result of scattering in the atmosphere.  If the scattering particles are large compared with the wavelength, such as dust particles or cloud droplets, the scattering process will be about the same across the visible spectrum, and the sky color will generally match the sun color if the sun is visible at all.  Even if the scattering particles absorb some wavelengths, such as red desert dust, the effect on sky color and star color will be quite similar – both will take on a reddish tinge.  The first photographs of the surface of Mars owe their sky color to this effect.

The sky color of Earth is due to scattering by the smallest available particles, the air molecules.  Very small particles scatter short wavelengths much more strongly than longer wavelengths, a process known as Rayleigh scattering.  Thus the shortest wavelengths produced by a star—the blue and violet—are scattered from the direct beam from the star and give the black sky an overlying blue radience while the star, because some of the blue has been removed, looks slightly reddish.  Because this is a true color contrast phenomenon, our eyes do not adjust to make everything look as if the illumination were white.   If the star has more blue and violet light (is more massive and hotter than the sun) there is more shortwave light to scatter, and the sky will be brighter and probably tend toward the violet in color.  A less massive, cool star, on the other hand, has very little blue or violet light to scatter, and will appear to shine in a relatively dark sky.  Such a sky could appear dark blue-green.  So even if the overall light appears “white” (in the sense that the light of an incandescent light bulb is white), sunlit areas will appear reddish and shadows green to blue.

Is a visually green sun possible?  Yes, but probably not as a common phenomenon on any planet.  Blue or green suns (and moons) are observed on rare occasions on Earth, when there are particles in the air which are uniformly of exactly the right size.  Far-traveled forest fire smoke, volcanic eruptions, and very fine desert dust have been known to produce blue or green suns in a orangy sky.  The process is called Mie scattering, and if the particles are just the right size they may scatter red light more than they scatter blue.  Thus the blue or green wavelengths from the sun reach our eyes directly, while the red and yellow wavelengths are scattered away, to reappear as sky color.  The needed particles, however, are of such a size that they do eventually fall out of the atmosphere.  Furthermore, there are not many processes that produce scattering particles all of exactly the same size.

Hmm – maybe some kind of life form that produced huge masses of spores of exactly the right size?  That could produce a green sun—but only during the season of spore production.

Next week the planet building series will end with a discussion of moons and their phases.

Author’s note: the entire planet building series, in order, will be put up on my author website, http://www.sueannbowling.com .