Expert: Philip Stahl - 1/6/2005

Question
What factors influence what color the sky will appear (if viewed from a planet or moon's surface) - assuming the planet has an atmosphere?  In other words, if Mars' atmosphere was composed of the same gases as earth's in the same ratio, would its sky appear to be blue?  What would be necessary for a sky to appear blue - is it the density of the atmosphere, the thickness, or the gas composition, water vapor or lack of, or a combination of these?


Answer
Hello.


Since the color of sky is determined by scattering (by particles in the atmosphere) then Rayleigh's law of scattering allows us to isolate the factors involved.

By Rayleigh's law (applicable to particles smaller than 0.1 of the wavelength of light):

S ~  (n - 1)^2/ (N* L^4)

Where n is the refractive index of the medium (e.g. atmosphere), N = particle density (no. per cubic centimeter), and L is the wavelength.

Thus the factors are: refractive index of the atmosphere, number density of particles and wavelength.

Note the proportionality in the above for degree of scattering, s ~ 1/ L^4 (wavelength to the fourth power).

This means as L decreases (smaller wavelengths) the scattering by small particles INCREASES. Referencing color of scattered light to wavelength - one finds a descending scale of wavelength toward violet - say for the VISIBLE spectrum:

red- orange -yellow- green- blue- violet

LONGER <---------------------> SHORTER


Thus, from this you can see the scattering must be most pronounced near the violet end of the spectrum.

The scattering, from observations, is clearly by actual molecules of AIR in the atmosphere, not just dust! One may test this if one lives in an area that's had a long drought, and much dust is in the atmosphere. At those times the sky appears much less blue and more "whitish".

In Barbados (West Indies) for example, where I used to live, we would regularly get Sahara dust blown over from W. Africa. At those times the sky always assumed a whitish-grey hue, because the dust particles preferentially scattered the sunlight that way. After a thunderstorm, when the air would clear of dust, the blue sky would return (and make the tourists happy!)

The same sort of abatement of the blue sky arrives when high cirrus clouds appear, since they contain millions of ice crystals - and it is these that now preferentially scatter the sunlight. Making the sky again, more "whitish" than blue.

Now, since at any given time the sky will contain a set of different types of particles (air molecules, ice crystals, dust etc.) it stands to reason that at any given time the sky color will be the SUM of all the different, contributing colors.

Thus, the usual color mix will be: blue, green, and very little yellow, red. The result being a blue sky, since the scattering - as I noted - is preferentially toward the VIOLET end of the spectrum all other things being equal (e.g. no dust storms etc)

If Mars had the same atmospheric gases as Earth, in the same ratio, there might be a chance of seeing blue sky at some time or other. The problem is the immense quantity of particulate dust on Mars, and the dust storms that continually erupt on its surface and sometimes blur much of the surface from view. Based on what I noted earlier, in terms of colors that arise from which particles, you may deduce the predominant color of Mars' sky would likely be a yellowish, or whitish hue (or 'mix' thereof)  but not so blue. Certainly this would apply so long as dust storms raged, and even suspended particulate dust can cause deviation from normal blue sky.