I read that mars is supposed to be a salmon colored desert with blue sky. I don't get it,if the sun hits a planet, wouldn't the atmosphere be blue like earth? Mars isn't *THAT* far away.

Originally posted by malakiem
I read that mars is supposed to be a salmon colored desert with blue sky. I don't get it,if the sun hits a planet, wouldn't the atmosphere be blue like earth? Mars isn't *THAT* far away.

Nope, not necessarily. The blue color comes from a process called Rayleigh scattering. This is the scattering of photons at molecules, ie particles that are smaller than the wavelength of the incoming light. Short wave light is scattered a lot more than long wavelength light. In numbers, the effect grows with 1/(wavelength to the power of four), in short (1/lambda^4). This means that "red light" is passing the atmosphere less disturbed than "blue light". If the sun's high above the horizon then blue light is scattered from all directions into the observer's eyes.
But the very same effect is responsible for the reddish sky at sunset or sunrise on earth. If the sun's near the horizon the light rays travel a much longer distance through the atmosphere and all the blue light is scattered away from the observer, only the long wavelength (red) light arrives at the observer's viewpoint.

In the full range of visible light (approx. 360 to 830 nm) the ratio of scattering strength is 1/(360^4) / (1/830^4) = 28.26
This means that the shortest visible wavelength is scattered 28 times stronger than the longest visible wavelength.

If you take the more commonly used range of 380 to 780 nm the ratio still is 17.75.

There are several factors that influence the appearance of the atmosphere of Mars. The atmosphere of Mars is a few orders of magnitude thinner and there's a lot of dust in the atmosphere. The amount of dust varies very much, depending on the weather conditions/storm activities.
Color appearance of light scattering at bigger particles (bigger than the wavelength of light) is described by the effect of Mie scattering.
On earth Mie scattering makes the sky look white/bleached near the sun and at the horizon. The color of the sky also changes considerably near big cities because of the vast amount of dust particles from industry and car exhaust. That#s why the sky can look brownish above big cities.
If you look closely you can see that on color measurment photographs taken by the MERs the sky can look slightly bluish around the sun.

Another factor changes the atmospherical appearance very much. If you're inside the atmosphere you're subject to a lot of stray light, indirect lighting which is visible only in certain directions (especially in the direction you're are standing: downwards, or better: away from the sun). Looking down to the planet from space the visual effect of this stray light is very different, as a lot of this light isn't scattered back to you but first towards the ground and then back to you. The way of the light through the atmosphere is longer (depending on the shallowness of the viewing angle) than viewing from inside the atmosphere.
On earth even the thickness of the ozone layer influences the color appearance of the atmosphere.

[edit on 17-1-2009 by ColorScientist]

[edit on 17-1-2009 by ColorScientist]