Blue Sky on Mars

[reeay]

If the Martian atmosphere were free of dust, the daytime sky would appear blue, because of Rayleigh scattering by the molecules (primarily carbon dioxide) that make up the atmosphere. Because the atmosphere is thinner than earth’s atmosphere, the Martian sky would be a darker blue than ours, much as the sky on earth appears at high altitudes with a similar density of air molecules. It is possible (though unlikely) that future missions to Mars will find a different sky color.

http://www.webexhibits.org/causesofcolor/14C.html

Didn't NASA have some mission to Mars in the 70s? They would probably have known how the coloring of the sky worked, no?

http://youtu.be/plWnm7UpsXk