And so the sky was changing color. Overhead it was a rich violet, and above the western hills it was whitish, shading up into lavender, and some color between lavender and violet that Sax didn’t have a name for. The eye could distinguish differences in light frequency of only a few wavelengths, so the few names for the colors between red and blue were totally inadequate to describe the phenomena. But whatever you called them, or didn’t, they were sky colors very unlike the tans and pinks of the early years. Of course a dust storm would always temporarily return the sky to that primeval ochre tone; but when the atmosphere washed out, its color would be a function of its thickness and chemical composition. Curious as to what they could expect to see in the future, Sax took his lectern from his pocket to try some calculations.

He stared at the little box, suddenly realizing that it was Sax Russell’s lectern— that if checked, it would give him away. It was like carrying around a genuine passport.

He dismissed the thought, as there was nothing to be done about it now. He concentrated on the color of the sky. In clean air, sky color was caused by preferential light scattering in the air molecules themselves. Thus the thickness of the atmosphere was critical. Air pressure when they had arrived had been about 10 millibars, and now it averaged about 160. But since air pressure was created by the weight of the air, creating 160 millibars on Mars had taken about three times as much air over any given spot than would have created such a pressure on Earth. So the 160 millibars here ought to scatter light about as much as 480 millibars on Earth; meaning the sky overhead ought to have something like the dark blue color seen in photos taken in mountains about 4,000 meters high.