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.

But the actual color filling the windows and skylight of their rover was much more reddish than that, and even on clear mornings after heavy storms, Sax had never seen it look anywhere near as blue as a Terran sky. He thought about it more. Another effect of Mars’s light gravity was that the air column lofted taller than Earth’s. It was possible that the smallest fines were effectively in suspension, and had been blown above the altitude of most clouds, where they escaped being scrubbed out by storms. He recalled that haze layers had been photographed that were as much as fifty kilometers high, well above the clouds. Another factor might be the composition of the atmosphere; carbon dioxide molecules were more efficient light scatterers than oxygen and nitrogen, and Mars, despite Sax’s best efforts, still had much more CO₂ in its atmosphere than Earth did. The effects of that difference would be calculable. He typed up the equation for Rayleigh’s law of scattering, which states that the light energy scattered per unit volume of air is inversely proportional to the fourth power of the wavelength of the illuminating radiation. Then he scribbled away on his lectern screen, altering the variables, checking handbooks, or filling in quantities by memory, or guesswork.

He concluded that if the atmosphere was thickened to one bar, then the sky would probably turn milk white. He also confirmed that in theory the present-day Martian sky ought to be a lot bluer than it was, with its scattered blue light about sixteen times the intensity of the red. This suggested that fines very high in the atmosphere were probably reddening the sky. If that was the correct explanation, one could infer that the color and opacity of the Martian sky would for many years be subject to very wide variation, depending on weather and other influences on the cleanness of the air…

And so he worked on, trying to incorporate into the calculation skylight radiance intensities, Chandrasekhar’s radiative transfer equation, chromaticity scales, aerosol chemical compositions, Le-gendre polynomials to evaluate the angular scattering intensities, Riccati-Bessel functions to evaluate the scattering cross sections, and so on — occupying the better part of the drive to Arena Glacier, concentrating hard and steadfastly ignoring the world around him and the situation in which he now found himself.

Early that afternoon they came to a small town called Bradbury, which under its Nicosia-class tent looked like something out of Illinois: treelined blacktop streets, screened-in porches fronting two-story brick houses with shingle roofs, a main street with shops and parking meters, a central park with a white gazebo under giant maples…

They headed west on a smaller road, across the top of Syrtis Major. The road was made of black sand that had been cleared of rocks and sprayed with a fixative. This whole region was very dark — Syrtis Major had been the first Martian surface feature spotted through Earth telescopes, by Christiaan Huygens on November 28, 1659, and it was this dark rock that had allowed him to see it. The ground was almost black, sometimes a kind of eggplant purple; the hills and grabens and escarpments that the road twisted through were black; the fretted mesas were black, the thulleya or little ribs were black, ridge after ridge after ridge of them; the giant ejecta erratics, on the other hand, were often rust-colored, reminding them forcibly of the color from which they had temporarily escaped.

Then they drove over a black bedrock rib and the glacier lay before them, crossing the world from left to right like a lightning bolt inlaid into the landscape. A bedrock rib on the far side of the glacier paralleled the one they were on, and the two ribs together looked like old lateral moraines, although really they were just parallel ridges that had channelized the outbreak flood.

The glacier was about two kilometers across. It appeared to be no more than five or six meters thick, but apparently it had run down a canyon, so there were hidden depths.

Parts of its surface were like ordinary regolith, just as rocky and dusty, with a kind of gravel surface that revealed no sign of the ice below. Other parts looked like chaotic terrain, except clearly made of ice, with knots of white seracs sticking up out of what looked like boulders. Some of the seracs were broken plates, bunched like the back of a stegosaurus, translucent yellow with the setting sun behind them.

All was motionless, to every horizon — not a movement to be seen anywhere. Of course not; Arena Glacier had been here for forty years. But Sax could not help remembering the last time he had seen such a sight, and he glanced involuntarily to the south, as if a new flood might burst out at any moment.

The Biotique station was located a few kilometers upstream, on the rim and apron of a small crater, so that it had an excellent view over the glacier. In the last part of sunset, as some of the regulars got the station activated, Sax went with Claire and the visitors from Armscor, including Phyllis, up to a big observation room on the top floor of the station, to look at the broken mass of ice in the waning moments of the day.

Even on a relatively clear afternoon like this one, the horizontal rays of the sun turned the air a burnished dark red, and the surface of the glacier sparked in a thousand places, the recently broken ice reflecting the light like mirrors. The majority of these scarlet gleams lay in a rough line between them and the sun, but there were a few elsewhere on the ice, where the reflecting surfaces stood at odd angles. Phyllis pointed out how much larger the sun looked, now that the soletta was in position. “Isn’t it wonderful? You can almost see the mirrors, can’t you?”

“It looks like blood.”

“It looks positively Jurassic.”

To Sax it looked like a G-type star about one astronomical unit away. Of course this was significant, as they were 1.5 astronomical units away. As for the talk of rubies, or dinosaur’s eyes …

The sun slipped over the horizon and all the points of red light disappeared at once. A great fan of crepuscular rays stretched across the sky, the pinkish beams cutting a dark purple sky. Phyllis exclaimed over the colors, which were indeed very clear and pure. She said, “I wonder what makes those magnificent rays,” and automatically Sax opened his mouth to explain about the shadows of hills or clouds over the horizon, when it occurred to him that a, it was a rhetorical question (perhaps), and b, to give a technical answer would be a very Sax Russell thing to do. So he shut his mouth, and considered what Stephen Lindholm would say in such a situation. This kind of self-consciousness was new to him, and distinctly uncomfortable, but he was going to have to say things, at least some of the time, because long silences were also fairly Sax Russellish, and not at all like Lindholm as he had been playing him so far. So he tried his best.