“So how did they do it?” Nadia asked, suddenly curious.
Ann thought about it and smiled. “I don’t know. Not very well, I suspect. Probably dead reckoning.”
Nadia became intrigued by the problem, and started working on it on a sketchpad. Geometry had never been her strong point, but presumably at the north pole on midsummer’s day, the sun would inscribe a perfect circle around the horizon, never getting higher or lower. If you were near the Pole, then, and it was near midsummer’s day, you might be able to use a sextant to make timed checks on the sun’s height above the horizon. . . . Was that right?
“This is it,” Ann said.
“What?”
They stopped the rover, looked around. The white plain undulated to the nearby horizon, featureless except for a couple of broad red contour lines. The lines did not form bull’s-eye circles around them, and it didn’t look like they were at the top of anything.
“Where, exactly?” Nadia asked.
“Well, somewhere just north of here.” Ann smiled again. “Within a kilometer or so. Maybe that way.” She pointed off to the right. “We’ll have to go over there a ways and check with the satellite again. A little bit of simple GPS and we should be able to hit it on the nose. Plus or minus a meter, anyway.”
“If we just took the time, we could make it plus or minus a centimeter!” Simon said enthusiastically. “Let’s pin it down!”
So they drove for a minute, consulted the radio, turned to right angles and drove again, made another consultation. Finally Ann declared they were there, or close enough. Simon instructed the computer to keep working on it, and they suited up and went out and wandered around a bit, to make sure they had stepped on it. Ann and Simon drilled a boring. Nadia kept walking, in a spiral that expanded away from the car. A reddish white plain, the horizon some four kilometers away; too close; it came to her in a rush, as during the black dune sunset, that this was alien— a sharp awareness of the tight horizon, the dreamy gravity, a world just so big and no bigger . . . and now she was standing right on its north pole. It was L8 = 92, about as near midsummer as you could ask, so if she stood facing the sun, and didn’t move, the sun would stay level and circle around her for all the rest of the day, or the rest of the week for that matter! It was strange. She was spinning like a top. If she stood still long enough, would she feel it?
Her polarized faceplate reduced the sun’s glare on the ice to an arc of crystalline rainbow points. It wasn’t very cold. She could just feel a breeze against her upraised palm. A graceful red streak of depositional laminae ran over the horizon like a longitude line. She laughed at the thought. There was a very faint ice ring around the sun, big enough that its lower arc just touched the horizon. Ice was subliming off the polar cap and gleaming in the air above, providing the crystals in the ring. Grinning, she stomped her boot prints into the north pole of Mars.
• • •
That evening they aligned the polarizers so that a very dimmed-down image of the white desert stood around them in the module windows. Nadia sat back with an empty food tray in her lap, sipping a cup of coffee. The digital clock flicked from 11:59:59 to 0:00:00, and stopped. Its stillness accentuated the quietness in the car. Simon was asleep; Ann sat in the driver’s seat, staring out at the scene, her dinner half-eaten. No sound but the whoosh of the ventilator. “I’m glad you got us up here,” Nadia said. “It’s been great.”
“Someone should enjoy it,” Ann said. When she was angry or bitter her voice became flat and distant, almost as if she were being matter-of-fact. “It won’t be here long.”
“Are you sure, Ann? It’s five kilometers deep here, isn’t that what you said? Do you really think it will completely disappear just because of black dust on it?”
Ann shrugged. “It’s a question of how warm we make it. And of how much total water there is on the planet, and how much of the water in the regolith will surface when we heat the atmosphere. We won’t know any of those things until they happen. But I suspect that since this cap is the primary exposed body of water, it’ll be the most sensitive to change. It could sublime away almost entirely before any significant part of the permafrost has gotten within fifty degrees of melting.”
“Entirely?”
“Oh, some will be deposited every winter, sure. But there’s not that much water, when you put it in the global perspective. This is a dry world, the atmosphere is superarid, it makes Antarctica look like a jungle, and remember how that place used to suck us dry? So if temperatures go up high enough, the ice will sublime at a really rapid rate. This whole cap will shift into the atmosphere and blow south, where it’ll frost out at nights. So in effect it’ll be redistributed more or less evenly over the whole planet, as frost about a centimeter thick.” She grimaced. “Less than that, of course, because most of it will stay in the air.”