“Lucky for me indeed.”

She trudged on ahead of him. There was no reason to hurry to say the next thing; they had all day. Wahram watched her walk ahead of him: her stride was graceful and long, she was in her home g and sinuous, efficient. Very quickly she could get well ahead of him. She did not seem ill yet. From behind he sometimes heard her having conversations with her qube. For whatever reason, she had shifted Pauline’s voice to exterior audibility; maybe she was keeping that little promise to him. The conversations between the two almost always sounded like arguments; Swan’s voice was clearer and more hectoring, but Pauline’s alto, slightly muffled by Swan’s own skin, was somehow mulishly contentious as well. Depending on how one programmed them, qubes could be real fiends for argument, quibblers to the highest degree. Once Wahram caught up enough to be able to eavesdrop on them, and came in on something that had been going on for a while; Swan was saying, “Poor Pauline, if I were you I would be so sad! I feel so sorry for you! It must feel terrible to be nothing but a packet of algorithms!”

Pauline said, “This is the rhetorical device called anacoenesis, in which one pretends to put oneself in the place of one’s opponent.”

“No, not at all,” Swan assured her. “I really am sympathetic. To be so few qubits, to be just algorithms grinding it out. I mean considering that, you do very well.”

Pauline said, “This is the rhetorical device called synchoresis, in which one makes a concession before renewing the assault.”

“Maybe you’re right. I don’t really know why I thought you were stupid, given the huge power of these arguments of yours. And yet—”

“This is both sarcasm and aporia in the bad sense I mentioned before, of a momentary expression of doubt, often faked, before renewing the attack.”

“And thisis the defense called casuistry, where when you’ve got nothingyou retreat into a cloud of verbiage. Maybe you’re right, maybe it’s just smart consciousness and stupid consciousness. That would explain a lot.”

Pauline did not seem to be deterred. “Happy to submit our speech acts to a double-blind study to see if any distinctions at all can be made between yours and mine.”

“Really?” Swan said. “Are you saying you can pass a Turing test?”

“It depends who’s asking the questions.”

Swan laughed scornfully, but she really was amused, Wahram could hear it. So at least the qube was good for that.

The two of them swapped the lead every half hour, just to mark time and change the view, such as it was. They did not always talk; it would have been impossible, he thought. In any case they hiked in silence for many minutes at a time. Over them the tunnel lights seemed to move independently backward, as if they walked at the top of a vast Ferris wheel, and only just kept pace with the backward sweep of the wheel. At the end of an hour Wahram’s feet were sore, and he was happy to sit down. They used their aerogel sleeping pads as cushions to sit on. Meals came from foil envelopes found in the emergency gear at the stations, and were bland for the most part. After a while they mostly only wanted to drink water, though there were some powders to mix in if one wanted.

In general their rests were about half an hour long. Any longer and Wahram began to stiffen up, and Swan became fretful. And the sunwalkers would get too far ahead. So Wahram would heave himself to his feet and take off again. “Do you think any of these stations have walking poles in them?”

“I doubt it. We can look at the next one. Maybe something could be used as canes.”

After a time of silence she would sometimes snap. “All right tell me something! Tell me about yourself! What’s your first memory?”

“I don’t know,” Wahram said, trying to locate it.

“My first memory,” she said, “comes from a time that my parents tell me I was three. My parents were part of a house that decided to move to the other side of the city. I think we were trading places north to south, in order to look at the other half of the countryside as we passed by it. Or maybe they just told me that. So a bunch of carts were there, and both houses were moving stuff back and forth. Everything my family owned could fit on the back of one battery cart and two handcarts. My mother took me back inside when the place was emptied, and it scared me. I think that’s why I remember it. My room looked much smaller when it was empty, and that seemed backward and scared me, like the world had shrunk. We fill rooms to make them bigger. Then we went back outside, and the other image that sticks with me, along with the empty room, was all the stuff in the bed of the cart, and everyone standing by it at the curb, under a set of trees. Above some trees I could see the Dawn Wall.”

She hiked on for a time in silence, and Wahram felt the empty grumble in his stomach that marked another mealtime’s approach.

“By now that’s all burned down,” she said.

But now her voice was unusually calm. She was no longer grieving in the same way, it seemed.

“When the sun got high enough that the city was out of the shade of the Dawn Wall,” she added, “it would go quick.”

“I know the tracks don’t melt on the brightside,” Wahram said. “Anything else?”

“The city infrastructure will be fine,” she conceded. “The shell. Some metals, ceramics, mixes of the two. Glassy metals. And then just ordinary tempered steel, stainless steel. Austenite steel. We’ll see. I suppose it will be interesting to see what it looks like when night falls on it again. Everything will have burned away except the frame, I guess. As soon as the sun hit, the plants would begin to die. They’ll be dead by now, all the plants and animals, even the bacteria and such. We’ll have to rebuild it.”

“Maybe,” he said.

“What do you mean?”

“Well, I think they’ll want to understand what happened to the tracks, and feel they are in a position to stop it happening again. Or else build to a different design. Free the city from its tracks, maybe, and roll over the landscape on wheels.”

“That would require some locomotion,” she pointed out. “As it is, the expansion of the tracks drives the city forward.”

“Well, it will be interesting to see what happens, then.” Wahram hesitated. “It would be pointless to rebuild and then have a recurrence.”

“If it was a low-likelihood accident, then a recurrence wouldn’t be likely.”

“I was under the impression that all such happenstances were already guarded against.”

“Me too. Are you suggesting that it was some kind of attack?”

“Yes, well, I’ve been thinking about that, anyway. I mean, consider what happened to us on Io.”

“But who would want to attack Terminator?” she demanded. “Attack it and yet miss it by a few kilometers, killing the town but leaving the people alive?”

“I don’t know,” Wahram said uneasily. “There’s been talk about the conflict between Earth and Mars, how it could even lead to war.”

“Yes,” she said, “but the talk always goes on to declare this impossible, because everyone is so vulnerable. Mutual assured destruction, as always.”

“I’ve always wondered about that,” Wahram admitted. “What if a first strike is made to look like an accident, and is so successful that no one knows who did it, and meanwhile the victim is mostly vaporized? A scenario like that might make one think there is not any certain mutually assured destruction.”

“Who would feel that way?” Swan asked.

“Almost any power on Earth could make the calculation. They’re safer than any of us. And Mars is notoriously self-absorbed, and also can’t be punctured with a single dart. No, I’m not convinced there can’t be a power out there that harbors a feeling of invulnerability. Or an anger so great they don’t care about consequences.”