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“So they have to keep moving,” he said. “How do they rest, eat, sleep?”

“We eat on our feet, and sleep in carts pulled by companions,” Swan said. “We take turns at that, and on it goes.”

He gave her a look. “So you have an inexorable spur to action. I can see the appeal.”

She almost laughed. “Do you need such a spur?”

“I think everyone does. Don’t you?”

“No. Not at all.”

“But you join these ferals,” he said.

“That’s just to do it. To see the land and the sun. I check out things I made, or do a little crack mining. I don’t need to find reasons to stay busy.”

This was exactly backward, she realized, and shut her mouth.

“You’re lucky,” he said. “Most people do.”

“Do you think?”

“Yes.” He gestured at the sunwalkers, whom they were rapidly leaving behind. “What happens if you run into an obstruction that keeps you from continuing westward?”

“You have to avoid those. In some places they’ve built little ramps that go up cliffs, or trail systems that get through chaotic terrain quickly. There are routes established. Some people stick to certain routes, some do them all. Others like to try new terrain. It’s pretty common to do a complete circumnavigation.”

“Have you done that?”

“Yes, but it’s too long for me. I usually go out for a week or two.”

“I see.”

It was pretty clear he didn’t.

“We were made to do this, you know,” she said suddenly. “Our bodies are nomads. Humans and hyenas are the two predators that chase their prey down by wearing it out.”

“I like walking,” he allowed.

“So what about you? What do you do to occupy your time?”

“I think,” he said promptly.

“And that’s enough for you?”

He glanced at her. “There’s a lot to think about.”

“But what do you do?”

“I suppose I read. Travel. Listen to music. Look at the visual arts.” He thought some more. “I work on the Titan project, that’s very interesting, I find.”

“And the Saturnian league, more generally, Mqaret tells me. System diplomacy.”

“Yes, well, my name came up in the lottery and I had to do my time, but it’s almost over now, and then I plan to return to Titan and get back to my waldo.”

“So… what were you and Alex working on?”

His pop eyes took on a look of alarm. “Well, some of it she wouldn’t want me to talk about. But she spoke of you often, and now that she’s gone, I just wondered if she might have left you a message. Or even arranged things such that you might be able to step in a bit in her absence.”

“What do you mean?”

“Well, you designed many of the terraria out there, and now they form the bulk of the Mondragon Accord. They would listen to you, perhaps, knowing you were one of Alex’s closest confidantes. So… possibly you could go out with me and meet some people.”

“What, to Saturn?”

“To Jupiter, actually.”

“I don’t want to do that. My life is here, my work. I traveled the system enough when I was young.”

He nodded unhappily. “And… you are quite certain Alex didn’t leave anything for you? Something to give to me, in case something happened to her?”

“Yes, I’m sure! There’s nothing! She didn’t do things like that.”

He shook his head. They sat in silence as the tram slid over the dark face of Mercury. To the north some hilltops were just sparking white with the rising sunlight. Then the top of Terminator’s dome appeared over the horizon, like the shell of a transparent egg. As it cleared the horizon, the city looked like a snow globe, or a ship in a bottle-an ocean liner on a black sea, caught in a bubble of green light. “Tintoretto would have liked your city,” Wahram said. “It looks like a kind of Venice.”

“No it doesn’t,” Swan said crossly, thinking hard.

TERMINATOR

Terminator rolls around Mercury just like its sunwalkers, moving at the speed of the planet’s rotation, gliding over twenty gigantic elevated tracks, which together hold aloft and push west a town quite a bit bigger than Venice. The twenty tracks run around Mercury like a narrow wedding band, keeping near the forty-fifth latitude south, but with wide detours to south and north to avoid the worst of the planet’s long escarpments. The city moves at an average of five kilometers an hour. The sleeves on the underside of the city are fitted over the track at a tolerance so fine that the thermal expansion of the tracks’ austenite stainless steel is always pushing the city west, onto the narrower tracks still in the shade. A little bit of resistance to this movement creates a great deal of the city’s electricity.