Even Rosenberg slowed, though, as they reached the edge of the city.

It was, she thought again, like walking into a downtown.

The structures here were grotesque spires of ice: some, she guessed, were more than a half-mile tall. The nearest was an octagonal pillar, tipped away from her, Pisa-like. The ground around its base was littered with irregular blocks of ice, some feet high. The surface beneath was smooth ice, as flat as a freeway. And slick, with a thin layer of surface water. Like an ice rink.

Machined.

She clambered past the worst of the ice blocks and walked forward, across a free stretch of floor, until she reached the wall of the structure. She looked up at it. The wall, one of the eight comprising this octagonal cylinder, narrowed as she peered up, merging at infinity with its neighbors into a crimson-grey line.

Suddenly, staring up at the pillar, she felt giddy, as if with reversed vertigo; some primitive primate fear, as Rosenberg would say, that the thing might tumble down and crush her seemed to be about to overwhelm her.

She put out her hand. She touched a cold, hard surface.

The ice was like rock, but there was a slickness to it. When she pulled away her palm, her skin was wet. And now she looked more closely she could see the edges of the building, between the huge facets, were smoothed over.

The building was melting.

She heard Rosenberg’s footsteps receding, so she hurried around the octagonal pillar and followed him, proceeding deeper into the city.

It was like walking through an ice-sculpture caricature of Manhattan. The buildings — spires and pillars, even some narrow, inverted cones — towered over her, their washed-out crimson-grey lines obscuring the sky. In some places she could see lacy bridges connecting the peaks of the structures, but there were a lot more stumps and broken arches than complete spans. The narrow, regular streets between the buildings were cluttered up with rubble, smashed-up ice fragments, some of them huge.

About all of this there was a sense of smoothing out: of rounded corners and edges, of melting. There were even icicles dangling down from the stumps of bridges. Most of the buildings seemed open, with immense arch-like doorways like cathedral entrances. When she peered inside she found nothing but scattered rubble.

The ammono beetles toiled in thin files towards and away from the dense center of the city. With what seemed an inexhaustible patience they worked their way around the innumerable ice-fall obstacles that cluttered up the orderly streets; if she watched for a while, Benacerraf observed that the ammonos always followed the same path around each obstacle, like ants following a biochemical trail.

She met Rosenberg at the center of a small square, bounded on all sides by elephantine ice walls. He was peering up at the huge buildings. There was water on his cheeks; it shone in the pink-grey light of the ice walls.

“All the damage is at ground level. See? That’s where the walls are smashed up and cracked…”

She looked at the building with new eyes. “You’re right, Rosenberg. So how did they get this way?”

“Isn’t it obvious? They fell, Paula.” His eyes were a red-rimmed mess, she noticed. Evidently his mood had crashed again. “Suppose you were building, here on Titan, in this one-seventh gravity and all this thick air… Wouldn’t you build up as high as your materials could go, huge Gothic structures, stilts and spires and bridges miles high? Why, you could pump your walls full of air and use buoyancy to get even more of a lift… But then the sun blew up, and the damn stuff just started melting.”

She walked up to him and took his hand. “Shit, Rosenberg. You’re crying again.”

He looked down at her. “Don’t you get it? Look around you: the ancient, ruined crystal city… This is Xi City. Maybe the houses turn to follow the sun—”

“What?”

“Didn’t you read Bradbury? This is the way the Solar System was supposed to be, Paula. This is why we went to the Moon, why we sent out the probes to Mars.” He walked a few paces ahead, and turned around, his arms outstretched to the huge, sculpted ice walls. “This was what we were looking for all the time. This! It’s just come billions of years too late, is all. Damn, damn…” He ran a hand over his face, smearing tears and snot. “I’m sorry.”

“I know. Come on, Rosenberg.”

Hand in hand, they walked on, deeper into the heart of the crystal city.

A few hundred yards further in, the buildings thinned out, and the crimson light grew brighter; it was like entering a clearing at the heart of a forest thicket.

Benacerraf led the way through the clutter at the base of the last of the buildings. When they stood at the edge of the clear area beyond, she could see across it to the buildings at the far side, maybe a quarter-mile away.

The floor here was clear of the debris of falling rubble. And there was a single structure, as far as she could see: a slim spire maybe twenty feet tall, at the geometric center of the clearing, dwarfed by its skyscraper cousins.

Ammonos moved in complex, interlacing files across the surface. The clearing was roughly circular, and the blank faces of structures walled it in on all sides, as if fencing off the now cloudless crimson sky.

The spire-like object stood at the center of an inner disc of ice, which was clear of even the smallest loose debris; in fact, she thought, it looked as if it had been repeatedly melted and refrozen.

She noticed that the ammono beetles studiously avoided the melt crater, even if they had to take a long detour to do so.

The spire was actually slimmer at the base than at its tip, and now she looked more closely she thought she could see some kind of opening at the top there, pointing up at the face of the sun.

Like an air-scoop mouth, she thought.

And at the base of the spire—

“Fins,” Rosenberg said beside her, pointing. “The thing has fins, Paula. Will you look at that.”

“It’s some kind of rocket, Rosenberg.”

He frowned up at the scoop. “Methane. That’s the propellant. Methane, scooped out of the atmosphere and burned in oxygen, mined from the water-ice.” Now he scratched his bald head. “God damn, Alan Nourse had it right after all.”

“Who?”

“Never mind… I think we’d better get out of here.”

“Huh? Why?”

“Look around.”

The ammono beetles had gone.

Rosenberg said, “The ammonos have built Cape Canaveral in the middle of Xi City. I guess I don’t want to be around when the ship goes up.”

He reached for her hand. Together they walked away from the methane rocket.

They found a valley, maybe a mile from Xi City. It was just a rough gouge in the ice, but it afforded some shelter from the wind. And on its floor there was a shallow, running stream, and clumps of grass-analogue, and some of the mushroom plants.

They zipped together their suits and huddled close beside each other. They sat facing Xi City, and munched mushroom flesh. “So how long do you think we have, Rosenberg?”

“How long?”

She waved a hand. “Before we lose all this. For instance, it’s too hot for Titan to retain an atmosphere now. How come the air doesn’t evaporate?”

“Oh, it is evaporating,” he said. “But it will take a while. The oxygen atoms at the top of the atmosphere must be bleeding steadily into space. But the mass is big… Paula, it will take tens of millions of years for all this air to leak away. It’s like melting the bedrock ice. It will take a million years or more to melt even a few miles of ice, and there are hundreds of miles under us. You have to think in terms of planetary masses, Paula. Nothing happens suddenly. Anyway, it makes no difference. The sun won’t keep still that long. I think it has some growing to go before it’s done with its red giant phase.”