…But now there seemed to be something approaching over the tight-curving horizon: a feature which didn’t chime with her memory. She leaned forward in her seat, trying to see ahead more clearly.

It was a ridge of ice, looming over the oceans, stretching from side to side of her field of view as it came over the edge of the world.

“Spinner — look.”

“I can’t quite make it out — it doesn’t seem to fit the maps…”

“Maps?” Louise muttered. “We may as well throw the damn things out.”

It was the rim of a crater — a crater so huge it sprawled like an immense scar around the curve of the planet. Within the mile-high walls of the crater, a new sea, deep and placid, lapped its huge low-gravity waves.

“Well, that wasn’t here before,” Spinner said. “It’s wiped out half the surface of the moon.”

Louise had Spinner download projections of the crater’s overall shape, the deep profile hidden from view by the circular methane ocean it embraced.

Beneath the ocean surface the crater was almost cylindrical, with sharp, vertical walls and a flat base.

“Volcanic, do you think?” Spinner asked.

“It doesn’t look like any volcano mouth I’ve ever seen,” Louise said slowly. “Anyway, Titan is inert.”

“Then what? Could it be an impact crater? Maybe when the moons got broken up — ”

“Look at it, Spinner,” Louise said impatiently. “The shape’s all wrong; this was no impact.”

“Then what?”

Louise sighed. “What do you think? We’ve come all this way to find another relic of war, Spinner-of-Rope. Now we know what happened to the people. When whatever caused that struck Titan, the whole surface of the moon must have convulsed. No wonder the cities were lost…”

She imagined the ice-ground cracking, becoming briefly liquid once more, swallowing communities whole; there must have been mile-high tidal waves in the low gravity methane seas, overwhelming the food ships in moments.

Spinner was silent for a while. Then, “You’re saying this was done deliberately?”

Louise smiled. Superet, reconstructing the future from the glimpses left by Michael Poole’s encounter with the Qax, had come across the concept of a starbreaker: a planet-smashing weapon wielded by the Xeelee — a weapon based on focused gravity waves. Superet had even had evidence that a starbreaker of limited power had been deployed inside the Solar System itself: by the Qax invaders from the future, during their failed onslaught on the craft of the Friends of Wigner.

She said to Spinner, “You ought to be getting used to this by now. We know the Xeelee had weaponry sufficient to destroy worlds. For some reason they spared Titan. Instead — they wiped it clean. Just as they did Callisto.”

Louise took the pod down to one of the largest individual islands, close to the rough rim of the Kuiper Sea. There was a soft crunch when she landed, as the pod crushed the friable-ice surface.

A small airlock blistered out of the side of the pod’s hull, and Louise climbed through it.

Instantly she was enclosed by a shell of darkness. In the murk of photochemical smog, her suit lights penetrated barely a few feet. Looking down she could only just make out the surface. Under a layer of thick frost, which creaked as it compressed under her boots, the ground was firm, flat. She lifted herself on her toes, trying her weight; she felt light, springy, under Titan’s thirteen percent gee. There was a soft wind which pushed at her chest.

Snow, drifting down from the huge atmosphere, began to lace across her faceplate; it was white and stringy, and — when she tried to wipe it off with her glove — it left clinging remnants. It was a snow of complex organic polymers, drifting down from the hundred-mile-thick chemical soup above her head.

“Louise? Can you still hear me?”

“I hear you, Spinner.”

She took a few steps forward, away from the gleaming pod; soon, its lights were almost lost in the polymer sleet.

“You know, we terraformed Titan,” Louise told Spinner. “There were ships to extract food and air from the seas. You could walk about on the surface in nothing more than a heated suit. We got the atmosphere clear, Spinner-of-Rope. You could see Saturn, and the rings. And the Sun. You knew you weren’t alone down here — that you were part of the System…”

Now, the terraforming had collapsed. Titan had reverted. It was as if humans had never walked Titan’s surface.

“There used to be a city here, Spinner. Port Cassini. Huge, glittering caverns in the ice; igloos on the surface… A hundred thousand people, at least.

“Mark was born here. Did you know that?” She looked around, dimly. “And as far as I can remember this was the site of his parents’ home…”

She tried to imagine how it must have been to stand here as the final defense around Titan fell, and the Xeelee onslaught began. The starbreaker beams — cherry-red, geometrical abstractions — burned down, through the hydrocarbon smog, from the invisible nightfighters far above the surface. Methane seas flash-evaporated in moments — and the ancient water-ice of the mantle flowed liquid for the first time in billions of years…

“Louise? Are you ready to go home, now?”

“Home?” Louise raised her face to the hidden sky and allowed the primeval, polymeric snow to build up over her faceplate; for a moment, tears, ancient and salty, blinded her. “Yes. Let’s go home, Spinner-of-Rope.”

“Helium flash,” Mark said.

Uvarov had been dozing; his dreams, as usual, were filled with birds: ugly carrion-eaters, with immense black wings, diving into a yellow Sun. When Mark spoke the dreams imploded, leaving him blind and trapped in his chair once more. He felt a thin, cold sensation in his right arm: another input of concentrated foodstuffs, provided by his chair.

Yum, he thought. Breakfast.

“Mark,” he whispered.

“Are you all right?”

“All the better for your cheery questioning, you — construct.” He spoke with a huge effort, fighting off his all-encompassing tiredness. “If you’re so concerned about my health, plug yourself into my chair’s diagnostics and find out for yourself. Now. Tell me again what you said. And what in Lethe it means…”

“Helium flash,” Mark repeated.

Uvarov felt old and stupid; he tried to assemble his scattered thoughts.

“We’ve heard from Lieserl. Uvarov, the birds are continuing to accelerate the evolution of the Sun.” Mark hesitated; his intonation had gone flat, a sign to Uvarov of his distraction. “I’ve put together Lieserl’s observations with a little extrapolation of my own. I think we can tell what’s going to come next… Uvarov, I wish I could show you. In pictures — a Virtual simulation — it would be easy.”

“Well, you can’t,” Uvarov said sourly, twisting his face from side to side. “Sorry to be so inconvenient. You’re just going to have to hook up a few more processor banks to enhance your imagination and tell me, aren’t you?”

“…Uvarov, the Sun is dying.”

For millions of years, the photino birds had fed off the Sun’s hydrogen-fusing core. Each sip of energy, by each of Lieserl’s birds, had lowered the temperature of the core, minutely.

In time, after billions of interactions, the core temperature had dropped so far that hydrogen fusion was no longer possible. The core had become a ball of helium, dead, contracting. Meanwhile, a shell of fusing hydrogen burned its way out of the Sun, dropping a rain of helium ash onto the core.

“The inert core has steadily got more massive — contracting, and heating up. Eventually the helium in the collapsing core became degenerate — it stopped behaving as a gas, because — ”

“I know what degenerate matter is.”

“All right. But you have to be clear about why that’s important, for what comes next. Uvarov, if you heat up degenerate matter, it doesn’t expand, as a gas would… Degenerate matter is not a gas; it doesn’t obey anything like the gas laws.”