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20

The pod slid, smooth and silent, down toward Titan.

Louise clutched at her seat. The hull was quite transparent, so that it felt as if she swathed in her environment suit, with a catheter jammed awkwardly inside her were suspended helplessly above the pale brown clouds of Titan.

Above her, the Xeelee nightfighter folded its huge wings.

Titan, Saturns largest satellite, was a world in itself: around three thousand miles across, larger than Earths Moon. As she descended, the cloudscape took on the appearance of an infinitely flat, textured plane. Huge low pressure systems in the photochemical smog spiraled around the world, and small, high clouds scudded across the stratosphere.

The first thin tendrils of air curled around the walls of the pod. Overhead, the stars were already misting out.

Suddenly the pod dropped, precipitously. She was jarred down into her seat. Then the little craft was yanked sideways, rocking alarmingly.

Lethe, Louise said ruefully, rubbing her spine.

Louise had left Spinner in the lounge, to follow the pods progress on the data desk. Are you all right? Spinner asked now.

Ive been better Im not hurt, Spinner-of-Rope.

You knew you had to expect this kind of treatment. Titans atmosphere is a hundred miles thick: plenty of scope for generating a lot of weather. And there are high winds, up there at the top of the atmosphere.

It was quite dark in the cabin now; the opaque atmosphere had enfolded the pod completely, leaving only the cabin lights to gleam from the transparent walls.

Spinner went on, And did you know Titan has seasons? Its spring; youve got to expect a lot of turbulence.

As the pod dropped further it shuddered against a new onslaught; this time Louise thought she actually heard its structure creak.

Spring, murmured Louise. Where are the songs of Spring? Ay, where are they?

Louise?

John Keats, Spinner-of-Rope. Never mind.

Now the buffeting of the little ship seemed to lessen; she must have passed through the high-wind stratosphere. She pulled out a little slack in the restraints which bound her to the seat. Beyond the hull, the cabin lights illuminated flakes of ammonia ice, and fine swirls of murky gas shot up past the pod and out of sight.

Its bloody dark, she muttered.

Louise, youre dropping into a mush of methane, ethane and argon. Its a smog of photochemical compounds, produced by the action of the Suns magnetosphere on the air I can see a lot of hydrogen cyanide, and

I know all that, Louise growled, gripping her seat as the pod lurched again. Dont read out the whole damn data desk to me. Photochemical compounds arent what I came down here to find.

What, then?

People, Spinner.

Once, this had been the most populous world outside the orbit of Jupiter: Titan had cradled mankinds most remote cities. Surely Louise had thought if anywhere had survived the devastation that had struck the inner worlds it would be here.

She needed to see what was going on. Louise punched at the control pad before her. The walls of the pod faded to pearly opacity. She called for a Virtual image, an amalgam constructed of radar and other data.

Below her, in the pods Virtual windows, the landscape of Titan assembled itself, as if from elements of a dream.

She banked the pod and took it skimming over the crude Virtual representation, fifty miles above the surface.

Titan had a core of rock at its heart, clad by a thick mantle of frozen water ice. Beneath the obscuring blanket of atmosphere, eighty percent of the solid ice surface was covered by oceans of liquid methane and ethane, richly polluted by hydrocarbons. The remaining fraction of dry ice-land was too sparse to form into sizeable continents; instead, ridges of water-ice, protruding above the methane, formed strings of islands and long peninsulas.

Well, the oceans were still here. Louise let the ancient, familiar names roll through her head: there was the Kuiper Sea, Galilei Archipelago, the Ocean of Huygens, James Maxwell Bay

But, of the humans who had once named this topography, there was no sign. In fact, it was as if they had never been.

Once, huge factory ships had sailed across these complex oceans, trailing high, oily wakes; enough food had been manufactured in those giant ships to feed all of Titan, and most of the other colony-moons in the Saturn system as well. There were no ships here now. Maybe, if she looked hard enough, she would find traces of huge metal carcasses, entombed in the ice floors of the chemical seas.

But now there seemed to be something approaching over the tight-curving horizon: a feature which didnt 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 cant quite make it out it doesnt 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 wasnt here before, Spinner said. Its wiped out half the surface of the moon.

Louise had Spinner download projections of the craters 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 doesnt look like any volcano mouth Ive 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 shapes all wrong; this was no impact.

Then what?

Louise sighed. What do you think? Weve 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, Youre saying this was done deliberately?

Louise smiled. Superet, reconstructing the future from the glimpses left by Michael Pooles 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 pods 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 Titans 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 werent 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 Titans 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. Lets 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 youre so concerned about my health, plug yourself into my chairs 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.

Weve 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. Ive put together Lieserls observations with a little extrapolation of my own. I think we can tell whats going to come next Uvarov, I wish I could show you. In pictures a Virtual simulation it would be easy.

Well, you cant, Uvarov said sourly, twisting his face from side to side. Sorry to be so inconvenient. Youre just going to have to hook up a few more processor banks to enhance your imagination and tell me, arent you?

Uvarov, the Sun is dying.

For millions of years, the photino birds had fed off the Suns hydrogen-fusing core. Each sip of energy, by each of Lieserls 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 thats important, for what comes next. Uvarov, if you heat up degenerate matter, it doesnt expand, as a gas would Degenerate matter is not a gas; it doesnt obey anything like the gas laws.

So we have this degenerate, dead core of helium, the burning shell around it. What next?

Now we start speculating. Uvarov, in a conventional giant, when the core mass is high enough about half a Solar mass the temperature becomes so high, a hundred million degrees or more, that a new fusion chain reaction starts up: the triple-alpha reaction, which

The fusion of the helium ash into carbon.

Yes. Suddenly thedead core is flooded with helium fusion energy. Now remember what I told you, Uvarov: the core is degenerate. So it doesnt expand, to compensate for all that heat

You turn condescension into an art form, Uvarov growled impatiently.

Because it cant expand, the core cant cool off. There is a runaway fusion reaction a helium flash lasting no more than seconds. After that, the core starts to expand again, and eventually a new equilibrium is reached

All right. Thats the standard story; now lets get back to the Sun. Sol isnt a conventional giant, whatever it is.

No. But its approaching its helium flash point.

Wont the action of the birds suppress this helium runaway the helium flash just as theyve suppressed hydrogen fusion, all this time?

No, Uvarov. Theyre not taking out enough energy to stop the flash Maybe they dont intend to. And, of course, the fact that the core of Sol is so unusually hydrogen-rich is going to make a difference to the outcome. Perhaps there will be some hydrogen fusion in there as well, a complex multiple reaction.

Mark. You said a new equilibrium will be reached, after the helium flash. Uvarov didnt like the sound of that. He wondered if it would be healthy to be around, while an artificially induced red giant struggled to find a new stability after the explosion of its core What will happen, after the helium flash?

Well, the pulse of heat energy released by the flash will take time some centuries to work its way through the envelope. The envelope will expand further, seeking a new balance between gravity and radiation pressure. And the energy released in the flash will be immense, Uvarov.

Immense?

Uvarov, there will be a superwind.


Superwind

The helium flash would blow away half the mass of the Sun, into an expanding shell ballooning outwards at hundreds of miles a second.

The core exposed, a shrunken thing of carbon-choked helium would become a white dwarf star: cooling rapidly, with half the mass of Sol but just a few thousand miles across, no larger than old Earth. The flocks of photino birds, insubstantial star-killers, would continue to swoop around the heart of Sols diminished gravity well.

At present before the flash Sol was a red giant around two astronomical units across. After the superwind the envelope would be blown into a globe twenty thousand times that size, a billowing, cooling cloud three hundred light days across.

The furthest planet from the heart of old Sol was only forty astronomical units out six light-hours. So the swelling envelope would, at last, smother all of Sols children.

Then, when the superwind was done, the dwarf remnant would emit a new wind of its own: a fizz of hot, fast particles which would blow at the expanding globe, pushing out the inner layers. The globe would become a planetary nebula a huge, cooling, hollow shell of gas, fluorescing in the light of the dying dwarf at its heart.


Mark said, At last, of course, the fusing helium in the core will be exhausted. Then the core will shrink once more, until the temperature of the regions around the core becomes high enough for helium fusion to start in a shell outside the core, but within the hydrogen-burning shell. And the helium fusion will deposit carbon ash onto the core, growing in mass and heating it up until the fusion of carbon begins

The cycle repeats, Uvarov. There will be carbon flashes and, later, flashes of oxygen and silicon At last, the giant might have a core of almost pure iron, with an onion-shell structure of fusing silicon, oxygen, carbon, helium and hydrogen around it. But iron is a dead end; it can only fuse by absorbing energy, not liberating it.

And all this will happen to the Sun?

Mark hesitated. Our standard models say that the reactions go all the way to iron only in stars a lot more massive than the Sun say, twelve Solar masses or more. He sighed, theatrically. Will we get onion-shell fusion in the heart of the Sun? I dont know, Uvarov. We may as well throw out our theoretical models, I guess. If the photino birds are as widespread as they seem to be, there may not be a single star in the Universe which has followed through astandard lifecycle.

Superwind, Uvarov breathed. How soon is Sols helium flash?

Lieserls observations are sketchy on this. But, Uvarov, the conditions are right. The flash may even have happened by now. The superwind could already be working its way out

How soon, damn you?

We have a few centuries. No more.

Uvarov swept his blind face around the saloon. He pictured the ruined Jovian system beyond these walls, the bloated star dominating the sky outside.

Then we cant stay here, he said.


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