Xeelee: An Omnibus: Raft, Timelike Infinity, Flux, Ring

by Stephen Baxter

  Above her, the Xeelee nightfighter folded its huge wings.

  Titan, Saturn’s largest satellite, was a world in itself: around three thousand miles across, larger than Earth’s Moon. As she descended, the cloudscape took on the appearance of an infinitely flat, textured plane. Huge low pressure systems in the photochemical smog spiralled 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 pod’s progress on the data desk. ‘Are you all right?’ Spinner asked now.

  ‘I’ve been better . . . I’m not hurt, Spinner-of-Rope.’

  ‘You knew you had to expect this kind of treatment. Titan’s 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? It’s spring; you’ve 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.

  ‘It’s bloody dark,’ she muttered.

  ‘Louise, you’re dropping into a mush of methane, ethane and argon. It’s a smog of photochemical compounds, produced by the action of the Sun’s 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. ‘Don’t read out the whole damn data desk to me. Photochemical compounds aren’t 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 mankind’s 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 pod’s 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 per cent 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 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. Paradoxa, 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. Paradoxa 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 per cent 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 l
eft 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 defence 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.’

  ‘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 the “dead” core is flooded with helium fusion energy. Now remember what I told you, Uvarov: the core is degenerate. So it doesn’t expand, to compensate for all that heat . . .’

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

  ‘Because it can’t expand, the core can’t 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. That’s the standard story; now let’s get back to the Sun. Sol isn’t a conventional giant, whatever it is.’

  ‘No. But it’s approaching its helium flash point.’

  ‘Won’t the action of the birds suppress this helium runaway - the helium flash - just as they’ve suppressed hydrogen fusion, all this time?’

  ‘No, Uvarov. They’re not taking out enough energy to stop the flash . . . Maybe they don’t 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 didn’t 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 Sol’s 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 Sol’s 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 ou
tside 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 don’t 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 a “standard” lifecycle.’

  ‘Superwind,’ Uvarov breathed. ‘How soon is Sol’s helium flash?’

  ‘Lieserl’s 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 can’t stay here,’ he said.

  21

  By the time she’d climbed to the top of the giant kapok tree her hand-grips were slick with sweat, and her lungs were pumping rapidly. Spinner-of-Rope took off her spectacles and wiped the lenses on a corner of her loincloth. Zero-gee or not, it still took an effort to haul her bulk around this forest . . . an effort that seemed to be increasing with age, despite all the AS treatment in the world.

  She was at the crown of the kapok. The great tree was a dense, tangled mass of branches beneath her. Seeds drifted everywhere, filling the rippling canopy with points of light - like roaming stars, she thought. Somewhere a group of howler monkeys shrieked out their presence. Their eerie ululation, rising and falling, reminded her of the klaxon which had once called the Undermen to their dreary work . . .