Titan

by Stephen Baxter

  She loosed her traces and leaned clumsily over the lip of the crack. She dipped one gloved hand in the water. Immediately the cold penetrated the layers of her glove, and the heater diamonds stung her flesh. Close to its freezing point, the water was a hundred and seventy degrees above the ambient temperature.

  Water was molten rock here. It was as if some suited monster had come to Earth, and dipped its hand into the scalding red-hot lava stream of a volcano. But she was the alien, here on Titan.

  She lifted out her hand. Away from the water surface the air temperature dropped quickly, and the droplets of oily water that clung to the fabric of her glove spread and froze, turning to frost patterns. When she closed her fist the frost crackled and broke away, hard ice fragments falling back to the water’s dark surface.

  She stepped over the crack in the ice and hauled the sled across.

  She came to more leads of open water, slick with hydrocarbons, opening and refreezing. Some of them were too wide to risk crossing, and she had to detour, tracing up and down between the leads. In some places the ice was so thin it was spongy and creaking, and if she stepped too close to an edge it would crumble away into the open water. She found a lead that was closing, its edges grinding noisily together. Where the two plates met, the ice was cracking, its sharp sounds ricocheting out across the emptiness, echoing from the iron-hard ice.

  The ice field stretched on; ridges and plates pushed out of the plain like pieces of gigantic, abandoned furniture. From all around her rang out the aching, grinding noise of moving ice, crackling like the shock waves from a Shuttle launch. The noises came together in great waves, punctuated by godlike silences.

  As she penetrated the field of frozen-over ejecta, the visibility opened out, the pervading gloom of Titan’s orange sky lifting a little. Thin methane clouds, dark and tangled, blew ahead of her, obscuring the tall orange sky. Perhaps the relative warmth of the water was clearing the air of some of the organic haze.

  At last she came to a place where the broken layer of methane clouds, ahead of her, grew still darker. The darkness—near to black—seemed to begin in a sharp discontinuity, almost a straight line, scraped across the sky.

  She smiled. Rosenberg had warned her to expect this.

  It was a water sky.

  There must be a wide stretch of open water, no more than a few miles away, reflecting darkly from the low methane clouds of Titan.

  She pitched her camp on a large plate of ice, hundreds of yards from any open leads. The air was so warm that she was able to strip off the outer layers of her suit. It felt like a great luxury, as she rubbed handfuls of half-melted ice over her bruised skin to clean herself.

  She drank her fill of cool comet water.

  That night, as she lay huddled against the tent’s plastic wall, she listened to the muffled groaning of the thin ice beneath her. She imagined the slow swell of the comet water, the big underground waves traveling back and forth across the ejecta sheet.

  At any moment this plate could crack, pitching her into the cold water, suit and all. But somehow that wasn’t a frightening prospect. She was, after all, made of water. Water was home.

  She slept, without dreaming, as well as she had done since Rosenberg’s death.

  She went through her waking ritual for the last time.

  She breakfasted on dried strawberries, crackers, and tiny, sweet lettuce leaves from the CELSS farm. She took a final dump, into an empty plastic food bag, and cleaned herself thoroughly.

  She blew her nose on a fragment of parachute fabric. It was the last time she’d be able to do that, even.

  There was a last time for everything, she thought: not just the grand actions, but the small, human things. It all counted.

  She pulled on her suit. She tucked her little packet of photos inside her suit, over her heart. She sealed her helmet and gloves, and turned the switch that powered up her PLSS. She heard the familiar high-pitched whine of the pumps and fans, the cool hiss of the oxygen blowing over her face.

  She packed away what she could: her food and waste bags, the power cell. Soon, the tent was as neat as she’d left Discovery.

  She pushed her way out of the tent’s cramped little airlock. Outside, standing on the thin, grinding ice, she tucked Rosenberg’s canister of spores under her arm, to keep it as warm as possible.

  She looked around her little outpost. The half-empty sled stood on the ice, its parachute-fabric cover loosely knotted over it. The tent, closed up, was compact and neat.

  She fixed her Hasselblad to the S-band antenna stand, and lined it up so it framed the tent and sled. She checked that the antenna was still aligned correctly on Cassini; it was possible the drifting of the pack ice during her sleep had pushed it off its line.

  Feeling self-conscious, she went to stand in the camera’s field of view. Standing there before her little camp, in her grimy, battered, much-repaired EVA suit, she held up her canister of spores, while the camera fired image after image up to Cassini.

  She hated these Armstrong poses. But maybe, she thought, this one was justified. After all, if Rosenberg was right, with this one act she might be shaping the future of a new biosphere.

  These might be the most important photographs ever taken.

  She wondered whether to smile or not.

  Her residual sense of orderliness made her walk around the camp once more, checking everything was intact and stowed away.

  Then she turned and strode off, across the ice, towards the water sky.

  A wind began to pick up, blowing off the broken ground in front of her, hard and piercing; she found she relished its resistance.

  She could feel her packet of photographs, a hard rectangle pressed against her chest by the suit.

  She felt as if she was discarding her life, in huge layers: first Earth itself, shrunken to a pinprick of light by the huge distance she’d traveled; then Tartarus Base, with its painfully assembled and repaired life-sustaining gadgets; and at last even the trappings of her own little encampment out here on the water ice. Now, she was left with nothing but her body, and the battered suit that was its last protection.

  The leads began to widen and interconnect.

  Soon the ice was broken up into isolated islands, some only a few feet across, separated by channels of gray, scummy water. Ahead, fragmented ice stretched in a loose mosaic. She could see the open water ahead of her, a dark band encroaching from the horizon, flecked with loose ice floes.

  She pressed on, climbing over the narrower channels, taking care to stick to the larger ice floes. But the ice was fragmenting rapidly. Soon, even the biggest floes were unstable beneath her feet.

  She couldn’t go any further. This would have to do.

  She kicked off her skis, and stacked them neatly to one side. She wouldn’t be needing them any more. She took a last sip of orange juice, from the worn plastic nipple inside her helmet.

  She walked to the edge of the ice. She took Rosenberg’s canister of spores, and dipped both her gloved hands in the water. The cold of the water was a thrilling shock, easily penetrating the feeble resistance of the gloves’ heating elements.

  Under the water, she opened Rosenberg’s canister, and shook out the spores, scattering them as widely as she could.

  When the canister packet was empty she withdrew it, shook it clear of ice, and tucked it neatly into a sample pocket, buttoning closed the flap.

  Then she stood straight. She looked around at the haze-drenched world around her: the cramped, close horizon, the scattered darkness of the methane clouds above, the shattered ice landscape, with that band of free water, just out of reach.

  She reached up and snapped the switch on her chest that shut down her PLSS.

  The sound of pumps and fans died immediately. The air stopped washing over her face, and felt thicker, more stale. The cold of Titan dug into her flesh through the pattern of heating elements. And she could hear the moan of the wind, a remote bass tone, and the deep crackling of the ice sea, emer
ging from all over the landscape.

  It was the first time she’d heard the music of Titan, unmasked by the man-made noises of her equipment.

  She walked forward, across this icy beach.

  Before she could reach the edge of the floe, the ice crumbled under her weight.

  There was a moment of falling—extended by Titan’s low gravity—long enough for a small stab of terror to dig into her consciousness. But then her feet and legs hit the thick, oily surface of the water. The meniscus rushed up her body, its cold mass enclosing, and joined over her head.

  Her suit made her more dense than the water, and so she sank into darkness.

  She fell slowly. She let her arms and legs relax, and she felt them drift away from her torso, separated by the flow of water.

  She turned slowly onto her back.

  Above her she could see the surface of the water, the dim orange glow of the sky above, huge oily ripples creasing the meniscus. But the surface receded, its detail lost, and soon the sky was invisible, save for the faintest of orange glows.

  The water felt comfortable as she fell deeper into it, as if she was returning to a kind of home.

  Now, at last, it was all gone. The Universe had collapsed down to the layer of water that pressed against the surface of her suit, the bubble of air in her helmet. There were no more choices, no decisions, no plans.

  Maybe this was mankind’s last moment, she thought, here on this remote beach, the furthest projection of human exploration. Maybe, in fact, the sole purpose of the human story, fifty thousand years of crying and living and loving and dying and building, had been to deliver her here, now, to this alien beach, the furthest extension of mankind, with her little canister of seeds.

  The cold dug deeper. For a while she found herself shivering, and she wrapped her arms around her torso. But that seemed to pass, and she felt comfortable again.

  She knew what was happening. This was hypothermia, her core body temperature falling, as her body heat leaked out through the suit’s unresisting layers into the giant welcoming mass of fluid beyond.

  It didn’t really matter.

  She thought she was unconscious for a time.

  It was hard to be sure.

  Then she thought she could see Columbia, far below, rising towards her.

  She smiled.

  The orbiter’s leading edges glowed, a faint orange. The floodlights in the payload bay glowed like a captive constellation. And beyond Columbia there were stars: thousands of them, easily visible to her dark-adapted eyes, like the blackest desert night on Earth. She could even see the great sweep of the Galaxy, the ragged edge of the dust-clouds at the core.

  The EVA was over. She reached up her hands, and started to take off her helmet.

  Book Six

  TITAN

  SUMMER

  Voyager One reached the boundary of the Solar System.

  This was the heliopause, the sheet in space where the wind of ionized particles from the sun grew so feeble it was overwhelmed by the broader stream of interstellar ions. Already Voyager was a hundred times Earth’s distance to the sun, ten times Saturn’s distance.

  When gushes of solar plasma hit the heliopause, immense radio blasts—a hundred trillion watts—were generated. Voyager’s instruments, almost overwhelmed, recorded this, and faithfully attempted to download the data back to Earth.

  Still there was no reply, no reassuring command stream.

  Even beyond the heliopause, the sun’s gravity held sway; there were clouds of objects out here—ice moons, a trillion comets, never observed by humans—circling the central star. Voyager soared through this new realm, its radioisotope power slowly fading.

  Voyager tried to contact Earth until its reaction gas failed, and it could no longer point its antenna. And by 2020 there was no longer sufficient power to drive the radio transmitter. Still the software cycled through its reacquisition algorithm, sending commands to inert attitude thrusters and radio transmitters, until the last trickle of power died.

  It took twenty thousand years for Voyager to cross the Oort Cloud, the sun’s immense swarm of comets. At last it was free of the Solar System, its final gravitational bonds broken.

  Its power and radio transmitter long dead, Voyager embarked on a new journey through the silent calm of interstellar space: an endless circling of the heart of the Milky Way galaxy.

  There was almost nothing here to damage the derelict craft. The stars were so sparsely scattered that Voyager would never encounter another stellar system…

  As time eroded, the logic of physics unfolded implacably.

  The sun was no longer young.

  Its core became denser and hotter, as it clogged with the accumulated helium ash of billions of years’ hydrogen fusion. The sun got brighter, at the rate of eight percent per billion years.

  But for a long time Earth’s surface temperature remained the same. Earth was protected by matter and energy feedback cycles maintained by living and geological processes. And as the temperature rose silicate rocks weathered more easily, absorbing carbon dioxide from the atmosphere.

  But it couldn’t last forever.

  Eventually the carbon dioxide concentration in the atmosphere fell so low that the plants and trees could no longer photosynthesize. That put an end to the biosphere’s carbon supply. The rocks continued to weather, and the carbon dioxide concentration fell still more rapidly, and Earth heated quickly.

  Maybe humans could have prevented this, with some huge feat of planetary engineering. There were no humans around to try.

  On the parched planet, one species after another faced what human biologists had called thermal barriers to their survival. The more complex plant and animal species diminished first, as Earth shed the biological complexity painfully gained over billions of years.

  After a billion and a half years the surface temperature averaged fifty degrees Centigrade, above which no animal, fish, crustacean or insect could survive. Most vascular plants and mosses succumbed as well, leaving the land and oceans empty save for microorganisms: multicelled animals like algae and fungi.

  But above sixty or seventy degrees the structural characteristics of even the simplest multicelled creatures—like membrane systems—could not be sustained. The survivors now were one-celled creatures, like cyanobacteria and some photosynthetic bacteria.

  Above seventy degrees photosynthesis ceased at last.

  The last survivor of Earth’s once-rich biosphere was a hardy bacterium, swimming through the sulfur-rich waters surrounding a black smoker ocean-floor vent. The story of life on Earth had come full circle, for the heat-loving archaebacteria were among the oldest life forms: they had arisen on a younger, hotter Earth, and become the progenitors of all subsequent life.

  After another hundred million years the oceans began to boil.

  Huge clouds of vapor were suspended in the atmosphere. A new greenhouse factor came into play, driving temperatures higher still, ever faster.

  The end came at two hundred and fifty degrees Centigrade, above which the very stuff of life—the giant molecules, nucleic acids and amino acids—was broken down.

  The water clouds did not last long. The water vapor was broken up by energetic sunlight and its hydrogen was driven off into space, leaving a planet baked permanently free of water.

  And the loss of all water stopped the weathering of silicate rock, the process which drew carbon dioxide out of the atmosphere. Volcanic carbon dioxide began to accumulate in the atmosphere. New clouds rose, and the planet began to bake…

  At that remote time, Venus and Earth became at last what humans had dreamed in ancient times: twin planets, alike in every significant detail—scorched dry, their surfaces cracked and flattened under a dense, sluggish atmosphere, utterly lifeless.

  It was different, for Titan.

  The heating of the sun ruined the old surface of Titan, the gumbo-streaked, icy landscape humans had explored. The ethane of Clear Lake boiled, evaporated. The gases dissolved
there—nitrogen, methane, hydrogen—thickened the atmosphere still further, adding to a greenhouse effect that accelerated the warming of the atmosphere.

  Eventually the comparatively thin shells of ice over the magma—the ancient ammonia oceans—melted, exposing the primal seas once more. Ammonia and water vapor enriched the air and boosted the greenhouse effect still further.

  It got warm.

  The remnants of ancient biologies stirred. A story of life, interrupted billions of years earlier, was able to resume.

  For a while. But it was a life which would not have long to flourish.

  Soon, the sun neared the end of its stable Main Sequence lifetime; it began its final, deadly bloom.

  When the sun’s core hydrogen was exhausted, the fusion fire there dimmed. For ten million years the core contracted. Then a shell of hydrogen outside the core started to burn. That started the expansion of the outer layers.

  The sun grew gigantic, its surface billowing towards what was left of the inner planets. Confronted by the huge face of the sun. Earth’s surface temperature reached three thousand degrees, only a little less than the surface of the expanding sun.

  Life on Titan shriveled, baked, as even the water ice bedrock began to melt.

  … Like a desiccated dragonfly corpse adrift on a breeze. Voyager One circled the heart of the Galaxy.

  At last the slow sublimation of metal caused the aluminum structure to weaken to the point where its ten-sided framework collapsed. The fragments of the spacecraft—instrument booms and power generator, pitted and tarnished, metal walls reduced to a paper thickness—drifted away from each other. The directional antenna, as thin as a dried autumn leaf, crumbled away from the curving spars that supported it, so that the ruin of the spacecraft was surrounded by a cloud of glittering aluminum dust.

  Voyager was a fragment of American technology, a thing of metal dug from the vanished Earth, some twenty thousand light years from the sun. It was the last human artifact in existence.

  … Rosenberg was lying on his back.