Titan

by Stephen Baxter

  “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 spirelike 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.”

  “How do you know?”

  “Because this place is so damn cold. The black-body temperature here will be closer to nine hundred degrees, when the giant phase reaches its climax…”

  “Shit.”

  “Yeah. The atmosphere will evaporate first. Then the ice mantle will melt, and boil away. Nothing left but the rocky core.”

  “How long?”

  He shrugged. “I’d say we have a hundred thousand years.”

  “A hundred thousand years. Not much.”

  He grunted around a mouthful of mushroom. “Only twice as long as the human species existed before we were born. You just don’t think big enough, Paula.”

  “No. Hell, I guess I never did. So,” she said. “What are we supposed to do now?”

  “I guess that’s up to us. We could try to talk to the ammonos. You know, I’ve been thinking about why we’re here.”

  “You have?”

  “Yeah. Think about it. They terraformed their own planet. They rebuilt our biosphere, or a copy of it, from what we left behind, as best they could. And they found us in the ice, and managed to … repair us. But I don’t think they understand what we are. They don’t react to us, except as some kind of animal, and they’ve made no attempt to communicate with us. Paula, they might not even know we’re intelligent. Yes, talking to the ammonos would be a hell of a challenge.” He looked up. “Maybe they could tell us what happened to Earth, to mankind. Maybe I could make a telescope. Grind some ice into lenses. It would be interesting to see what else is out there.”

  “What else?”

  “We could fly here.”

  “We could?”

  “The light gravity, the thick air… Da Vinci flying machines would work.” He frowned. “Maybe some kind of winged bicycle would be the best solution. Hell, it would be easy. You could glide most of the way. I’ve seen it done. And then we could think about making our own methane rockets. Maybe we could even borrow some of the ammonos’ technology. Paula, this is a moon, but a big damn moon. We can explore it from pole to pole…”

  After a time, Benacerraf sat back. “Plans and schemes. Busy, busy, busy. But what’s it for?”

  “Huh?”

  “Rosenberg, this isn’t some dumb camping trip. It’s not even an EVA. We’re the last survivors of the human race, stuck here in the far future. Are we supposed to repopulate the planet?”

  He coughed, spraying out mushroom. “Sorry,” he said, wiping fragments off their joined suits. “I wasn’t expecting that. I sure as hell am no Adam.”

  “And I ain’t no Eve,” she said firmly.

  Anyhow, the phrase reminded her uncomfortably of Bill Angel.

  “I don’t think we need to,” Rosenberg said. “I think I know what that rocket ship is for.”

  “It’s pretty damn small,” she said.

  “Huh? The rocket?” He looked puzzled. “Small for what?”

  “For an evacuation. Titan is doomed, right? But you wouldn’t get a single ammono beetle in that thing.”

  He laughed. “You’re thinking like a human, Paula.”

  “What do you expect?”

  “That’s not a human artifact. And what lies behind it isn’t a human motivation. You have to learn to think like an ammono. We’re dealing here with a race who, when confronted with the destruction of their world, retreated into their worldhouse, and rebuilt their moon to accommodate us. Terrestrial life. Can you imagine humans doing the same?”

  “What are you saying?”

  “I’m saying these guys think big. Bigger than we ever did. But in a different way. I think they are trying to save their biosphere. And ours. But they’re doing it the way we should have done it. And could have, if anybody had provided the funding.” He looked up at the sun’s diseased face. “But we weren’t smart enough, Paula. We blew it. We dropped a fucking rock on ourselves. We lost ten billion years. We might have covered the Galaxy by now. But we blew it.”

  “I think we did okay, Rosenberg,” she said gently. “We’re here, aren’t we? We came to Saturn, and in the end, we found something wonderful. And if you’re right, because of us, Earth life is going to live on, to survive even the death of the sun… Do you think this is what it was all about? All those millennia of struggling, the whole bloody human story, just to deliver the two of us, here, to the end of time?…”

  The light around her changed. She looked up, to the east.

  The sun, a broad, r
uddy disc, was descending towards Saturn’s limb. The grand, slow eclipse had started, she saw, with a perfectly circular arc of darkness bitten out of the sun’s swollen face, and red sunlight glimmering around the rim of Saturn, the layers of atmosphere there. She thought she could see the shadow of Saturn sweeping like a wing across the plains of Cronos towards her, and the air grew dark and subdued. She thought she could see a fine, glittering line stretching up towards the zenith: perhaps the remnants of the rings.

  … Hey, Paula. Scuttlebutt from home. Some double-dome from JPL is saying he’s found life on Titan…

  Benacerraf could feel the elemental human warmth of Rosenberg’s bare skin, all along her flank, from shoulder through hip to ankle.

  They planned further.

  Today they should try again to build a fire, she said. With a fire they could warm themselves, heat up some water, maybe try cooking some of the vegetable life and see if that improved its flavor.

  And beyond that they ought to think about a shelter. Maybe they could construct some kind of log cabin from the wood-analogue of the trees here. But it might prove difficult to cut the wood. Ripping off small branches for a fire was one thing; carpentry for a serious construction would be something else, without metals to work into tools.

  Rosenberg started talking longer term. There might be metals to be extracted, from meteorites embedded in craters in the ice…

  To the east, over the shadowed ruins of Xi City, white rocket light flared.

  Epilogue

  The mirror array drifted through the rubble of what had been Saturn’s ring system, the ruddy light of bloated Sol casting sharp highlights from its structure. The array was a hundred yards long. Six cup-shaped minors, each a yard across, were spaced along a spider-web boom.

  The mirrors were pointed away from Sol. The array was looking for planets, of other stars.

  For three months now, it had maintained its focus on a young blue-white star, as bright as any in the sky: twenty-seven light years from Sol, fifty times as luminous as Sol in its remote heyday. The six mirrors gathered the star’s scattered photons and focused them on a single collector.

  The design was subtle. The collector operated in the infrared part of the spectrum, where planets shone most brightly. Even so, the star was still millions of times brighter than any planet; but light waves arrived at the six mirrors slightly out of phase and canceled each other out, allowing planetary light to shine through.

  The images formed were ghostly, faint, building up layer by layer.

  There proved to be twelve major planets in the new system: three gas giants, the rest rocky or icy worlds. Of the smaller worlds, two lay in the habitable zone for Earth-like life—seven times as far as Earth from Sol—and one lay further out, in a region which might support ammono-like life.

  The subtle collectors, slow and persistent and patient, detected spectroscopic traces of atmospheric gases: carbon dioxide, oxygen, water, ammonia, methane.

  These worlds, it was decided, were valid targets.

  The sail spread like a flower, its silvered surface capturing blood-red pools of sunlight.

  It was five hundred yards across. The payload at its heart, a mere two hundred pounds, was a small, black pod

  The probe would not carry much on-board intelligence. The only passengers were microscopic life forms, engineered either for Earth-like conditions, or for Titan summer.

  Slowly, slowly, the sail billowed out, driven by the energy-thin drizzle of photons from the fat, faded sun. The probe, still orbiting Saturn, began to spiral outward, fine lines hauling at the sail so that it tacked in the unwavering breeze of light.

  It took a thousand years to achieve solar escape velocity.

  The journey took twenty thousand years.

  The cruise was uneventful. The minuscule acceleration reduced as the light pressure from Sol dwindled with distance, and the interstellar medium—hydrogen atoms and ions—exerted a tiny but constant drag at the sail.

  Each capsule contained diverse species. Many were extremophiles, able to adapt to extremes of temperature, pressure, acidity. Those landing on the Earth-like worlds contained organisms similar to blue-green algae. Most of the species were single-celled, but some were multicellular eukaryotes. Eukaryotes were more fragile. But there was evidence that on both Titan and Earth the progression to multicellular forms had formed an evolutionary bottleneck, of such low probability that on many worlds it might never happen. If eukaryotes could be protected and prosper, billions of years of evolution could be shortcut.

  But the microorganisms traveled through a deeply hostile environment.

  They had to be shielded against ionizing particles and ultraviolet radiation. And the organisms were engineered to withstand heavy radiation fluxes; what was carried amounted to spores: biologically inert, free of water or ammonia.

  At the midpoint of the twenty-seven light year journey there was a shift in polarization, so that the sail’s silvered surface was now directed towards the new star, the darkened side towards the diminished red blur that was Sol.

  With the mirrored sail reversed, a long deceleration began.

  There was a variety of designs, of strategies.

  Some of the probes from Titan headed for clouds where new stars were being formed. Some of them were designed to colonize comets; at closest approach to a parent star the comet would spew spores into interplanetary space, for later capture by planets.

  And so on.

  This was panspermia: the delivery of life forms to other worlds, other star systems.

  There were some on Titan who hypothesized that the worlds of Sol had themselves been seeded, in the remote past, by an early starfaring race. If that were true, the resulting life forms were morally obliged to continue, to spread life further, as far as possible.

  On the other hand, if i: were not true, if Sol life was the first, then the moral imperative to spread, to propagate, was all the greater.

  So a cloud of solar sails drifted outward from Titan, like thistledown on the light of dying Sol, fleeing the doomed world. A wind of life, blowing between the stars.

  The star was the heart of a young, vigorous system. A disc of protoplanetary debris still encircled it, through which its planets swam.

  On arrival, the sail was ejected.

  The probe entered a neat elliptical orbit around the brilliant central star. The outer edge of the ellipse touched the orbits of the Titan-like planets, the inner edge the orbits of the Earth-like worlds. At the inner and outer points of its orbit, the probe ejected a multitude of tiny parcels: hundreds of thousands of them, shielded against ultraviolet radiation, each containing thousands of organisms.

  Over twenty years, the parcels distributed themselves into sparse rings around the central star. The parcels were coated with a substance whose reflectivity varied with the intensity of the light falling on it. Thus, each parcel oscillated between the limits of its habitable zone, maximizing the probability of capture.

  The target planets moved through the rings, sweeping up capsules

  Many of the capsules, entering at urfavorable geometries, were burned up in the thick atmospheres of their target worlds But some survived, and drifted down through cloud layers of water vapor or ammonia to settle like silvered snowflakes on land, or oceans.

  The thin metal coating of the capsules corroded. The parcels in which the microorganisms arrived were egglike, containing a small amount of prepackaged nutrients This helped the organisms survive as they adapted to local conditions.

  The awakening microorganisms, released, began to disperse and evolve. They were adapted to rapid and efficient mutation.

  Biological processes began

  The surfaces of these worlds bore the scars of recent, and continuing, planetesimal bombardment. This would not be an easy place in which to survive. But on each planet, a handful of organisms survived. And began to breed.

  Together, the children of Sol began to remake the worlds of a new star.

&nbs
p; Afterword

  The untimely death of Carl Sagan (1934–1996), who has a cameo role in Book Two of this novel, was a sad footnote to a year full of scientific wonders.

  Sagan was an astronomer and planetary scientist, and author of accessible and uplifting nonfiction and science fiction. As a scientist, Sagan played an active role on spaceprobes such as Mariner 9 to Mars—Sagan ensured the probe was positioned to photograph Mars’s moons—and Pioneer 10 to Jupiter and beyond, on which Sagan was responsible for placing a message to alien life. Sagan’s speculations on terraforming Venus—the first serious scientific speculation on the subject—on the possibility of permafrost on Mars, and on conditions on Titan, helped influence the thinking of subsequent workers and writers—including myself.

  Like H.G. Wells, Sagan seems to have believed that the future of mankind would be a race between education and catastrophe. In 1984 he co-authored the concept of nuclear winter which may, perhaps, have helped avert that very catastrophe from befalling us. As we near the end of a millennium still largely gripped by the madnesses which dominated its opening, we cannot afford to lose Sagan’s brand of clear-thinking, cheerful, communicative rationality.

  Carl Sagan’s death was announced after I had drafted his appearance in Titan. So, sadly, this book is already alternate history. But I decided Sagan should stay in.

  —Stephen Baxter

  Great Missenden

  January 1997

  Arrival

  After seven years of flight, after traveling a billion miles from Earth, the human spacecraft Cassini reached Saturn…

  A fat pie-dish shape, ten feet across, clung to the side of the Cassini stack. It was a combined aeroshell and heat shield for a separate spacecraft, called Huygens, which was designed to land on Saturn’s largest moon, Titan.