Manifold: Space

by Stephen Baxter

  The wall looked carved. She saw basins, valleys, little mountains and ridges, all on the scale of meters. It was like flying over a miniaturized landscape at some theme park.

  "...The central structure is obviously a power source," Nemoto was saying. "There is deuterium in there. Fusion, perhaps. A miniature Sun, suspended at the center of this hollow world. And from the topography of that inner surface it seems that the moonlet's basins and valleys have been carved to take a liquid. Water? A miniature Sun, model rivers and seas – or at least, lakes. Perhaps the moonlet was spun up to provide artificial gravity... This is a bubble world, Carole, designed to support some form of life, independent of the outside universe."

  "But that makes no sense," Carole replied. "We're orbiting Venus. There's a gigantic Sun just the other side of that wall, pumping out all the energy anybody could require. Why would anybody hide away in this... cave?"

  But Nemoto, time-delayed, kept talking, of course, oblivious to her questions.

  Carole stopped a meter or so short of the wall. She deployed her portable lab, letting its laser shine on the wall.

  She stroked the wall's surface. The texture was nothing like the lunar-surface rock and regolith of the moonlet's exterior. Instead there seemed to be an underlay of crystalline substances that glinted and sparkled – quartz perhaps. Here and there, clinging to the crystalline substrate, she found a muddy clay. Though the "mud" was dried out in the vacuum, she saw swirls of color, complex compounds mixed in with the basic material. It reminded her of the gloopy mud of a volcanic hot spring.

  The first results of her lab's analysis began to chatter across its surface. Quartz, yes, and corundum – aluminum oxide. And everywhere, especially in those clay traces, she found traces of sulphuric acid.

  Nemoto understood immediately.

  "...Sulphuric acid. Of course. That is the key. What if these artificial lakes and rivers were once filled with acid? An acid biosphere is not as unlikely as it sounds. Sulphuric acid stays liquid over a temperature range three times that of water. Of course the acids dissolve most organic compounds – have you ever seen a sugar cube dropped in acid? But alkanes – simple straight-chain hydrocarbons – can survive. Or perhaps there is a biochemistry based on silicones, long-chain molecules based on silicon-oxygen pairs... Only a few common minerals can resist an acidic environment: quartz, corundum, a few sulphates. These walls have been weathered. Your mother would have understood... Venus is full of acid, you see. The clouds are filled with floating droplets of it. This is a good place to be, if what you need is acid..."

  Carole gazed into the empty lake basins and tried to imagine creatures whose veins ran with acid. But this toy world, Nemoto had said, was hundreds of millions of years old. If any of their descendants survived they must be utterly transformed by time, she thought, as different from those who built this moonlet as I am from my mindless Mesozoic ancestors.

  And if we found them – if we ever touched – we would destroy each other.

  "...This bubble world is surely not meant to stay here, drifting around Venus, forever. We may presume that this was merely the construction site, Venus a resource mine. The bubble is already on a near-escape orbit; a little more energy and it could have escaped Venus altogether – perhaps even departed the Sun's gravity field. You see?"

  "I think so..."

  "This rogue moonlet could travel to the nearer stars in a few centuries, perhaps, with its occupants warmed against the interstellar chill by their miniature interior Sun..."

  They had been migrants to the Solar System, born in some remote, acidic sea. Perhaps they had come in a single, ancient moonlet, a single spore landing here as part of a wider migration. They had found raw materials in Venus's orbit – perhaps a moon or captured asteroids – to be dismantled and worked. They had made more bubble worlds, filled them with oceans of sulphuric acid mined from Venus's clouds, and sent them on their way – thousands, even millions of moon-ships, the next wave of colonization, continuing the steady diffusion of their kind.

  "It's a neat method," Nemoto said. "Efficient, reliable. A low-technology way to conquer the stars..."

  "Could it have been the Gaijin?" Carole asked.

  "...But how convenient," Nemoto was saying, "that these sulphur-eaters should arrive in the Solar System and find precisely what they needed: a planet like Venus whose clouds they could mine for their acid oceans, a convenient moon to dismantle. And where did the energy come from for all this?... Oh, no, Carole, these weren't Gaijin. Whatever the secrets of this sulphuric-acid biology, it is nothing like the nature of the Gaijin. And this is all so much older than the Gaijin."

  Not the Gaijin, Carole thought, chilled. An earlier wave of immigrants, hundreds of millions of years in the past. The Gaijin weren't even the first.

  "We can't know why they stopped before they had completed their project," Nemoto said softly. "War. Cataclysm. Who knows? Perhaps we will find out on Venus. Perhaps that is what the Gaijin are here to discover."

  My mother's generation grew up thinking the Solar System was primordial – basically unmodified by intelligence, before we crawled out of the pond. And now, though we had barely started looking, we found this: the ruin of a gigantic colonization and emigration project, ancient long, long before there were humans on Earth.

  "You expected to find this," she said slowly. "Didn't you, Nemoto?"

  "...Of course," Nemoto said at last. "It was logically inevitable that we would find something like this – not the details, but the essence of it – somewhere in the Solar System. The violation. And the secretive activities of the Gaijin drew me here, to find it.

  "One more thing," Nemoto whispered. "Your data has enabled me to make a better estimate of the artifact's age. It is eight hundred million years old." Nemoto laughed softly. "Yes. Of course it is."

  Carole frowned. "I don't understand. What's the significance of that?"

  "Your mother would have known," Nemoto said.

  Chapter 12

  Sister Planet

  Four hundred kilometers high, Carole was falling toward Venus. The lander had no windows; the conditions it had to survive were much too ferocious for that. But the inner walls were plastered with softscreens, to show Carole what lay beyond the honeycombed metal that cushioned her. Thus, the capsule was a fragile windowed cage, full of light, and her universe was divided into two: stars above, glowing planet below.

  Her descent would be a thing of skips and hops and long glides as she shed her orbital energy. The sensation was so gentle, the panorama so elemental, that it was almost like a virtual simulation back on Earth. But this was no game, no simulation; she was really here, alone in this flimsy capsule, like a stone thrown into the immense air ocean of Venus, a hundred million kilometers from any helping hand.

  Still she fell. The cloud decks below her remained featureless, but they were flattening to a perfect plain, like some geometric demonstration. Looking up, she could see a great cone of shining plasma trailing after her lander as it cut into the high air. She imagined seeing herself from space, a fake meteorite shining against the smooth face of Venus.

  As her altitude unraveled the air thickened, and the bites of deceleration came hard and heavy, the buffeting more severe. Now the noise began; a thin screaming of tortured air, molecules broken apart by the heat of her descent, and there were flashes of plasma light at her virtual windows, like flashbulb pops. The temperature of the thin air outside rose to Earthlike levels, twenty or thirty centigrade.

  But the air was not Earthlike. Sulphuric acid was already congealing around her, tiny droplets of it, acid formed by the action of sunlight on sulphur products and traces of oxygen that leaked up from the pool of air below.

  At seventy kilometers she fell into the first clouds.

  The stars winked out, and thick yellow mist closed around her. Soon even the Sun was perceptibly dimming, becoming washed out, as if seen through high winter clouds on Earth. Still the bulk of Venus's air ocean lay beneath her.
But she was already in the main cloud deck, twenty kilometers thick, the opaque blanket that had, until the age of space probes, hidden Venus's surface from human eyes.

  The buffeting became still more severe. But her capsule punched its way through this thin, angry air, and soon the battering of the high superstorms ceased.

  Her main parachute blossomed open; she was briefly pushed back hard in her seat, and her descent slowed further. There was a rattle as small unmanned probes burst from the skin of her craft and arced away, seeking their own destiny.

  The visibility was better than she had expected: perhaps she could see as far as one, even two kilometers. And she could make out layers in the cloud, sheets of stratumlike mist through which she fell, one by one.

  Now came a patter against the hull: gentle, almost like hail, just audible under the moaning wind noise. She glimpsed particles slapping against the softscreen window: long crystals, like splinters of quartz. Were they crystals of solid sulphuric acid? Was that possible?

  The hail soon disappeared. And, still fifty kilometers high, she dropped out of the cloud layer into clear air.

  She looked up at the rigging, giant orange parachutes. The capsule was swaying, very slowly, suspended from the big parachute system. The clouds above were thick and solid, dense, with complex cumulus structures bulging below like misty chandeliers, almost like the clouds of Earth. The Sun was invisible, and the light was deeply tinged with yellow, fading to orange at the blurred horizon, as if she were falling into night. But there was still no sign of land below, only a dense, glowing haze.

  With a clatter of explosive bolts her parachutes cut away, rippling like jellyfish, lost. She dropped further, descending into thickening haze. The lower air here was so dense it was more like falling into an ocean: Venus was not a place for parachutes.

  The light was dimming, becoming increasingly more red.

  Telltales lit up as her capsule's protective systems came online. The temperature outside was rising ferociously, already far higher than the boiling point of water – though she was still twice as high as Earth's highest cirrus clouds. The lander's walls were a honeycomb, strong enough to withstand external pressures that could approach a hundred atmospheres. And the lander contained sinks, stores of chemicals like hydrates of lithium nitrate, which, evaporating, could absorb much of the ferocious incoming heat energy. But the real heat dump was a refrigeration laser; every few minutes it fired horizontally, creating temperatures far higher even than those of Venus's air.

  I'm floating in a sea of acid, she thought, in a mobile refrigerator. It all seemed absurd, a system of clunky gadgetry. It was hard to believe the Gaijin would do it this way.

  And yet it was all somehow wonderful.

  Now there was a fresh pattering against the hull of the ship. More hail? No, rain – immense drops slamming against her virtual walls, streaking and quickly evaporating. This was true acid rain, she supposed, sulphuric acid droplets formed kilometers above. The rain grew ferocious, a sudden storm rattling against her walls, and the drops streaked and ran together, blurring her vision. For a brief moment she felt frightened, adrift in this stormy sky.

  But, as quickly as it had begun, the rain tailed off. It was so hot now the rain was evaporating. A little deeper the intense heat would destroy the acid molecules themselves, leaving a mist of sulphur oxides and water.

  Abruptly the haze cleared below her. As if she were peering down toward the bed of some orange sea, she made out structure below: looming forms, shadows, what looked like a river valley.

  Land.

  Suspended from a balloon, she drifted over a continent.

  "This is Aphrodite," Nemoto murmured from the distant Moon. "The size of Africa. Shaped like a scorpion – look at the map, Carole; see the claws in the west, the stinging tail to the east? But this is a scorpion fourteen thousand kilometers long, and stretching nearly halfway around the planet's equator..."

  Carole – in her refrigerated balloon-lifted lander, still very high – was drifting from the west, past the claws of the scorpion. She saw a monstrous plateau: nearly three thousand kilometers across, she learned, its surface some three kilometers above the surrounding plains, to which it descended sharply. But the surface of the plateau was far from smooth. She saw ridges, troughs, and domes, a bewildering variety of features, all crowded within a landscape that was blocky, jumbled, cut by intersecting ridges and gouges.

  "The land looks as if it's been cracked open," she said. "And then reassembled. Like a parquet floor."

  "...Yes," Nemoto whispered at last. "This is the oldest landscape on Venus. It shows a history of great heat, of cataclysm. We will see much geological violence here."

  Everywhere she looked the world was murky red, both sky and land, still, windless. The sky above was like an overcast Earth sky, the light a somber red, like a deep sunset – brighter than she had expected, but more Marslike, she thought, than Earthly. The Sun itself was invisible save for an ill-defined glare low on the horizon. The "day" here would last more than a hundred Earth days, a stately combination of Venus's orbit around the Sun and its slow rotation – the "day" here was longer than Venus's year, in fact.

  Beyond the great plateau, she crossed a highland region that was riven by immense valleys – spectacular, stunning, and yet forever masked by the kilometers of cloud above, hidden away on this blasted planet where no eyes could see it. The easternmost part of Aphrodite was a broad, elongated dome, obviously volcanic, with rifts, domes, lava flows, and great shield volcanoes. But the most spectacular feature was a huge volcanic formation called Maat Mons: the largest volcano on Venus, three hundred kilometers wide and eight kilometers high. It was a twin to Mauna Loa, Earth's largest volcano, stripped of concealing ocean.

  This was a world of volcanism. The vast plains were covered by flood basalts – frozen lakes of lava, like the maria of the Moon – and punctured by thousands of small volcanoes, shield-shaped, built up by repeated outpourings of lava. But there were some shield structures – like the Hawaiian volcanoes, like Maat Mons – that towered five or eight kilometers above the plains, covered in repeated lava flows.

  As she drifted farther east, away from Aphrodite and over a smooth basalt lowland, Carole learned to pick out features that had no counterparts on Earth. There were steep-sided, flat-topped domes formed by sticky lava welling up through flaws in the crust. There were volcanoes with their flanks gouged away by huge landslides that left ridges like protruding insect legs. There were domes surrounded by spiderweb patterns of fractures and ridges. There were volcanoes with flows that looked like petals, pushing out across the plains.

  And, most spectacular, there were coronae: utterly unearthly, rings of ridges and fractures. Some of these were thousands of kilometers across, giant features each big enough to straddle much of the continental United States. Perhaps they were formed by blobs of upwelling magma that pushed up the crust and then spread out, allowing the center to implode, like a failed cake. To Carole the rings of swollen, distorted, and broken crust looked like the outbreak of some immense chthonic mold from Venus's deep interior.

  There were even rivers here.

  Her balloon ship drifted over valleys kilometers wide and thousands of kilometers in length, unlikely Amazons complete with flood plains, deltas, meanders, and bars, here on a world where no liquid water could have flowed for billions of years – if ever. One of these, called Baltis Vallis, was longer than the Nile, and so it was the longest river valley in the Solar System. Perhaps the rivers had been cut by an exotic form of lava – for example, formed by a salty carbon-rich rock called carbonatite – that might have flowed in Venus's still-hotter past.

  Suspended from a balloon, Carole would drift over this naked world for a week while her eyes and the lander's sensors probed at the strange landscapes below. And then, perhaps, she would land.

  She was, despite herself, enchanted. Venus had no water, no life; and yet it was a garden, she saw: a garden of volcanism and sculptur
ed rock. My mother would have understood all this, Carole thought with an echo of her old, lingering guilt. But my mother isn't here. I'm here.

  But Nemoto, coldly, told her to look for patterns. "You are not a tourist. Look beyond the spectacle, Carole. What do you see?"

  What Carole saw was wrinkles and craters.

  Wrinkles: the ground was covered with ridges and cracks, some of them running hundreds of kilometers, as if the whole planet were an apricot left too long in the Sun.

  And craters: they were everywhere, hundreds of them, spread evenly over the whole of the planet's surface. There were few very large craters – and few very small ones too; hardly any less than five or ten kilometers across.

  "...Violence, you see," Nemoto said. "Global violence. Those wrinkles in the lowlands, like the tesserae cracks of the highlands, are proof that the whole of the lithosphere, the outer crust of this planet, was stretched or compressed – all at the same time. What could do such a thing?

  "And as for the craters, there is little wind erosion here, Carole; the air at the bottom of this turgid ocean of gas is very still, and so the craters have remained as fresh as when they were formed. Few are small, for that thick air screens out the smaller impactors, destroying them before they reach the ground. But, conversely, few of the craters are large. Certainly none of them compare with the giant basins of the Moon. But those immense lunar basins date back to the earliest days of the Solar System, when the sky was still full of giant rogue planetesimals. And so we can tell, you see – "

  " – that these craters are all young," Carole said.

  " – that no crater is much older than eight hundred million years," Nemoto said, not yet hearing her. "In fact, no feature on the surface of this planet appears older than that. Eight hundred million years: it might seem an immense age to you, but the planets are five times older still. Carole, eight hundred million years ago, something happened to Venus – something that distorted the entire surface, wiping it clean of older features, destroying four billion years of geologic heritage. We can never know what was lost, what traces of continents and seas were brutally melted..."