Vacuum Diagrams

by Stephen Baxter

  She looked up, following his pointing arm. There, just above the far horizon, was a small blue star.

  She had her face plate magnify the image. The star exploded into a compact sculpture of electric blue threads, surrounded by firefly lights: the Thoth construction site.

  Thoth was a habitat to be placed in orbit close to Sol. Irina Larionova was the consulting engineer contracted by Superet to oversee the construction of the habitat.

  At Thoth, a Solar-interior probe would be constructed. The probe would be one Interface of a wormhole, loaded with sensors. The Interface would be dropped into the Sun. The other Interface would remain in orbit, at the center of the habitat.

  Thoth's purpose was to find out what was wrong with the Sun.

  Irina Larionova wasn't much interested in the purpose of Thoth, or any of Superet's semi-mystical philosophizing. It was the work that was important, for her: and the engineering problems posed by Thoth were fascinating.

  The electric-blue bars she could see now were struts of exotic matter, which would eventually frame the wormhole termini. The sparks of light moving around the struts were GUTships and short-haul flitters. She stared at the image, wishing she could get back to some real work.

  Irina Larionova had had no intention of visiting Mercury herself. Mercury was a detail, for Thoth. Why would anyone come to Mercury, unless they had to? Mercury was a piece of junk, a desolate ball of iron and rock too close to the Sun to be interesting, or remotely habitable. The two Thoth exploratory teams had come here only to exploit: to see if it was possible to dig raw materials out of Mercury's shallow — and close-at-hand — gravity well, for use in the construction of the habitat. The teams had landed at the South Pole, where traces of water-ice had been detected, and at the Caloris Basin, the huge equatorial crater where — it was hoped — that ancient impact might have brought iron-rich compounds to the surface.

  The flitters from Thoth actually comprised the largest expedition ever to land on Mercury.

  But, within days of landing, both investigative teams had reported anomalies.

  Larionova tapped at her suit's sleeve-controls. After a couple of minutes an image of Dolores Wu appeared in one corner of Larionova's face plate. Hi, Irina, she said, her voice buzzing like an insect in Larionova's helmet's enclosed space.

  Dolores Wu was the leader of the Thoth exploratory team in Caloris. Wu was Mars-born, with small features and hair grayed despite AntiSenescence treatments. She looked weary.

  "How's Caloris?" Larionova asked.

  Well, we don't have much to report yet. We decided to start with a detailed gravimetric survey...

  "And?"

  We found the impact object. We think. It's as massive as we thought, but much — much — too small, Irina. It's barely a mile across, way too dense to be a planetesimal fragment.

  "A black hole?"

  No. Not dense enough for that.

  "Then what?"

  Wu looked exasperated. We don't know yet, Irina. We don't have any answers. I'll keep you informed.

  Wu closed off the link.

  Standing on the corona-lit wall of Chao Meng-Fu Crater, Larionova asked Kevan Scholes about Caloris.

  "Caloris is big," he said. "Luna has no impact feature on the scale of Caloris. And Luna has nothing like the Weird Country in the other hemisphere..."

  "The what?"

  A huge planetesimal — or something — had struck the equator of Mercury, five billion years ago, Scholes said. The Caloris Basin — an immense, ridged crater system — formed around the primary impact site. Whatever caused the impact was still buried in the planet, somewhere under the crust, dense and massive; the object was a gravitational anomaly which had helped lock Mercury's rotation into synchronization with its orbit.

  "Away from Caloris itself, shock waves spread around the planet's young crust," Scholes said. "The waves focused at Caloris' antipode — the point on the equator diametrically opposite Caloris itself. And the land there was shattered, into a jumble of bizarre hill and valley formations. The Weird Country... hey. Dr. Larionova."

  She could hear that damnable grin of Scholes's. "What now?" she snapped.

  He walked across the summit towards her. "Look up," he said.

  "Damn it, Scholes—"

  There was a pattering against her face plate.

  She tilted up her head. Needle-shaped particles swirled over the wall-mountain from the planet's dark side and bounced off her face plate, sparkling in corona light.

  "What in Lethe is that?"

  "Snow," he said.

  Snow... on Mercury?

  In the cool darkness of the tunnel, the people clambered over each other; they bumped against the Ice walls, and their muttering filled the water with criss-crossing voice-ripples. Cilia-of-Gold swam through and around the crowd, coaxing the people to follow her will.

  She felt immensely weary. Her concentration and resolve threatened continually to shatter under the Seeker's assault. And the end of the tunnel, with the deadly Heads beyond, was a looming, threatening mouth, utterly intimidating.

  At last the group was ready. She surveyed them. All of the people — except the very oldest and the very youngest — were arranged in an array which filled the tunnel from wall to wall; she could hear flukes and carapaces scraping softly against Ice.

  The people looked weak, foolish, eager, she thought with dismay; now that she was actually implementing it her scheme seemed simple-minded. Was she about to lead them all to their deaths?

  But it was too late for the luxury of doubt, she told herself. Now, there was no other option to follow.

  She lifted herself to the axis of the tunnel, and clacked her mandibles sharply.

  "Now," she said, "it is time. The most important moment of your lives. And you must swim! Swim as hard as you can; swim for your lives!"

  And the people responded.

  There was a surge of movement, of almost exhilarating intent. The people beat their flukes as one, and a jostling mass of flesh and carapaces scrapped down the tunnel.

  Cilia-of-Gold hurried ahead of them, leading the way towards the tunnel mouth. As she swam she could feel the current the people were creating, the plug of cold tunnel water they pushed ahead of themselves.

  Within moments the tunnel mouth was upon her.

  She burst from the tunnel, shooting out into the open water of the cavern, her carapace clenched firm around her. She was plunged immediately into a clammy heat, so great was the temperature difference between tunnel and cavern.

  Above her the Ice of the cavern roof arched over the warm chimney mouth. And from all around the cavern, the helmet skulls of Heads snapped around towards her.

  Now the people erupted out of the tunnel, a shield of flesh and chitin behind her. The rush of tunnel water they pushed ahead of themselves washed over Cilia-of-Gold, chilling her new.

  She tried to imagine this from the Heads' point of view. This explosion of cold water into the cavern would bring about a much greater temperature difference than the Heads' heat-sensor skulls were accustomed to; the Heads would be dazzled, at least for a time: long enough — she hoped — to give her people a fighting chance against the more powerful Heads.

  She swiveled in the water. She screamed at her people, so loud she could feel her cilia strain at the turbulent water. "Now! Hit them now!"

  The people, with a roar, descended towards the Heads.

  Kevan Scholes led Larionova down the wall-mountain slope into Chao Meng-Fu Crater.

  After a hundred yards they came to another rover. This car was similar to the one they'd abandoned on the other side of the summit, but it had an additional fitting, obviously improvised: two wide, flat rails of metal, suspended between the wheels on hydraulic legs.

  Scholes helped Larionova into the rover and pressurized it. Larionova removed her helmet with relief. The rover smelled, oppressively, of metal and plastic.

  While Scholes settled behind his controls, Larionova checked the rover's data desk
. An update from Dolores Wu was waiting for her. Wu wanted Larionova to come to Caloris, to see for herself what had been found there. Larionova sent a sharp message back, ordering Wu to summarize her findings and transmit them to the data desks at the Chao site.

  Wu acknowledged immediately, but replied: I'm going to find this hard to summarize, Irina.

  Larionova tapped out: Why?

  We think we've found an artifact.

  Larionova stared at the blunt words on the screen.

  She massaged the bridge of her nose; she felt an ache spreading out from her temples and around her eye sockets. She wished she had time to sleep.

  Scholes started the vehicle up. The rover bounced down the slope, descending into shadow. "It's genuine water-ice snow," Scholes said as he drove. "You know that a day on Mercury lasts a hundred and seventy-six Earth days. It's a combination of the eighty-eight-day year and the tidally locked rotation, which—"

  "I know."

  "During the day, the Sun drives water vapor out of the rocks and into the atmosphere."

  "What atmosphere?"

  "You really don't know much about Mercury, do you? It's mostly helium and hydrogen — only a billionth of Earth's sea-level pressure."

  "How come those gases don't escape from the gravity well?"

  "They do," Scholes said. "But the atmosphere is replenished by the Solar wind. Particles from the Sun are trapped by Mercury's magnetosphere. Mercury has quite a respectable magnetic field: the planet has a solid iron core, which..."

  She let Scholes' words run on through her head, unregistered. Air from the Solar wind, and snow at the South Pole...

  Maybe Mercury was a more interesting place than she'd imagined.

  "Anyway," Scholes was saying, "the water vapor disperses across the planet's sunlit hemisphere. But at the South Pole we have this crater: Chao Meng-Fu, straddling the Pole itself. Mercury has no axial tilt — there are no seasons here — and so Chao's floor is in permanent shadow."

  "And snow falls."

  "And snow falls."

  Scholes stopped the rover and tapped telltales on his control panel. There was a whir of hydraulics, and she heard a soft crunch, transmitted into the cabin through the rover's structure.

  Then the rover lifted upwards through a foot.

  The rover lurched forward again. The motion was much smoother than before, and there was an easy, hissing sound.

  "You've just lowered those rails," Larionova said. "I knew it. This damn rover is a sled, isn't it?"

  "It was easy enough to improvise," Scholes said, sounding smug. "Just a couple of metal rails on hydraulics, and vernier rockets from a cannibalized flitter to give us some push..."

  "It's astonishing that there's enough ice here to sustain this."

  "Well, that snow may have seemed sparse, but it's been falling steadily — for five billion years... Dr. Larionova, there's a whole frozen ocean here, in Chao Meng-Fu Crater: enough ice to be detectable even from Earth."

  Larionova twisted to look out through a viewport at the back of the cabin. The rover's rear lights picked out twin sled tracks, leading back to the summit of the wall-mountain; ice, exposed in the tracks, gleamed brightly in starlight.

  Lethe, she thought. Now I'm skiing. Skiing, on Mercury. What a day.

  The wall-mountain shallowed out, merging seamlessly with the crater plain. Scholes retracted the sled rails; on the flat, the regolith dust gave the ice sufficient traction for the rover's wide wheels. The rover made fast progress through the fifty miles to the heart of the plain.

  Larionova drank coffee and watched the landscape through the view ports. The corona light was silvery and quite bright here, like Moonlight. The central peak loomed up over the horizon, like some approaching ship on a sea of dust. The ice surface of Chao's floor — though packed with craters and covered with the ubiquitous regolith dust — was visibly smoother and more level than the plain outside the crater.

  The rover drew to a halt on the outskirts of the Thoth team's sprawling camp, close to the foothills of the central peak. The dust here was churned up by rover tracks and flitter exhaust splashes, and semitransparent bubble-shelters were hemispheres of yellow, homely light, illuminating the darkened ice surface. There were drilling rigs, and several large pits dug into the ice.

  Scholes helped Larionova out onto the surface. "I'll take you to a shelter," he said. "Or a flitter. Maybe you want to freshen up before—"

  "Where's Dixon?"

  Scholes pointed to one of the rigs. "When I left, over there."

  "Then that's where we're going. Come on."

  Frank Dixon was the team leader. He met Larionova on the surface, and invited her into a small opaqued bubble-shelter nestling at the foot of the rig.

  Scholes wandered off into the camp, in search of food.

  The shelter contained a couple of chairs, a data desk, and a basic toilet. Dixon was a morose, burly American; when he took off his helmet there was a band of dirt at the base of his wide neck, and Larionova noticed a sharp, acrid stink from his suit. Dixon had evidently been out on the surface for long hours.

  He pulled a hip flask from an environment suit pocket. "You want a drink?" he asked. "Scotch?"

  "Sure."

  Dixon poured a measure for Larionova into the flask's cap, and took a draught himself from the flask's small mouth.

  Larionova drank; the liquor burned her mouth and throat, but it immediately took an edge off her tiredness. "It's good. But it needs ice."

  He smiled. "Ice we got. Actually, we have tried it; Mercury ice is good, as clean as you like. We're not going to die of thirst out here, Irina."

  "Tell me what you've found, Frank."

  Dixon sat on the edge of the desk, his fat haunches bulging inside the leggings of his environment suit. "Trouble, Irina. We've found trouble."

  "I know that much."

  "I think we're going to have to get off the planet. The System authorities — and the scientists and conservation groups — are going to climb all over us, if we try to mine here. I wanted to tell you about it, before—"

  Larionova struggled to contain her irritation and tiredness. "That's not a problem for Thoth," she said. "Therefore it's not a problem for me. We can tell Superet to bring in a water-ice asteroid from the Belt, for our supplies. You know that. Come on, Frank. Tell me why you're wasting my time down here."

  Dixon took another long pull on his flask, and eyed her.

  "There's life here, Irina," he said. "Life, inside this frozen ocean. Drink up; I'll show you."

  The sample was in a case on the surface, beside a data desk. The thing in the case looked like a strip of multicolored meat: perhaps three feet long, crushed and obviously dead; shards of some transparent shell material were embedded in flesh that sparkled with ice crystals.

  "We found this inside a two-thousand-yard-deep core," Dixon said.

  Larionova tried to imagine how this would have looked, intact and mobile. "This means nothing to me, Frank. I'm no biologist."

  He grunted, self-deprecating. "Nor me. Nor any of us. Who expected to find life, on Mercury?" Dixon tapped at the data desk with gloved fingers. "We used our desks' medico-diagnostic facilities to come up with this reconstruction," he said. "We call it a mercuric, Irina."

  A Virtual projected into space a foot above the desk's surface; the image rotated, sleek and menacing.

  The body was a thin cone, tapering to a tail from a wide, flat head. Three parabolic cups — eyes? — were embedded in the smooth "face," symmetrically placed around a lipless mouth... No, not eyes, Larionova corrected herself. Maybe some kind of sonar sensor? That would explain the parabolic profile.

  Mandibles, like pincers, protruded from the mouth. From the tail, three fins were splayed out around what looked like an anus. A transparent carapace surrounded the main body, like a cylindrical cloak; inside the carapace, rows of small, hairlike cilia lined the body, supple and vibratile.

  There were regular markings, faintly visible, in th
e surface of the carapace.

  "Is this accurate?"

  "Who knows? It's the best we can do. When we have your clearance, we can transmit our data to Earth, and let the experts get at it."

  "Lethe, Frank," Larionova said. "This looks like a fish. It looks like it could swim. The streamlining, the tail—"

  Dixon scratched the short hairs at the back of his neck and said nothing.

  "But we're on Mercury, damn it, not in Hawaii," Larionova said.

  Dixon pointed down, past the dusty floor. "Irina. It's not all frozen. There are cavities down there, inside the Chao ice cap. According to our sonar probes—"

  "Cavities?"

  "Water. At the base of the crater, under a couple of miles of ice. Kept liquid by thermal vents, in crust-collapse scarps and ridges. Plenty of room for swimming... we speculate that our friend here swims on his back—" he tapped the desk surface, and the image swiveled "—and the water passes down, between his body and this carapace, and he uses all those tiny hairs to filter out particles of food. The trunk seems to be lined with little mouths. See?" He flicked the image to another representation; the skin became transparent, and Larionova could see blocky reconstructions of internal organs. Dixon said, "There's no true stomach, but there is what looks like a continuous digestive tube passing down the axis of the body, to the anus at the tail."

  Larionova noticed a threadlike structure wrapped around some of the organs, as well as around the axial digestive tract.

  "Look," Dixon said, pointing to one area. "Look at the surface structure of these lengths of tubing, here near the digestive tract."

  Larionova looked. The tubes, clustering around the digestive axis, had complex, rippled surfaces. "So?"

  "You don't get it, do you? It's convoluted — like the surface of a brain. Irina, we think that stuff must be some equivalent of nervous tissue."

  Larionova frowned. Damn it, I wish I knew more biology. "What about this thread material, wrapped around the organs?"

  Dixon sighed. "We don't know, Irina. It doesn't seem to fit with the rest of the structure, does it?" He pointed. "Follow the threads back. There's a broader main body, just here. We think maybe this is some kind of parasite, which has infested the main organism. Like a tapeworm. It's as if the threads are extended, vestigial limbs..."