Phoresis

by Greg Egan

  This was it: she was touching the atmosphere. And she was coming in fast enough for this rarefied upper layer to heat the glider, even before it delivered any perceptible force. She contemplated the gentle heat, refusing to let it alarm her. Joanna had taken her through the calculations: if every scrap of the difference in potential energy between the top of the tower and the surface of Tvíburi was used to raise the temperature of her body, the effect would be about the same as holding her hand a bit too close to a lamp for comfort. To be that hot for too long would be intolerable, then injurious, and eventually unsurvivable—but once there was any kind of wind to cool her, the actual heat she retained would start to fall short of that hypothetical limit. If she could have ensured that all the inflows and outflows of energy were averaged over the descent, she would have had a guarantee that she’d be fine. In reality, it would all be down to details of Tvíburi’s atmosphere that no one had known how to measure in advance.

  Rosalind fixed her gaze on the ice, picturing the chilly weather below as the heat shifted from cheerful to unpleasant. The glider began to tremble, then pitch; it oscillated unsteadily, then settled with its nose toward the horizon, leaving her facing straight down. A thin, hot breeze flowed over her body; somehow she’d expected the wind to be cool, as if the air’s two roles were separable, and this merciful intervention would be like an independent bystander coming to her aid. But the truth was good enough: the balance was shifting, and as the hot wind blew more strongly, it also grew less fierce. As the air thickened, and slowed her more and more, it also had the capacity to absorb a greater share of the energy burden itself, and to carry more heat away.

  Her view had shrunk to a region much smaller than any of her maps, but she’d retained a sense of the position of the landmarks that had gone out of sight, so she did not feel lost. As the ice field loomed toward her, she picked up lateral speed, flipping through the ever-expanding storybook, glossing over the details of the icy monologue, hoping to reach the end before dark. The air was merely warm now; she opened her throat and took a tentative breath. It was thicker than she’d expected, and carried a strange dusty aftertaste, but it seemed to satisfy her lungs. She waited a few moments, in case there was some delayed adverse reaction—as if her caution really mattered, when in the end she’d have no choice. But when she felt an unambiguous surge of energy spreading to her limbs, she inhaled again, deeply. The warmth and the strange smell made her cough, but the realization that Tvíburi seemed to be welcoming her filled her with equal parts elation, and shock at the stark reminder that the result could easily have been different.

  The ice was a blur now, but the glider remained steady; Rosalind couldn’t recall a flight as calm as this. The thicker air could only react more forcefully to her encroachment, but apparently its greater density also helped dampen out turbulence. If the glider broke apart, it wasn’t going to do it in flight. Everything would depend on the landing.

  The cool breeze she’d been longing for suddenly arrived, sending her clothes fluttering. She saw the glider’s shadow racing over the ice, unable to outrun its pursuer, and she braced herself for the inevitable meeting.

  The runners struck the ground, sliding forward at an impossible speed. Rosalind stared down at the ice, terrified that the frame would break apart while she was moving so fast that the abrasive surface would take all her skin off the instant she touched it. But if anything, the ride kept growing smoother. Maybe the runners had grown so hot that they were simply melting away the rough patches. But if the smallest obstacles could vanish, anything larger would still be fatal.

  She kept her body rigid, gripping the handle bars tightly but prepared to reach out and grab one of the bracing rods if the glider deformed and the structure betrayed her.

  The betrayal never came. Friction did its work, uninterrupted, and the glider came to a halt, intact.

  Rosalind unstrapped herself and knelt on the ground, trembling. Then she crawled out from under the glider and surveyed her surroundings.

  The ice field stretched almost as far as she could see in most directions, though if she squinted at the horizon she could make out one of the geysers she’d noticed from on high, and some hills to the north. Tvíburi’s orbit had carried the whole world westward beneath her as she approached through the void, so she’d expected to land well east of the prime meridian; once she explored those hills, she could confirm exactly where she was.

  The sun had moved a short way past Tvíbura, but every part of the home world that she could see was still in night. There was something comforting about the utter familiarity of the configuration; she could have looked up on any other afternoon and seen a near-identical sight. Nothing was back-to-front here, nothing was reversed or deranged, on its own terms. By day she saw Tvíbura’s night, and her east was Tvíbura’s west—but if two friends standing face-to-face could accommodate the meaning of left and right, the cartographic version should cause no greater confusion.

  Something small and dark in the sky caught her attention. For a moment she wondered if it might be one of the gliders from the tower, but its motion was both too slow and too complicated.

  As the thing came lower, she realized that it was a kind of lizard. But instead of holding its limbs outspread, stretching the membrane between them to act as a natural glider, it was moving them in a way that was making the membrane flutter. The action looked bizarre, and utterly counterproductive, but rather than sending the creature plummeting, these strange flutters seemed to be controlling its flight. And when it dropped toward her, instead of continuing to the ground—as every lizard she’d ever observed before would have been compelled to do—it rose up into the air again in a wide, helical trajectory, ascending so high that it disappeared from sight.

  Rosalind began weeping with joy. Not only was the air here breathable, there were animals—strange, vigorous animals, who evidently suffered no lack of food. Whatever the absence of grasslands meant, this world had to be far from barren.

  And they’d come with seeds, they’d come with tools, they’d come with generations of farmers’ knowledge. All this time, Tvíburi had been waiting to feed her sister’s children.

  7

  By sunset, five members of the expedition had joined Rosalind at the base of the hills. Anya had chanced on one of the supply drops along the way, and used the enclosed cart to drag the crate with her. The contents included blankets and a small tent, so they set up the tent and crowded in for the night.

  There was no sign of Sigrid or Joanna, but Rosalind wasn’t too worried yet. The protocol “head for the nearest landmark to the north” had sent six of them from the ice field to the same hills, but it was not a foolproof recipe for convergence—and when the six who’d met up so far had done their best to mark their own landing sites on a map, it had been clear that chance variations in atmospheric conditions had had as much of an effect on where people hit the ground as the timing of their departure and the systematic libration of the two worlds. It wouldn’t be hard to guess the most likely places where the missing pair might have ended up, and if they followed the rules and waited to be found by a search party sent by the majority, everyone would be reunited before long.

  Sleep proved impossible, but Rosalind resisted the temptation to walk out of the tent and start exploring. Many of the plans they’d made together before departing were sure to prove ill-conceived, but she wasn’t going to start her new life with a frivolous rebellion against the sensible consensus that everyone should rest and regain their strength on the first night after the journey.

  So she settled for exploring from her blanket: pondering the strange smell of the new world, the effort it took to breath the thicker air and the greater reward it offered, the unfamiliar insect chirps, the deeper tones of the wind.

  In the middle of the night, it started raining. No one had thought to set up a container to catch the ethane, so Rosalind went out and did it herself.

  When she checked, around dawn, the container was almost
three-quarters full—more than enough to keep them all healthy for days. She looked across the ice field and saw two figures in the distance, approaching slowly, arms around each other’s shoulders.

  She ran to meet them. Joanna was uninjured, but Sigrid had broken her foot.

  “How bad is it?” Rosalind asked, stepping in to support her on the other side to Joanna as they continued on toward the camp.

  “It’ll heal,” Sigrid insisted. “It’s a clean break, it just needs a splint.”

  “All right.” They had splints, bandages, and disinfecting ointments in every crate.

  When Sigrid had been tended to, they left her in the tent to rest and began their sweep of the area to try to locate the remaining supply drops. Rosalind strode across the ice, diligently scanning the ground ahead for anything from an undamaged glider to the sparsest trail of debris, fighting the urge to keep looking up to check that it really was Tvíbura above her.

  She was hungry, but that was more out of habit than need. The food they’d brought would have to be rationed carefully, with one meal every second day, but the air alone gave her a sense of vigor.

  Around mid-morning, she found one of the supply drops. The glider was intact, apart from a small tear in one panel. She disassembled it and stacked the panels on top of the crate. As she started back toward the camp, she saw another of the flying lizards wheeling above her.

  By noon, the travelers had recovered seven crates in total. By nightfall, eleven. They took a vote and decided unanimously to move on to the next stage of the plan; if they ended up desperately short of anything, they could always come back and search again for the twelfth crate.

  But Rosalind was hoping that the glider in question had torn itself apart high above the ice field, leaving its cargo to plummet to the ground far short of all the other landing sites. Their luck could not be perfect, and if the cost of that was a broken foot for Sigrid and one missing box of supplies, that would leave her much less nervous than eight travelers entirely unscathed, twelve crates recovered, and some unknown price yet to be paid.

  The nearest soil deposit was about two days’ walk north-west of the hills. They decided to send an advance party of four, traveling light, rather than lugging all of their supplies to a destination that might turn out to be unsuitable. Sigrid willingly forfeited her place, then the rest of them drew lots. Kate, Joanna, Rosalind and Anya picked the red tokens.

  It was still early when they set out. As they skirted around the base of the hills, Rosalind found herself shooing away mites—or some kind of tiny black insect that fled from her approaching hand, but was too numerous for this discouragement to have much effect. Unlike the ones back home, none seemed to want to bite Tvíburans, but they were still curious enough to spend time exploring the foreigners’ skin, and however many got the message that there was nothing palatable for them here, they clearly had no ability to share that knowledge with the rest of the swarm.

  “What are they eating?” she wondered. She had yet to see anything that she recognized as vegetation.

  “Each other?” Kate suggested.

  “Very funny.”

  “There must be something growing up in the hills,” Anya decided. “Some fungus that can draw nutrients out of the rock. Or maybe there are patches of soil that blew in and got trapped.”

  Joanna coughed, then burst out laughing.

  “What?” Kate demanded.

  “I think I just ate one of the insects.” She probed the inside of her mouth with her tongue, then added, “Yeah, it went down, and it’s not coming back. So if I’m still alive tomorrow, we’ll know what we can use for food if we ever get desperate.”

  They set up camp at nightfall, with their destination a smudge on the horizon. Rosalind was surprised that they’d spotted it so early; either they’d walked faster than she’d expected, or the mass of soil rose even higher above the ice than she’d estimated when she’d examined its shifting shadow through the telescope.

  Halfway through the night, she woke to the sound of something pushing against the tent. She tried to clear her head, wondering if it might have been the wind. Then she heard it again. The fabric wasn’t rustling in the breeze. It was being prodded.

  The light of Tvíbura coming through the weave of the tent was bright enough to show her a small inward bulge in the wall, close to the ground, but this offered no real clues about the would-be intruder. Rosalind woke her companions, and took a knife from her pack.

  She unlaced the entrance and stepped out. As she came around to the side of the tent, she saw a low, dark shape fleeing across the ice, moving with a rapid, elegant lope, heading north-west.

  Joanna appeared beside her.

  “Did you get a look?” she asked Rosalind.

  “It wasn’t very big. Maybe some kind of cat.”

  “Curious enough to give something strange and motionless a poke,” Joanna mused, “but too shy to stick around when an animal of our size emerges. If they’re the kind of neighbors we’re going to have, I can live with that.”

  “Let’s hope that it’s our strangeness they’re shy of,” Rosalind replied. “Not a resemblance to something they’ve already learned to fear.”

  Their ancestors had hunted Tvíbura’s predators to extinction, but that battle had relied on numbers and resources that could not be mustered at short notice here. Still, all they’d seen so far were a few lizards, aloof in the sky, some insects disinclined to bite them, and one timid cat. Rosalind lowered the knife, which she’d been holding up instinctively. One more day’s walk, and they might at least discover what lay at the very bottom of the food chain.

  As they drew nearer to their destination, it resolved into a high plateau. The hills where they’d met probably hadn’t been much taller, but the color of this material was completely different: a rich brown, just like the most coveted soil back home. The gray of the hills here matched, almost exactly, the gray of the hills and mountains on Tvíbura, so why should other comparisons not hold? Rosalind couldn’t explain why a giant mound of soil with no grass to bind it hadn’t simply blown away, but she refused to believe that they were headed toward nothing but a useless slab of rock.

  By mid-afternoon the wind was growing dustier, and the ice around them less pristine. Rosalind ran a finger over the ground then put it in her mouth; it tasted of soil. If she could breath Tvíburian air, belched up from Tvíburian oceans, what was to stop the geysers here from delivering soil that the plants of her home world could feed upon?

  The closer they came to the plateau, the less it looked like desiccated, ancient rock. Kate said, “That’s ripe for farming. I can smell it!”

  Joanna broke into a run, and this time Rosalind joined her. They sprinted together over the brown muddy ice, shouting exuberant taunts at each other as they took turns gaining the lead.

  When they’d almost reached their destination, Rosalind stopped and glanced back toward Kate and Anya, who were proceeding at their usual unhurried pace. She felt slightly foolish, but she didn’t care. She had no more patience left.

  She turned to examine the steep incline ahead. Where the approach to the plateau met the ice, the soil had spilled out and left a thin, loose coating, but once it started to rise it became pitted and clumpy, not at all like the side of a sandpile. She caught up with Joanna at the base of the slope.

  “Are these some kind of roots?” Joanna wondered, squatting down to examine the tangle of pale, fibrous strands that poked out through the soil.

  “That’s what they look like,” Rosalind agreed. “But the roots of what?”

  They started up the slope, supporting each other, judging each step carefully as they negotiated the treacherously porous surface. The soil kept crumbling beneath their feet, but never catastrophically; their weight seemed to be collapsing a succession of small, air-filled spaces, but the roots were so tightly woven through the soil that it was impossible to start an avalanche.

  When they came to an opening that might have been either a
cavern formed by chance from a gap in the roots, or the mouth of a burrow dug by an animal, they skirted around it; this wasn’t the time to start pestering the neighbors. The ascent was growing arduous, but Rosalind had no intention of resting or retreating. There was not much daylight remaining, and she did not trust the light of Tvíbura to reveal everything they needed to know.

  Finally, they staggered up onto the top of the plateau. The surface ahead of them stretched into the distance, roughly level as far as Rosalind could see, but as she stepped gingerly forward it was clear that it was no smoother or less porous than the slope. She knelt down and probed the ground with her fingers. The “roots” were still tangled with the soil, but they seemed oblivious to their change of circumstance. There were no stems, or flowers, or leaves protruding from them into the air; in the distance, the ground looked lifeless, just as it had through the telescope from Tvíbura, but each time Rosalind took a few more steps and checked again, she found the same tough, pale filaments locking up the soil.

  Joanna said, “So this is Tvíburi’s natural vegetation? Its idea of grassland?”

  “Apparently.” Rosalind didn’t think these strands could be something like Yggdrasil roots—part of an organism that drew its nourishment from below the ice, and which had merely trapped the soil by chance. This plant—or colony of plants—was living off the bounty that the geysers had rained down onto the ice, stabilizing it much as the grasses on Tvíbura would have done, but feeding so well on the soil itself that it had no real interest in sunlight.

  “So how do we clear a field, out of this?” Joanna asked. “A scythe, a plow?”

  Rosalind took the knife from her pack and knelt down again. There was so much soil here: enough for a thousand farms. And nothing to fight the crops for their view of the sky. But when she plunged her knife into the ground, it was impossible to move the blade sideways. The soil’s existing tenants—invisible from a few strides away, let alone from across the void—were nonetheless so numerous, and so strong, that it was like trying to carve into solid rock.