Shelma thought it over. “That sounds like as good a choice as any.”
3
Azar persisted with her doorway metaphor, and walked through an “airlock” from Mologhat Station into the robot insect as if the two were docked together. Amused by the conceit, Shelma followed behind her, but she couldn’t resist a mild rebuke. “The poor balloon doesn’t even rate a mention?”
Azar shuddered. “Please, heights make me dizzy.” Only gamma rays had the bandwidth to transmit their software in a reasonable amount of time, but gamma rays couldn’t penetrate far through a planetary atmosphere. So the nanotech on Tallulah’s surface had built a small hydrogen balloon, which had risen high enough into the stratosphere to receive their transmission and transcribe the data into a densely encoded molecular memory, before deflating and descending.
The scape Azar had constructed inside the insect resembled the kind of transparent-domed flight deck found in sight-seeing aircraft back on Hanuz. Shelma would be perceiving some very different furnishings, but at least the two of them shared the same view of the jungle beyond the windshield; Shelma’s vision had always stretched into the far infrared, and now Azar had chosen to match her.
The insect was perched on a broad, flat leaf, one of dozens of papery structures sprouting from a slender trunk. The leaf’s veins glowed with the heat of warm sap, and a hot mist wafted up from the blotchy hexagonal pores that dotted the surface. When Azar looked up into the sky, the stars were barely visible through the fog.
Scout mites had already crawled up and down this plant and begun deciphering its strange biochemistry. Sap that was cooled and concentrated by evaporation in the leaves was pumped down to the roots, where it was diluted in chambers of fresh water. The increase in entropy that the dilution entailed allowed enzymes in the sap to drive an endothermic reaction, absorbing heat from the ground while synthesizing sugars from dissolved carbon dioxide.
The plant’s heritable replicator was a carbohydrate polymer known as C3, which had been found on many other worlds. Once they’d built up a database of sequences from a sufficient number of species, they could start trying to construct an evolutionary tree, as well as looking for signs of technological tinkering.
Azar took hold of a joystick and flew their host across to another plant, a small bush adorned with twigs that sprouted leaves like radial cooling fins. They landed on a twig while the scout mites burrowed and sampled.
“There’s not much sap in this one,” Shelma noted. “The leaves just look like mats of fiber.” There were no pores here, no steamy exudations.
Azar watched a display of the scouts’ discoveries. There were long, fibrous structures running all the way from the leaves to the tips of the roots, and they were packed with interlocking polymers. In some fibers the polymers were rich in mobile electrons; in others they had positive “holes”, electron deficits that could shuffle along the molecule’s backbone from site to site.
“Thermoelectric diffusion?” she guessed. The electrons and holes would conduct heat from the ground up into the leaves, and in doing so they’d set up an electric potential, which in turn could be used to drive chemical reactions.
As the details came through, they confirmed her suspicion. The plant was a living thermocouple, with the heat-pumped currents in the polymers shuffling electrons to and from the enzymes that synthesized carbohydrates.
The thermocouple bush had no easily digestible nutrients above ground, so Azar flew back to the entropy tree and thrust the insect’s proboscis into a vein, drawing out a tankful of sugary sap. There was no free atmospheric oxygen to help metabolize the sugars, but like the plant itself, their robot could make use of nitrate ions in the sap as an oxidizing agent, reducing them to ammonia in the process. Scout mites were still hunting for the organisms responsible for creating the nitrates in the first place.
Shelma said, “So where are the insects? Where are the animals?” So far, they’d seen nothing moving in the jungle.
“Maybe the Ground Heaters didn’t have time to tweak any animals for the new conditions,” Azar suggested. “If they were about to get ejected from their solar system, their priorities would have been a new form of energy, and a primary food source that could exploit it. The old animals just died out, and nobody had the heart to try to create new ones.”
“Maybe,” Shelma conceded. “But wouldn’t your first response to the prospect of losing your sun be to build a few domed arks: sealed habitats with artificial heat and light that preserved the original ambient conditions, and as much of the original biosphere as possible?”
Azar said, “And then you’d slowly begin modifying the species from the arks to live off the new energy source. Still, they might have started with the plants but got no further.”
The scout mites collected more C3 sequences, and as the numbers reached the point where comparisons became meaningful it grew increasingly clear that these genomes were natural, not engineered. Even the genes responsible for building the gloriously technomimetic thermocouple fibers had the same messy, incrementalist, patchwork character of all the others.
Stranger still, the genetic analysis pointed to a common ancestor for all these plants just two hundred million years ago, long after Tallulah had been orphaned.
As Azar reviewed descriptions of other C3 worlds, pulling the data down from Mologhat’s library, she realized that in a couple of hours the station would set below the horizon. The time lag for her queries was already ponderous, and re-routing everything around Tallulah through the limited-bandwidth microprobes would only make that worse.
“We should clone the station’s library,” she suggested to Shelma. The library was far bigger than their personal software, and there was no room for it in their present insect host, but they could at least bring it down into the stratosphere, making the data far more accessible than it was from Mologhat’s distant orbit.
Shelma agreed. They set the nanotech to work fitting out the balloon for a new flight, then continued exploring the jungle.
As in many communities of plants there was competition for access to the sky, but here it was all about shedding heat rather than catching sunlight. The healthiest plants had their roots deep in the ground and their leaves exposed to the darkness of space. To be caught in too warm a cranny, sentenced to uniform tepidity, was fatal. The only exceptions to that rule were parasites: vampiric vines that stretched over trunks, branches and leaves, their barbed rootlets anchoring them to their victims and drawing out nutritious sap.
As they moved through the jungle, the new sequence data that came in from the scouts only shored up their original conclusions: the life they were seeing was entirely natural, and this branch of it was relatively young.
“Suppose,” Shelma ventured, “that the Ground Heaters didn’t need to engineer anything to live this way.”
“You mean there were species that exploited thermal gradients all along?” Azar frowned. “How do you evolve to use
that
as your energy source? A single cell can never do it alone; you need to be a certain minimum size to access a useful temperature difference.”
“I’m not saying that the very first lifeforms used it,” Shelma replied. “They might have relied on chemosynthesis, extracting energy from volcanic gases or mineral-rich geysers.”
“Right.” That was how Azar’s own ancestral lineage had begun, back on Earth; photosynthesis had come much later. “So they grew to a certain size using chemosynthesis, then found they could switch to thermal effects. But this is all before the Ground Heaters have even evolved, so what’s keeping the surface rocks so hot?”
Shelma pondered this. “Tidal heating? What if Tallulah was orbiting close to a cool red dwarf, or even a brown dwarf. With such weak sunlight, tidal heating might have been a far more potent energy source than photosynthesis.”
“But it can’t last,” Azar protested. “Eventually the planet would end up tidally locked.” The energy used to stretch and squeeze the rock, heating it up by internal friction, would ultimately be extracted from Tallulah’s spin, slowing its rotation until its day matched its year and one hemisphere faced forever sunwards.
“Eventually, yes. But what if the Ground Heaters evolved before that happened? They would have been facing a slow, predictable decline in their energy source over millennia. So instead of responding frantically to a sudden catastrophe, they could have spent centuries perfecting a replacement.”
“And the ejection from their star came much later, but by then there was nothing they needed to do. They’d already made themselves independent.” Azar laughed, delighted. The artificial seasons and the variation in heat with latitude would still make sense: tidal heating would have been strongest at the equator, and at higher latitudes it would have been affected by seasonal changes in the angle between the planet’s axis and the direction of the tidal force.
What this elegant hypothesis didn’t explain was why the plants here were so young. Nor did it shed any light on exactly what the Ground Heaters had done to achieve their independence.
The data-collecting balloon was in place again. Before Mologhat vanished below the horizon, Azar instructed the station to send down a copy of its library.
As she was checking the interface with the cloned library, a message arrived from the microprobes. Thousands of kilometers away, something had exploded on the ocean floor and hurled a few billion tonnes of water skywards.
Azar turned to Shelma, still watching the satellite vision with her mind’s eye. “What’s happening? Some glitch with the heat source?” For a system that had survived for a billion years, this hiccup packed a mighty punch: the eruption was already high above the atmosphere, steam turning to ice like a cometary impact in reverse.
Shelma looked nervous. “Mologhat saw no vulcanism anywhere on the planet, in the last three years. Do you think we’ve annoyed someone?”
“If we have, why are we still alive? It’s not the ground beneath our feet that’s exploded.” The balloon clearly wasn’t the target, nor were any of the microprobes – and though the water missile was heading roughly in Mologhat’s direction, it wouldn’t reach anywhere near that far. But when Azar tried to contact the station, the microprobes replied that it was not responding.
Shelma said, “Don’t jump to any conclusions. Mologhat might have imposed a communications blackout; if it thought it was under fire it would shift orbits and try not to do anything to give its position away.”
Azar felt sick. “You think the gamma ray transmission was mistaken for some kind of attack?” Nothing had happened when she and Shelma had arrived the same way, but that burst had been considerably shorter, and it had come down almost vertically. The second beam had come from close to the horizon, giving it a longer path through the upper atmosphere – which would have made it more noticeable, and easier to trace back to its source.
Within minutes, the microprobes had reported six more eruptions, from underwater sites scattered around the planet. It made no sense to Azar; these gigatonnes of water were rising into orbits about a thousand kilometers up, but if they were meant as weapons, who were they aimed at? The microprobes were much lower down, and Mologhat was a hundred times further away. A direct hit with a solid iceberg could have done a lot of damage to any intruder, but these glistening snowballs weren’t even holding together; Tallulah was just shrouding itself with a tenuous halo of tiny ice crystals.
“This isn’t warfare!” she declared. “They don’t think they’re under attack. They saw the gamma rays, and thought:
antimatter
. They’re afraid they’re drifting into a cloud of antimatter. The ice is to tell them if there’s any more around.”
Shelma considered this. “I think you’re right. They picked up a flash of annihilation radiation, and jumped to the conclusion that it was a natural source.”
Never mind that there was no natural source of bulk antimatter anywhere in the galaxy; if you’d spent a billion years in space without encountering another civilization, perhaps a small cloud of antihydrogen seemed like a far less extravagant hypothesis than alien visitors using proton-antiproton gamma rays for communications.
“So they still don’t even know we’re here?” Azar wondered. “All those radio messages came to nothing. What do we have to do to get noticed – tattoo the binary digits for pi across the stratosphere?”
Shelma said, “I wouldn’t advise that. But it’s not even clear to me that there’s anyone home; this might just be a non-sentient device that’s outlived the people it’s meant to be protecting.”
The water missiles had stopped; the absence of any answering flash of radiation must have made it clear that if there was antimatter around, it was far too thinly spread to pose any kind of hazard.
Azar tried calling Mologhat again, but it was still not responding. “They must have hit it,” she said. “Whatever they thought it was, they must have launched something small and fast, and knocked it out before the ice storm even started.” She felt numb.
So much for the doorway to the stars.
Shelma touched her arm reassuringly. “It might yet reply – but even if it’s gone, we’re not stranded.”
“No?” The microprobes had nowhere near enough power for interstellar transmissions, or even the raw materials to build the kind of hardware they’d need. And the data-ferrying balloon couldn’t send them anywhere; their return path to Mologhat would have involved a gamma-ray mirror on the balloon, modulating and reflecting radiation coming down from the station itself.
Azar slumped into her seat.
How had she ever imagined she could do this? Travel fifteen hundred light years as if it was nothing?
There was no magic gate leading home, just fourteen quadrillion kilometers of vacuum.
Shelma said, “We have plenty of resources down here.”
Azar rubbed her eyes and tried to concentrate. “That’s true.” Given time, the nanotech could build them almost anything – and they didn’t even have to reach all the way back to Hanuz or Bahar; they just had to connect to the Amalgam’s network. Still, the nearest node was seven hundred light years away; getting a signal that far was a daunting prospect. “Can we do this from the ground?”