by Greg Egan
“If they want to be left in peace, we have no right to disturb them.” Azar hoped that this declaration was redundant, but she wanted the ground rules absolutely clear.
“Of course,” Shelma agreed. “But if they insist on playing dead to perfection, all they’ll get are the rights of the dead. Which, while not negligible, are somewhat diminished.”
Once a civilization became extinct – not merely mutating into something new, but leaving no sentient heirs whatsoever – it was widely accepted that its history devolved into a common legacy that anyone was entitled to investigate. If sovereignty really had ceased to be an issue, Tallulah was certainly worth exploring. Tens of thousands of orphan planets had been found in the past, but only a few dozen had shown signs of habitation, and those worlds had yielded nothing but sad ruins buried beneath the permafrost. In the age of the Amalgam – the meta-civilization that now ringed the galaxy – the extinction of an entire world was unthinkable; if a catastrophe could not be averted, people who already had a robust digital form could be evacuated in seconds, and even those who had chosen purely biological modes could be scanned in a matter of days at the most.
The people of Tallulah, it seemed, had been halfway in between. When some cosmic mishap tossed them from their stellar hearth they had been unwilling or unable to evacuate, but they had not stood by and watched the air around them fall to the ground like snow. Whether trapped by their fate or just stubbornly resolved to ride it out, they had found a way to survive it. If they had since succumbed to some other tragedy, or merely surrendered to the passage of time, Azar saw no disrespect in digging up their secrets. Their achievements had endured for a billion years; they deserved recognition and understanding.
2
Mologhat’s orbit was a discreet hundred thousand kilometers from Tallulah, but it had dispatched a swarm of microprobes into smaller, faster orbits of various inclinations, providing complete coverage of the surface. If there had been any lingering suspicion that the heating of the crust might have been due to some freakish natural process, the details put that idea to rest: not only was the temperature modulated by latitude, diminishing toward the planet’s rotational poles, the records showed that it cycled over a period of about three months, creating imitation seasons. These nostalgic echoes of a long-lost circumstellar orbit were so clear that Azar was surprised they’d put the heat source in the ground at all, rather than launching an artificial sun.
“Not only would that have given them light from above,” she suggested to Shelma as they strolled through Mologhat’s library, “they could have kept the old diurnal rhythm too.” Heat conduction from deep in the crust would have washed out any cycle as short as a typical planetary day.
Shelma said, “It’s a lot of extra work to make a microsun efficient – to keep it from pouring energy out into space.”
“That’s true.”
“And perhaps they were insecure as well,” Shelma added, sliding out an image from the stack beside her that showed an animated model of Tallulah’s weather patterns. “They were already on the verge of losing one sun. They might have preferred to keep their energy source buried, rather than risk being parted from this one too.”
“Yeah. Still, it’s interesting that they tweaked the biosphere for such a radical shift – ground heat replacing sunlight – but kept the seasons.”
Shelma smiled. “Days, seasons, you’ve got to have something. People go mad without change.” Both she and Azar had chosen to retain sleep cycles, their software following the dictates of their ancestral phenotypes. But Azar knew that the Baharis’ ancestors were nocturnal; what Azar perceived as the station’s night would be day to Shelma, and vice versa.
Azar pulled out a map of vegetation density. Using synthetic aperture methods, the microprobes had resolved details on Tallulah’s surface down to about a tenth of a meter, and even at that coarse resolution they had identified thousands of different kinds of plants. Spectroscopy could not untangle the detailed biochemistry from orbit, but the biosphere was clearly carbon-based/anaerobic, with the plants synthesizing carbohydrates but releasing no free oxygen.
Shelma spread her arms to take in the whole collection of data around them. “Everything here is open to interpretation. We’re going to need to make landfall to get any further.”
“I agree.” Azar was nervous, but relieved by the verdict. She was glad she hadn’t traveled this far just to find that Tallulah was clearly occupied by hermits, and there was nothing left to do but abandon them to their solitude.
“The question, then,” Shelma said, “is how we want to do it.” She began reeling off options. They could sprinkle a few nanotech spores on the surface, then sit back and wait for the army of robot insects they built to scour the planet. Or they could leave Mologhat and travel to the surface themselves, in various ways. Of course they could always combine the two, delegating most of the exploration while still being in the thick of it.
Azar had studied all of these methods before her departure, but Shelma sounded too blasé to be merely reciting theoretical knowledge. “You’ve done this kind of thing before, haven’t you?”
“Dozens of times.” Shelma hesitated. “This is your first time out-of-system?”
“Yes.” That wasn’t a lucky guess; everyone knew about the dearth of travelers from Hanuz. “It’s hard for us,” Azar explained. “Leaving everyone we know for hundreds of years. You don’t mind doing that?”
“My ancestors were solitary for part of their life cycle,” Shelma said, “and sociable for the rest. Now we’re flexible: we can switch between those modes at will. What I don’t understand is why you don’t just travel in packs, if that would make things easier.”
Azar laughed. “I know some people do that, but our social networks are so tangled that it’s hard to find a truly self-contained group – least of all a group where everyone can agree on a single destination. And if they do, they’re more likely to emigrate than to take a trip and come home again.”
“I see.”
“Anyway, forget about Hanuz. We need to make some decisions.” Azar wasn’t going to sit around on Mologhat while robots had all the fun, but there were practical limits on how far she could go just to get some dirt beneath her fingernails. If she had her own standard body reconstructed down on the surface, tweaked to survive the local conditions, she’d spend all her time foraging for food. Mologhat had only a few micrograms of its original antimatter store left; the few hundred megajoules that would generate were enough for its own modest needs, but pilfering any of it to power a sixty-kilogram behemoth would be insane; she could burn up the whole lot in a month. If Tallulah had had a reasonable abundance of deuterium she could have powered her body with D-D fusion, but the isotope was rare here.
“What if we build a high-capacity processor into one of the explorer insects?” Azar suggested. “Then we download into that. We get to see the world firsthand and make some real-time decisions, but we don’t waste energy or leave a big footprint.” If Tallulah turned out to be inhabited after all, the difference between being perceived as friend or enemy might easily hang on something as simple as the amount of local resources they’d used, or how physically intrusive their presence had been.
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.
