"Maybe all of this was engineered from scratch," Yatima mused. "For decorative purposes. Maybe Swift was sterile and airless, and someone came along and built this ecosystem, molecule by molecule. Using heavy isotopes to make it last a little longer. Like sculpting in gold, to avoid corrosion."
"No. Wherever the Transmuters are now, this must have been their native biosphere." Orlando seemed grimly convinced, as if the alternative was too decadent and frivolous to contemplate. "They would have substituted the isotopes slowly, feeding them into the existing atmosphere over millennia. It was a mark of respect that they didn't wrap their home in a protective sphere, or shift its orbit, or modify its sun. They slipped in a change at the lowest possible level, underneath the biochemistry."
Yatima guided vis car over the puddle. Vivid green eels several millimeters long undulated by, much faster than the probe. A red-and-yellow twelve-legged spider walked upside-down on the membrane, picking out the flatslugs that lived embedded in it. Yatima didn't have much sympathy for the prey; they blithely fed on the protective polymers that almost every other species took the trouble to synthesize and excrete. Then again, it was a niche begging to be filled, and none of these creatures did anything with a conscious purpose.
"If they cared so much about their biological cousins, they can't have been expecting Lacerta. There's no sign of any built-in protection against a gamma ray burst."
Orlando was unswayed. "Maybe the only thing they could have done that would have made a difference were anathema to them. And they must have known that even if there were massive extinctions, they'd given the biosphere enough general resilience to recover."
They'd found few fossils on Swift, so it was difficult to judge the extent to which life had been disrupted by the burst. Models showed that most of the existing species would have coped relatively well, but that was hardly surprising; they were the ones that had survived, not a representative sample of pre-Lacerta life. The heritable material here cycled between five different molecular coding schemes in successive generations; some species used a "pure" scheme, all Alpha leading to all Beta, Gamma, Delta, and Epsilon, while others had mixtures o all five in every generation. Some biologists claimed to have identified a genetic bottleneck due to Lacerta, but Yatima wasn't convinced that anyone understood Swift's biochemistry well enough yet to say what a normal level of diversity would have been.
"So where are they now? Have they been swallowed by an Introdus, or scattered by a Diaspora? If you can read their minds about everything else, that ought to be an easy question to answer."
Orlando replied with sublime confidence, "Would I be here, if I thought I was wasting my time?" His tone was ironic, but Yatima didn't believe he was entirely joking.
They'd scoured the planet from orbit, looking for cities, for ruins, for mass anomalies, for buried structures. But a civilization as advanced as the Transmuters could have miniaturized their polises beyond any chance of detection. One faint hope was that since they'd bothered to intervene in the fate of Swift's organic life, they might show themselves at the oases now and then. Yatima wasn't optimistic. If they were still on the planet they could hardly be unaware of their visitors, but they hadn't chosen to make contact. And if they didn't want to be seen, they were unlikely to send big, clumsy, millimeter-wide drones plowing through these puddles. Yatima watched a rare translucent creature swim by beneath the probe, propelled by a jet of water it created by contracting its whole hollow body. Ve'd thought ve'd be prepared to study a world like this, patiently helping the biologists extract the kind of insights into evolutionary principles offered by even the most modest extraterrestrial biosphere. There were no spectacular new body plans or life cycles here, no strategies for feeding or reproduction that hadn't been tried out back on Earth, but at a molecular level everything worked differently, and there was a vast labyrinth of utterly novel biochemical pathways to be mapped. Yet the Transmuters made it almost impossible to care. Their absence—or their perfect camouflage—monopolized everyone's attention, transforming the intricate machinery of the biosphere into a very long footnote to a far more mesmerizing blank page.
Ve turned to Orlando. "I don't think they're in hiding. How shy could they be, after giving the atmosphere a spectrum that screams, 'Civilization! Come and visit!' We only noticed it close up, but it wouldn't take a huge technological advance to spot it from thousands of light years away.
Orlando didn't reply; he'd been staring down into the puddle, and he continued to watch a swarm of crimson larvae molting, and eating each other's discarded skins. Yatima understood the stake he had in making contact with the Transmuters. By the end of the Diaspora, when his scattered clones had reconverged, the Earth would be habitable again—but he could never feel secure about returning to the flesh until Lacerta had been explained. Any Coalition theory was likely to remain as suspect as the original belief that Lac G-1's neutron stars would take seven million years to collide. But if the Transmuters had firsthand knowledge of the galaxy's dynamics on a timescale of millions of years—and were beneficent enough to transform this planet's atmosphere, atom by atom, just to save their distant relatives from extinction—surely they wouldn't begrudge an infant civilization a little information and advice on its own long-term survival.
"Okay." Orlando looked up. "Maybe the spectrum was meant to stand out like a beacon. Maybe that's the whole point. They could have preserved the atmosphere in a thousand other ways, but they chose a method that would get them noticed."
"You mean they went out of their way to attract attention? Why?"
"To bring people here."
"Then why are they being so unsociable? Or are they just waiting to ambush us?"
"Very funny." Orlando met vis gaze. "You're right, though: they're not hiding from us, that's absurd. They're gone. But they must have left something behind. Something they wanted us to see."
Yatima gestured at the oasis.
Orlando laughed. "You think they built this as an ornamental pond, and invited the whole galaxy to come and admire it?"
"It doesn't look like much now," Yatima admitted. "But even loaded with deuterium and oxygen-18 it's been drying out slowly. Six billion years ago it might have been spectacular."
Orlando was not persuaded. "Maybe we're both wrong about the biosphere. Maybe there was no life here at all when the Transmuters left; it could have evolved later. The persistence of water vapor might he nothing but a side effect of the method they chose to make Swift stand out to anyone with a decent spectroscope and a glimmering of intelligence."
"And we just haven't searched hard enough for whatever it is we were meant to find? The lure wasn't exactly subtle, so the payoff should be just as hard to miss. Either it's turned to dust, or we're looking at the dregs of it right now."
Orlando was silent for a moment, then he said bitterly, "Then they should have used a beacon that turned to dust, too."
Yatima resisted pointing out the technical problems with choosing isotopes with suitable half-lives. Ve said, "They might have visited other planets, and left something more enduring. The next C-Z to arrive might find some kind of artifact…" Ve trailed off, distracted. Another possibility was hovering on the edge of consciousness; ve waited a few tau, but it wouldn't break through. Keeping vis icon in the Swift scape—along with vis linear input, in case Orlando spoke—ve shifted vis gestalt viewpoint to a map of vis own mind.