"You got it. Well, we knew it was here. We even have samples taken by automated probes. Though we never spotted those birds before." She hefted the stuff, films of it draped over her gloved hand, and looked at me. "I wonder if you understand how exotic this stuff is. I'm pretty sure this is silane life. That is, based on a silicon chemistry, rather than carbon. . ."

The things on the lake did indeed look like jet black lilies. But they were not lilies, or anything remotely related to life like my own.

Life of our chemical sort is based on long molecules, with a solute to bring components of those molecules together. Our specific sort of terrestrial life, which Miriam called "CHON life," after its essential elements carbon, hydrogen, oxygen, and nitrogen, uses water as its solute, and carbon-based molecules as its building blocks: carbon can form chains and rings, and long stable molecules like DNA.

"But carbon's not the only choice, and nor is water," Miriam said. "At terrestrial temperatures silicon bonds with oxygen to form very stable molecules."

"Silicates. Rock."

"Exactly. But at very at very low temperatures, silicon can form silanols, a.n.a.logous to alcohols, which are capable of dissolving in very cold solutes-say, in this ethane lake here. When they dissolve they fill up the lake with long molecules a.n.a.logous to our organic molecules. These can then link up into polymers using silicon-silicon bonds, silanes. They have weaker bonds than carbon molecules at terrestrial temperatures, but it's just what you need in a low-energy, low-temperature environment like this. With silanes as the basis you can dream up all sorts of complex molecules a.n.a.logous to nucleic acids and proteins " low temperatures, silicon can form silanols, a.n.a.logous to alcohols, which are capable of dissolving in very cold solutes-say, in this ethane lake here. When they dissolve they fill up the lake with long molecules a.n.a.logous to our organic molecules. These can then link up into polymers using silicon-silicon bonds, silanes. They have weaker bonds than carbon molecules at terrestrial temperatures, but it's just what you need in a low-energy, low-temperature environment like this. With silanes as the basis you can dream up all sorts of complex molecules a.n.a.logous to nucleic acids and proteins "

"Just what we have here."

"Exactly. Nice complicated biomolecules for evolution to play with. They are more commonly found on the cooler, outer worlds Neptune's moon Triton for example. But this lake is cold enough. The energy flow will be so low that it must take a lo-ong a lo-ong time for anything much to grow or evolve. But on t.i.tan there is plenty of time." She let the filmy stuff glide off her manipulator scoop and back into the lake. "There's so much we don't know. There has to be an ecology in there, a food chain. Maybe the films are the primary producers an equivalent of the plankton in our oceans, for instance. But where do they get their energy from? And how do they survive the annual drying-out of their lakes?" time for anything much to grow or evolve. But on t.i.tan there is plenty of time." She let the filmy stuff glide off her manipulator scoop and back into the lake. "There's so much we don't know. There has to be an ecology in there, a food chain. Maybe the films are the primary producers an equivalent of the plankton in our oceans, for instance. But where do they get their energy from? And how do they survive the annual drying-out of their lakes?"

"Good questions," I said. "I wish I cared."

She stowed her sample bottles in her pack. "I think you care more than you're prepared to admit. n.o.body as intelligent as you is without curiosity. It goes with the territory. Anyhow we should get back to the gondola."

I hesitated. I hated to prove her right, that there was indeed a grain of curiosity lodged in my soul. But I pointed at the enigmatic black form lying further along the beach. "Maybe we should take a look at that first."

She glanced at it, and at me, and headed that way without another word.

It turned out, as I had suspected, that the crumpled form was a bird. I recalled one hitting our gondola during their a.s.sault and falling away; perhaps this was that very casualty.

It was a block of ice, about the size of my head, wrapped up in a torn sheet of black film. With great care Miriam used her manipulator arm to pick apart the film, as if she was unwrapping a Christmas present. The ice ma.s.s wasn't a simple lump but a mesh of spindly struts and bars surrounding a hollow core. It had been badly damaged by the fall. Miriam took samples of this and of the film.

"That ice lump looks light for its size," I said. "Like the bones of a bird."

"Which makes sense if it's a flying creature." Miriam was growing excited. "Jovik, look at this. The filmy stuff, the wings, look identical to the samples I took from the surface of the lake. It has to be silane. But the ice structure is different." She broke a bit of it open, and turned on a suit lamp so we could see a ma.s.s of very thin icicles, like fibers. It was almost sponge-like. Inside the fine ice straws were threads of what looked like discolored water. "Rich in organics," Miriam said, glancing at a data panel on her manipulator arm. "I mean, our sort of organics, CHON life, carbon-water-amino acids, a kind of DNA. There are puzzles here. Not least the fact that we find it here it here, by this lake. CHON life has been sampled on t.i.tan before. But it's thought carbon-water life can only subsist here in impact-melt crater lakes, and we're a long way from anything like that..."

Her pa.s.sion grew, a trait I have always found attractive.

"I think this is a bird, one of those we saw flying at us. But it seems to be a composite creature, a symbiosis of these hydrocarbon wings and the ice lump-saline life cooperating with CHON life. Just remarkable. You wonder how it came about in the first place ... but I guess there are examples of just as intricate survival strategies in our own biosphere. Give evolution enough time and anything is possible. I wonder what it is they both want both want, though, what the two sides in this symbiosis get out of the relationship...

"It's a genuine discovery, Jovik. n.o.body's seen this before-life from two entirely different domains working together. And I wouldn't have noticed it if not for you." She held out the ice lump to me. "They'll probably name it after you."

Her enthusiasm was fetching, but not that much. "Sure. But my concern right now is how much power we have left in these suit heaters. Let's get back to the gondola."

So she stowed away the remaining fragments of the t.i.tan bird, Jovik Emry's contribution to System science, and we retraced our path back to the gondola.

IX Gondola

The days are very long on t.i.tan, and by the time we got back to the gondola nothing seemed to have changed about the landscape or the sky, not a diffuse shadow had shifted. We found Poole and Dzik happily fixing big balloon wheels to axles slung beneath the crumpled hull.

When they were done, we all climbed back aboard. Poole had reset some of the interior lamps so they glowed green, yellow and blue; it was a relief to be immersed once more in bright Earth light.

We set off in our gondola-truck for the next part of our expedition. We were making, I was told, for an impact crater believed to hold liquid water, which itself was not far from a cryovolcano, another feature of interest for the science types. This site was only perhaps 100 kilometers from where we had come down.

Miriam transferred her samples to cold stores, and ran some of them through a small onboard science package. She jabbered about what she had discovered. Poole encouraged her more than Dzik did, but even that wasn't much.

Dzik and Poole were more interested in that moment with playing with the gondola. Like overgrown boys they sat at an improvised driver's console and fussed over gear ratios and the performance of the big tires. Poole even insisted on driving the bus himself, though t.i.tan was so flat and dull for the most part he could easily have left the ch.o.r.e to the onboard systems. That proved to me the fallacy of not bringing along specialist biologists on a jaunt like this. It was only Miriam who seemed to have a genuine pa.s.sion for the life systems we were supposed to be here to study; Dzik and Poole were too easily distracted by the technology, which was, after all, only a means to an end.

They had however rearranged the interior to make it feel a little less cramped. The couches had been separated and set up around the cabin, so you could sit upright with a bit of elbow room. The cabin was pressurized, so we could remove our helmets, and though the expandable walls didn't work any more there was room for one at a time to shuck off his or her exosuit. Poole ordered us to do so; we had already been inside the suits for a few hours, and the suits, and ourselves, needed some maintenance. Poole had set up a curtained-off area where we could let our discarded suits perform their self-maintenance functions while we had showers of water recycled from our urine and sweat, which was deemed a lot safer than melt from the ice moon. Poole himself used the shower first, and then Miriam. She was hasty, eager to get back to her work, and kept talking even while she cleaned up.

After Miriam was out of the shower I took my turn. It was a miserable drizzle and lukewarm at that, but it was a relief to let my skin drink in the water. I was quick, though; with the unknown dangers of t.i.tan only centimeters away beyond the gondola's fragile metal walls, I didn't want to spend long outside the security of the suit.

After me, Bill Dzik followed, and it was an unlovely stink his suit released. I was spitefully glad that for all his bl.u.s.ter his reaction to the terrors of our landing must have been just as ign.o.ble as mine.

After a couple of hours we reached our destination. Safely suited up, I sat in my couch and peered over Miriam's and Poole's shoulders at the landscape outside. That cryovolcano was a mound that pushed out of the landscape some kilometers to the west of us. It had the look of a shield volcano, like Hawaii or Mons Olympus, a flat-profiled dome with a caldera on the top. It wasn't erupting while we sat there, but I could see how successive sheets of 'lava' had plated its sides.

That lava was water ice, heavily laced with ammonia, which had come gushing up from this world's strange mantle, a sea of ammonia and water kilometers down beneath our tires.

As for the crater lake I saw nothing but a plain, flatter and even more featureless than the average, covered with a thin scattering of ice sand. But the lake was there, hidden. Poole extracted radar images which showed the unmistakeable profile of an impact crater, right ahead of us, kilometers wide. Such is the vast energy pulse delivered by an infalling asteroid or comet or, in Saturn's system, perhaps a ring fragment or a bit of a tide-shattered moon the water locally can retain enough heat to remain liquid for a long time, thousands of years. Such a lake had formed here, and then frozen over with a thin crust, on top of which that skim of sand had been wind-blown. But the briny lake remained, h.o.a.rding its heat.

And, studded around the lake's circular rim, were more sponge-like ma.s.ses like the one we had discovered wrapped up in silane film at the sh.o.r.e of the polar lake. These ma.s.ses were positioned quite regularly around the lake, and many were placed close by creva.s.ses which seemed to offer a route down into the deep structure of the ice rock beneath us. Miriam started gathering data eagerly.

Meanwhile Poole was puzzling over some images returned from the very bottom of the crater lake. He had found motion, obscure forms laboring. They looked to me like machines quarrying a rock deposit. But I could not read the images well enough, and as Poole did not ask my opinions I kept my mouth shut.

Miriam Berg was soon getting very agitated by what she was finding. Even as she gathered the data and squirted it up to Harry Poole in the Crab Crab, she eagerly hypothesised. "Look I think it's obvious that t.i.tan is a junction between at least two kinds of life, the silanes of the ethane lakes and the CHON sponges. I've done some hasty a.n.a.lysis on the CHON tissues.

They're like us, but not identical. They use a subtly different subset of amino acids to build their proteins; and they have a variant of DNA in there a different set of bases, a different coding system. The silanes, meanwhile, are like the life systems we've discovered in the nitrogen pools on Triton, but again not identical, based on a different subset of silicon-oxygen molecular strings.

"It's possible both forms of life were brought here through panspermia the natural wafting of life between the worlds in the form of something like spores, blasted off their parent world by impacts and driven here by sunlight and gravity. If the System's CHON life arose first on Earth or Mars, it might easily have drifted here and seeded in a crater lake, and followed a different evolutionary strategy. Similarly the silanes at the poles found a place to live, and followed their own path, independently of their cousins..."

The transfer of materials from the oily ethane lakes to the water crater ponds might actually have facilitated such creations. You need membranes to make life, something to separate the inside of a cell from the outside. As water and oil don't mix, adding one to the other gives you a natural way to create such membranes.

She shook her head. "It seems remarkable that here we have a place, this moon, a junction where families of life from different ends of Sol System can coexist."

"But there's a problem," Bill Dzik called from his shower. "Both your silanes and your sponges live in transient environments. The ethane lakes pretty much dry up every t.i.tan year. And each crater lake will freeze solid after a few thousand years."

"Yes," Miriam said. "Both forms need to migrate. And that's how, I think, they came to cooperate..."

She sketched a hasty narrative of the CHON sponges emerging from the crater lakes, and finding their way to the summer pole. Maybe they got there by following deep creva.s.ses, smashed into t.i.tan's ice crust by the impacts that dug out crater lakes like this one in the first place. Down there they would find liquid water, kilometers deep and close to the ammonia ocean. It would be cold, briny, not to terrestrial tastes, but it would be liquid, and survivable. And at the pole they would find the silane lilies floating on their ethane seas. The lilies in turn needed to migrate to the winter pole, where their precious life-stuff ethane was raining out.

Miriam mimed, her fist touching her flattened palm. "So they come together, the sponges and the lilies "

"To make the t.i.tan birds," I said.

"That's the idea. They come flapping up out of the lake, just as we saw, heading for the winter pole. And meanwhile, maybe the sponges get dropped off at fresh crater lakes along the way. It's a true symbiosis, with two entirely different spheres of life intersecting and cooperating, for without the migration neither form could survive alone." She looked at us, suddenly doubtful. "We're all amateurs here. I guess any competent biologist could pick holes in this the size of the center of Saturn's rings."

Dzik said, "No competent biologist would even be hypothesising this way, not with so few facts."

"No," Virtual Harry said tinnily. "But at least you've come up with a plausible model, Miriam. And all without the need to evoke even a sc.r.a.p of sentience. Good job."

"There are still questions," Miriam said. "Maybe the sponges provide the birds' intelligence, or at least some kind of directionality. But what about power? The lilies especially are a pretty low-energy kind of life form . . ."

Michael Poole said, "Maybe I can answer that. I've been doing some a.n.a.lysis of my own. I can tell you a bit more about the silane lilies' energy source. Believe it or not even on a world as murky as this I think they're photosynthesising." And he ran through the chemistry he thought he had identified, using entirely different compounds and molecular processing pathways from the chlorophyll-based green-plant photosynthesis of Earth life.

"Of course," Miriam said. "I should have seen it. I never even asked myself what the lilies were doing were doing while they were lying around on the lake's surface . . . Trapping sunlight!" while they were lying around on the lake's surface . . . Trapping sunlight!"

Harry was growing excited too. "Hey, if you're right, son, you may already have paid for the trip. Silane-based low-temp photosynthesisers would be hugely commercially valuable. Think of it, you could grow them out of those nitrogen lakes on Triton and go scudding around the outer System on living sails." His grin was wide, even in the reduced Virtual image.

Poole and Miriam were smiling too, staring at each other with a glow of connection. Theirs was a strange kind of symbiosis, like silane lily and CHON sponge; they seemed to need the excitement of external discovery and achievement to bring them together.

Well, there was a happy mood in that grounded gondola, the happiest since we had crashed. Even Bill Dzik as he showered was making grunting, hog-like noises of contentment.

And just at that moment there was a crunching sound, like great jaws closing over metal, and the whole bus tipped to one side.

Poole and Miriam staggered and started shouting instructions to each other. I had my helmet over my head in a heartbeat.

Then there was another crunch, a ripping sound-and a scream, gurgling and suddenly cut off, and an inward rush of cold air that I felt even through my exosuit. I turned and saw that near the shower part.i.tion, a hole had been ripped in the side of the gondola's flimsy hull, revealing t.i.tan's crimson murk. Something like a claw, or a huge version of Miriam's manipulator arm, was working at the hull, widening the breach.

And Bill Dzik, naked, not meters from the exosuit that could have saved him, was already frozen to death.

That was enough for me. I flung open the hatch in the gondola roof and lunged out, not waiting for Miriam or Poole. I hit the t.i.tan sand and ran as best I could, the exosuit laboring to help me. I could hear crunching and chewing behind me. I did not look back.

When I had gone 100 meters I stopped, winded, and turned. Poole and Miriam were following me. I was relieved that at least I was not stranded on t.i.tan alone.

And I saw what was becoming of our gondola. The machines that had a.s.sailed it and they were machines, I had no doubt of it-were like spiders of ice, with lenticular bodies perhaps ten meters long, and each equipped with three grabber claws attached to delicate low-gravity limbs. Four or five of these things were laboring at the wreck of our gondola. I saw that they had gone for the wheels first, which was why we had tipped over, and now were making a fast job of ripping the structure apart. Not only that, beyond them I saw a line of similar-looking beasts carrying silvery fragments that could only be pieces of the gondola off up the rising ground towards the summit of the cryovolcano. Some of the larger components of the wreck they left intact, such as the GUT engine module, but they carried them away just as determinedly.

In minutes, I saw, there would be little left of our gondola on the ice surface-not much aside from Bill Dzik, who, naked, sprawled and staring with frozen eyeb.a.l.l.s, made an ugly corpse, but did not deserve the fate that had befallen him.

Harry Poole's head popped into Virtual existence before us. "Well," he said, "that complicates things."

Michael swatted at him, dispersing pixels like flies.

X Spiders

"Dzik is dead," I said. "And so are we." I turned on Michael Poole, fists bunched in the thick gloves. "You and your absurd ambition-it was always going to kill you one day, and now it's killed us all."

Michael Poole snorted his contempt. "And I wish I'd just thrown you into a jail back on Earth and left you to rot."

"Oh, Lethe," Miriam said with disgust. She was sifting through the scattered debris the spiders had left behind. "Do you two have any idea how ridiculous you look in those suits? Like two soft toys facing off. Anyhow you aren't dead yet, Jovik." She picked up bits of rubbish, rope, a few instruments, some of her precious sample flasks, enigmatic egg-shaped devices small enough to fit in her fist and food packs.

Michael Poole's curiosity snagged him. "They didn't take everything."

"Evidently not. In fact, as you'd have noticed if you weren't too busy trading insults with your pa.s.senger, they didn't take us us. Or Bill."

"What, then?"

"Metal. I think. Anything that has a significant metal component is being hauled away."

"Ah." Poole watched the spiders toiling up their volcano, bits of our ship clutched in their huge claws. "That makes a sort of sense. One thing this moon is short of is metal. Has been since its formation. Even the core is mostly light silicate rock, more like Earth's mantle than its iron core. Which maybe explains why every surface probe to t.i.tan across 1600 years has disappeared without a trace-even the traces of your illegal sample-collectors, Emry. They were taken for the metal."And," he said, chasing the new idea, "maybe that's what we saw in the radar images of the deeps of the crater lake. Something toiling on the floor, you remember, as if quarrying? Maybe it was more of those spider things after the metallic content of the meteorite that dug out the crater in the first place."

"Well, in any event they left useful stuff behind," said Miriam, picking through the debris. "Anything ceramic, gla.s.s fiber, plastic. And the food packs. We won't starve, at least."

Poole had homed in on theory, while she focused on the essentials that might keep us alive. That tells you everything about the man's lofty nature.

"But they took the GUT engine, didn't they?" I put in sharply. "Our power source. Without which we'll eventually freeze to death, no matter how well fed we are."

"And, incidentally," Miriam said, "the ident.i.ty-backup deck. We cached the backups in the GUT engine's own control and processing unit, the most reliable store on the gondola. If we lose that, we lose the last trace of poor Bill too."

I couldn't help but glance at Dzik's corpse, fast-frozen on the ice of t.i.tan.

Not Poole, though. He was watching those receding spiders. "They're heading down into the volcano. Which is a vent that leads down into the mantle, the ammonia sea, right? Why? What the h.e.l.l are those things?"

Miriam said, "One way to find out." She hefted one of those ceramic eggs in her right hand, pressed a stud that made it glow red, and hurled it towards the nearest spider. It followed a low-gravity arc, heavily damped in the thick air, and it seemed to take an age to fall. But her aim was good, and it landed not a meter from the spider.

And exploded. Evidently it had been a grenade. The spider shattered satisfactorily, those ugly claws going wheeling through the air.

Miriam had already started to run towards the spider. You couldn't fault her directness. "Come on."

Poole followed, and I too, unwilling to be left alone with Bill's frozen remains. Poole called, "What did you do that for?"

"We want to know what we're dealing with, don't we?"

"And why are we running?"

"So we can get there before the other spiders get rid of it."

And sure enough the other spiders, still laden with bits of the gondola, had already turned, and were closing on their shattered fellow. They didn't seem perturbed by the sudden destruction of one of their kind, or of our approaching presence. They seemed to perceive only what was essential to them only what was metallic.

We got there first, and we squatted around the downed spider in a splash of suit light. The spider hadn't broken open; it was not enclosed by a hull or external carapace. Instead it had shattered into pieces, like a smashed sculpture. We pawed at the debris chunks, Miriam and Poole talking fast, a.n.a.lyzing, speculating. The chunks appeared to be mostly water ice, though Poole speculated it was a particular high-pressure form. The internal structure was not simple; it reminded me of a honeycomb, sharp-edged chambers whose walls enclosed smaller cl.u.s.ters of chambers and voids, on down through the length scales like a fractal. Poole pointed out threads of silver and a coppery color the shades were uncertain in t.i.tan's light. They were clearly metallic.

"So the spiders at least need metal," Miriam said. "I wonder what the power source is."

But we weren't to find out, for the other spiders had closed in and we didn't want to get chomped by accident. We backed off, dimming our suit lights.

Miriam asked, "So, biological or artificial? What do you think?"

Poole shrugged. "They seem dedicated to a single purpose, and have these metallic components. That suggests artificial. But that body interior looks organic. Grown."

I felt like putting Poole in his place. "Maybe these creatures transcend your simple-minded categories. Perhaps they are the result of a million years of machine evolution. Or the result of a long symbiosis between animal and technology."

Poole shook his head. "My money's on biology. Given enough time, necessity and selection can achieve some remarkable things."

Miriam said, "But why would their systems incorporate metal if it's so rare here?"

"Maybe they're not native to t.i.tan," I said. "Maybe they didn't evolve here." But neither of them were listening to me. "The real question is," I said more urgently, "what do we do now?"