The head of Harry Poole, projected somehow by our suit's comms systems, popped into existence, the size of an orange, floating in the air. The small scale made his skin look even more unnaturally smooth. "And that," he said, "is the first intelligent question you've asked since we pressganged you, Jovik. You ready to talk to me now?"

Michael Poole glared at his father, then turned and sucked water from the spigot inside his helmet. "Tell us how bad it is, Harry."

"I can't retrieve you for seven days," Harry said.

I felt colder than t.i.tan. "But the suits "

"Without recharge our suits will expire in three days," Poole said. "Four at the most."

I could think of nothing to say.

Harry looked around at us, his disembodied head spinning eerily. "There are options."

"Go on," Poole said.

"You could immerse yourselves in the crater lake. The suits could withstand that. It's cold in there, the briny stuff is well below freezing, but it's not as cold as the open air. Kept warm by the residual heat of impact, remember. Even so you would only stretch out your time by a day or two."

"Not enough," Miriam said. "And we wouldn't get any work done, floating around in the dark in a lake."

I laughed at her. "Work? Who cares about work now?"

Poole said, "What else, Harry?"

"I considered options where two people might survive, rather than three. Or one. By sharing suits."

The tension between us rose immediately.

Harry said, "Of course those spiders also left you Bill's suit. The trouble is the power store is built into the fabric of each suit. To benefit you'd have to swap suits. I can't think of any way you could do that without the shelter of the gondola; you'd freeze to death in a second."

"So it's not an option," Poole said.

Miriam looked at us both steadily. "It never was."

I wasn't sure if I was relieved or not, for I had been determined, in those few moments when it seemed a possibility, that the last survivor in the last suit would be myself.

"So," Poole said to Harry, "what else?"

"You need the gondola's GUT engine to recharge your suits," Harry said. "There's just no alternative."

I pointed at the toiling spiders on the cryovolcano. "Those beasts have already thrown it into that caldera."

"Then you'll have to go after it," Harry said, and, comfortably tucked up in the Crab Crab, he grinned at me. "Won't you?"

"How?" I was genuinely bewildered. "Are we going to build a submarine?"

"You won't need one," Harry said. "You have your suits. Just jump in .. ."

"Are you insane? You want us to jump into the caldera of a volcano, after a bunch of metal-chewing monster spiders?"

But Miriam and Poole, as was their way, had pounced on the new idea. Miriam said, "Jovik, you keep forgetting you're not on Earth. That 'volcano' is just spewing water, lava that's colder than your own bloodstream." She glanced at Harry. "The water's very ammonia-rich, however. I take it our suits can stand it?"

"They're designed for contact with the mantle material," Harry said. "We always knew that was likely. The pressure shouldn't be a problem either."

Poole said, "As for the spiders, they will surely leave us alone if we keep away from them. We know that. We might even use them in the descent. Follow the spiders, find the engine. Right?"

Harry said, "And there's science to be done." He displayed data in gleaming Virtual displays-cold summaries only meters away from Bill Dzik's corpse. Harry said that his preliminary a.n.a.lysis of our results showed that the primary source of the atmosphere's crucial methane was nothing in the air or the surface features, but a venting from the cryovolcanoes. "And therefore the ultimate source is somewhere in the ammonia sea," Harry said. "Biological, geological, whatever it's down there."

"OK," Poole said. "So we're not going to complete the picture unless we go take a look."

"You won't be out of touch. I'll be able to track you, and talk to you all the way in. Our comms link has a neutrino-transmission basis; a few kilometers of ice or water isn't going to make any difference to that."

A few kilometers few kilometers? I didn't like the sound of that.

"So that's that," Miriam said. "We have a plan."

"You have a shared delusion," I said.

They ignored me. Poole said, "I suggest we take an hour out. We can afford that. We should try to rest; we've been through a lot. And we need to sort through these supplies, figure out what we can use."

"Yeah," said Miriam. "For instance, how about nets of ice as ballast?"

So he and Miriam got down to work, sorting through the junk discarded by the spiders, knotting together cable to make nets. They were never happier than when busy on some task together.

And there was Bill Dzik, lying on his back, stark naked, frozen eyes staring into the murky sky. I think it tells you a lot about Michael Poole and even Miriam that they were so focussed on their latest goal that they had no time to consider the remains of this man whom they had worked with, apparently, for decades.

Well, I had despised the man, and he despised me, but something in me cringed at the thought of leaving him like that. I looked around for something I could use as a shovel. I found a strut and a ceramic panel from some internal part.i.tion in the gondola, and used cable to join them together.

Then I dug into the soil of t.i.tan. The blade went in easily; the icy sand grains didn't cling together. As a native of Earth's higher gravity I was over-powered for t.i.tan, and lifted great shovelfuls easily. But a half-meter or so down I found the sand was tighter packed and harder to penetrate, no doubt some artefact of t.i.tan's complicated geology. I couldn't dig a grave deep enough for Bill Dzik. So I contented myself with laying him in my shallow ditch, and building a mound over him. Before I covered his face I tried to close his eyes, but of course the lids were frozen in place.

All the time I was working I clung to my anger at Michael Poole, for it was better than the fear.

XI Volcano

So we climbed the flank of the cryovolcano, paralleling the trail followed by the ice spiders, who continued to toil up there hauling the last useful fragments of our gondola. We were laden too with our improvised gear-rope cradles, bags of ice-rock chunks for ballast, food packs. Miriam even wore a pack containing the pick of her precious science samples.

It wasn't a difficult hike. When we had risen above the sand drifts we walked on bare rock-ice, a rough surface that gave good footing under the ridges of our boots. I had imagined we'd slip walking up a bare ice slope, but at such temperatures the ice under your feet won't melt through the pressure of your weight, as on Earth, and it's that slick of melt.w.a.ter that eliminates the friction.

But despite the easy climb, as we neared the caldera my legs felt heavy. I had no choice but to go on, to plunge into ever greater danger, as I'd had no real choice since being pressganged in the first place.

At last we stood at the lip of the caldera. We looked down over a crudely carved bowl perhaps half a kilometer across, water-ice rock laced with some brownish organic muck. Most of the bowl's floor was solid, evidently the cryovolcano was all but dormant, but there was a wide creva.s.se down which the spiders toiled into darkness. The spiders, laden as they were, clambered nimbly down the sides of this creva.s.se, and Poole pointed out how they climbed back up the far side, unladen. If you listened carefully you could hear a crunching sound, from deep within the creva.s.se.

This was what we were going to descend into.

"Don't even think about it," Miriam murmured to me. "Just do it."

But first we needed a tame spider.

We climbed a few paces down the flank, and stood alongside the toiling line. Miriam actually tried to la.s.so a spider as it crawled past us. This was a bit over-ambitious, as the thick air and low gravity gave her length of cable a life of its own. So she and Poole worked out another way. With a bit of dexterity they managed to snag cable loops around a few of the spider's limbs, and Poole threw cable back and forth under the beast's belly and over its back and tied it off, to make a kind of loose net around the spider's body. The spider didn't even notice these activities, it seemed, but continued its steady plod.

"That will do," Poole said. "All aboard!" Grasping his own burden of pack and ballast nets he made a slow-motion leap, grabbed the improvised netting, and set himself on the back of the spider. Miriam and I hurried to follow him.

So there we were, the three of us sitting on the back of the beast with our hands wrapped in lengths of cable. The first few minutes of the ride weren't so bad, though the spider's motion was jolting and ungainly, and you always had the unpleasant awareness that there was no conscious mind directing this thing.

But then the lip of the caldera came on us, remarkably quickly. I wrapped my hands and arms tighter in the netting.

"Here we go!" Michael Poole cried, and he actually whooped as the spider tipped head first over the lip of the creva.s.se, and began to climb down its dead vertical wall. I could not see how it was clinging to the sheer wall-perhaps with suckers, or perhaps its delicate limbs found footholds. But my concern was for myself, for as the spider tipped forward we three fell head over heels, clinging to the net, until we were hanging upside down.

"Climb up!" Poole called. "It will be easier if we can settle near the back end."

It was good advice, but easier said than done, for to climb I had to loosen my grip on the cable to which I was clinging. I was the last to reach the a.r.s.e end of the descending spider, and find a bit of respite in a surface I could lie on.

And all the while the dark of the chasm closed around us, and that dreadful crunching, chewing noise from below grew louder. I looked up to see the opening of this chimney as a ragged gash of crimson-brown, the only natural light; it barely cast a glow on the toiling body of the spider. Impulsively I ordered my suit to turn on its lights, and we were flooded with glare.

Poole asked, "Everybody OK?"

"Winded," Miriam said. "And I'm glad I took my claustrophobia pills before getting into the gondola. Look. What's that ahead?"

We all peered down. It was a slab of ice that appeared to span the creva.s.se. For an instant I wondered if this was as deep as we would have to go to find our GUT engine. But there was no sign of toiling spiders here, or of the pieces of our gondola, and I feared I knew what was coming next. That sound of crunching grew louder and louder, with a rhythm of its own.

"Brace yourselves," Poole said-pointless advice.

Our spider hit the ice floor. It turned out to be a thin crust, easily broken that was the crunching we had heard, as spider after spider smashed through this interface. Beyond the broken crust I caught one glimpse of black, frothy water before I was dragged down into it, head first.

Immersed, I was no colder, but I could feel a sticky thickness all around me, as if I had been dropped into a vat of syrup. My suit lamps picked out enigmatic flecks and threads that filled the fluid around me. When I looked back, I saw the roof of this vent already freezing over, before it was broken by the plunging form of another spider, following ours.

Michael Poole was laughing. "Dunked in molten lava, t.i.tan style. What a ride!"

I moaned, "How much longer? How deep will we go?"

"As deep as we need to. Have patience. But you should cut your lights, Emry. Save your power for heating."

"No, wait." Miriam was pointing at the ice wall that swept past us. "Look there. And there!"

And I made out tubular forms, maybe half a meter long or less, that clung to the walls, or, it seemed, made their purposeful way across it. It was difficult to see any detail, for they quickly shot up and out of our field of view.

"Life?" Poole said, boyishly excited once more.

Miriam said, "It looks like it, doesn't it?" Without warning us she loosened one hand from the net, and grabbed at one of the tubes and dragged it away from its hold on the wall. It wriggled in her hand, pale and sightless, a fat worm; its front end, open like a mouth, was torn.

"Ugh," I said. "Throw it back!"

But Miriam was cradling the thing. "Oh, I'm sorry. I hurt you, didn't I?"

Poole bent over it. "Alive, then."

"Oh, yes. And if it's surviving in this ammonia lava, I wouldn't mind betting it's a cousin of whatever's down below in the sea. More life, Michael!"

"Look, I think it's been browsing on the ice. They are cl.u.s.tered pretty thickly over the walls." And when I looked, I saw he was right; there the tube-fish were, browsing away, working their way slowly up the vent. "Maybe they actively keep the vent open, you think?"

Poole took a small science box from Miriam's pack, and there, together, even as we rode that alien back down into the throat of the volcano, they briskly a.n.a.lyzed the beast's metabolism, and the contents of the water we were immersed in, and sent the results back to the Hermit Crab. Crab. Harry's Virtual head popped up before us, grinning inanely, even in that extreme situation. Harry's Virtual head popped up before us, grinning inanely, even in that extreme situation.

I had seen enough. With a snap, I made my suit turn its lights off. I had no desire to sit shivering in the dark as invisible ice walls plummeted past me. But I was gambling that curiosity would get the better of Poole and Miriam, and I was right; soon it was Poole whose suit glowed, spending his own precious power to light me up, as they labored over their pointless science.

"So I was right," Miriam breathed at last. "This vent, and the mantle ocean, host a whole other life domain-a third on t.i.tan, in addition to the silanes and the CHON sponges. Ammono life..." on t.i.tan, in addition to the silanes and the CHON sponges. Ammono life..."

t.i.tan's liquid mantle is thought to be a relic of its formation, in a part of the solar nebula where ammonia was common. t.i.tan was born with a rocky core and a deep open ocean, of water laced with ammonia. The ocean might have persisted for a billion years, warmed by greenhouse effects under a thick primordial atmosphere. A billion years is plenty of time for life to evolve. With time the ocean surface froze over to form an icy crust, and at the ocean's base complex high-pressure forms of ice formed a deep solid layer enclosing the silicate core. Ice above and below, but still the liquid ocean persisted between, ammonia-rich water, very alkaline, very viscous. And in that deep ocean a unique kind of life adapted to its strange environment, based on chemical bonds between carbon and nitrogen-hydrogen chemical groups rather than carbon-oxygen, using ammonia as its solute rather than water: "ammono life," the specialists call it.

"Yes, a third domain," Miriam said. "One unknown elsewhere in Sol System so far as I know. So here on t.i.tan you have a junction of three entirely different domains of life: native ammono life in the mantle ocean, CHON life in the crater lakes blown in from the inner system, and the silane lilies wafting in from Triton and the outer cold. Incredible."

"More than that," Harry said tinnily. "Michael, that tube-fish of yours is not a methanogen it doesn't create methane-but it's full of it. Methane is integral to its metabolism, as far as I can see from the results you sent me. It even has methane in its flotation bladders."

Miriam looked at the tube-fish blindly chewing at the ice walls. "Right. They collect it somehow, from some source deep in the ocean. They use it to float up here. They even nibble the cryovolcano vent walls, to keep them open. They have to be integral to delivering the methane from the deep ocean sources to the atmosphere. So you have the three domains not just sharing this moon but cooperating in sustaining its ecology."

Harry said, "Quite a vision. And as long as they're all stupid enough, we might make some money out of this d.a.m.n system yet."

Miriam let go of her tube-fish, like freeing a bird; it wriggled off into the dark. "You always were a realist, Harry."

I thought I saw blackness below us, in the outer glimmer of Poole's suit lamps. "Harry. How deep is this ice crust, before we get to the mantle ocean?"

"Around 35 kilometers."

"And how deep are we now? Can you tell?"

"Oh, around 35 kilometers."

Michael Poole gasped. "Lethe. Grab hold, everybody."

It was on us almost at once: the base of the vent we had followed all the way down from the cryovolcano mouth at the surface, a pa.s.sage right through the ice crust of t.i.tan. I gripped the net and shut my eyes.

As we pa.s.sed out of the vent, through the roof of ice and into the mantle beneath, I felt the walls recede from me, a wash of pressure, a vast opening-out.

And we fell into the dark and cold.

XII Ocean

Now that the walls were gone from under its limbs I could feel that the spider was swimming, or perhaps somehow jetting, ever deeper into that gloopy sea, while the three of us held on for our lives. Looking up I saw the base of t.i.tan's solid crust, an ice roof that covered the whole world, glowing in the light of Poole's lamps but already receding. And I thought I saw the vent from which we had emerged, a much eroded funnel around which tube-fish swam languidly. Away from the walls I could more easily see the mechanics of how they swam; lacking fins or tails, they seemed to twist through the water, a motion maybe suited to the viscosity of the medium. They looked more like vast bacteria than fish.

Soon we were so far beneath the ice roof that it was invisible, and we three and the crab that dragged us down were a single point of light falling into the dark. And Poole turned off his suit lamps!

I whimpered, "Lethe, Poole, spare us."