Stephen Baxter - Xeelee - Endurance

And I missed the next great shock suffered by our dysfunctional little crew when the base of Titan’s underground ocean, an ice floor three hundred kilometres beneath the surface, at last hove into view. The strange landscape of this abyssal deep, made of folded high-pressure ices littered by bits of meteorite rock, was punctured by vents and chasms, like an inverted mirror image of the crust far above us. And the spider we rode did not slow down. It hurled itself into one of those vents, and once more its limbs began to clatter down a wall of smooth rock-ice.

Harry warned Miriam and Poole that this latest vent looked as if it penetrated the whole of this inner layer of core-cladding ice – Ice VI, laced by ammonia dihydrate – a layer another five hundred kilometres deep. At the base of this vent there was only Titan’s core of silicate rocks, and there, surely, the spiders’ final destination must lie.

There was nothing to be done but to endure this extension of the ride. It would take perhaps a further day. So Poole and Miriam allowed the spider to drag us down. More tube-fish, of an exotic high-pressure variety, grazed endlessly at the icy walls. Miriam popped me another vial to keep me asleep, and fed me intravenous fluids. Harry fretted about the exhaustion of our power, and the gradual increase of pressure; beneath a column of water and ice hundreds of kilometres deep, we were approaching our suits’ manufactured tolerance. But they had no choice but to continue, and I, unconscious, had no say in the matter.

When the ride was over, when the spider had at last come to rest, Miriam woke me up.

I was lying on my back on a lumpy floor. The gravity felt even weaker than it had on the surface. Miriam’s face hovered over me, illuminated by suit lamps. Smiling, she said, ‘Jovik. Look what we found.’

I sat up. I felt weak, dizzy, hungry. Beside me, in their suits, Miriam and Poole sat watching my reaction. Then I remembered where I was and the fear cut in.

I looked around quickly. Even by the glow of the suit lamps I could not see far. The murkiness and floating particles told me I must be still immersed in the water of Titan’s deep ocean. I saw a roof of ice above me – not far above, a hundred metres or so. Below me was a surface of what looked like rock, dark and purple-streaked. I was in a sort of ice cavern, then, whose walls were off in the dark beyond our bubble of light. I learned later that I was in a cavern dug out beneath the lower icy mantle of Titan, between it and the rocky core, eight hundred kilometres below the icy plains where I had crash-landed days before. Around us I saw ice spiders, toiling away at their own enigmatic tasks, and bits of equipment from the gondola, chopped up, carried here and deposited. There was the GUTengine! My heart leapt; perhaps I would yet live through this.

But even the engine wasn’t what Miriam was smiling about. She repeated, ‘Look what we found.’

I looked.

Set in the floor, in this rocky core of a world, was a hatch.

They allowed me to eat and drink, and void my bladder. Moving around was difficult, the cold water dense and syrupy; every movement I made was accompanied by the whir of servomotors as the suit laboured to assist me. I was reassured to know that the GUTengine was still functioning, and that my suit cells had been recharged. In principle I could stay alive long enough to get back to the Hermit Crab. All I had to do was find my way out of the core of this world, up through eight hundred kilometres of ice and ocean . . . I clung to the relief of the moment, and put off my fears over what was to come next.

Now that I was awake, Michael Poole, Miriam Berg, and Virtual Harry rehearsed what they had figured out about methane processing on Titan. Under that roof of ice, immersed in that chill high-pressure ocean, they talked about comets and chemistry, and all the while the huge mystery of the hatch in the ground lay between us, unaddressed.

Harry said, ‘On Earth, ninety-five per cent of the methane in the air is of biological origin. The farts of animals, the rot of vegetation. So could the source be biological here? You guys have surveyed enough of the environment to rule that out. There could in principle be methanogen bugs living in those ethane lakes, for instance, feeding off reactions between acetylene and hydrogen, but you found nothing significant. What about a delivery of the methane by infalling comets? It’s possible, but then you’d have detected other trace cometary gases, which are absent from the air. Only one plausible possibility remains . . .’

When Titan was young, still warm from its birth and before it froze, its ammonia-water ocean had extended all the way to the rocky core. There, chemical processes could have produced plentiful methane: the alkaline water reacting with the rock would liberate hydrogen, which in turn would react with sources of carbon, monoxide or dioxide or carbon grains, to manufacture methane. But that process would have been stopped as soon as the ice layers plated over the rock core, insulating it from liquid water. What was needed, then, was some way for chambers to be kept open at the base of the ice, where liquid water and rock could still react at their interface. And as for a way for the methane produced in the depths to be brought up through the ice to the ocean, and then released in the atmosphere . . .

‘The tube-fish,’ I said.

‘And their relatives, yes.’

Looking up at the ice ceiling above me, I saw how it had been shaped and scraped, as if by lobster claws. ‘So the spiders keep these chambers open, to allow the methane-creating reactions to continue. And the tube-fish carry the methane to the ocean, through vents they in turn help keep open.’

‘That’s it,’ Michael Poole said, wonder in his voice. ‘They do it to keep a supply of methane pumping up into the atmosphere. And they’ve been doing it for billions of years. Have to have been, for the ecologies up there to have evolved as they have – the tube-fish, the CHON sponges, the silanes. This whole world is an engine , a very old engine. It’s an engine for creating methane, for turning what would otherwise be just another nondescript ice moon into a haven, whose purpose is to foster the life forms that inhabit it.’

‘Ha!’ I barked laughter. ‘So all this is technology. Therefore the spiders are clearly sentient – or their makers are, or were. You don’t even need this hatch in the ground to make that case. You have found precisely what you were afraid of , haven’t you, Michael Poole? Sentience at the heart of Titan. You will never be allowed to open it up for exploitation now. So much for your commercial ambitions!’

‘Which you were going to share in,’ Harry reminded me, scowling.

I sneered. ‘Oh, I’d only have wasted the money on drugs and sex. To see you world-builders crestfallen is worth that loss. So,’ I said. ‘To the final mystery. What’s under the hatch?’

They glanced at each other. ‘The final answers, we hope,’ Michael Poole said.

Miriam said, ‘We’ve no idea what’s under there. We’ve put off looking until we brought you round, Jovik.’

Poole said, ‘We need everybody awake, ready to react. We might even need your help.’ He looked at me with faint disgust. ‘And,’ he said more practically, ‘it’s probably going to take three of us to open it. Come see.’

We all floated through the gloopy murk.

The hatch was a disc of some silvery metal, perhaps three metres across, set flush in the rocky ground. Spaced around its circumference were three identical grooves, each maybe ten centimetres deep. In the middle of each groove was a mechanism like a pair of levers, hinged at the top.