Free Xeelee: Endurance by Stephen Baxter

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 clustered pretty thickly over the walls.’
    And when I looked, I saw he was right; there the tube-fish were, nibbling away, working their way slowly up the vent.
    Poole speculated, ‘Maybe they actively keep the vent open?’ He took a small science box from Miriam’s pack, and there, together – even as we rode that alien’s back down into the throat of the volcano – they briskly analysed the beast’s metabolism, and the contents of the water we were immersed in, and sent the results back to the Hermit Crab. Even Harry’s Virtual head popped up before us, grinning inanely, 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 laboured over their pointless science.
    At length they came to some conclusion. ‘So I was right,’ Miriam breathed at last. ‘This vent, and the mantle ocean, host a whole other domain – a third on Titan, in addition to the silanes and the CHON sponges. Ammono life . . .’
     
    The moon’s liquid mantle is thought to be a relic of its formation, in a part of the solar nebula where ammonia was common.
    Titan was born with a rocky core and a deep ocean, of water laced with ammonia. The ocean might have stayed open for a billion years, warmed by greenhouse effects under a thick primordial atmosphere. A billion years is plenty of time for life to evolve. Eventually 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 had emerged 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 solvent rather than water: ‘ammono life’, the specialists call it.
    ‘Yes, a third kind of life,’ Miriam said. ‘One unknown elsewhere in the Solar System so far as I know. So here on Titan 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, up through the crevices in the ice cap and 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 damn system yet.’
    Miriam let go of her tube-fish, like freeing a bird; it wriggled off into the