‘But it is those organic traces that made Titan so interesting,’ the ColU said. ‘On a land of ice rock, where volcanoes belch ammonia-rich water, a rain of methane falls, carving river valleys and filling seas. And in those seas—’

‘The probes found life. I remember the reports. Some kind of slow-moving bugs in the methane lakes.’

‘Yes, life based – not on carbon, as ours is – but on silicon. Just as carbon-carbon bonds, the backbone of your chemistry, Stef Kalinski, can be made and broken in room-temperature water, so silicon-silicon bonds can be made in the cold methane of Titan’s lakes. A form of life not so very unlike ours superficially, but with a different biochemistry entirely – and very slow-moving, low in energy, slow to reproduce and evolve. We found nothing but simple bugs on Titan, simpler than most bacteria – not much more complicated than viruses.

‘But Titan is not the only cold world. Here at Proxima, while the Earth-like Per Ardua was the planet that caught all the attention—’

‘Ah. Proxima d.’

‘Yes. That was a Mars-sized world just outside the zone that would have made it habitable for humans, like Per Ardua.’

‘So far as I know it was never even given a decent name. Nobody cared about it – or the other Proxima worlds.’

‘They did not. But it was very like Titan – another common template for a world, it seems. And room for another kind of life.

‘Stef Kalinski, Earthshine has spoken of a panspermia bubble, of worlds like Earth and Per Ardua linked by a common chemistry carried by rocks between the stars, worlds with cousin life forms. But there could be other bubbles, worlds with different kinds of climate, different kinds of biochemistries, yet linked in the same way. Maybe one bubble could even overlap another, you see – for clearly a stellar system may contain more than one kind of world.’

Stef was starting to understand. ‘You always speak in riddles, ColU, whether you intend to or not. But I think I see. The sample I brought you—’

‘The pyramid-beast over there has a silicon-based biochemistry very similar to that recorded on Titan, but not identical. Maybe it is a visitor from Proxima d, do you think? Somehow hardened to withstand what must be for it a ferocious heat, even here on Per Ardua’s dark side. As if a human had landed on Venus. But it is here, and surviving. And with more time still …’

‘Yes, ColU?’

‘Stef Kalinski, we have seen that, given billions of years, life forms from across the same panspermia bubble can integrate, grow together.’

‘The Earth ants in the Arduan stromatolite.’

‘Exactly. Now, is it possible that given tens of billions, hundreds of billions of years, even different kinds of life could mix and merge? Your fast, quick kind, and the slow-moving Titanian over there? Could that be the next stage in the evolution of the cosmos itself? You already share a world, you see.’

‘It’s a fantastic thought,’ she said slowly. ‘But it’s never going to happen. Is it, ColU? Because this is the End Time, according to you. There will be no tens or hundreds of billions of years—’

‘I’m afraid not, Colonel Kalinski. Here on the dark side I have been able to make quite precise assays of the sky: the state of the stars, the proximity of Andromeda – even the background glow of the universe as a whole, which contains warp-bubble clues to its future.’

‘Hmm.’ She looked up into the dark. ‘Well, it is marvellous to see, for an astronomer. And you’ve come to a conclusion, have you?’

‘I have. And a precise estimate of the time remaining.’

Stef felt chilled, as if she’d been given bad news by a doctor. ‘You’re going to have to explain all this to the others, you know. In language they can understand.’

‘Yes, Colonel Kalinski. Of course. And the importance of finding Earthshine soon, by the way, is only increased.’

Stef could hear the others returning, father and daughter laughing, full of life and energy. And she looked across at the silicon-life explorer from Proxima d, the ice giant. ‘I wonder if that thing can see us … Just tell me,’ she said. ‘How long have we got?’

‘A year,’ the ColU said flatly. ‘No more. The data’s still chancy.’

And Stef immediately thought of Mardina, and the baby.

She pursed her lips and nodded. ‘A year, then. For now, not a word. Come on, let’s get ready to go on.’

CHAPTER 67

The party gradually penetrated deeper into the cold of the Per Ardua farside.

The forty-minute tunnel hops all felt much the same to Beth, but in the short intervals during which they trekked from one tunnel exit to the entrance of the next, always following trails carefully scouted out by Titus with Clodia or Chu, Beth did get glimpses of parts of her world she had never seen before. After all, during the years she’d spent growing to a young adult on Per Ardua, she had never gone further than the tall forests that screened the terminator zone.

Stef and the ColU had made such a journey as this once before, with Beth’s father and Liu Tao, in a purloined ISF rover. That party had followed a more or less direct course to the antistellar, cutting over the ice surface of a frozen ocean. The gravity tunnels, however, naturally enough, stuck to continental land, detouring around the shores of frozen oceans. As a result the journey was longer than a direct route, and was taking longer than the handful of days that Titus and the ColU had first estimated – but still it would be brief enough.

And while Stef in that earlier party had spent mind-numbing days crossing geometrically perfect ice plains, now Beth saw more interesting features. Eroded mountain ranges from which glaciers spilled like huge, dirty tongues. Places where earthquakes or other geological upheavals had raised and cracked the ice cover, creating frozen cliffs that gleamed a deep blue in the light of their torches.

Yet even these features were probably impressively old, the ColU said. There would always be a lot of weather activity at the terminator, where the warm air and water from the day side spilled into the cold of the night. But here in the dark, weather would always be desperately rare: no clouds, no fresh falls of snow or hail. Even meteor impacts would be infrequent in such an elderly system as this, with much of the primordial debris left over from the planets’ formation long since swept up. So they drove across a sculpted but static landscape – and a landscape bathed in the complex, red-tinged light of an ageing Andromeda.

Sometimes they saw more ‘Titanians’, enigmatic, sharp-edged pyramids standing like mute monuments. But the ColU assured them that the Titanians, in their way, on their own timescale, could be exploring just as vigorously as the humans.

Beth noticed, however, that Stef barely glanced at the sky, or the icebound landscape, or even the ‘Titanians’. As they travelled, and in the ‘evenings’ as they rested, Stef sat huddled with the ColU at the back of their sled-cart, or in a corner of their shelter, talking softly, Stef making occasional notes on the glowing face of her slate. Everybody knew what they were discussing: the ColU’s ideas about the fate of the world. Beth tried to read Stef’s expression. There was nothing to be discerned from the ColU’s neutral tone.

At last, one evening, after they had cleared away their meal, with them all bundled in their warmest clothes, their feet swathed in layers of socks, gathered around the warmth of the kernel stove, Stef announced that they needed to talk about the End Time.

‘In a way,’ Stef began cautiously, ‘the idea that the world will have an end – that the universe itself will end, and relatively soon – ought to feel natural to us.

‘We have no direct experience of infinity, of eternity. Our own lives are short. And the scientists in my culture proved quite definitively that eternity doesn’t lie behind us, that our universe had a beginning, a birth in a cataclysmic outpouring of energy. Why, then, should we imagine that eternity lies ahead of us, an unending arena for life and mind?’