Stef and the ColU both kept a careful watch on the temperature outside; it was dropping, of course, but not dramatically quickly. Under thicker cloud it could even rise above freezing. ‘Thus proving the theory,’ said Stef, ‘that a thick atmosphere on a world like this is enough of a thermal blanket to transport sufficient heat around to the dark side to keep everything from freezing up.’

‘That and the fact that all the air didn’t freeze up in great bergs of solid oxygen and nitrogen on the far side a billon years ago,’ Liu said drily. ‘That and the fact that we are still breathing.’

‘But it’s always good to have observational confirmation.’

As they pushed on the cloud cover broke up, quite abruptly, to reveal a star-crowded sky. The temperature plummeted, and frost gathered.

During one rest stop Yuri bundled himself up in thermal underwear and padded coat and over-trousers, and went out with the others to look at the sky.

‘Funny thing,’ he said. ‘I’ve never seen much of the stars, one way or another. When I was a kid, before the cryo, the night sky of Earth was a wash-out. Full of space mirrors and other orbital clutter, even away from the glow of city lights and the smog. You could see the stars from Mars, but we weren’t let out of the domes. And then, here on Per Ardua, the sun never set at all.’

‘Drink it in, my friend,’ Liu said. ‘Drink it in. You can’t beat the Alpha suns, can you?’ A dazzling pair of diamonds, their light bright enough to cast shadows – bright enough, the ColU thought, to power some feeble photosynthesis.

Stef, meanwhile, was staring east. ‘Look. Can you see that?’ It was a brilliant star, hanging low on the horizon.

‘I see it,’ murmured the ColU. ‘But a star of that magnitude does not feature in the constellation maps I have stored in my memory. A nova, perhaps?’

‘We’d have heard of that,’ Stef said. ‘I guess we’ll find out . . .’

CHAPTER 76

They drove on, over ground that was permanently frozen now. The ice was gritty and old, Stef pointed out; away from the terminator region, where the warm air spilled over into the dark and quickly dumped its vapour, fresh precipitation must be rare.

Ten days past the terminator zone the ColU called a halt, on an otherwise unremarkable plain of ice. ‘We are no longer over dry land,’ it announced simply.

‘I can confirm that,’ Stef said quickly, inspecting ghostly radar images of a crumpled hidden surface beneath them. ‘This is the ocean, about where the Ad Astra maps indicated the shore should be. Just here it’s solid ice all the way to the ocean floor, which is no more than a dozen metres or so beneath us. Further out where the ocean is deeper we’re expecting liquid water under a crust of ice. The next landmass, and the antistellar, are thataway,’ she said, pointing. ‘The driving should be easy, but let’s take it carefully.’

They drove on into the silent dark, the light of their floods splashing ahead. They adopted a new driving strategy now, out on the ice, for safety in these different conditions. The ColU and the rover drove not in a convoy but in parallel, with maybe a quarter of a kilometre between them. That way neither of them would fall into the same crevasse, at least, and they could better triangulate the position of any obstacles. The landscapes they had crossed, with all their intricate detail, had been replaced now by a smooth plain of ice. Cloud and mist were rare, and the brilliant, unwavering starlight hung over them. It was an eerie, featureless, timeless phase of the journey, Yuri thought. With barely a vibration from the rover’s smooth-running engine, with no sense of the ground transmitted through the vehicle’s sturdy suspension, for long periods it felt as if they weren’t moving at all.

But then they began to see icebergs, like tremendous ships, bound fast in the frozen sea. ‘Evidently,’ the ColU said, ‘there are times when the ice melts enough for bergs to float across the open surface. During exceptional volcanic warming pulses, perhaps. But then the sea refreezes, trapping the bergs . . .’

Stef and the ColU seemed exhilarated by the confirmation of liquid water persisting under the ice. ‘It had to be there,’ Stef said. ‘There is probably a global system of deep ocean currents, transporting heat right around the planet. Part of the water cycle too, probably, restoring some of the mass lost to the unending rain at the terminator. Had to be there. But it’s ground truth; you don’t know for sure until you see it.’

The workings of an invisible ocean were less than captivating for Yuri and Liu. In these changeless hours they dozed, watched the stars, and for the first time in his life Yuri learned to play chess.

Stef, meanwhile, spent a lot of time watching that strange eastern star rise in the sky, tracking its motion using images captured by her slate. More brilliant than any star save the Alpha twins, it was rising too quickly, she said. ‘So not a star at all,’ she murmured. ‘Then what?’

On the seventy-first day the ColU called for a cautious slowdown. ‘We are approaching landfall . . .’

This was the single landmass they would encounter before the antistellar point, an Australia-size island continent that, according to the Ad Astra maps, lay between their terminator crossing point and the antistellar. They crept forward over the last of the sea pack ice, wary of its thinning, and then rolled up a shallow beach onto the land. Their floods picked out grimy ice beneath their treads, and low, eroded-looking hills, icebound, were shadows against the starry sky. They swung north and east, travelling in convoy once more.

Stef said, ‘There has to be an ice cap in the middle of this continent, even if it doesn’t show up in the Ad Astra data set. So we’re going to keep to the coastal fringe. If we get stuck we can always duck out onto the sea ice again.’

‘Actually the air temperature is rising,’ reported the ColU blandly.

After another half-day they came to a stretch of open, ice-free landscape, and they clambered out of the rover to explore. It was some kind of volcanic province, Yuri saw, with hot mud pools, and slicks of heat-loving bacteria that showed up a brilliant purple and green in their lights. So this was where the local warmth came from. Their breath steamed in the chill air, but Yuri could feel the warmth of the ground under his booted feet. They all wore head flashlights, which made them look like ghostly alien visitors in this calm, dreaming place. The ColU and Stef happily took samples and made images.

‘Life all over,’ Liu said.

‘Everywhere you go,’ Stef agreed. ‘There’s surely life even under the ice, on the bottom of the covered ocean, wherever there are hot springs, mineral seeps. The same as on Earth.’

‘And stromatolites,’ the ColU said.

‘What?’ Stef straightened up, sample bottle in hand. ‘Impossible. Not in the dark. You need photosynthesisers to build stromatolites.’

‘But here they are,’ the ColU said mildly.

It was true. Rising to the west of the bacteria garden, the landscape was covered with shapes like huge mushrooms, with broad tops and wide, deep stems anchored firmly to the ground.

They walked over. Stef stabbed a sampling tube into one big specimen, an unhesitating gesture that made Yuri wince, and extracted a cross-section sample that she inspected by the light of her head flashlight. ‘You’re right, ColU,’ she said. ‘Kind of. This is a stratified bacterial community. The upper layers do look like they are photosynthesising – by Alpha light presumably, it must be a very slow process. But further down I think we have mineral chompers, like the heat lovers in the mud pools. Call it a stromatolite, then, but of a strange, complex sort.’

‘And unimaginably ancient,’ the ColU said. ‘There would be nothing to disturb them here. No predators. And all of this must be a kind of surface expression of the deeper community, the deep hot biosphere, which won’t care if it’s on the day side or the dark.’