Yet even the space between the oases was not altogether empty of life; there were hardier creatures who had dared its rigours. Often swimming overhead were the Europan analogues of fish – streamlined torpedoes, propelled by vertical tails, steered by fins along their bodies. The resemblance to the most successful dwellers in Earth's oceans was inevitable; given the same engineering problems, evolution must produce very similar answers. As witness the dolphin and the shark – superficially almost identical, yet from far distant branches of the tree of life.

There was, however, one very obvious difference between the fish of the Europan seas and those in terrestrial oceans; they had no gills, for there was hardly a trace of oxygen to be extracted from the waters in which they swam. Like the creatures around Earth's own geothermal vents, their metabolism was based on sulphur compounds, present in abundance in the near-volcanic environment.

And very few had eyes. Apart from the flickering glow of the rare lava outpourings, and occasional bursts of bioluminescence from creatures seeking mates, or hunters questing prey, it was a lightless world.

It was also a doomed one. Not only were its energy sources sporadic and constantly shifting, but the tidal forces that drove them were steadily weakening. Even if they developed true intelligence, the Europans must perish with the final freezing of their world.

They were trapped between fire and ice.

37 – Estrangement

'I'm truly sorry, old friend, to be the bearer of such bad news, but Caroline has asked me, and you know how I feel about you both.

'And I don't think it can be such a surprise. Some of the remarks you've made to me over the last year have hinted at it... and you know how bitter she was when you left Earth.

'No, I don't believe there's anyone else. If there was, she'd have told me... But sooner or later – well, she's an attractive young woman.

'Chris is fine, and of course he doesn't know what's happening. At least he won't be hurt. He's too young to understand, and children are incredibly... elastic? – just a minute, I'll have to key my thesaurus... ah, resilient.

'Now to things that may seem less important to you. Everyone is still trying to explain that bomb detonation as an accident, but of course nobody believes it. Because nothing else has happened, the general hysteria has died down; we're left with what one of your commentators has called the "looking-over-the-shoulder syndrome".

'And someone has found a hundred-year-old poem that sums up the situation so neatly that everybody's quoting it. It's set in the last days of the Roman Empire, at the gates of a city whose occupants are waiting for invaders to arrive. The emperor and dignitaries are all lined up in their most costly togas, ready with speeches of welcome. The senate has closed, because any laws it passes today will be ignored by the new masters.

'Then, suddenly, a dreadful piece of news arrives from the frontier. There aren't any invaders. The reception committee breaks up in confusion; everyone goes home muttering disappointedly, "Now what will happen to us? Those people were a kind of solution."

'There's just one slight change needed to bring the poem up to date. It's called "Waiting for the Barbarians" – and this time, we are the barbarians. And we don't know what we're waiting for, but it certainly hasn't arrived.

'One other item. Had you heard that Commander Bowman's mother died only a few days after the thing came to Earth? It does seem an odd coincidence, but the people at her nursing home say that she never showed the slightest interest in the news, so it couldn't possibly have affected her.'

Floyd switched off the recording. Dimitri was right; he was not taken by surprise. But that made not the slightest difference; it hurt just as badly.

Yet what else could he have done? If he had refused to go on the mission – as Caroline had so clearly hoped – he would have felt guilty and unfulfilled for the remainder of his life. That would have poisoned his marriage; better this clean break, when physical distance softened the pain of separation. (Or did it? In some ways, it made things worse.) More important was duty, and the sense of being part of a team devoted to a single goal.

So Jessie Bowman was gone. Perhaps that was another cause for guilt. He had helped to steal her only remaining son, and that must have contributed to her mental breakdown. Inevitably, he was reminded of a discussion that Walter Curnow had started, on that very subject.

'Why did you choose Dave Bowman? He always struck me as a cold fish – not actually unfriendly, but whenever he came into the room, the temperature seemed to drop ten degrees.'

'That was one of the reasons we did select him. He had no close family ties, except for a mother he didn't see very often. So he was the sort of man we could send on a long, open-ended mission.'

'How did he get that way?'

'I suppose the psychologists could tell you. I did see his report, of course, but that was a long time ago. There was something about a brother who was killed – and his father died soon afterward, in an accident on one of the early shuttles. I'm not supposed to tell you this, but it certainly doesn't matter now.'

It didn't matter; but it was interesting. Now Floyd almost envied David Bowman, who had come to that very spot a free man unencumbered by emotional ties with Earth.

No – he was deceiving himself. Even while the pain gripped his heart like a vice, what he felt for David Bowman was not envy, but pity.

38 – Foamscape

The last beast he saw, before he left the oceans of Europa, was much the largest. It closely resembled one of the banyan trees from Earth's tropics, whose scores of trunks allow a single plant to create a small forest sometimes covering hundreds of square metres. The specimen, however, was walking, apparently on a trek between oases. If it was not one of the creatures that had destroyed Tsien, it certainly belonged to a very similar species.

Now he had learned all that he needed to know – or, rather, all that they needed to know. There was one more moon to visit; seconds later, the burning landscape of Io lay below him.

It was as he had expected. Energy and food were there in abundance, but the time was not yet ripe for their union. Around some of the cooler sulphur lakes, the first steps had been taken on the road to life, but before any degree of organization had occurred, all such bravely premature attempts were thrown back into the melting pot. Not until the tidal forces that drove Io's furnaces had lost their power, millions of years later, would there be anything to interest biologists on that seared and sterilized world.

He wasted little time on Io, and none at all on the tiny inner moons that skirted Jupiter's ghostly rings – themselves only pale shadows of the glory that was Saturn's. The greatest of worlds lay before him; he would know it as no man had ever done, or ever would.

The million-kilometre-long tendrils of magnetic force, the sudden explosions of radio waves, the geysers of electrified plasma wider than the planet Earth– they were as real and clearly visible to him as the clouds banding the planet in multihued glory. He could understand the complex pattern of their interactions, and realized that Jupiter was much more wonderful than anyone had ever guessed.

Even as he fell through the roaring heart of the Great Red Spot, with the lightning of its continent-wide thunderstorms detonating around him, he knew why it had persisted for centuries though it was made of gases far less substantial than those that formed the hurricanes of Earth. The thin scream of hydrogen wind faded as he sank into the calmer depths, and a sleet of waxen snowflakes – some already coalescing into barely palpable mountains of hydrocarbon foam – descended from the heights above. It was already warm enough for liquid water to exist, but there were no oceans there; that purely gaseous environment was too tenuous to support them.