But those very factors also made it a planetary engineer's paradise. Here was the only world, except for the arid and much less hospitable Mars, on which men might one day walk unprotected beneath an open sky. Ganymede had ample water, all the chemicals of life, and – at least while Lucifer shone – a warmer climate than much of Earth.

Best of all, full-body spacesuits were no longer necessary; the atmosphere, though still unbreathable, was just dense enough to permit the use of simple face-masks and oxygen cylinders. In a few decades – so the microbiologists promised, though they were hazy about specific dates – even these could be discarded. Strains of oxygen-generating bacteria had already been let loose across the face of Ganymede; most had died but some had flourished, and the slowly rising curve on the atmospheric analysis chart was the first exhibit proudly displayed to all visitors at Dardanus.

For a long time, van der Berg kept a watchful eye on the data flowing in from Europa VI, hoping that one day the clouds would clear again when it was orbiting above Mount Zeus. He knew that the odds were against it, but while the slightest chance existed he made no effort to explore any other avenue of research. There was no hurry, he had far more important work on his hands – and anyway, the explanation might turn out to be something quite trivial and uninteresting.

Then Europa VI suddenly expired, almost certainly as a result of a random meteoric impact. Back on Earth, Victor Willis had made rather a fool of himself – in the opinion of many – by interviewing the 'Euronuts' who now more than adequately filled the gap left by the UFO-enthusiasts of the previous century. Some of them argued that the probe's demise was due to hostile action from the world below: the fact that it had been allowed to operate without interference for fifteen years – almost twice its design life – did not bother them in the least. To Victor's credit, he stressed this point and demolished most of the cultists' other arguments; but the consensus was that he should never have given them publicity in the first place.

To van der Berg, who quite relished his colleagues' description of him as a 'stubborn Dutchman' and did his best to live up to it, the failure of Europa VI was a challenge not to be resisted. There was not the slightest hope of funding a replacement, for the silencing of the garrulous and embarrassingly long-lived probe had been received with considerable relief.

So what was the alternative? Van der Berg sat down to consider his options. Because he was a geologist, and not an astrophysicist, it was several days before he suddenly realized that the answer had been staring him in the face ever since he had landed on Ganymede.

Afrikaans is one of the world's best languages in which to curse; even when spoken politely, it can bruise innocent bystanders. Van der Berg let off steam for a few minutes; then he put through a call to the Tiamat Observatory – sitting precisely on the equator, with the tiny, blinding disc of Lucifer forever vertically overhead.

Astrophysicists, concerned with the most spectacular objects in the Universe, tend to patronize mere geologists who devote their lives to small, messy things like planets. But out here on the frontier, everyone helped everyone else, and Dr Wilkins was not only interested but sympathetic.

The Tiamat Observatory had been built for a single purpose, which had indeed been one of the main reasons for establishing a base on Ganymede. The study of Lucifer was of enormous importance not only to pure scientists but also to nuclear engineers, meteorologists, oceanographers – and, not least, to statesmen and philosophers. That there were entities which could turn a planet into a sun was a staggering thought, and had kept many awake at night. It would be well for mankind to learn all it could about the process; one day there might be need to imitate it – or prevent it.

And so for more than a decade Tiamat had been observing Lucifer with every possible type of instrumentation, continually recording its spectrum across the entire electromagnetic band, and also actively probing it with radar from a modest hundred-metre dish, slung across a small impact crater.

'Yes,' said Dr Wilkins, 'we've often looked at Europa and Io. But our beam is fixed on Lucifer, so we can only see them for a few minutes while they're in transit. And your Mount Zeus is just on the dayside, so it's hidden from us then.'

'I realize that,' said van der Berg a little impatiently. 'But couldn't you offset the beam by just a little, so you could have a look at Europa before it comes in line? Ten or twenty degrees would get you far enough into dayside.'

'One degree would be enough to miss Lucifer, and get Europa full-face on the other side of its orbit. But then it would be more than three times further away, so we'd only have a hundredth of the reflected power. Might work, though: we'll give it a try. Let me have the specs on frequencies, wave envelopes, polarization and anything else your remote-sensing people think will help. It won't take us long to rig up a phase-shifting network that will slew the beam a couple of degrees. More than that I don't know – it's not a problem we've ever considered. Though perhaps we should have done so – anyway, what do you expect to find on Europa, except ice and water?'

'If I knew,' said van der Berg cheerfully, 'I wouldn't be asking for help, would I?'

'And I wouldn't be asking for full credit when you publish. Too bad my name's at the end of the alphabet; you'll be ahead of me by only one letter.'

That was a year ago: the long-range scans hadn't been good enough, and offsetting the beam to look on to Europa's dayside just before conjunction had proved more difficult than expected. But at last the results were in; the computers had digested them, and van der Berg was the first human being to look at a mineralogical map of post-Lucifer Europa.

It was, as Dr Wilkins had surmised, mostly ice and water, with outcroppings of basalt interspersed with deposits of sulphur. But there were two anomalies.

One appeared to be an artefact of the imaging process; there was an absolutely straight feature, two kilometres long, which showed virtually no radar echo. Van der Berg left Dr Wilkins to puzzle over that; he was only concerned with Mount Zeus.

It had taken him a long time to make the identification, because only a madman – or a really desperate scientist – would have dreamed that such a thing was possible. Even now, though every parameter checked to the limits of accuracy, he still could not really believe it. And he had not even attempted to consider his next move.

When Dr Wilkins called, anxious to see his name and reputation spreading through the data banks, he mumbled that he was still analysing the results. But at last he could put it off no longer.

'Nothing very exciting,' he told his unsuspecting colleague. 'Merely a rare form of quartz – I'm still trying to match it from Earth samples.'

It was the first time he had ever lied to a fellow scientist, and he felt terrible about it.

But what was the alternative?