As the floods of molten sulphur spread out from volcanoes and vents, the versatile element changed through a narrow spectrum of reds and oranges and yellows when, chameleon-like, it was transformed into its vari-coloured allotropes. Before the dawn of the Space Age, no one had ever imagined that such a world existed. Fascinating though it was to observe it from his comfortable vantage point, Poole found it hard to believe that men had ever risked landing there, where even robots feared to tread... His main interest, however, was Europa, which at its closest appeared almost exactly the same size as Earth's solitary Moon, but raced through its phases in only four days. Though Poole had been quite unconscious of the symbolism when he chose his private landscape, it now seemed wholly appropriate that Europa should hang in the sky above another great enigma – the Sphinx.

Even with no magnification, when he requested the naked-eye view, Poole could see how greatly Europa had changed in the thousand years since Discovery had set out for Jupiter. The spider's web of narrow bands and lines that had once completely enveloped the smallest of the four Galilean satellites had vanished, except around the poles. Here the global crust of kilometre-thick ice remained unmelted by the warmth of Europa's new sun: elsewhere, virgin oceans seethed and boiled in the thin atmosphere, at what would have been comfortable room temperature on Earth.

It was also a comfortable temperature to the creatures who had emerged, after the melting of the unbroken ice shield that had both trapped and protected them. Orbiting spysats, showing details only centimetres across, had watched one Europan species starting to evolve into an amphibious stage: though they still spent much of their time underwater, the 'Europs' had even begun the construction of simple buildings.

That this could happen in a mere thousand years was astonishing, but no one doubted that the explanation lay in the last and greatest of the Monoliths – the many-kilometre-long 'Great Wall' standing on the shore of the Sea of Galilee.

And no one doubted that, in its own mysterious way, it was watching over the experiment it had started on this world – as it had done on Earth four million years before.

MISS PRINGLE

FILE INDRA

My dear Indra – sorry I've not even voice-mailed you before – usual excuse, of course, so I won't bother to give it.

To answer your question – yes, I'm now feeling quite at home at the Grannymede, but am spending less and less time there, though I've been enjoying the sky display I've had piped into my suite. Last night the Io flux-tube put on a fine performance – that's a kind of lightning discharge between Io and Jupiter – I mean Lucifer. Rather like Earth's aurora, but much more spectacular. Discovered by the radio astronomers even before I was born.

And talking about ancient times – did you know that Anubis has a Sheriff? I think that's overdoing the frontier spirit. Reminds me of the stories my grandfather used to tell me about Arizona... Must try some of them on the Medes...

This may sound silly – I'm still not used to being in the Bowman Suite. I keep looking over my shoulder...

How do I spend my time? Much the same as in Africa Tower. I'm meeting the local intelligentsia, though as you might expect they're rather thin on the ground (hope no one is bugging this). And I've interacted – real and virtual – with the educational system – very good, it seems, though more technically oriented than you'd approve. That's inevitable, of course, in this hostile environment...

But it's helped me to understand why people live here. There's a challenge – a sense of purpose, if you like – that I seldom found on Earth.

It's true that most of the Medes were born here, so don't know any other home. Though they're – usually – too polite to say so, they think that the Home Planet is becoming decadent. Are you? And if so, what are you Terries – as the locals call you – going to do about it? One of the teenage classes I've met hopes to wake you up. They're drawing up elaborate Top Secret plans for the Invasion of Earth. Don't say I didn't warn you...

I've made one trip outside Anubis, into the so-called Night Land, where they never see Lucifer. Ten of us -Chandler, two of Goliath's crew, six Medes – went into Farside, and chased the Sun down to the horizon so it really was night. Awesome – much like polar winters on Earth, but with the sky completely black... almost felt I was in space.

We could see all the Galileans beautifully, and watched Europa eclipse – sorry, occult – Io. Of course, the trip had been timed so we could observe this...

Several of the smaller satellites were just also visible, but the double star Earth-Moon was much more conspicuous. Did I feel homesick? Frankly, no – though I miss my new friends back there...

And I'm sorry – I still haven't met Dr Khan, though he's left several messages for me. I promise to do it in the next few days – Earth days, not Mede ones!

Best wishes to Joe – regards to Danil, if you know what's happened to him – is he a real person again? – and my love to yourself.

STORE TRANSMIT

Back in Poole's century, a person's name often gave a clue to his/her appearance, but that was no longer true thirty generations later. Dr Theodore Khan turned out to be a Nordic blond who might have looked more at home in a Viking longboat than ravaging the steppes of Central Asia: however, he would not have been too impressive in either role, being less than a hundred and fifty centimetres tall. Poole could not resist a little amateur psychoanalysis: small people were often aggressive over-achievers – which, from Indra Wallace's hints, appeared to be a good description of Ganymede's sole resident philosopher. Khan probably needed these qualifications, to survive in such a practically-minded society.

Anubis City was far too small to boast a university campus – a luxury which still existed on the other worlds, though many believed that the telecommunications revolution had made it obsolete. Instead, it had something much more appropriate, as well as centuries older – an Academy, complete with a grove of olive trees that would have fooled Plato himself, until he had attempted to walk through it. Indra's joke about departments of philosophy requiring no more equipment than blackboards clearly did not apply in this sophisticated environment.

'It's built to hold seven people,' said Dr Khan proudly, when they had settled down on chairs obviously designed to be not-too-comfortable, 'because that's the maximum one can efficiently interact with. And, if you count the ghost of Socrates, it was the number present when Phaedo delivered his famous address...'

'The one on the immortality of the soul?'

Khan was so obviously surprised that Poole could not help laughing.

'I took a crash course in philosophy just before I graduated – when the syllabus was planned, someone decided that we hairy-knuckled engineers should be exposed to a little culture.'

'I'm delighted to hear it. That makes things so much easier. You know – I still can't credit my luck. Your arrival here almost tempts me to believe in miracles! I'd even thought of going to Earth to meet you – has dear Indra told you about my – ah – obsession?'

'No,' Poole answered, not altogether truthfully.

Dr Khan looked very pleased; he was clearly delighted to find a new audience.

'You may have heard me called an atheist, but that's not quite true. Atheism is unprovable, so uninteresting. Equally, however unlikely it is, we can never be certain that God once existed – and has now shot off to infinity, where no one can ever find him... Like Gautama Buddha, I take no position on this subject. My field of interest is the psychopathology known as Religion.'