Nevertheless, the words spoken at the time conveyed much of the drama of that first landing on the moons of Jupiter. The commentary broadcast from the approaching Leonov by Heywood Floyd served admirably to set the scene, and there were plenty of library shots of Europa to illustrate it:

'At this very moment I'm looking at it through the most powerful of the ship's telescopes; under this magnification, it's ten times larger than the Moon as you see it with the naked eye. And it's a really weird sight.

'The surface is a uniform pink, with a few small brown patches. It's covered with an intricate network of narrow lines, curling and weaving in all directions. In fact, it looks very much like a photo from a medical textbook, showing a pattern of veins and arteries.

'A few of these features are hundreds – or even thousands – of kilometres long, and look rather like the illusory canals that Percival Lowell and other early-twentieth-century astronomers imagined they'd seen on Mars.

'But Europa's canals aren't an illusion, though of course they're not artificial. What's more, they do contain water – or at least ice. For the satellite is almost entirely covered by ocean, averaging fifty kilometres deep.

'Because it's so far from the Sun, Europa's surface temperature is extremely low – about a hundred and fifty degrees below freezing. So one might expect its single ocean to be a solid block of ice.

'Surprisingly, that isn't the case because there's a lot of heat generated inside Europa by tidal forces – the same forces that drive the great volcanoes on neighbouring Io.

'So the ice is continually melting, breaking up and freezing, forming cracks and lanes like those in the floating ice sheets in our own polar regions. It's that intricate tracery of cracks I'm seeing now; most of them are dark and very ancient – perhaps millions of years old. But a few are almost pure white; they're the new ones that have just opened up, and have a crust only a few centimetres thick.