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Water and organic chemicals make Jupiter’s tiny moon the most exciting destination in the solar system, says Michael Hanlon.

Where there is water, there might be life. That is the mantra that has guided NASA’s astrobiology programme ever since it became respectable to talk about little green men, or rather microbes, back in the Nineties. If we want to find out whether we are alone, we need to find somewhere with an ocean.

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Water is a perfect solvent (just about everything, even gold, dissolves in water to some extent), allowing the complex suite of biochemical reactions that drive living processes. Other liquids have been proposed as the basis of life, but most biologists believe that water-plus-carbon is the most likely scenario for life outside Earth.

The trouble is, liquid water is a rare commodity in the Solar System. Frozen water is everywhere. Comets are mostly made of water ice. Mars is covered with the stuff. The outer planets and their moons are rich in water. But large quantities of the liquid stuff are rare.