“Follow the water.” That’s been a guiding principle in the modern search for extraterrestrial life, based on the overwhelming evidence that all living things on Earth — no matter how exotic or extreme — require water to survive. For the past two decades, this emphasis on water has focused attention on Mars, where NASA has intently sought evidence of ancient rivers or modern trickles. Each eroded pebble and layer of sediment there has been heralded as an important clue.
Lately, though, the celestial dowsing process has been pointing in a sharply different direction, away from the majestic deserts of the Red Planet and toward a motley assortment of small, frozen bodies. The shift began in the late 1990s, when the Galileo spacecraft gathered evidence that Jupiter’s moon Europa has a thick layer of water beneath its icy crust. No need to mince words: “Thick layer of water” is another way of saying “a vast, global ocean,” one that just happens to be sequestered underground (or rather, under ice). Europa may hide twice as much water as all Earth’s oceans put together.
Any thought that Europa was an anomaly soon evaporated with the discovery of geysers on Enceladus, a 300-mile-wide satellite of Saturn. Then the, er, floodgates opened as scientists began reporting evidence of water sloshing around inside Jupiter’s moon Ganymede, Saturn’s Titan and Mimas, and perhaps Neptune’s Triton. There may be water layers inside Pluto and the dwarf planet Ceres, too.
