The largest and deepest body of water known to us has never been sailed upon; it has no islands or shores, no wind-churned waves, no sunlight-silvered surface. This dark ocean can’t be found on any map of Earth – it’s more than 300 million miles away, on Europa, one of at least 69 moons that orbit Jupiter. Data from the Galileo spacecraft, which flew past Europa 11 times between 1995 and 2003, revealed that an immense salty sea lies beneath this moon’s smooth icy surface. Estimated to be 60 miles deep – about eight times the maximum depth of the Pacific – it has two to three times as much water as all of Earth’s oceans combined.
And Europa isn’t some singularly soggy outlier. At least two additional Jovian moons – Ganymede and Callisto – have subsurface oceans. Titan and Mimas, which orbit Saturn, probably do, too. And there’s no doubt that another Saturnian moon, Enceladus, harbours water beneath its frozen crust, probably a volume comparable to the Great Lakes. Astonishing and irrefutable evidence for Enceladus’s briny deep came in 2005 when the Cassini space probe captured images of geysers spouting ice and water vapour hundreds of miles into space. Cassini even flew right through the geysers in October 2015, skimming within 30 miles of the moon’s surface to sample their contents.
To say that the abundance and ubiquity of liquid water in the outer solar system completely upended scientists’ expectations doesn’t do justice to the discoveries. Before the revelations provided by Cassini, Galileo and other probes, the consensus was stark: the moons around Jupiter and Saturn would look much like our own or those of Mars – rocky, crater-pocked wastelands utterly hostile to life. ‘Nobody expected that there were subsurface oceans,’ says Seth Shostak, an astronomer with the SETI Institute in Mountain View, California. ‘It extends our concept of habitability and where you might find life to worlds that we hadn’t considered before. We always assumed that it had to be on a planet. I reckon there are seven other places in our solar system where we have reason to think there might be life – at least the conditions for life. Seven! And most of them are moons!’
With so much water in our own neck of the cosmos, it’s almost certain that countless planets around other stars must have oceans too, not to mention retinues of wet moons. Astronomers have already tentatively identified a few ‘water worlds’ beyond our solar system – planets with no dry land at all. ‘It’s mind-boggling,’ says Christopher Glein, a Cassini mission scientist at the Southwest Research Institute in San Antonio, Texas. ‘It’s like inventing a new field of oceanography.’