The most interesting oceans in the solar system are the ones we cannot see. Beneath the ice shells of Jupiter's moon Europa and Saturn's tiny Enceladus lie global seas of liquid saltwater, kept warm by tidal flexing and in contact with rock — the same three ingredients that, on Earth, gave rise to life at hydrothermal vents. This year the case for those hidden oceans grew stronger: a reanalysis of data from the Cassini probe confirmed that fresh ice grains flung from Enceladus's south-polar geysers carry complex organic molecules, straight from the ocean below and not weathered by long exposure to space. The building blocks are there, in the plume, waiting to be sampled.
The problem is reaching them. An ocean sealed beneath kilometres of ice is not a place you land and drill, so the plan is to let the moon deliver its ocean to the spacecraft — fly through the plumes and catch what erupts. NASA has built an autonomous life-detection suite for exactly that, and the European Space Agency has named an Enceladus orbiter-and-lander a long-term flagship, to launch in the 2040s. In the meantime the prototypes are being tested where Earth most resembles an ice moon: under the Antarctic ice shelf, where a robotic probe barely ten centimetres across is being trialled this year, melting through the ice to swim in the black water beneath — exactly as its descendants would on Europa.
The uncertainty is worth holding honestly. Europa's plumes, once glimpsed, have not been reliably seen again, and some of the evidence that helped launch these missions now carries a question mark. Even a null result would matter — to find every condition for life present, and life itself absent, would be its own profound discovery. Either way, the instruments now being lowered beneath Antarctic ice are a reminder that the search for life elsewhere runs straight through the study of our own ocean. They are the same science, pointed in different directions.
Sources
Organics in the plume: the European Space Agency on the Cassini reanalysis confirming complex organic molecules in Enceladus's freshly ejected ice grains. The ingredients for life: NASA on Enceladus's ocean, chemistry and internal heat. The missions and the instruments: the European Space Agency on its Enceladus flagship; Science on the Europa life-detection effort and the under-ice testing. The life-detector: Space.com on NASA's Ocean Worlds Life Surveyor.
Further Reading
The sceptical footnote: Science on why Europa's once-detected plumes have not been reliably seen since — a caution against over-reading the evidence that sold the missions.