Europa holds an ocean under the ice

Jupiter’s moon Europa has a shell of ice over a global ocean of liquid water. Magnetic measurements from Galileo are the evidence. The ocean may hold more water than all of Earth’s seas.

Europa’s surface is ice, cracked and ridged and almost without craters, which means the face is young and something has been repaving it. When Galileo flew past in the late 1990s, the magnetometer found that the moon was disturbing Jupiter’s field in the way a conducting layer would. Salt water conducts. Margaret Kivelson and colleagues made the case in 2000: a global ocean of salty water under the ice fits the magnetic signature, and a dry interior does not. The ocean has not been seen, tasted, or drilled. It has been inferred, twice over, from a field that changes as the moon moves.

How much water depends on how thick the ice is and how deep the ocean goes, neither of which has been measured directly. Plausible shells of ten to thirty kilometres, over an ocean tens of kilometres deep, give a volume larger than Earth’s oceans. That comparison is a model, and models move. What has held since Galileo is the existence of the layer. A moon a quarter of Earth’s width is the leading candidate for the largest body of liquid water in this neighbourhood, and the water is under ice, in the dark, with no surface to freeze from.