Under the ice of Jupiter's moon Europa lies a hidden ocean.
Europa, a moon slightly smaller than ours, is the smoothest solid object known in the Solar System. Its young, cracked ice sits on top of what scientists broadly agree is a salty ocean of liquid water, kept warm by Jupiter's tides. That makes it one of the best places to look for life beyond Earth.
Galileo Galilei discovered Europa in January 1610, along with Jupiter's other three large moons. It is a little smaller than Earth's Moon, and its surface is a pale shell of ice criss-crossed by tan cracks and streaks. There are almost no mountains or large craters, which suggests the surface is geologically young and constantly renewed.
Most planetary scientists think the reason is a global ocean. Estimates put Europa's outer layer of water at about 100 kilometres thick, part frozen crust and part liquid beneath. The key evidence came from the Galileo spacecraft, which orbited Jupiter from 1995 to 2003 and found that Europa has a magnetic field induced by Jupiter's own. That points to a conductive layer under the ice, most likely salty water.
What keeps it liquid, when the surface is around −160°C at the equator? Tides. Europa's slightly stretched orbit, kept that way by the gravitational rhythm of the neighbouring moons, means Jupiter's pull flexes the moon constantly. That kneading generates heat, and it may drive activity on the seafloor similar to undersea volcanoes on Earth. How thick the ice is remains debated, from a few kilometres to much more.
The Hubble Space Telescope has seen signs of water vapour plumes, and a 2018 reanalysis of Galileo data supported the idea. If plumes really vent the ocean into space, a spacecraft could sample it without landing. Salts on the surface may also have come up from below. No probe has yet landed, but NASA's Europa Clipper, launched in October 2024, is designed to study the moon closely.
Source: Wikipedia — Europa (moon) · Text summarised from Wikipedia (CC BY-SA 4.0)
