Europa

Europa is a little smaller than our Moon, and from a distance it looks like a ball of cracked white and rust-brown ice. Galileo Galilei discovered it in 1610 as one of the four large moons of Jupiter. Its surface has been photographed by spacecraft from Voyager to Juno. What lies beneath it never has: an ocean of salt water, probably 60 to 150 kilometres deep, holding about twice as much water as all of Earth’s oceans.

How we know the ocean is there

Nobody has seen Europa’s ocean. Its existence was worked out from Jupiter’s magnetic field. As Europa moves through that field, a layer of salty, electrically conducting water would produce a magnetic field of its own. In January 2000 the Galileo spacecraft flew 351 kilometres above Europa and measured exactly such a response, best explained by salt water within the outer hundred kilometres.

The surface tells the same story. It has very few craters, so it is young, and it is crossed by long cracks and ridges. In places called chaos terrain, the ice looks broken into blocks that have drifted and frozen again, as if something warm came close to the surface. How thick the ice is remains uncertain: NASA’s estimate is 15 to 25 kilometres, and in 2025 the Juno spacecraft’s microwave measurements gave 29 kilometres, give or take ten, for the region it saw.

Salt and carbon from below

In 2019 Hubble found sodium chloride, ordinary table salt, on Europa’s surface, most strongly in a chaos region called Tara Regio. In 2023 the James Webb Space Telescope found carbon dioxide concentrated in the same place. The researchers concluded that the carbon most likely came up from the ocean, although they could not rule out that it formed at the surface. Carbon is one of the ingredients life on Earth needs, which is why Europa is near the top of the list of places to search.

Plumes, or not

In 2013 a team using Hubble reported water vapour above Europa’s south pole, rising perhaps 200 kilometres. More possible plumes followed in Hubble images in 2016 and 2017, and in 2019 the Keck Observatory detected water vapour on one night out of 17. But the James Webb telescope saw no plume in 2023, and in 2026 a re-analysis of all Hubble’s ultraviolet images of Europa from 1999 to 2020 found no localised emission at all, not even in the original 2013 data. For now, Europa’s plumes are unconfirmed.

Europa Clipper

NASA’s Europa Clipper launched on 14 October 2024 and is due to reach Jupiter in April 2030. From 2031 it will fly past Europa 49 times, some passes only 25 kilometres above the ice, to map the shell, look for water beneath it and measure the ocean’s salt. It is not designed to detect life, only to find out whether Europa could support it. Europe’s JUICE spacecraft will add two flybys of its own in July 2032.