Solar Exploration

Europa is the smallest of Jupiter's four large Galilean moons, but scientifically it's one of the most important objects in the solar system: beneath its smooth, icy crust, scientists believe there is a global saltwater ocean containing roughly twice as much liquid water as all of Earth's oceans combined. That hidden ocean is kept liquid not by sunlight — far too weak at Jupiter's distance — but by tidal heating, as Jupiter's immense gravity flexes and warms Europa's interior with every orbit.

Europa's surface is famously smooth by planetary standards, crisscrossed with long reddish-brown cracks called lineae, thought to form as the icy shell shifts over the ocean below. Very few impact craters are visible, suggesting the surface is geologically young and constantly renewed — strong indirect evidence of an active, possibly life-friendly, environment underneath.

Because Europa orbits deep inside Jupiter's radiation belts, its icy surface is constantly bombarded by charged particles, which split water molecules in the ice and produce a razor-thin oxygen atmosphere — far too sparse to breathe, but a tantalizing sign of ongoing chemistry. Some observations suggest plumes of water vapor occasionally erupt through cracks in the ice, similar to the geysers seen on Saturn's moon Enceladus, potentially offering a way to sample the subsurface ocean without ever drilling through the ice.