Subsurface ocean

An ocean of liquid water thought to lie beneath the ice shell of an icy moon. A leading candidate for where to look for life.

The moons of Jupiter and Saturn, far from the Sun, are frozen at the surface. But a moon constantly kneaded by its planet's gravity warms inside, and liquid water can persist under the ice. In 2005 the Cassini spacecraft found water vapour and ice grains jetting from the south pole of Enceladus, a small moon of Saturn^[1]. It is strong evidence of liquid water below.

Jupiter's moon Europa, too, is thought to hold a great deal of water under its ice. Given liquid water, a source of energy and organic compounds, life might exist even where sunlight never reaches. The search for life has widened from Mars to these oceans under the ice. Europa Clipper, due at Jupiter in 2030, will study the thickness of the ice and the make-up of the ocean.