Could there be oceans (and even life) hiding beneath the icy surfaces of the moons of Jupiter and Saturn?

For most of the history of planetary science, the search for habitable worlds focused on planets close to the Sun, where surface water can stay liquid. But data from NASA’s Cassini spacecraft at Saturn and the Galileo spacecraft at Jupiter has overturned that assumption, revealing that several icy moons may be hiding vast oceans of liquid water beneath their frozen crusts. In this Discourse, Michele Dougherty — the first female Astronomer Royal — takes us through the Cassini mission’s most extraordinary discovery: a plume of water vapour erupting from the south pole of Saturn’s tiny moon Enceladus, and what it revealed about the possibility of life beyond Earth. She then looks ahead to ESA’s JUICE mission, launched in 2023, which is now on its way to spend years studying Jupiter’s giant moons Ganymede, Callisto and Europa — all suspected to hide oceans of their own.