Sniffing out life on Titan (Image: NASA/JPL/University of Arizona/SPL) Plumes of icy material streaming from Enceladus hint at the presence of a subsurface sea (Image: NASA/JPL/Space Science Institute)

IF LIFE is to be found beyond our home planet, then our closest encounters with it may come in the dark abyss of some extraterrestrial sea. For Earth is certainly not the only ocean-girdled world in our solar system. As many as five moons of Jupiter and Saturn are now thought to hide seas beneath their icy crusts.

To find out more about these worlds and their hidden oceans, two ambitious voyages are now taking shape. About a decade from now, if all goes to plan, the first mission will send a pair of probes to explore Jupiter’s satellites. They will concentrate on giant Ganymede and pale Europa, gauging the depths of the oceans that almost certainly lie within them.

A few years later, an even more audacious mission will head towards Saturn to sniff the polar sea spray of its snow-white moon Enceladus. It will also visit Titan, which has perhaps the most astonishing extraterrestrial landscape in our solar system. To explore this giant moon, the spacecraft will send out two seemingly antique contraptions: a hot-air balloon to fly over the deserts and mountains, and a boat that will float on a sea of liquid hydrocarbons.

This plan for ocean exploration was announced in February, when the science chiefs of NASA and the European Space Agency decided to press ahead with the planning stages of both missions. Jupiter is the destination that tops the schedule, probably because the Europa …