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 Jupiter System Mission relies on well-tested space technology. The plan is for EJSM to lift off in early 2020, in two pieces. NASA's contribution, the Jupiter Europa Orbiter (JEO), and ESA's Jupiter Ganymede Orbiter (JGO) will be launched within a month of each other and plot parallel courses for Jupiter, arriving after six years. They will then engage in a complex dance, visiting various moons before each probe homes in on its prime target.

JEO has the tougher task. It will have to spend a long time in the inner reaches of Jupiter's radiation belts, where it will come under intense bombardment by high-energy electrons that would quickly disable an ordinary spacecraft. Though JEO will be built using electronics hardened against radiation, it will have to be clad in aluminium armour to survive in this hostile region.

When it finally goes into orbit around Europa, JEO's instruments will explore not just the moon's surface, but its depths too. The first hints that this moon's crust hides a liquid water ocean came from Voyagers 1 and 2, which saw a flat landscape criss-crossed with cracks when they flew by in 1979. This was confirmed in the 1990s by NASA's Galileo spacecraft. Galileo also found that Europa distorts Jupiter's magnetic field, which could be accounted for by an electrically conducting layer below the moon's ice. Most planetary scientists see this as compelling evidence for a subsurface sea of salt water.

JEO will map the magnetic field of Europa in even finer detail, and also measure the shape of its gravitational field. Putting the two together will give us further insights into the moon's structure - especially the thickness of its ice crust and the depth of its ocean....

(New Scientist)