The search for life beyond Earth “follows the water,” reports the Economist (since water is vital for earth’s lifeforms, and the laws of chemistry are universal). “For most of the space age that insight led scientists to Mars.” But…

More and more, though, planetary scientists are following the water to other places — and in particular to the so-called “icy moons” that orbit Jupiter, Saturn, Neptune and Uranus, the solar system’s quartet of giant gas planets. Many of those moons are either known or suspected to have oceans beneath their icy shells, kept liquid by gravitational squeezing from the planets they orbit.

On April 13th, if all goes well, a new spacecraft will blast off from French Guiana en route to Jupiter with the aim of investigating some of those watery moons up close. The European Space Agency’s Jupiter Icy Moons Explorer (given the slightly contrived acronym “JUICE “) will slingshot once around Venus and three times around Earth before arriving at Jupiter in 2031…. JUICE will investigate three of the so-called Galilean moons — Callisto, Europa and Ganymede, all of which are thought to have subsurface oceans. (The fourth, Io, is arid, and so not of interest.)

Ganymede is the probe’s primary target. Despite being a moon, it is bigger than the planet Mercury. Its subsurface ocean may contain more water than all of Earth’s oceans combined. The probe’s cameras will add much more detail to the existing, low-resolution maps of Ganymede’s surface. An ice-penetrating radar will scan several kilometres below the ground. A magnetometer will take advantage of the fact that Ganymede, apparently uniquely among the solar system’s moons, has a weak magnetic field that interacts with the much bigger field generated by Jupiter itself. The subtleties of that magnetic field were an early clue for the existence of an ocean, hinting at the presence of a large chunk of conductive fluid — such as salty water — beneath the surface. Better readings of the magnetic field will help scientists estimate just how big the ocean is….

Nor is JUICE the only probe on its way to Jupiter. Next year NASA will launch Europa Clipper, focused, as its name suggests, on Europa. Despite its later launch, it will take a quicker route to Jupiter, arriving a few months before JUICE . And, because there are limits to what can be discerned from orbit, both NASA and the Europeans are sketching plans for future landers that would descend to the surface of such moons to sample the seawater directly.