Over the last two decades we have come to recognise that the icy, airless, frozen moons of the distant gas giant planets Jupiter and Saturn are among the most likely places in our Solar System to host extra-terrestrial life.

Robotic spacecraft, such as NASA’s Galileo and Cassini missions, have shown that several icy moons contain vast oceans of liquid water deep below their icy surfaces, where conditions may be suitable for life.

Two of the best-studied of these icy moons are Europa, one of Jupiter’s moons, and Enceladus, a moon of Saturn.

Enceladus (Figure 1) is relatively small; approximately 500 km in diameter. But despite its small stature, Enceladus has captivated planetary scientists. The Cassini spacecraft, which studied Enceladus in detail between 2005 and 2016, discovered the existence of enormous plumes of water vapour, gases, salts and ice that shoot hundreds of kilometres out into space (Figure 1). Data collected by Cassini showed that the plumes of Enceladus originate from its global subsurface ocean and are powered by interactions between the water and the hot rocky core deep within the moon.

Figure 1: Plumes rising from Enceladus's South Polar Region.

Although the ocean of Enceladus is cut off from sunlight, we have good reason to believe that it is habitable in the present day. Water-rock (hydrothermal) interactions generate a plethora of chemical reactions that can supply the ocean with the ingredients needed to support life. On Earth, there are deep-sea hydrothermal vents where whole ecosystems survive without any energy from sunlight. The basis of the food-chain in these ecosystems is bacteria that use chemical energy from the hydrothermal vents to convert carbon dioxide into sugars and other biologically useful molecules, a role normally fulfilled by photosynthesis in plants on the Earth’s surface. Studies using data from the Cassini mission have shown that some of the microorganisms that live in hydrothermal vents deep in Earth’s oceans could survive in Enceladus’s ocean, using hydrogen as their main source of energy.

If micro-organisms exist within Enceladus’s ocean, whole cells, or the molecules they produce, might be ejected into space in the plumes. This means that a spacecraft need not sample the ocean – or even land on the surface at all – to search for life at Enceladus; it could simply fly through the plumes to capture ejected materials. Cassini did exactly this and encountered large organic molecules, although there’s no evidence yet that they have a biological origin. Detecting evidence of life in Enceladus’s ocean from within the plumes will require a follow-up mission with more sensitive instrumentation.

Jupiter’s moon Europa (Figure 2) is another tantalising prospect in the search for life beyond Earth. Like Enceladus, Europa harbours an ocean of liquid water beneath its icy surface and a rocky interior that is heated thanks to gravitational interactions between Europa, Jupiter’s other large moons, and Jupiter itself. This ‘tidal heating’ could mean that, as observed at Enceladus, hydrothermal water-rock interactions may be occurring at the seafloor, releasing potentially life-giving chemical energy into the ocean.

Figure 2: Jupiter's moon Europa, imaged by NASA's Galileo spacecraft in the late 1990s.

The best visual signs that there is internal heating on Europa are the great cracks on its surface, which show that the ice shell is flexing, fracturing and shifting (Figure 2). Frozen water from the ocean below can make its way to the surface through these cracks, where it can be studied by spacecraft.

Several observations of Europa, both up-close by NASA’a Galileo spacecraft and from a distance by telescopes such as the Hubble Space Telescope, have revealed evidence of salts at the surface that probably originate from the ocean. In 2019, the discovery of sodium chloride – table salt – on Europa’s surface generated excitement. Because this salt is the dominant chemical in Earth’s oceans, the discovery suggested that the chemical composition of Europa’s ocean may be more familiar than once thought. There is also evidence that, like Enceladus, Europa has plumes erupting from its surface that might eject some sub-surface liquid up through the ice.

Although it seems likely that processes of exchange between ocean and icy shell are occurring at Europa, we don’t yet know how widespread they are, or where to look for evidence of water close to the surface. But that will soon change. Two exploration missions are close to launch: NASA’s Europa Clipper and the European Space Agency’s JUpiter Icy Moons Explorer; JUICE for short. JUICE is planned to launch in 2022, and Europa Clipper in 2024. They will both take around seven years to reach Jupiter.

These missions will study Europa and its neighbours Ganymede and Callisto, both of which might also have oceans locked under their icy surfaces. The spacecraft will observe the surfaces of these moons, hunting for evidence of frozen ocean water transported to the surface in the form of salts or plumes. They will also use radar to look for pockets of liquid water at shallower depths, within the ice itself, which are thought to be responsible for forming ‘chaos terrain’ on Europa’s surface.

JUICE will conduct two flybys of Europa, taking high-resolution images, then place itself into orbit around Ganymede, Jupiter’s largest moon. It will also take infrared images of both moons, which will help us to identify salts on the surfaces that may originate from the ocean below.

Figure 3: Artist's impression of the Jupiter Icy Moons Explorer, JUICE.

Europa Clipper will focus on Europa. It will stay in a wide orbit around Jupiter and fly by Europa 44 times to build a global map of the moon. Clipper will carry instruments that can catch dust thrown up from Europa’s surface from impacts, or by potential plumes, and look at its chemical composition in detail. If evidence of life deep below the ice on Europa is being ejected into space, Clipper may be able to detect it.

Figure 4: 2019 Europa Clipper Spacecraft (Artist's Concept).

These missions will help to pinpoint locations where the ocean could have recently interacted with the surface, paving the way for future lander missions that will directly search for evidence of life. Together, they stand to revolutionise our understanding of Jupiter’s icy moons, providing us with an unprecedented ‘window’ through the ice to look into the oceans deep below and uncover the secrets harboured there.