For most of the past 200 years, were you to ask an astronomer where the most likely place in the solar system is to find life, the answer will have been Mars. The red planet and its potential inhabitants have captured our collective imagination for centuries, transforming from an imaginary canal-building civilisation in the 19th century to the much more scientifically plausible microbes of today. But now, the thinking is different. In the past few decades, astronomers have been increasingly drawn to the deeper, darker realms of the solar system. Specifically, they have become fascinated by the icy moons of Jupiter and Saturn. Years of research have all but proved that some of these moons contain vast oceans of liquid water below their frozen surfaces. On Earth, water is the number one prerequisite for supporting life. So could these icy moons be habitable too? In April, the European Space Agency (Esa) will launch a mission designed to find out. The Jupiter Icy Moons Explorer (Juice) is now at Esa’s spaceport in Kourou, French Guiana. It is undergoing final testing and fitting to its launch vehicle, an Ariane 5 rocket similar to the one that propelled the James Webb space telescope into orbit in December 2021. Yet even to those in the field of astrobiology there is a sense of incredulity about the idea of investigating planetary habitability on Jupiter’s moons. “If you said to someone 50 years ago, I’m gonna go and look for life in the outer solar system around the gas giant planets, people would have thought you were mad because there was no reason to think that it was a reasonable proposition at all,” says Charles Cockell, one of the directors of the UK Centre for Astrobiology at Edinburgh University. What changed was a set of measurements from a Nasa mission in the 1990s that, at first, made no sense. Every time the Galileo spacecraft drew near to Europa, one of Jupiter’s icy moons, readings from the magnetometer instrument indicated that something inside the moon was interfering with the mighty magnetic field generated by Jupiter. To collect more of these strange readings, Margaret Kivelson, the instrument’s principal investigator, and others helped persuade Nasa planners to keep extending the mission. Eventually, after 12 close flybys, there was enough data for her team to present an answer. The signals suggested the presence of salty water beneath Europa’s icy surface. The currents in this water were generating their own small magnetic fields that were then interfering with Jupiter’s, producing the unexpected readings. Additionally, images from those close flybys showed ice floes on Europa’s surface. One region in particular, called the Conamara Chaos, in honour of a rugged coastline in Ireland, became the poster child portrait of the moon. But the biggest surprise was the amount of water that the theoreticians needed to account for the signals: more than twice the amount of water found in all of Earth’s oceans. Because Europa is only a quarter of Earth’s diameter, the water must be spread around the moon in a global ocean, 25 to 95 miles deep, beneath a 10- to 15-mile-thick ice crust. “On Earth, the ocean looks very substantial. Its deepest part is 11km [seven miles]. But compared with Europa, it’s actually a thin veneer,” says Cockell. And it is not just Europa. Jupiter possesses three other large moons: Io, Ganymede and Callisto. Galileo data also showed that Jupiter’s moon Ganymede and maybe even Callisto have interior oceans. At Saturn, the Esa/Nasa Cassini-Huygens mission revealed something similar for the moons Enceladus and Titan. Meanwhile, at Mars, the search for water was going in the opposite direction. Esa’s Mars Express mission was using a radar to probe for large bodies of subsurface water but was coming up empty. “We looked but we didn’t find anything,” says Esa’s Olivier Witasse, who was project scientist of Mars Express at the time. Now, he has shepherded Juice through the construction phase, a process that began in 2015, to its imminent launch. Scheduled for launch on 13 April, Juice will carry the most sophisticated set of scientific instruments yet into the outer solar system. Witasse’s excitement at the prospect is palpable. “In terms of science, this mission is just incredible,” he says. Although Juice will visit Europa and Callisto, its main target is Ganymede. The largest moon of the solar system, if Ganymede were in orbit around the sun it would be classed as a planet because it is larger in diameter than the inner planet Mercury. Yet because Europa became such a strong focus for the Galileo probe, Ganymede remains tantalisingly mysterious – and potentially even more fascinating than Europa. “There may be six to eight times more water on Ganymede than on Earth but we don’t know any details,” says Witasse. It’s possible that Ganymede’s putative ocean is much deeper beneath the surface. Estimates suggest that the ocean begins 60-90 miles below the surface. As with the study of Europa, the main investigation will be undertaken using the magnetometer instrument. The principal investigator is Michele Dougherty from Imperial College London, who was also in charge of the magnetometer on the Cassini mission that discovered the global ocean at Enceladus. “Our readings come from the interiors of these worlds. It’s almost like being able to see inside them. So, I’m only a quarter joking when I say magnetometers are the most important instruments in the world,” says Dougherty with a cheeky smile. Beyond Ganymede, Juice will investigate Jupiter itself. The largest planet in the solar system, Jupiter is a whopping 11 times larger than Earth at its equator, and more than five times further from the sun. Although Jupiter takes 12 years to complete an orbit, its day flies by in just under 10 hours. Known as a gas giant planet, Jupiter has no solid surface; the extensive atmosphere of hydrogen and helium simply gets denser the further down you go until it behaves more like the liquid metal mercury than rock. From this churning interior comes the strong magnetic field that revealed its moons’ oceans. Ganymede is the only moon in the solar system that generates its own magnetic field. This makes it fascinating in its own right but severely complicates the task of disentangling the magnetic interference that will provide data on the ocean’s characteristics, such as depth, extent and even salinity. “It is terrifying what we are trying to do,” says Dougherty. “Detecting those tiny signals is what keeps me awake at 2am. It’s like finding needles in a haystack.” The closer the spacecraft can get to Ganymede, the stronger those elusive signals will become, and so the easier the job will be. That is why there is a plan for Juice to enter orbit around Ganymede in 2034. This will be the first time a spacecraft has ever held an orbit around a moon other than our own. If all goes to plan it will orbit at an altitude of 310 miles for at least a year, with a further plan to extend the mission and take the spacecraft into a 125-mile orbit. This would certainly help when interpreting the data but requires there to be spare fuel left on the spacecraft. The spacecraft’s main power source comes from its solar panels. The array of panels is the largest ever used on an interplanetary mission, which is necessary since the sunlight that reaches Jupiter is just 4% of that found at Earth. While sufficient for running the instruments and communicating with Earth, it cannot generate enough power to move the spacecraft. For that, Juice relies on engines and thrusters that require fuel. Once this fuel is exhausted, the mission is effectively over – and how much fuel the spacecraft will have to spare depends on when they launch in April. Launch delays because of last-minute technical issues are frequent. In the case of Juice, the timing is critical because Jupiter is a moving target. Each day of delay after the launch window opens means that more fuel will have to be expended to reach the target. The more fuel it takes to get to Jupiter, the less likely they are to get really close to Ganymede. The difficulty means that no one should expect quick results. “We’re not going to solve this on our first flyby of Ganymede,” says Dougherty. “Only at the end of the mission will we have enough data to separate everything out.” She also stresses that it will be a combination of magnetic and other data from the spacecraft that will inform the interpretation of the results. But the mission is worthwhile because the challenge of understanding Jupiter and its icy moons goes beyond the understanding of our own solar system. It will also help us to assess the habitability of the wider galaxy. Around the same time that Nasa’s Galileo was painstakingly collecting its data about Europa, astronomers around the world were discovering the first planets around other stars. What started as a trickle of discoveries has turned into a flood. Now, more than 5,300 exoplanets are known to exist around other stars. Of these, around 1,600 belong to a new class of planet that is entirely absent from our own solar system. Called super-earths, they have between two and 15 times the mass of the Earth. “We think that some of these super-earths might have interiors that are similar to the moons of Jupiter and Saturn,” says Giovanna Tinetti of University College London. This is because in some cases astronomers, when measuring the bulk density of these large planets, have discovered intermediate densities, meaning they can’t be solid rock or completely gaseous. Instead they have densities that suggest a large quantity of ice – or water – in their interiors. “We speculate that some of these super-earths might have an interior composition more similar to the moons of the giant planets. So basically, layers of ices, rocky material and also maybe oceans, both at the surface or in their interiors,” says Tinetti. She is currently overseeing the construction of the Ariel (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) mission for Esa. This space telescope will target super-earths for more detailed study upon its launch in 2029. So, the more information that Juice can return about the interiors of Jupiter’s icy moons, the more clues we may have to the interiors of these distant worlds, and whether or not they may be capable of supporting life. “It is important that with the objects we can reach, we go there and study them,” says Tinetti. “Because we can then use that information to connect to other worlds that we cannot visit anytime soon.” Nasa’s sister mission Juice was originally envisioned as a joint mission with Nasa. Soon after the Galileo mission ended in 2003, scientists in the US and Europe began working on a follow-up mission to further explore Jupiter’s remarkable icy moons. Named Laplace, the mission was worked on for several years before political breakdown forced the teams to go their separate ways. Nasa claimed Europa as the main focus for its mission, leaving the other icy moons for Esa to investigate. Juice is the first to make it to the launch pad but Nasa is not far behind. Nasa’s Europa Clipper is scheduled for launch late next year, yet it will arrive at Jupiter in 2030, one year before Juice. Because Europa is deeper in Jupiter’s magnetic field than Ganymede, the radiation environment is much more dangerous to spacecraft electronics. So instead of orbiting Europa, Clipper will dart in and out, making 44 close flybys during a three-and-a-half-year mission. Some of the flybys will be as low as 15 miles above Europa’s surface. Together Juice and Clipper will explore the icy moons. While they are technically separate missions, the collegiate spirit that blossomed between the scientists in the early days continues today. “We have a very good relationship with our colleagues at Nasa,” says Olivier Witasse, Esa’s project scientist for the Juice mission. A joint steering committee for the two missions has already highlighted many opportunities for the two spacecraft to work together once they are both in orbit. For example, one of Juice’s flybys of Europa will be followed a few hours later by a similar manoeuvre by Clipper. This will allow the teams to compare their data and calibrate their instruments, bringing their overall datasets more in line with each other. “We have already a great collaboration, and a lot of opportunities to do science together,” says Witasse. This article was amended on 5 March 2023. The photograph of Juice in Toulouse, France, is at an ESA facility, not its headquarters as an earlier version said.