JUICE: What secrets lie beneath the icy surface of Jupiters moons? – BBC Science Focus Magazine

Deep beneath the salty ocean, the seafloor is cracked. Hot gases from the layers below bubble into the water, sustaining colonies of microbial life that are eking out an existence far from the sunkissed surface.

This may sound like a scene from the bottom of Earths vast oceans, but its actually a possible description of Europa one of the icy moons orbiting Jupiter. And thanks to the upcoming Jupiter Icy Moons Explorer (JUICE) mission, we may finally have the opportunity to find out how accurate that description is.

Astrobiologists scientists who look for signs of life beyond the confines of our planet have long adhered to a simple mantra: follow the water. Thats because every living thing on Earth, from the tiniest bacterium to the mighty blue whale, needs liquid water to survive. While alien life without water may be possible, looking for that molecular marriage between hydrogen and oxygen is an excellent place to start.

In the hunt for HO, much has been made of the habitable zone the narrow ring around a star where the temperature is just right for liquid water. Earth sits in this region, so the majority of our water neither freezes nor boils. But the habitable zone is an imperfect concept.

At least five objects in the outer Solar System have sub-surface oceans, says Dr Mark Fox-Powell, an astrobiologist at the Open University. All are far beyond the outer reaches of the traditional habitable zone. Three of these oceans can be found beneath the surfaces on a trio of Jupiters moons: Europa, Ganymede and Callisto. Jupiter has a habitable zone of its own. The required heat isnt coming from the Sun, but from the gravity of Jupiter. It expands and contracts the moons, warming them up like squash balls.

Scores and fractures criss-cross Europas icy crust. The red-brown material is thought to be salt and sulphur compounds that have been modified by radiation Science Photo Library

While we have been to the Jovian system many times, these moons have rarely been the main attraction. The last time we were there studying them directly was with the Galileo spacecraft in the 1990s, says Fox-Powell. Instead, the focus has tended to fall on the giant planet itself. But now theres JUICE, a dedicated mission heading for its icy satellites.

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At the heart of the JUICE mission is a spacecraft thats been built by the European Space Agency (ESA). It bears a slight resemblance to a giant bird, with solar panel wings stretching out on either side of the spacecrafts main body. The sunlight at Jupiter is 30 times dimmer than the light that reaches Earth, so the panels need to be big. They cover an area equal to 85m, or about half the size of a volleyball court.

Its three-metre-diameter antenna will send the data JUICE collects back to mission control, although it will take almost two hours to travel the more than half a billion kilometres to Earth.

A scale model of the RIME antenna undergoes testing at the Hertz facility in the Netherlands ESA/ M Cowan

ESA had been working towards launching JUICE in 2022 until the coronavirus pandemic hit. Instead of the planned liftoff, this coming year will now see frantic activity as ESA scrambles to claw back the time lost during lockdown, and make the final preparations needed to ready the landmark mission for its rescheduled launch in 2023.

The original plan was for JUICE to take a convoluted route, involving five flybys of Earth, Venus and Mars to use the planets gravitational might to slingshot the spacecraft towards Jupiter, a journey that was set to take 7.5 years. ESA has yet to reveal exact details of the new timeline, but JUICE should arrive at Jupiter at the start of the 2030s. Once there, it will spend at least three years exploring Europa, Ganymede and Callisto. It will be joined by NASAs Europa Clipper mission, currently scheduled for launch in 2024 and arrival in April 2030.

Weve known about these moons for a long time. Along with Io the most volcanically active place in the Solar System Europa, Ganymede and Callisto make up the so-called Galilean moons, first seen by the Italian astronomer Galileo at the start of the 17th Century.

Of the trio that JUICE will focus on, Europa tends to steal the limelight. Its definitely the poster child of the Galilean moons, says Fox-Powell. Thats because beneath its icy crust sits an ocean that contains more liquid water than all of Earths seas, lakes and rivers combined. If theres life swimming around in our oceans, could the same be true of Europa?

Part of the problem is that the ocean is hiding beneath a thick, icy surface. We cant access it directly, says Fox-Powell. Thankfully, scientists think the icy crust and the water are interacting, a bit like the molten rock beneath Earths surface that breaks through during volcanic activity. It means we can use material on the surface to study the oceans indirectly, Fox-Powell says.

Plumes of water from the ocean below often erupt through Europas icy surface, as illustrated here. Science Photo Library

We may even be able to collect a sample of that material, despite JUICE being unable to land on Europa. The spacecraft is carrying 10 high-precision instruments to Jupiter, including the Particle Environment Package (PEP). Its designed to study dust and other molecules that have been kicked up from the surface, says Fox-Powell. Its not impossible that, if that material came from the oceans, it could contain molecules that are indicative of life.

If there are organisms in Europas oceans, then theyll need a source of energy. Hidden beneath the icy crust, they cant get that energy from the Sun. Fox-Powell sees two potential options. The Jovian system is an environment flooded with intense levels of radiation as Jupiters magnetic field slings and funnels high-energy particles around.

Any ocean material that ends up on the surface is going to be irradiated, Fox-Powell says. That changes the chemistry of the ice. One likely scenario is that the radiation is breaking water into hydrogen and oxygen, with that oxygen potentially seeping back down into the ocean below. Other potential by-products include compounds containing the element sulphur. On Earth, theyre known to support microbial life, says Fox-Powell. JUICE will help us to learn more about that ocean-surface boundary and to what extent the conditions are suitable for biology.

Alternatively, life may have colonised the ocean floor. On Earth, there are whole communities of organisms that thrive on the seabed without any sunlight whatsoever. The source of their energy is hydrothermal vents cracks in the boundary between the ocean and Earths hot interior. JUICE could help us see how geologically active Europas interior is.

While Europa grabs the lions share of public attention, its not JUICEs main target. The mission will only fly by Europa twice but will buzz past Callisto on 12 occasions. Callisto is the outermost of the four Galilean moons, so is least affected by Jupiters gravity and radiation. In contrast to Europa, whose surface is constantly reshaped by material welling up from beneath the ice, Callisto has the oldest surface in the Solar System. Unchanged for billions of years, its pockmarked by more impact craters than any other body orbiting the Sun.

Astronomers suspect that a 200km-deep ocean lies beneath Callistos ancient surface. This is where JUICEs Radar for Icy Moons Exploration (RIME) instrument will come into its own. It will transmit radio waves that can penetrate the icy shells of the Galilean moons down to a depth of around nine kilometres. From the way the radio waves are reflected back, we should be able to learn more about the moons internal structures.

Another approach will be provided by the Gravity and Geophysics of Jupiter and Galilean Moons (3GM) instrument. It will measure the gravitational fields of Callisto and the other icy moons, which will reveal how different layers of material including water are stacked up inside them.

JUICE will also use Callisto for a leg-up. Mission controllers will use the gravity of the moon to increase the spacecrafts inclination by about 30 so it can get a better look at Jupiters polar regions the source of Jupiters vast and intense magnetic field.

It is magnetism that determined where JUICE will spend the bulk of its time: Ganymede. Along with a dozen fly-bys, the spacecraft will also go into orbit around Ganymede and stay there for eight months. It will be the first time that a spacecraft from Earth has orbited a moon other than our own.

Ganymede is the most exciting body in the Solar System, says Prof Michele Dougherty, from Imperial College London. For one thing, its bigger than any other moon. In fact, its bigger than the dwarf planet Pluto and the planet Mercury. Like Europa, its also thought to have a sub-surface ocean that contains more water than we have on Earth.

Yet its Ganymedes magnetism thats the main attraction. Its unique among the moons of the Solar System for having a magnetic field of its own. Dougherty is the principal investigator for J-MAG an instrument on JUICE for measuring magnetic fields. J-MAG is located at the end of a 10.6m-long boom to keep it away from magnetic interference from the main spacecraft. Its sensitive electronics are locked inside a lead-lined vault to protect them from Jupiters intense radiation.

Auroral activity on Ganymede holds clues as to the magnetic influence of Jupiter NASA/ ESA

Dougherty wants to measure Ganymedes magnetic field in detail, including how it interacts with Jupiters own magnetic field. Astronomers using the Hubble Space Telescope have spotted auroral activity on Ganymede. The equivalent of the northern and southern lights on Earth, the auroras should wobble around Ganymedes poles due to the influence of Jupiters magnetism.

That they dont, suggests a sub-surface ocean of salty water on Ganymede thats conducting electricity and counter-balancing Jupiters magnetic might. Studying Ganymedes magnetic field could provide further clues about the size and nature of this ocean. In turn, that could help us understand if its a place that may be home to alien life.

Separating Ganymedes magnetic field from Jupiters is far from straightforward, though, particularly given how much the planet dominates its surrounding satellites. Its like trying to find needles in a haystack, Dougherty says, but theyre changing size, shape and colour all the time. Still, shes confident the team can pull it off. The flybys will be used to practise, with the really important data coming once JUICE settles into orbit around Ganymede. The results are going to be spectacular, Dougherty says.

If shes right, itll be the crowning achievement at the end of a long and winding road. Dougherty was previously involved in another flagship spacecraft: the Cassini mission to Saturn. Discussions about JUICE began in earnest in 2008, when Cassini had already been at Saturn for four years.

It was one of Saturns moons Enceladus that made people sit up and take notice. My team was instrumental in discovering that Enceladus has plumes of water vapour, Dougherty says. Water from a sub-surface ocean was being spat out into space, showing that its possible to find water beyond the traditional habitable zone.

The discoveries at Enceladus showed us that focusing on moons of the outer planets was a good thing to do. Soon a plan was hatched to get a closer view of Jupiters icy moons. Not that its all been plain-sailing. At one point during the pandemic, with labs closed, Doughertys team was building parts of J-MAG on their kitchen tables. Building an instrument is always stressful, but the pandemic took that stress to the next level, she says.

A simplified mock-up of the JUICE spacecraft is used to carry out tests Airbus/ Rolf Schwark

That effort is all the more remarkable given that the team will eventually destroy all of that hard work. Sometime in 2034, the spacecraft is likely to run out of propellant. Without any fuel, scientists will no longer be able to manoeuvre it around the Jovian system. So the team will do whats been done before with spacecraft like Cassini and the MESSENGER mission to Mercury: deliberately crash it.

By smacking into the surface of Ganymede, JUICE will provide one final experiment to see what this gargantuan moon is made of. Its days of exploring Jupiters icy moons will be over, but scientists will continue to pore over JUICEs collection of valuable data for a long time afterwards. In 20 years time, our understanding of these moons will be different, says Fox-Powell. JUICE is going to provide a real revolution. It could, finally, tell us whether or not were alone in this vast and often surprising Solar System.

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JUICE: What secrets lie beneath the icy surface of Jupiters moons? - BBC Science Focus Magazine