Launching soon: ESA's Juice to probe Jupiter's moons for signs of possible life

We speak to professor deeply involved in multi-year, billion-plus-euro mission

Feature On December 4, 1973, NASA's Pioneer 10 became the first spacecraft to observe Jupiter up close, zipping by some 82,000 miles above the alien world's swirling clouds. Its twin probe, Pioneer 11, followed a year later, coming within a third of that distance to the gas giant.

Now the European Space Agency is about to try for an even closer look at the biggest planet in our locale, eventually at a distance of about 6,000 miles – and as low as 120 miles of Jupiter's moons.

Images from the earlier spacecraft captured ripples of red, orange, and brown stripes on Jupiter's surface, along with its Great Red Spot. Observations revealed new details about Jupiter's environment, including its huge and powerful magnetic field that trapped charged particles and accelerated them close to the speed of light. 

Electromagnetic energy emitted by the moving particles were concentrated into regions around the planet to form intense radiation belts. Three more spacecraft, Voyager 1, 2, and Ulysses, also flew by Jupiter and spotted its faint ring system, and never-before-seen moons. Astronomers began to realize Jupiter's satellites were a lot more complex when they detected hints of volcanic activity and signs of water on the natural satellites.

NASA sent its Galileo probe to orbit Jupiter in 1989, and it managed to pass by four of its major moons – Io, Europa, Ganymede, and Callisto – over the course of nearly eight years. Data collected by the spacecraft's instruments showed just how different and in some ways alike they were to one another. Scientists found evidence of oceans hidden beneath the icy crusts of Europa and Callisto. Volcanoes erupted on Io, and Ganymede generates its own magnetic field - the only moon in the Solar System to have one. 

Two other spacecraft, including Cassini and New Horizons, have flown by Jupiter since. Although NASA's Juno probe is still orbiting Jupiter, neither will study the Jovian moons in as much detail as ESA's Jupiter Icy Moons Explorer (Juice) scheduled for launch Thursday at 1215 UTC (1315 BST, 0815 EDT).

Update... The Juice launch was set back a day, to April 14, due to inclement weather. It successfully took off and is on its way to Jupiter.

"Juice's overarching theme is the emergence of habitable worlds around gas giants," the Euro space agency said.

Scientists are examining Jupiter and its moons as a mini solar system in its own right. They want to understand what conditions are needed for planets to form and life to emerge. Are Jupiter's moons potentially habitable, or were they ever? Could they support any kind of life, or did they?

It's hoped Juice's readings can help answer those questions.

Oceans on distant moons

"We are interested in whether the right conditions are present – and the key points are the presence of water, an energy source, the evidence of complex chemistry and stability in the system over time," Emma Bunce, professor of planetary plasma physics at the University of Leicester in England and a co-investigator working on Juice's magnetometer and ultraviolet spectrograph instruments, told The Register this week.  

"Previously, the NASA Galileo mission found evidence of subsurface oceans at Europa, Ganymede, and Callisto – but with a fairly small number of flybys and a scientific payload which was designed for much more general science objectives." 

Bunce said Juice, which will carry ten instruments and is the most advanced scientific payload flown to the outer Solar System yet, will be able to analyze whether the moons are habitable or not in greater detail. ESA is paying most attention to Ganymede; it's larger than the planet Mercury, and the most massive moon in the Solar System.

Ganymede has an internal liquid iron core surrounded by an outer shell of rock. In between its rock mantle and icy surface lies what could well be a hidden ocean that scientists believe could carry more liquid than all of the Earth's oceans combined. 

"Now with Juice we can go into orbit around Ganymede, which will make gathering evidence much easier. We can make a number of key measurements: magnetic field measurements relating to a global conducting ocean; and gravity field measurements using radio science experiments that can tell us about the mass and density distribution of different distinct layers in the interior," Bunce told us.

"Laser altimeter measurements can help to identify the shape of the moon and the tidal deformation, the source of energy that allows liquid water to be present underneath the surface; and near infrared spectroscopy can identify the chemistry of the surface in great detail."

Bunce wants to know why Ganymede is the only moon that has an internally driven magnetic field, and how deep and salty its oceans might be. The moon's magnetic field interacts with its host planet's field, and auroras are created in Ganymede's atmosphere.

"Earth has a magnetic field and a magnetosphere which plays an important role in protecting us from cosmic radiation and harmful effects of high energy particles emitted by the Sun. I am interested to learn about Ganymede's similarities and differences to our own planet's space environment," she said.

The second ocean world Juice will study is Europa, and it will analyze the moon's chemical composition and hunt for any biosignatures – chemicals or features that potentially indicate past or present signs of life.

Images of plumes of water vapor shooting from Europa's surface have been snapped by other spacecraft, and Juice will map any pools of liquid water underneath its icy surface.

The spacecraft won't spend much time around that moon, however, to avoid frying its instruments since the radiation levels in its environment are particularly harsh. NASA's upcoming Europa Clipper probe, expected to launch next year, will also observe the moon in closer detail if it gets there.

Juice can then spend more time on its secondary target: Callisto. The second-largest Jovian moon has a dark exterior mottled with white spots. Its surface is covered with craters from impacts with asteroids and comets.

Callisto doesn't seem to be geologically active, and is tidally locked with Jupiter. Astronomers believe its surface hasn't changed much over time, meaning it could betray secrets as to how Jupiter's system and other similar gas giant systems formed. 

Finally, Juice will also study Io and some of the smaller moons, such as Metis, Adrastea, Amalthea and Thebe. But since we haven't yet detected liquid water on those natural satellites, the spacecraft won't focus on them as much as Ganymede, Europa, and Callisto. 

Io is volcanically active, with more than 400 volcanoes belching gas, dust, and ionized matter into its atmosphere. All of this material gets entangled with Jupiter's magnetic field. Io can develop 400,000 volts of electric potential across it, and three million amperes of electric current, across its body, and generates lightning in Jupiter's upper atmosphere. 

Astronomers want to find out what Io's surface is made out of, how it interacts with Jupiter, and how the planet's smaller moons formed and if they change over time. 

Juice won't arrive at Jupiter until 2031

The €1.6 billion ($1.76 billion) Juice mission is ESA's largest and most ambitious mission in its Cosmic Vision program that will last until 2025.

Looking like an overly large washing machine with huge solar panel wings, Juice will be the first spacecraft to ever orbit around a moon other than our own. It will be the final probe, however, to be launched atop an Ariane 5 rocket as the European agency begins rolling out its next generation Ariane 6 rocket later this year.

Scientists and space fans eager for discoveries about Jupiter and its main moons will have to be patient. The voyage toward the gas giant from our home planet is long. To save on fuel, Juice will perform multiple gravity assists flying by the Earth and Moon in 2024, Venus in 2025, and Earth in 2026, and again in 2029, before it hopefully arrives at Jupiter in July 2031.

The Jovian system is treacherous. Jupiter's magnetic field is 20,000 times stronger than Earth's and stretches to a size nearly 15 times bigger than the Sun, making it one of the largest structures in the Solar System. Radiation levels are intense, and engineers have taken extra precautions to shield Juice's instruments and harden its electronics. At distances more than 484 million miles from the Sun, sunlight is significantly weaker than it is here on Earth, and Juice will have to try and collect as much energy as it can using its big solar arrays. 

Juice's instruments are: an optical camera; a visible and infrared spectrometer; a UV imaging spectrograph; a sub-millimetre wave instrument; a radar sounder; laser altimeter; its radio science experiment; a magnetometer; a particle environment package; and a radio and plasma instrument. It also supports an extra system dubbed the Planetary Radio Interferometer and Doppler Experiment that will allow scientists to study its position and speed in space using ground-based telescopes on Earth. 

The spacecraft will be in hibernation for most of its journey, and will only begin science operations about six months before it gets to Jupiter, where it'll spend another six months orbiting the gas giant. Next, it'll move on to observe Callisto and Europa between 2032 and 2034. Then, it'll orbit Ganymede for the remainder of its life before mission control sends its final commands to crash into the moon's surface to be destroyed in 2035. All in all, Juice will perform a total of 35 flybys around Jupiter's watery moons if everything goes to plan. 

The Juice mission milestones ... Source: ESA

Italian astronomer Galileo Galilei is credited with discovering the gas giant's four largest moons, Io, Europa, Ganymede, and Callisto, with a homemade telescope in 1610. He initially thought they were planets but later realized they were satellites. Through his observations, and following work by the likes of Copernicus, Galilei advanced the theory that astronomical objects orbited the Sun, challenging ancient beliefs that Earth was special and must be at the center of the Solar System.

Over four centuries later, astronomers have observed enough of the universe to realize that Earth is probably just one of many planets that are habitable, and that maybe moons might be, too. ®