Imagine extraterrestrial micro monsters swimming, growing, and reproducing in an immense ocean of liquid water, trapped under an ice crust. This may sound like science fiction, but this could be happening right now —not in a far, far away galaxy, but in our own solar system. When Galileo Galilei discovered four Jupiter’s moons, he probably didn’t think he looked at entire habitable worlds. More than 400 years later, astronomers have gathered sufficient evidence to consider Ganymede and Europa great candidates to host life.  

Ganymede, Europa, and Callisto surfaces are characterized by thick ice crusts. Observations from the Hubble Space Telescope and NASA’s Galileo probe indicate that Ganymede, Europa, and Callisto may host great oceans of salty liquid water, each containing more water than all the Earth’s oceans combined. Moreover, these observations revealed the presence of water vapor in Ganymede’s atmosphere and spectacular plumes, presumably of water coming out of the bowels of Europa.     

These and other fascinating findings and other habitable conditions, such as Ganymede’s magnetosphere, motivate scientists from the European Space Agency to kick off the largest and most powerful mission to the Jovian system: Jupiter ICy moons Explorer (JUICE). The European probe will focus on Jupiter and its Galilean-icy moons: Ganymede, Europa, and Callisto. It will search for evidence of those interior oceans and their chemistry, characterizing the conditions that may have led to the emergence of habitable environments, as well as the diversity of processes in the Jovian system. 

ESA’s Jupiter Icy Moons Explorer, Juice, being unpacked from its shipping container in the Hydra clean room at ESA’s European Space Research and Technology Centre, ESTEC in the Netherlands on 30 April 2020. Credit: ESA.

“We won’t investigate the presence of life directly, because for that we would probably need a spacecraft that lands on the moons’ surface —such as the Huygens probe that actually landed on Titan— or even an ocean-explorer. However, JUICE has instruments powerful enough to investigate the conditions for emergency of life,” clarifies Dr. Miriam Rengel, astrophysics at the Max Planck Institute for Solar System Research.

In the solar system, searching for life as we know it makes sense. The ingredients of such life forms are liquid water, stable environments, the right organic chemistry, and some form of energy. If there is liquid water, there are temperature conditions for life. Still, the right chemistry and stability are needed for life to emerge. 

Galilean super moons, a collaborative system

JUICE will spend at least three years performing a series of flybys around Jupiter and its Galilean moons. But beyond investigating them individually, the mission will also focus on their collaborative complex behavior. 

The Jovian system is far from the sun. Yet, its icy moons potentially have liquid water oceans, and one of the reasons is Jupiter’s gravity. 

The gravitational interaction of the icy moons with Jupiter provides energy in the form of heat. Jupiter’s gravity is much stronger on the near side than on the far side of the moons. This difference—called tidal forces— deforms the moons, and this deformation produces friction that transforms into thermal energy. 

Another stunning example is how Ganymede offers protection from radiation to its sister Europa. For instance, although Europe has no permanent magnetic field, it enjoys the induced one by the powerful Ganymede’s and Jupiter´s magnetospheres. 

Ganymede’s and Europa’s gravitational pull also helps stabilize Io’s orbit, the closest moon to Jupiter.    

These interactions tell a story of the Jovian system’s evolution and capacity to sustain life. JUICE will investigate this interplay and its role in the emergence of the icy moons’ habitable environments. 

Complex interactions within the Jovian system. Credit: ESA

JUICE, the most complex and powerful mission ever launched to the Jovian system

JUICE will operate ten state-of-the-art instruments, some of which will measure in situ —for the first time in a mission to Jupiter—molecular and atomic quantum features such as vibrational and rotational spectra and isotopes ratios from the atmospheres and ice caps. “Isotope ratios provide a powerful tool  for understanding the origins and history of the planets and moons. With this data, we compare them to other objects across the solar system to understand if they were formed in the same way or differently,” Dr. Rengel sheds light. 

JUICE payload comprises cameras for imaging and other instruments such as spectrometers, sensors, a radar sounder, an altimeter, and a radio interferometer, which will detect radiation ranging from ultraviolet to microwave for sophisticated geophysical and meteorological measurements. Other measurements will include gravitational and magnetic fields.  

Dr. Miriam Rengel.

“With the Sub-millimetre Wave Instrument (SWI), we will investigate key isotope ratios and the presence of important molecules such as methane and water vapor in Jupiter’s stratosphere, unveiling the dynamics, circulation, and chemistry and their interactions in this poorly studied part of Jupiter’s atmosphere. We will also characterize the tenuous atmospheres of the Galilean moons in a unique and unprecedented manner, in terms of their horizontal distribution and vertical structure. We will be able to investigate satellite surface and subsurfaces and correlate them with atmospheric properties and geological features too, and even will be able to penetrate the ice caps up to around 14 cm. And, with this data, we will create models of their interiors,” explains Dr. Rengel, who works specifically with the SWI.

But what do we know so far of these extraordinary neighboring moons? Let’s surf on each of them.   

Ganymede, the largest moon

Ganymede is the largest moon in the solar system: it is way larger than our Moon and Pluto; it is even larger than Mercury and only three-quarters the size of Mars. 

Hubble Space Telescope Observations showed water molecules in the moon’s atmosphere. During its rotations, the temperature of the moon wildly varies from night and day. During the day, at the equator, the surface is hot enough to release water molecules into the atmosphere.

Ganymede is the only Jovian moon known to have a permanent magnetic field. This magnetic field causes auroras in the north and south poles. Because of its proximity to Jupiter, the gas giant’s magnetic fields embrace Ganymede, affecting Ganymede’s auroras behavior.  

Europa, the most promising habitable world  

According to Dr. Rengel, Europa is the strongest candidate until now to meet the four current astrobiologist habitability criteria or prerequisites for life to be sustained: liquid water, a stable environment, the right chemistry, and energy sources. 

Europa’s ice crust is much thinner than Ganymede’s, only 15 to 25 km. The vast ocean beneath presumably holds twice as much water as the Earth’s oceans. Hubble has collected evidence of plumes of water erupting from cracks in Europa. One of the main JUICE goals is to directly detect these water plumes and confirm water coming from under the thick layer of ice, where life may await to be unraveled.

Due to the thinner nature of its ice caps, Europa’s ocean probably has been exposed to interesting molecules and chemicals from other moons, especially from Io’s volcanoes.  

Researchers also believe that the plumes come from pockets of melted water in the ice crust, which can also be candidates for cozy homes for microorganisms. 

 Simulations predict a very rich geological activity in Europe. Observation shows that its surface is constantly reshaping, presumably by the movement of tectonic plates.  

Io, the Jovian hell, and Callisto, the freezing cold world

Io and Calisto appear to be less favorable for life as we know it.

Hosting hundreds of volcanoes and lakes of molten lava, Io is the most geologically violent world in the Solar System. Fountany eruptions have been spotted from Earth-based observatories. Although it is the smallest of the Galilean moons, it is still larger than our Moon. This powerful environment has permeated popular culture. Its fury was artistically depicted in the arrival of the astronauts to the Discovery spacecraft scene in the movie A Space Odyssey—2010, the 2001 sequel. JUICE will monitor Io’s volcanic activity and measure its surface composition. 

On the opposite side, Callisto is the outermost and second-largest Galilean satellite. It is the third-largest moon in the Solar System, after Ganymede and Titan. Moreover, it is larger than Mercury in size, but with much less mass. Although it is thought that Callisto may also have a large amount of water, it is not close enough to Jupiter to induce heat from its tidal gravitational force.  JUICE will investigate Callisto’s cratered surface in detail to find out more about its past activity.

“Overall, JUICE is a technically complex mission. It’s one of the largest and most complex missions ever launched into the Jovian system, with the most powerful instruments ever flown to the outer Solar System; it is larger and more complex than Juno (NASA) and, of course, more advanced than Galileo [the probe]. Technology has advanced tremendously since Galileo and JUICE’s instruments are powerful and have very high resolution, designed to investigate many aspects from water, oxygen, etc; and isotopologues containing carbon, nitrogen and sulfur for example,” Miriam Rengel expresses with enthusiasm.

JUICE will be launched in 2023. After a seven-year (7.6 years) journey, it will help confirm many of these models and probably surprise us with revolutionized knowledge, reshaping what we have learned about these icy worlds. 

Juice’s journey and Jupiter system tour. Credit: European Space Agency.

“The formation mechanisms of the atmospheres and water on these moons are believed to differ from those of Jupiter. An important goal of the JUICE mission is to understand the formation and evolution mechanisms of these moons and, in general, of the gas giants in the solar system. Moreover, the mission not only will help us investigate Jupiter and its moons, increasing our knowledge of the solar system, but also give us tools to understand other similar systems and the habitability of icy worlds outside the solar system. The Jovian system is an archetype to understand other similar systems, the basic conditions and the environments that can harbor life. JUICE is going to help us answer those kinds of fundamental questions,” Miriam Rengel concludes. 


Juice. (n.d.). Retrieved September 6, 2022, from

Scientists interviewed: Dr. Miriam Rengel, astrophysics at the Max Planck Institute for Solar System Research. She works with JUICE Sub-millimetre Wave Instrument (SWI).

Featured image: Artist composition of Jupiter and its Galilean moons with images of NASA Galileo probe and Voyager 1. Credit: NASA.

I tell science stories in English and Spanish. The same passion for science that once drove me to obtain a Ph.D. in physics, and work as a research scientist for fifteen years, inspired me to steer my career towards the popularization of science. Since 2014, I have been writing, editing, and managing outreach projects for several organizations. In 2021, I joined United Academics Magazine as Editor-in-chief. My favorite topics are theoretical physics, particle physics, climate change, astronomy, and space exploration.


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