Jupiter and Icy Moons Explorer

2022 will see ESA launch a mission
to explore Jupiter's moons. Ian Moss notes that
not all moons are the same…



Quick links to sections below:-
                    Launch vehicle and trajectory
                    The long wait
                    The JUICE scientific instruments
                    Previous missions to the gas giants


Luna: our moon. The very name evokes a sense of mystery, beauty and romance. A beacon in the night that waxes, wanes and exercises unseen power over the ocean tides on Earth.

Close-up it is a cratered rock; mountainous, arid, lifeless and unchanging. In the 400 or so years since Galileo first saw the larger moons of Jupiter, we have discovered many further moons associated with other planets in our solar system. Until recent years, they were thought of as “just moons”, over-shadowed in our minds by their spectacular home planets. However, several missions* to the gas giants have shown us that the picture is more complex. The largest moons of Jupiter: Ganymede, Europa, Callisto and Io, are not simply rocky satellites of the planet but are worlds in their own right with complex geology, meteorology and perhaps, biology.

Since 2012 the European Space Agency (ESA) has been developing the Jupiter Icy Moons Explorer (JUICE). This craft will undertake, from orbit, an extensive survey of Jupiter and its three largest moons: Callisto, Europa and Ganymede. The scientific mission objectives include:

  • Mainly for Ganymede and Callisto, investigation of the composition and properties of ocean layers and their icy crusts, as well as attempts to detect sub-surface water reservoirs.
  • Study of Ganymede’s magnetic field and how it interacts with Jupiter’s magnetosphere.
  • For Europa, the focus is on chemistry that is essential to life including analysis of the composition of non-water ice and measurement of the thickness of the icy crusts.
  • A detailed study of Jupiter’s atmosphere; including its circulation, meteorology and chemistry. These studies may provide insight into how the planet affects the stability of the environment of the three moons over an extensive, geological time scale.

The broader implication of all these investigations is that the Jovian system may provide a model for habitable worlds that are satellites of gas-giant planets in other star systems.


Ariane 6.  © ESA.


Launch vehicle and probe craft trajectory
Launch is planned for mid-2022. The launch vehicle will be one of the redoubtable Ariane series taking off from French Guiana. The probe craft will be released into a trajectory which will provide five gravity assists from inner planets. One each from Mars and Venus and three from Earth. This phase will take about four years; the craft will then spend five years heading to the Jovian system. It will spend two and a half years carrying out its scientific mission before running out of fuel and crashing on the surface of Ganymede.

JUICE probe craft. © ESA/ATG medialab.

The JUICE craft carries its own motors and propellant to enable it to manoeuvre around Jupiter and the Galilean moons. Electrical power for the craft and the science instruments is provided by a solar panel array as shown above. Each panel is about 3.5 x 2.5 metres.


Overview of Jupiter and the Galilean moons

Jupiter imaged from London, August 2021 by the writer. © I. Moss, 2021.



More than twice the size of all the other planets in our solar system combined, Jupiter easily owns the title of gas giant. With a year length of more than 4000 days, Jupiter orbits the Sun at a range of about 800 billion kilometres. The familiar characteristic bands and spots can be seen, even with small telescopes on Earth, that are caused by swirling ammonia and water in an atmosphere largely composed of hydrogen and helium. The famous “great red spot” is a storm that has been raging for hundreds of years, though in recent times it appears to be getting smaller. The nature of Jupiter’s core is unclear at present but it seems likely that it is a dense mixture of elements surrounded by liquid hydrogen. Jupiter’s gravity and radiation profoundly affect its moons; analysis of data from the JUICE probe may provide further insight into these interactions.

Callisto. © ESA.

Callisto, only slightly smaller than the planet Mercury, is the second largest of the Galilean moons. Its orbit is about 1,900,000 km from Jupiter. Comprised of roughly equal amounts of rock and frozen material it is extensively cratered. The largest crater, named Valhalla, is 3,000 km in diameter. Calisto differs from the other three moons in that it does not share their orbital resonance system with Jupiter and so does not experience tidal heating to the same extent.







Europa. © ESA.


Europa, slightly smaller than our moon, orbits Jupiter at around 671,000 km. It is largely composed of silicate rock and, intriguingly, has a thin atmosphere composed mainly of oxygen. On the surface there are red-brown coloured regions, thought to be caused by sulphur. There is also an extensive covering of frozen water, beneath which there may be liquid water. These findings have encouraged speculation about discovering extra-terrestrial life. The JUICE probe should give us further insight into this possibility.



Ganymede. © ESA.

Larger than the planet Mercury, Ganymede is the largest moon in the solar system. It orbits Jupiter at a distance of about 1,070,000 km. Like Europa, Ganymede is a world of siliceous rocks and water ice over a core of liquid iron. It is believed that a liquid, salt-water, ocean sandwiched between layers of ice exists 200 km below the surface. The thin atmosphere is substantially comprised of oxygen: atomic oxygen (O), molecular oxygen (O2) and possibly also ozone (O3) Atomic hydrogen (H) is also present in relatively low amounts.



Io. © ESA.

Although not a primary target of the JUICE mission, we should not forget Io. Slightly larger than our moon, Io is a world of Himalayan-scale mountains and volcanoes with an atmosphere of sulphur dioxide. No coating of ice for Io, it is mostly siliceous rock over a core of molten iron or iron sulphide. As if that were not sufficiently inhospitable, it is also the nearest moon to Jupiter which it orbits at around 422,000 km. Consequently, Io is heavily subjected to Jupiter’s gravity, magnetic field and radiation.




The long wait
Interpreting the data that the JUICE craft sends back to Earth will go on a lot longer than the scheduled ten years of mission time. We may eventually discover that life-sustaining conditions exist in some regions of one or more of the Jovian moons but direct evidence of extant, or past, life seems too much to ask for. A logical next step would be to land craft to perform in situ analyses of geological layers containing liquid or frozen water.

In decades to come, it may be possible to send samples back to Earth. That would require a major technological leap in order to repatriate a craft from such a vast distance with all the complexities of the Jovian environment to overcome. Even these problems will be minor compared to the difficulties of sending and returning a manned mission. A scale of life-times rather than decades would be a more realistic vision of achieving this.


The JUICE Scientific Instruments

Janus is an optical camera that will image the landscape of the moons and map the clouds of Jupiter.

MAJIS (Moons and Jupiter Imaging Spectrometer)
Majis is a spectrometer operating in the infra-red and visible region of the electromagnetic spectrum. It will analyse the clouds of Jupiter and the minerals and frozen material on the moons.

UVS (UV imaging Spectrograph)
Moving into the ultra-violet, this spectrometer will be used to study the composition of the upper Jovian atmosphere and its aurorae.

SWI (Sub-millimetre Wave Instrument)
This instrument will also be used to study the atmospheres of the planet and its moons using sub-millimetre wavelengths (commonly called terahertz) to investigate temperature, structure and composition. It will also look at the icy surfaces of the moons.

GALA (Ganymede Laser Altimeter)
The Gala Laser Altimeter will principally investigate the tidal deformation of Ganymede’s surface and will also be used to study the surface features of Europa and Callisto.

RIME (Radar for Icy Moons Exploration)
This is an ice penetrating radar capable of providing information about the moons’ sub-surfaces down to a depth of 9km.

J-MAG (Juice Magnetometer)
J-MAG is a magnetometer that will be used to study the sub-surface oceans of the icy moons and also the Jovian magnetic field and its interaction with the internal magnetic field of Ganymede.

RPWI (Radio & Plasma Wave Investigation) & PEP (Particle Environment Package)
These two instruments will investigate radio emissions and the plasma environment of the planet and the moons.

3GM (Gravity & Geophysics of Jupiter and Galilean Moons)
Gravity and geophysics will be studied using a transponder operating in the microwave (Ka) region of 26.5–40 GHz. This will provide information on the atmospheres of the planet and moons and on the extent of the internal oceans of the icy moons.

PRIDE (Planetary Radio Interferometer & Doppler Experiment)
Further study of the gravity fields of Jupiter, using precise information about the position and velocity of the JUICE spacecraft, will be conducted with Very Long Baseline Interferometry and the telecommunication system of the spacecraft.


The interpretation of the data from these instruments will make the 2030s an interesting time.


*Previous missions to the gas giants
(and their launch dates)
          - Pioneer 10 (1972)
          - Pioneer 11 (1973)
          - Voyager 1 (1977)
          - Voyager 2 (1977)
          - Ulysses (1990)
          - Galileo (1989)
          - Cassini (1997)
          - New Horizons (2006)
          - Juno (2011)

Ian Moss

Ian Moss is retired from a career in biotechnology.  He is an amateur astronomer with a particular interest in imaging planets and deep-space objects, mostly from his home in a light-polluted London suburb!  Ian is a member of Havering Astronomical Society which has a membership spanning from beginners to professionals.  The Society is active in out-reach to the local community.


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