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 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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