Ganymede: The Icy World of Many Records

A moon of superlatives

Ganymede, discovered in 1610 by Galileo Galilei (1564-1642) along with the other three Galilean moons, holds several records in the solar system:

• Largest moon: Diameter of 5,262 km (larger than Mercury and Pluto)
• Only moon with its own magnetic field (discovered in 1996 by Galileo)
• Most water: Its subsurface ocean contains more water than all Earth's oceans combined
• Only moon with an ionosphere (discovered by Hubble in 1996)

Orbiting at 1,070,400 km from Jupiter (orbital period: 7.15 days), Ganymede is in orbital resonance with Europa and Io (ratio 1:2:4), which influences its geological activity.

Internal structure and composition

Data from the Galileo (1995-2003) and Juno (since 2016) missions have established a detailed model of Ganymede's internal structure:

• Ice crust: 800 km thick, mainly composed of water ice (H₂O) with traces of CO₂ and SO₂
• Subsurface ocean: 100-200 km deep, estimated salinity of 5 g/L (similar to Earth's oceans)
• Silicate mantle: 1,300 km thick, rich in olivine and pyroxene
• Metallic core: 500-600 km radius, composed of iron and sulfur (Fe-FeS)

The average density of 1.936 g/cm³ suggests a composition of 46% water ice, 34% silicates and 20% metallic materials.

The unique magnetic field

Ganymede is the only moon in the solar system to possess an intrinsic magnetic field, discovered in 1996 by the Galileo probe. Its characteristics:

• Intensity: 719 nT (at the equator), about 1/10th of Earth's magnetic field
• Inclination: 10° relative to the rotation axis
• Origin: Probably generated by a liquid metallic core in convection (dynamo model)
• Interaction with Jupiter: Creates a mini magnetosphere nested within Jupiter's

This magnetic field partially protects the surface from energetic particles in Jupiter's magnetosphere, creating polar auroras observed by Hubble.

The subsurface ocean and its habitability potential

Multiple evidence supports the existence of a subsurface ocean on Ganymede:

1. Magnetic data: Variations in the magnetic field detected by Galileo, compatible with a conductive saltwater layer beneath the ice
2. Aurora observations: Their movement suggests the presence of a salty ocean influencing the magnetic field (Saur et al., 2015)
3. Thermal models: Heating by tidal forces and radioactive decay could maintain the ocean in liquid state
4. Topography: Faults and chaos terrains suggest movements of the ice crust

This ocean, buried under 150 km of ice, could contain up to 25 times the volume of Earth's oceans. However, its salinity and chemical composition remain poorly understood.

Surface geology: A world of contrasts

Ganymede's surface shows a striking dichotomy between:

The grooves (sulci) are unique formations on Ganymede: parallel bands 5 to 20 km wide, thousands of kilometers long, probably caused by stretching of the ice crust.

Thin atmosphere and space environment

Ganymede has an extremely tenuous exosphere (pressure: 10^-9 bar), mainly composed of:

• Atomic oxygen (O) and molecular oxygen (O₂)
• Water vapor (H₂O)
• Hydrogen (H)

This atmosphere is produced by:

1. Sublimation of ice under solar radiation
2. Bombardment by energetic particles from Jupiter's magnetosphere
3. Micrometeorite impacts

Ganymede is also surrounded by a dust cloud detected by Galileo, probably resulting from micrometeorite impacts on its icy surface.

Exploration missions

The JUICE mission: A new era of exploration

The JUICE (JUpiter ICy moons Explorer) mission of the ESA, launched in April 2023, will mark a turning point in the study of Ganymede:

• 12 flybys of Ganymede between 2031 and 2034
• Final orbit around Ganymede in 2034 (first probe to orbit a moon other than our own)
• Key instruments:
  • RIME: Radar to probe the ice crust up to 9 km deep
  • MAJIS: Spectrometer to study surface composition
  • GALA: Laser altimeter to map topography
  • 3GM: Radio instrument to study gravity field and ocean
• Scientific objectives:
  • Characterize the subsurface ocean and its habitability potential
  • Study the dynamics of the magnetic field
  • Analyze the composition and geology of the surface
  • Understand the interaction with Jupiter's magnetosphere

Habitability potential and search for life

Although less publicized than Europa, Ganymede also presents exobiological interest:

• Advantages:
  • Potentially more stable ocean than Europa's (less subject to tidal forces)
  • Possible presence of organic matter detected by Galileo
  • Possible energy sources: radioactivity, tidal interactions
• Challenges:
  • Thickness of the ice crust (150 km) making ocean access difficult
  • Very low surface temperatures (-113°C to -193°C)
  • Poorly known ocean composition (salinity, pH, dissolved elements)

Scientists envision scenarios where microbial life forms could exist in Ganymede's ocean, particularly around potential hydrothermal vents at the ocean floor, similar to those found on Earth.

Comparison with other Galilean moons