Galilean Moons Lecture Notes

Galilean Moons Overview

• The four largest moons of Jupiter are known as the Galilean moons: Io, Europa, Ganymede, and Callisto.

  • Mnemonic for remembering their order: I Eat Graham Crackers

• Discovered by Galileo Galilei around 1610 using a telescope with magnification of approximately 20.

• The moons are visible with a typical backyard telescope today.

• Galileo’s observations supported the Copernican (heliocentric) model, showing that moons orbit around Jupiter.

• Formed from material in an accretion disk around Jupiter during its formation.

• Evidence for this formation:

  • All moons follow nearly circular orbits in Jupiter's equatorial plane.

  • Orbits align with Jupiter's counterclockwise (CCW) rotation.

Orbital Dynamics and Properties

• The moons exhibit a 4:2:1 orbital resonance:

  • For every 1 orbit of Ganymede around Jupiter, Europa completes 2 orbits, and Io completes 4 orbits.

• Orbital resonance causes periodic gravitational interactions that:

  • Change their orbits to be more elliptical over time.

  • Lead to tidal forces, providing internal energy for some moons.

• The inner moons (Io, Europa, Ganymede) are tidally locked to Jupiter, with synchronous rotation—same rotation period as orbital period.

  • Tidal heating is significant for Io, Europa, and Ganymede, causing geological activity.

  • Callisto is too far for significant tidal flexing, making it geologically inactive.

Surface Characteristics

Io

• Noted for smooth surfaces and over 400 active volcanoes—most volcanically active body in the solar system.

• Interior:

  • Consists of a metallic core and a rocky mantle.

  • Estimated mantle temperature: approximately 1200 ext{ °C}.

• Volcanism driven by:

  • Tidal forces from Jupiter create heat through friction, leading to volcanic activity.

• Surface colors are influenced by sulfur compounds.

Europa

• High reflectivity (albedo ~ 0.7) indicates:

  • Surface primarily composed of water ice.

  • Relatively young surface owing to resurfacing processes.

• Evidence for a subsurface ocean:

  • Lack of impact craters, smoother surface with long fractures implying movement of water.

  • Estimated ocean depth: up to 100 ext{ km}.

• Surface ice thickness: between 15 – 25 km.

• Home to cryovolcanism; water plumes suggest subsurface activity.

Ganymede

• Largest moon: 50% larger in radius than Earth’s Moon.

• Unique for having a global magnetic field, suggesting a liquid, conducting interior.

• Interior structure:

  • Core radius of 500 ext{ km}, estimated temperature around 1700 ext{ K}.

  • Possible subsurface ocean, thickness of 100 ext{ km}, possibly containing more water than Earth's oceans.

Callisto

• Ancient, heavily cratered surface with a history of little geological alteration.

• Density is uniformly distributed, indicating a lack of metallic core and a homogeneous mix of rock and ice in its interior.

• Considered geologically dead but may possess a liquid layer beneath its icy surface.

Notable Missions

Galileo Spacecraft

• Operational from 1989 - 2003, studying Jupiter and its moons.

• First spacecraft to visit asteroids and provide direct observation of comet impacts.

• Major discoveries:

  • Volcanism on Io, evidence of a subsurface ocean on Europa, and a magnetic field around Ganymede.

New Horizons

• Launched January 19, 2006, primarily to study Pluto and the Kuiper Belt.

• Conducted gravity assists and scientific studies at Jupiter in 2007.

Conclusion

• The Galilean moons present varied geological processes influenced by their dynamics and interactions with Jupiter, making them significant objects of study in understanding planetary formation and the potential for life beyond Earth.