Lecture 28: Galilean satellites: Subterranean oceans, massive volcanism etc.

Galilean Satellites Overview

  • Focus on Ganymede, Callisto, Europa, and Io.

  • Key features: subterranean oceans, volcanism, geologic activity.

Ganymede

  • Fully differentiated structure:

    • Iron core with layers of rock and ice.

    • Possible salt-water ocean ~200 km below the surface.

  • Reasons for differentiation:

    1. Faster accretion: Leads to heat burial.

    2. Greater tidal heating: Impacts Ganymede’s interior heating.

  • Evidence of differentiation:

    • Low moment of inertia indicates a specific internal structure.

Evidence of Subsurface Oceans

  • Induced magnetic fields detected:

    • Generated when a conductive material (e.g., liquid water) moves through magnetic fields.

  • Ganymede and Europa show variability in magnetic fields.

  • Connection to electrical conductivity:

    • Saltwater conducts electricity better than ice.

Orbital Dynamics of the Galilean Moons

  • Orbital Resonance:

    • Io: 4 orbits, Europa: 2 orbits, Ganymede: 1 orbit.

    • Resonance leads to increased eccentricity, enhancing tidal heating.

  • Callisto's orbit is almost circular, receiving less tidal heating.

  • Libration: Caused by variations in orbital speed, creating friction and heating.

    • Example: Comparing to kneading clay.

Europa

  • Proximity to Jupiter enhances tidal heating.

  • Possible ocean of liquid water beneath a thin icy crust (~30 km thick).

  • Surface features:

    • Lightly cratered with complex networks of cracks, suggesting potential tectonic activity.

  • Cryovolcanism: Some surface structures may indicate historical volcanic activity.

  • Importance of potential subsurface ocean for life:

    • Similar to Earth’s hydrothermal vents.

Ongoing Missions to Study Europa

  • Europa Clipper:

    • Launched October 2024, arrives April 2030.

    • Goals: Analyze icy shell, search for cryovolcanism, study subsurface ocean composition.

  • ESA JUICE Mission:

    • Launched April 2023, arrives July 2031.

    • Focus on subsurface oceans of multiple moons including Europa.

Io

  • Most volcanically active body in the solar system.

  • High density (3.53 g/cm³) indicates a rocky composition, lacking ice.

  • Surface constantly renewed by volcanism:

    • No impact craters due to constant resurfacing.

  • Eruption details:

    • Volcanic plumes can extend hundreds of km into space.

    • Volcanism predicted by tidal heating calculations.

  • Interior properties:

    • Fully differentiated with a hot mantle (~10-20% molten).

Saturn’s Rings

  • Rings composed of small icy particles, primarily water ice (~93%).

  • Formation theories:

    1. Ancient remains from a moons’ destruction.

    2. Material from original accretion disk.

    3. Disruption of moons by impact.

    4. Young features from recent impacts.

  • Roche Limit:

    • Distance where moons are torn apart by tides (about 2.5x larger body’s radius).

  • Rings’ cleanliness suggests a recent formation related to catastrophic impacts.

Ring Dynamics

  • Rings clump, potentially leading to moon formation;

  • Collisions between moons can cause further disruption and ring formation.

  • Observation Importance:

    • Clear rings indicate they might not be as old as the solar system due to the absence of dark material.

Upcoming Exploration

  • Continuing explorations of Saturn's moons and their potential features.

  • Suggested readings include Wikipedia entries on Saturn’s rings and its moons.