Outer Planets and Their Moons

Overview of Outer Planet Moons

  • Number of Moons: Over 200 moons discovered in the solar system.
  • Location: Most orbiting planets and dwarf planets beyond the frost line.
    • Examples in the inner solar system:
    • Moon (Earth)
    • Phobos and Deimos (Mars)
  • Num. of Moons for Jovian Planets:
    • Jupiter: 79
    • Saturn: 82
    • Uranus: 27
    • Neptune: 14
    • Pluto (dwarf planet): 5 moons

Classification of Moons

  • Regular Moons:
    • Formed during planet formation, likely from the accretion disk.
    • Orbit in the equatorial plane, same direction as the planet's rotation.
  • Irregular Moons:
    • Likely captured by a planet.
    • Characteristics:
    • Non-equatorial orbital planes.
    • Opposite orbital directions compared to planet's spin.
    • Eccentric orbits.
    • Smaller than regular moons.
    • Usually irregularly shaped (often resembling potatos).
    • Process of capture may involve a third body or atmosphere to slow down the massive object.

Mimas (Saturn)

  • Discovery: By William Herschel in 1789.
  • Size: Radius ~ 200 km (125 mi).
  • Specific Gravity: 1.17 (mostly ice).
  • Notable Feature: Herschel crater (140 km wide).
    • Impact larger than this would have shattered the moon.
    • Central peak and distinct crater walls.

Enceladus (Saturn)

Orbital Characteristics:

  • Average orbit radius: 238,000 km
  • Eccentricity: 0.0047 (nearly circular orbit).
  • Little to no axial tilt relative to Saturn's equator.
  • Synchronous rotation with orbital period: 1.37 d.

Physical Characteristics:

  • Radius ~ 250 km (20% larger than Mimas).
  • Specific Gravity: 1.24 (mostly ice and rock).
  • Surface Features:
    • High albedo, few impact craters on surface.
    • “Tiger stripes” in the southern hemisphere indicating ice geysers.
    • Young surface underlies potential subsurface water.

Cryovolcanism:

  • Mechanism for resurfacing:
    1. Liquid seepage through cracks.
    2. Cryovolcanism: volcanism involving melted ice.

Internal Structure:

  • Ice outer shell, low-density rocky core, and a possible subsurface ocean (6 miles deep).
  • Evidence of active geysers ejecting water vapor and particles (400 m/s).
  • Exploration revealed organic compounds and gases in plumes.

Titan (Saturn)

Overview:

  • Largest moon of Saturn and slightly larger than Mercury.
  • Specific Gravity: 1.5 (indicating ice presence).

Atmospheric Characteristics:

  • Only moon with a substantial atmosphere.
  • Key Components:
    • Primarily nitrogen (N2), no oxygen.
    • Hydrocarbons (methane, ethane, etc.).

Huygens Probe:

  • Launched by Cassini, first probe to land in the outer solar system.
  • Took 2.5 hours to land, transmitting data for over an hour.
  • Surface temperature about 100 K (-280 °F).

Geological Features:

  • Ice “rocks” likely due to erosion by fluid (liquid methane) activity.
  • Evidence of liquid lakes and rivers of methane/ethane.

Miranda (Uranus)

  • Size: Radius ~ 240 km (150 mi).
  • Composition: About half water ice and half silicate rock.

Hypotheses for Surface Appearance:

  1. Result of colossal collision leading to haphazard reassembly.
  2. Surface melting from impact, allowing water to freeze upon exposure.

Exploration and Importance

  • Moons like Enceladus and Titan are of interest due to subsurface oceans and potential for life.
  • Ongoing research seeks to understand geological and hydrological processes on these moons, and their implication for astrobiology.