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.