Astronomy 161: An Introduction to Solar System Astronomy Prof. Richard Pogge, MTWThF 2:30

Lecture 40:
The Saturn System

Key Ideas:

• 1 Giant Moon: Titan
• 59 Smaller Moons, mostly icy
• Those larger than 300km in diameter are round

Enceladus

• Young surface with water-ice fountains

Titan

• Thick Nitrogen & Methane atmosphere
• Lakes of liquid Methane

The Moons of Saturn

• 1 Giant Moon: Titan

59 smaller moons:

• Range in size from 2 to ~1500 km in diameter.
• Moons larger than 300 km in diameter are spherical.
• Moons smaller than 300 km in size are irregularly shaped.

• Mostly icy, or mixes of rock & ice, with mean densities of 0.3 g/cc to 1.5 g/cc.

I've chosen two moons for closer scrutiny in this lecture: Enceladus and Titan.

For the current census of the Saturnian Moon System, see Scott Sheppard's Saturn Satellite Page.

Enceladus

• Surface is covered with a layer of fresh, clean ice

• Makes for a thin atmosphere of water vapor with traces of Carbon Dioxide (CO₂), Nitrogen (N₂), and Methane (CH₄).
• Repaves the surface with fresh ices

Cryovolcanism

• One of only 3 geologically active moons (others are Io and Triton).

Source of heat for the cryovolcanism on Enceladus is Tidal Heating

• Enceladus is in a 2:1 orbital resonance with the large moon Dione (Enceladus completes 2 orbits for every 1 orbit of Dione).
• Heat melts subsurface water, which geysers out through cracks in the surface ice.

Ices from the fountains of Enceladus are responsible for building up the faint E-ring around Saturn.

Titan

• Radius: 2575 km (2^nd largest after Ganymede)
• Mean Density: ~1.9 g/cc
• Icy mantle over a rocky core.

Titan has a heavy atmosphere:

• Only Solar System moon with a heavy atmosphere.
• Cold enough to retain a heavy atmosphere despite it smaller size

Titan's Atmosphere

• 90-97% N₂ (nitrogen)
• ~1.6% CH₄ (methane)
• Mix of argon and hydrocarbons like Ethane

Cold, dense atmosphere:

• Temperature: 90 K (-297° F)
• Surface pressure: ~1.6 Earth atmospheres.
• Covered in a thick brown haze layer composed of photochemical aerosols ("tholins").
• Clouds of methane.

The Surface of Titan

• Cassini imaging, plus Huygens probe descent imaging and radar imaging.

Varied terrain:

• Smooth, dark plains (methane mud flats)
• Rugged highlands
• Extensive dune fields
• Drainage channels
• A few large impact basins (e.g. Circus Maximus)
• Liquid methane lakes

Methane is Titan's water

• 94 K is between between the boiling & freezing points of Methane
• Get gaseous methane in atmosphere.

• Images of the surface reveal dendritic ("branching") drainage flow channels carved by flowing liquid (methane or maybe ethane).
• The Huygens probe seems to have landed in a mud flat; unconsolidated ground made of liquid methane and water ice grains (the latter of which act like sand does on Earth).
• Liquid methane lakes have been seen in radar maps of the polar regions, some as large as the Caspian Sea or Great Lakes on Earth.

Supplement: Cassini & Huygens

• Went into orbit around Saturn in July 2004.
• Baseline mission is 4-year study through late-2008

Huygens Titan probe (January 2005):

• Carried by Cassini to Titan, and descended on 2005 Jan 15
• 2.5 hour descent by parachute through Titan's atmosphere, imaging and analyzing the composition on the way down.
• Landed on Titan's surface, returning pictures & chemical analysis of the surface before losing contact about 3 minutes after landing.