An Introduction to Solar System Astronomy
Prof. Scott Gaudi
The Moons of Saturn
Saturn has Bright Rings and 61 known moons
- 1 Giant Moon: Titan
- 60 Smaller Moons, mostly icy
- Those larger than 300km in diameter are round
- Young surface with water-ice fountains
- Thick Nitrogen & Methane atmosphere
- Lakes of liquid Methane
Rings of Saturn
- Orbiting chunks of ice & rock
The Moons of Saturn
To date, 60 moons have been found orbiting Saturn
59 smaller moons:
The composition of the smaller moons is mostly ices:
- 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.
All are heavily cratered, but Enceladus stands out for also having
very young terrain.
- 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 here: Enceladus and Titan.
For the current census of the Saturnian Moon System, see Scott
Enceladus has the brightest (shiniest) surface of any body
the solar system.
The fresh ice comes from the interior of Enceladus, fed by water-vapor
fountains spraying out of cracks in the surface.
- 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
The water fountains on Enceladus are an
example of Cryovolcanism, or ice volcanos.
- 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.
Saturn's only giant moon:
- Radius: 2575 km (2nd 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
- ~80% N2 (nitrogen)
- ~3% CH4 (methane)
- Mix of argon and hydrocarbons like Ethane
Cold, dense atmosphere:
Hazy clouds of methane, nitrogen, & suspended aerosols hide the
surface at visible wavelengths.
- Temperature: 94 K (-290° F)
- Surface pressure: ~1.6 Earth atmospheres.
- Clouds of methane & N2 ices
- High layer of hydrocarbon "smog"
Methane is Titan's water
Methane appears to play the same role on Titan that water does on the
High pressures allows Methane to condense into clouds that could
rain liquid methane?
- 94 K is between between the boiling & freezing points
- Get gaseous methane in atmosphere.
The most spectactular evidence is the recent (early 2006) discovery
of lakes on Titan revealed by the Cassini radar mapping experiment.
These could be lakes of liquid methane.
- Images of the surface reveal drainage flow channels
- The Huygens probe seems to have landed in a mud of methane and water ice.
The Surface of Titan
Mostly covered by clouds, but we can peer through at infrared
wavelengths and with radar:
Young surface with few impact craters.
- Cassini imaging, plus Huygens probe descent imaging
and radar imaging.
Cassini will make many passes by Titan during its multi-year mission,
using a combination of visible and IR cameras and radar imaging. Over
the next few years, a more complete picture of this strange world should
begin to appear from behind its clouds.
- Smooth, dark plains (methane mud flats or liquid methane seas?)
- Rugged highlands
- Drainage channels (methane rivers?)
- A few large impact basins (e.g. Circus Maximus)
The Rings of Saturn
Saturn's rings are not solid, but instead are composed of
billions of tiny chunks of ice and rock:
- Chunks orbit independently around Saturn like tiny ice
- Vary in size from a few centimeters up to many meters
- All packed close together, continually colliding, giving
them worn, irregular shapes.
- Shiny ices make the rings appear very bright.
We will study planetary rings in more detail in a separate lecture.
Supplement: Cassini & Huygens
Cassini spacecraft was launched in Oct 1997 on ambitious mission
to orbit and study Saturn, its rings, and its moons, specifically the
- Went into orbit around Saturn in July 2004.
- Baseline mission was 4-years; now has a two-year extension.
Huygens Titan probe (January 2005):
- Carried by Cassini to Titan, and descended in January 2005.
- 2+ hour descent by parachute through Titan's
atmosphere, imaging and analyzing the composition on the
- Landed on Titan's surface, returning pictures &
chemical analysis of the surface before losing contact.
The Cassini/Huygens mission is a joint US/European effort. Cassini
was built by NASA (JPL), and Huygens by ESA (European Space Agency). You
can learn more about Cassini at the Cassini-Huygens Mission to Saturn
web page at JPL. Information on
the Huygens probe is on the Huygens page at ESA.
Return to [
Unit 6 Index
Astronomy 161 Main Page
Copyright © Scott Gaudi,
All Rights Reserved.