1. 12b. Saturn Jupiter & Saturn data Jupiter & Saturn seen from the Earth
Jupiter & Saturn rotation & structure
Jupiter & Saturn clouds
Jupiter & Saturn atmospheric motions
Jupiter & Saturn rocky cores
Jupiter & Saturn magnetic fields
Discovering Saturn’s rings
Structure of Saturn’s rings
Rings & shepherd satellites

2. Saturn Data (Table 12-2)

3. Saturn Data: Numbers Diameter: 120,000.km 9.26 . Earth
Mass: kg Earth
Density: water Earth
Orbit: km AU
Day: 10h.13m 59s Earth
Year: years Earth

4. Saturn Data: Special Features
Saturn is the second Jovian planet from the Sun
Saturn is the second largest Jovian planet
Saturn is dominated by a huge bright ring system
Saturn has no solid surface
~ 85% Jupiter’s diameter but only ~ 30% Jupiter’s mass
Saturn has a visually dull yet dynamic atmosphere
Great White Spot, belts & zones…
Saturn’s interior probably consists of three layers…
Atmosphere: Liquid molecular hydrogen
Mantle: Liquid metallic hydrogen
Core: “Metal” & “rock”
Saturn has 1 large & 17 small known moons
Titan has a dense, opaque nitrogen atmosphere

5. Saturn’s Rings are Easily Seen
Galileo Galilei 1610
His poor-quality telescope showed “handles” on Saturn
These disappeared by 1612
These re-appeared by 1613
He was unable to correctly identify these features
Christiaan Huygens 1655
His good-quality telescope showed thin, flat rings
Rings edge-on They become invisible
Rings tilted They become visible
Gian Domenico Cassini 1675
Dark band between the A & B rings Cassini division
Johann Franz Encke 1838
Dark band within the A ring Encke gap

6. Axial Tilt Gives Different Viewpoints
Saturn’s axis is tilted ~ 27° to its orbital plane
The ring system orbits in Saturn’s equatorial plane
Saturn orbits the Sun once in ~ 29.4 years
Every ~ 14.7 years, Saturn’s rings are seen edge-on
1995–1996
2008–2009
Every ~ 14.7 years, Saturn’s rings are seen at maximum tilt
2002–2003 We see the South side of the ring system
2015–2016 We see the North side of the ring system

7. Saturn Through a 1.5 m Telescope

8. Saturn’s Rings As Seen From Earth

9. Saturn’s Rings are Icy Fragments
Theory
James Clerk Maxwell 1857
The rings would be torn apart had they ever been a solid sheet
Observation
James Keeler 1895
Observed the Doppler effect on different parts of the rings
Confirmed that the rings obey Newton’s laws
Saturn’s rings have an albedo of ~ 0.80
Saturn’s clouds have an albedo of ~ 0.46
Ring particles range from 0.01 m to 5.00 m in diameter
Modal particle size is ~ 0.1 m in diameter Softball

10. Details of Saturn’s Ring System

11. The Roche Limit Context Competing gravitational forces
Applies only to objects held together by mutual gravity
Competing gravitational forces
Simple gravity between two objects
Traditionally measured from the center of mass
Differential gravity due to tidal forces
Traditionally measured from opposite sides
The theoretical Roche limit
Simple & differential gravitational forces are equal
Closer to the parent object Two objects are torn apart
Farther from the parent object Two objects stay together
The actual Roche limit
Saturn’s ring system is closer than the Roche limit

12. Saturn’s Rings are Thousands of Ringlets
The main ring system
The A & B rings look like a grooved phonograph record
The Cassini division is a very wide nearly empty band
The Encke gap is a very narrow nearly empty band
The F ring was discovered by Pioneer 11
Several intertwined stands ~ 10 km wide
A different perspective
Backscattering Our normal perspective from Earth
Relatively empty spaces look dark
Relatively full spaces look bright
Forward scattering Only possible from beyond Saturn
Relatively empty spaces look bright
Few particles are available to block transmission of sunlight
Relatively full spaces look dark
Many particles are available to block transmission of sunlight

13. Forward Scattering by Saturn’s Rings

14. Color Variations in Saturn’s Rings
All ring particles are very nearly pure white
The is expected of pure ices
Different sections of different rings exhibit color
The shades of color are very subtle
Computer enhancement increases saturation of these colors
Molecules causing the color are unidentified
Ringlet orbits must be rather stable
The colors show up in relatively wide bands

15. Enhanced Ring Color Variations

16. Saturn’s Inner Moons Affect the Rings
Independent satellites Mimas
Saturn’s moon Mimas orbits Saturn every 22.6 hours
A Cassini division particle orbits Saturn every 11.3 hours
Orbital resonance clears particles from the Cassini division
Similar to resonance between Jupiter’s Io, Europa & Ganymede
Shepherd satellites Pandora & Prometheus
These two moons shepherd F ring particles
Imbedded satellites Pan
Pan orbits Saturn within—& creates—the Encke gap
The countless ringlets probably have similar satellites

17. The F Ring’s Two Shepherd Moons

18. Saturn’s Atmospheric Properties
Visible features
Differential rotation
Much less color than Jupiter’s clouds
Possibly caused by additional atmospheric haze
Presence of belts [descending air] & zones [rising air]
Occasional short-lived storms
“White spots”
Three cloud layers Farther apart than Jupiter’s
Ammonia ice crystals
Ammonium hydrosulfide ice crystals
Water ice crystals
Extremely high wind speeds
~ 500 m . sec–1 near the equator
~ 67% the speed of sound in Saturn’s atmosphere

19. Saturn’s True Colors Seen By HST
1994

20. Cloud Layers of Jupiter & Saturn

21. Saturn’s Interior is Similar to Jupiter’s
Saturn is the most oblate of all planets
A total of ~ 9.8% shorter polar than equatorial diameter
Greater if Jupiter & Saturn had same interior structures
Jupiter has ~ 2.6% of its mass in a rocky core
Saturn has ~ 10% of its mass in a rocky core
Saturn has relatively little liquid metallic hydrogen
Too little mass to compress hydrogen enough
Saturn’s magnetosphere is relatively weak & empty
Not enough liquid metallic hydrogen
Saturn has no volcanic satellite
Few ions in Saturn’s magnetosphere

22. The Interiors of Jupiter & Saturn

23. Auroral Rings on Saturn From HST

24. Saturn Generates Its Own Energy
Two observations
Saturn emits more energy than it gets from the Sun
~ 25% more per kg than Jupiter
Saturn is distinctly deficient in helium
13.6% for Jupiter but only 3.3% for Saturn
One possible explanation
Helium is cold enough the condense in Saturn’s air
Helium precipitation falls to lower levers
Gravitational energy is converted into heat energy
Helium is permanently removed from Saturn’s upper atmosphere
This energy conversion equals Saturn’s excess heat

25. Saturn’s Moon Titan Has an Atmosphere
Titan data
Second largest satellite in the Solar System 5,150 km
Only satellite with a substantial atmosphere
Gerard Kuiper detects methane absorption spectrum 1944
Overall composition is ~ 90% N2
~ 1.5 x Earth’s pressure with ~ 10 x Earth’s gas
Weaker gravity does not compress gas as much
Titan is perpetually cloud covered
Titan’s surface is probably comparable to moonlight on Earth
Some implications
Hydrocarbon fog obscures visibility
Titan’s surface may be covered with hydrocarbon “goo”
Titan’s surface may have liquid hydrocarbon oceans
InfraRed radiation penetrates Titan’s clouds to “see” below

26. Saturn & Titan’s Atmosphere

27. Possible Hydrocarbon Seas on Titan

28. Saturn’s Six Icy-Surfaced Satellites
Mimas & Enceladus
Small
Tethys & Dione
Medium
Rhea & Iapetus
Large

29. Cassini/Huygens on Earth

30. Cassini/Huygens at Saturn

31. Cassini & Huygens Will Explore Saturn
A mission en route
Launched 15 Oct by a Titan IVB/Centaur rocket
Largest, heaviest & most complex US interplanetary spacecraft
Multiple gravity-assist maneuvers
Earth —> Venus —> Venus —> Earth —> Jupiter —> Saturn
The Cassini orbiter
Science observations begin 1 January 2004
Saturn Orbit Insertion 30 June 2004
Nominal end of science observations 1 July 2008
Extended mission ? ? ? ? ?
The Huygens lander
Lander separates from orbiter 25 December 2004
Lander enters Titan’s atmosphere 14 January

32. The Huygens Scientific Instruments
 Aerosol Collector & Pyrolyser (ACP)
Collect aerosols for chemical-composition analyses
Descent Imager/Spectral Radiometer (DISR)
Images & spectral measurements over a wide spectral range
A lamp in order to acquire spectra of the surface material
Doppler Wind Experiment (DWE)
Uses radio signals to deduce atmospheric wind properties
Gas Chromatograph & Mass Spectrometer (GCMS)
Identify & quantify various atmospheric constituents
High-altitude gas analyses
Huygens Atmosphere Structure Instrument (HASI)
Physical & electrical properties of the atmosphere
Surface Science Package (SSP)
Physical properties & composition of the surface

33. Important Concepts Saturn data Visually dominated by the ring system
~ 69% as dense as water
Saturn would float in a huge ocean
~ 30% Jupiter’s mass
Proportionally larger rocky core
~ 85% Jupiter’s diameter
Weaker gravity can’t compress gas
Visually dominated by the ring system
Countless mini-moons in “ringlets”
Very subtle colors in wide bands
The Roche limit
Tidal force = Mutual gravity force
Can break up comets & moons
Saturn’s moons
Independent, shepherd & imbedded
Almost all affect ringlet structures
Titan is largest in the Solar System
Dense & perpetually cloud-covered
Very rich in hydrocarbons
Saturn’s atmosphere
Same cloud layers as Jupiter
Spread out much more vertically Noticeably deficient in helium
Helium precipitation falls downward
Extremely high wind speeds
More excess heat per kg than Jupiter
Produced by falling helium droplets
Saturn’s interior
Generally similar to Jupiter
Much less liquid metallic hydrogen
Much weaker magnetosphere
Saturn’s moon Titan
Target of the Huygens probe
Enter Titan’s atmosphere Nov. 2004