12b. Saturn
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Saturn
data
Saturn seen from the Earth
Saturn rotation & structure
Saturn
clouds
Saturn atmospheric motions
Saturn
rocky cores
Saturn
magnetic fields
Discovering Saturn’s rings
Structure of Saturn’s rings
Rings & shepherd satellites
Saturn Data (Table 12-2)
Saturn Data: Numbers
• Diameter: 120,000.km
9.26 ⋅ Earth
• Mass:
5.7 ⋅ 1026 kg
• Density:
0.7 ⋅ water
0.13 ⋅ Earth
• Orbit:
1.4 ⋅ 109 km
9.53 AU
95.3 ⋅ Earth
• Day:
10h.13m 59s
0.43 ⋅ Earth
• Year:
29.41 years
29.41 ⋅ Earth
Saturn Data: Special Features
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Saturn is the 2nd Jovian planet from the Sun
Saturn is the 2nd largest Jovian planet
Saturn is dominated by a bright ring system
Saturn has no solid surface
– ~ 85% Jupiter’s diameter but ~ 30% Jupiter’s mass
• Saturn has a bland yet dynamic atmosphere
– Great White Spot, belts & zones…
• Saturn interior consists of three layers
– Atmosphere:
– Mantle:
– Core:
Liquid molecular hydrogen (H2)
Liquid metallic hydrogen (H2)
“Metal” & “rock”
• Saturn has 1 large & 61 confirmed small moons
– Titan has a dense, opaque 98.4% N2 atmosphere
Saturn’s Rings are Easily Seen
• Galileo Galilei
1610
– Poor-quality telescope showed “handles” on Saturn
• They disappeared by 1612
• They re-appeared by 1613
– Galileo was unable to identify these features
• Christiaan Huygens
1655
– Good-quality telescope showed thin, flat rings
• Rings seen edge-on become invisible
• Rings seen tilted become visible
• Gian Domenico Cassini
1675
– Dark band between the A & B rings Cassini division
• Johann Franz Encke
– Dark band
within the A ring
1838
Encke
gap
Axial Tilt Gives Different Viewpoints
• Saturn’s axis is tilted ~ 27° to its orbital plane
– Rings are precisely in Saturn’s equatorial plane
– Saturn orbits the Sun once in ~ 29.4 years
• Every 14.7 years, Saturn’s rings are
edge-on
– 1995 – 1996
– 2008 – 2009
– 2023 – 2024
• Every 14.7 years, Saturn’s rings are 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
Saturn Through a 1.5 m Telescope
Jupiter & Saturn: A Comparison
Saturn’s Rings As Seen From Earth
Saturn’s Rings are Icy Fragments
• Hypothesis
– James Clerk Maxwell
1857
• Rings would be torn apart if they were a solid sheet
• Observation
– James Keeler
1895
• Measured 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 particle diameters from 0.01 m to 5.00 m
• Modal particle size is ~ 0.1 m in diameter
Softball
Details of Saturn’s Ring System
The Roche Limit
• Context
– Applies only to objects bound 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
parent object
• Farther from parent object
• The
actual
Two objects are torn apart
Two objects stay together
Roche limit
– Saturn’s ring system is closer than the Roche limit
The Rings are Thousands of Ringlets
• The main ring system
– 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
Normal perspective from Earth
• Relatively empty spaces look dark
• Relatively full spaces look bright
– Forward scattering
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
Forward Scattering by Rings
Color Variations in Saturn’s Rings
• All ring particles are very nearly pure white
– This is expected of pure ices
• Different sections of different rings exhibit color
– The shades of color are very subtle
• Computer enhancement increases color saturation
– Molecules causing the color are unidentified
– Ringlet orbits must be rather stable
• The colors show up in relatively wide bands
Enhanced Ring Color Variations
Saturn’s Inner Moons Affect Rings
• Independent satellites
Mimas
– Saturn’s moon Mimas
orbits Saturn in 22.6 hours
– Cassini division particle orbits Saturn in 11.3 hours
• Orbital resonance clears Cassini division particles
• 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
– Countless ringlets probably have similar satellites
• Probably < 1 km in diameter
The F Ring’s Two Shepherd Moons
Saturn’s Atmospheric Properties
• Differential rotation
• Much less color than Jupiter’s clouds
– Possibly caused by additional atmospheric haze
• Presence of belts [falling 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
Saturn’s True Colors Seen By HST
1994
Cloud Layers of Jupiter & Saturn
Saturn’s Interior is Like Jupiter’s
• Saturn is the most oblate of all the planets
– ~ 9.8% shorter polar than equatorial diameter
– Greater if Jupiter & Saturn had same 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 H2
– Too little mass to compress very much hydrogen
• Saturn’s magnetosphere is relatively weak
– Not enough liquid metallic hydrogen
– Saturn has no volcanic satellite
• Few sulfur ions in Saturn’s magnetosphere
The Interiors of Jupiter & Saturn
Auroral Rings on Saturn From HST
Saturn Generates Its Own Energy
• Two observations
– Saturn emits more energy than it gets from the Sun
• ~ 25% more per kg than Jupiter
– Saturn’s atmosphere is distinctly deficient in helium
• 13.6% for Jupiter but only 3.3% for Saturn
• One possible process
– Helium is cold enough the condense in Saturn’s air
• Helium precipitation falls to lower levers
– Gravitational energy is converted into heat energy
– Helium permanently removed from Saturn’s upper atmosphere
– Energy conversion equals Saturn’s excess heat
Saturn’s Moon Titan’s Atmosphere
• Titan data
– Second largest Solar System satellite
5,150 km
– Only satellite with a substantial atmosphere
• Gerard Kuiper detects CH4 absorption spectrum
• Overall composition is ~ 98.4% N2
• ~ 1.5 x Earth’s pressure with ~ 10 x Earth’s gas
1944
– Weaker gravity does not compress gas as much
– Titan is perpetually cloud covered
• Titan’s surface comparable to full moonlight on Earth
• Some implications
– Hydrocarbon fog & rain obscure surface visibility
– Surface may be covered with hydrocarbon “goo”
– Surface has liquid hydrocarbon oceans
• InfraRed radiation penetrates clouds to “see” surface
Saturn & Titan’s Atmosphere
Hydrocarbon Seas on Titan
Saturn’s Six Icy-Surfaced Satellites
• Mimas & Enceladus
– Small
• Tethys & Dione
– Medium
• Rhea & Iapetus
– Large
Cassini/Huygens on Earth
Cassini/Huygens at Saturn
Cassini & Huygens Explore Saturn
• The overall mission
– Launched 15 Oct. 1997 by a Titan IVB/Centaur
• Largest, heaviest, most complex interplanetary spacecraft
– Multiple gravity-assist maneuvers
• Earth ⇒ Venus ⇒ Venus ⇒ Earth ⇒ Jupiter ⇒ Saturn
• The Cassini orbiter
– Science observations began
– Saturn Orbit Insertion
– Nominal end of science observations
– Extended mission
1 Jan 2004
30 Jun 2004
1 Jul 2008
?????
• The Huygens lander
– Lander separated from orbiter
– Lander entered Titan’s atmosphere
25 Dec 2004
14 Jan 2005
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
Important Concepts
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Saturn data
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– ~ 69% as dense as water
– Same cloud layers as Jupiter
• Saturn would float in a huge ocean
• Spread out much more vertically
Noticeably deficient in helium
• Helium precipitation falls downward
– ~ 30% Jupiter’s mass
• Proportionally larger rocky core
– Extremely high wind speeds
– ~ 85% Jupiter’s diameter
• More excess heat per kg than Jupiter
• Produced by falling helium droplets
• Weaker gravity can’t compress gas
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Visually dominated by the ring system
– Countless mini-moons in “ringlets”
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• Much less liquid metallic hydrogen
• Much weaker magnetosphere
– The Roche limit
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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 interior
– Generally similar to Jupiter
• Very subtle colors in wide bands
• Tidal force = Mutual gravity force
• Can break up comets & moons
Saturn’s atmosphere
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Saturn’s moon Titan
– Target of the Huygens probe
• Enter Titan’s atmosphere Nov. 2004