12b. saturn saturn data (table 12-2)...
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12b. Saturn • 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 95.3 ⋅ Earth • Density: 0.7 ⋅ water 0.13 ⋅ Earth
• Orbit: 1.4 ⋅ 109 km 9.53 AU
• Day: 10h.13m 59s 0.43 ⋅ Earth
• Year: 29.41 years 29.41 ⋅ Earth
Saturn Data: Special Features • 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: Liquid molecular hydrogen (H2) – Mantle: Liquid metallic hydrogen (H2) – Core: “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 1838 – Dark band within the A ring 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
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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 Two objects are torn apart • Farther from parent object Two objects stay together
• The actual Roche limit – Saturn’s ring system is closer than the Roche limit
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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
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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
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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 1944 • Overall composition is ~ 98.4% 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 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
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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 1 Jan 2004 – Saturn Orbit Insertion 30 Jun 2004 – Nominal end of science observations 1 Jul 2008 – Extended mission ? ? ? ? ?
• The Huygens lander – Lander separated from orbiter 25 Dec 2004 – Lander entered Titan’s atmosphere 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
• Saturn data – ~ 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
Important Concepts