12b. Saturn

1. 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

2. Saturn Data (Table 12-2)

3. Saturn Data: Numbers • Diameter: 120,000.km 9.26 ⋅ Earth • Mass: 5.7 ⋅ 1026 kg 95.3 ⋅ Earth • Density: 0.7 ⋅ water0.13 ⋅ Earth • Orbit: 1.4 ⋅ 109 km 9.53 AU • Day: 10h.13m 59s0.43 ⋅ Earth • Year: 29.41 years 29.41 ⋅ Earth

4. 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… • Saturninterior 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

5. 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 • GianDomenico Cassini 1675 • Dark band between the A & B rings Cassini division • Johann Franz Encke1838 • Dark band within the A ring Encke gap

6. 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

7. Saturn Through a 1.5 m Telescope

8. Jupiter & Saturn: A Comparison

9. Saturn’s Rings As Seen From Earth

10. 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

11. Details of Saturn’s Ring System

12. 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

13. The Rings are Thousands of Ringlets • The main ring system • A & B rings look like a grooved phonograph record • The Cassini division is a verywidenearly 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

14. Forward Scattering by Rings

15. 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

16. Enhanced Ring Color Variations

17. 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

18. The F Ring’s Two Shepherd Moons

19. 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

20. 1994 Saturn’s True Colors Seen By HST

21. Cloud Layers of Jupiter & Saturn

22. 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

23. The Interiors of Jupiter & Saturn

24. Auroral Rings on Saturn From HST

25. 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

26. 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

27. Saturn & Titan’s Atmosphere

28. Hydrocarbon Seas on Titan

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

30. Cassini/Huygens on Earth

31. Cassini/Huygens at Saturn

32. Cassini & HuygensExplore 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 14Jan2005

33. TheHuygens 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

34. 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