12b. Saturn Saturndata Saturnseen from the Earth Saturnrotation & structure Saturnclouds Saturnatmospheric motions Saturnrocky cores Saturnmagnetic fields Discovering Saturn’s rings Structure of Saturn’s rings Rings & shepherd satellites

Saturn Data (Table 12-2)

Saturn Data: Numbers Diameter:120,000.km9.26 ⋅ Earth Mass:5.7 ⋅ kg95.3 ⋅ Earth Density:0.7 ⋅ water0.13 ⋅ Earth Orbit:1.4 ⋅ 10 9 km9.53 AU Day:10 h.13 m 59 s 0.43 ⋅ Earth Year:29.41 years29.41 ⋅ Earth

Saturn Data: Special Features Saturn is the 2 nd Jovian planet from the Sun Saturn is the 2 nd 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:Liquidmolecularhydrogen (H 2 ) –Mantle:Liquidmetallichydrogen (H 2 ) –Core:“Metal” & “rock” Saturn has 1 large & 61 confirmed small moons –Titan has a dense, opaque 98.4% N 2 atmosphere

Saturn’s Rings are Easily Seen Galileo Galilei1610 –Poor-quality telescope showed “handles” on Saturn They disappeared by 1612 They re-appeared by 1613 –Galileo was unable to identify these features Christiaan Huygens1655 –Good-quality telescope showed thin, flat rings Rings seenedge-onbecome invisible Rings seentiltedbecome visible Gian Domenico Cassini1675 –Dark band between the A & B ringsCassini division Johann Franz Encke1838 –Dark band within the A ringEncke 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 areedge-on –1995 – 1996 –2008 – 2009 –2023 – 2024 Every 14.7 years, Saturn’s rings areat maximum tilt –2002 – 2003We see the South side of the ring system –2015 – 2016We 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 Maxwell1857 Rings would be torn apart if they were a solid sheet Observation –James Keeler1895 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 diameterSoftball

Details of Saturn’s Ring System

The Roche Limit Context –Applies only to objects bound by mutual gravity Competing gravitational forces –Simplegravity between two objects Traditionally measured from the center of mass –Differentialgravity due to tidal forces Traditionally measured from opposite sides ThetheoreticalRoche limit –Simple & differential gravitational forces are equal Closer toparent objectTwo objects are torn apart Farther fromparent objectTwo objects stay together TheactualRoche 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 divisionis a very wide nearly empty band The Encke gapis a very narrow nearly empty band –The F ring was discovered by Pioneer 11 Several intertwined stands ~ 10 km wide A different perspective –BackscatteringNormal perspective from Earth Relativelyemptyspaces look dark Relativelyfullspaces look bright –Forward scatteringPossible from beyond Saturn Relativelyemptyspaces look bright –Few particles are available to block transmission of sunlight Relativelyfullspaces 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 satellitesMimas –Saturn’s moon Mimasorbits Saturn in 22.6 hours –Cassini division particleorbits Saturn in 11.3 hours Orbital resonance clears Cassini division particles Resonance between Jupiter’s Io, Europa & Ganymede Shepherd satellitesPandora & Prometheus –These two moons shepherd F ring particles Imbedded satellitesPan –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 –Ammoniaice crystals –Ammonium hydrosulfideice crystals –Waterice 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 H 2 –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 satellite5,150 km –Only satellite with a substantial atmosphere Gerard Kuiper detects CH 4 absorption spectrum1944 Overall composition is ~ 98.4% N 2 ~ 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

Cassini/Huygens at Saturn

Cassini & Huygens Explore Saturn The overall mission –Launched 15 Oct 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 began1 Jan 2004 –Saturn Orbit Insertion30 Jun 2004 –Nominal end of science observations1 Jul 2008 –Extended mission? ? ? ? ? The Huygens lander –Lander separated from orbiter25 Dec 2004 –Lander entered Titan’s atmosphere14 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 Important Concepts