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 10 26 kg 95.3 Earth Density: 0.7 water 0.13 Earth Orbit: 1.4 10 9 km 9.53 AU Day: 10 h .13 m 59 s 0.43 Earth Year: 29.41 years 29.41 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 & zonesSaturn interior consists of three layers – Atmosphere: Liquid molecular hydrogen (H 2 ) – Mantle: Liquid metallic hydrogen (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 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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Page 1: 12b. Saturn Saturn Data (Table 12-2) 10/7/13teaching.up.edu/sci163/Phy163_2013_2Fa_12b_Saturn_Prn.pdf10/7/13! 1! 12b. Saturn • Saturn data • Saturn seen from the Earth • Saturn

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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