12b. saturn
DESCRIPTION
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). - PowerPoint PPT PresentationTRANSCRIPT
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: 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 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 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 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
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
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
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