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Lecture #27: Saturn The Main Point • Saturn: – General properties. Saturn is a large Jovian-class planet with a composition very much like the Sun, and is encircled by a beautiful and elaborate system of rings and other small satellites. – Atmosphere. Atmosphere – Interior. – Origin and evolution. • Reading: Chapters 7.1 (Saturn) and 11.1. Astro 102/104 1 Astro 102/104 The Jovian Planets Basic Properties of Saturn • • • • • • • • • • • • Astro 102/104 2 6 Average Distance from Sun: 1.43 billion km (a=9.54 AU). Orbital period: 29.46 years; eccentricity: 0.056. Period of Spin around axis: ~10 hours (varies with lat.) Tilt of Saturn's spin axis: 26.7° (seasons and tilted rings). Mass: 5.7x10 5 7x1026 kg = 94 ME ; Radius: 60,270 60 270 km = 9 9.4 4 RE Density = 0.7 g/cm3 (mostly H, He, small "rocky" core?) No "surface" like the terrestrial planets: only clouds visible. Gravity = 10.5 m/sec2 (1.07 times Earth's). Cloud-top temperature: 95 K (-178°C). Atmosphere: Mostly H, He (plus CH4, NH3, H2O, NH4HS, ...) Moons: 60+ known as of ’08 (many discovered since 2000!) Rings! Billions of icy moonlets all orbiting together. Astro 102/104 7 1 Observations Space Missions • Saturn is the most distant planet that can be seen without a telescope (if you know where to look...) • Through a small telescope, Saturn's yellow clouds and spectacular rings can be seen seen. • Features in Saturn's atmosphere are much more subtle than in Jupiter's. • 3 robotic spacecraft have flown past Saturn. • Cassini started orbiting Saturn in 2004. Mission Pioneer 11 Voyager 1 Voyager 2 Dates 1979 1980 1981 Goals and Results First Saturn flyby; studied magnetic field, clouds. Flyby; high resolution imaging and other studies. Flyby; high resolution imaging and other studies. Cassini 1997-2008 Orbiter; imaging, spectroscopy, magnetic fields, satellite radar mapping; Huygens entry probe landed on Titan 1/14/05. Astro 102/104 8 Astro 102/104 Atmospheric Circulation Features in Saturn's Atmosphere • Saturn has stronger equatorial winds than Jupiter. • Wind patterns & "shear" different than Jupiter's. • Zones, belts, and storms like Jupiter. • But smaller, lower contrasts, and shorter lifetimes. Astro 102/104 9 10 Astro 102/104 max > 900 mph! 11 2 Atmospheric Structure Atmospheric Composition • Determined by spectroscopy from telescopes and spacecraft. • Similar to Jupiter, but less H2O, more C–H. Name Hydrogen Helium Methane Ammonia Deuterated Hydrogen Deuterated Methane Ethane Phosphine Acetylene Propane Symbol H2 He CH4 NH3 HD CH3D C2H6 PH3 C2H2 C3H8 % Volume 94 6 8 x 10-4 2 x 10-4 5 x 10-5 2 x 10-5 5 x 10-6 1 x 10-6 2 x 10-8 1 x 10-10 } Thicker haze layer may explain lower contrast of cloud features on Saturn. The composition of Saturn is also very close to the composition of the Sun 100 0 Minor trace gases Astro 102/104 200 -100 H2O? 12 T (K) Astro 102/104 13 Saturn's Interior Saturn's Interior • Hydrogen and helium at immense pressures • Estimated using same methods as Jupiter... • Shape of Saturn particularly revealing: and temperatures. – Saturn is very oblate (a flattened sphere). Astro 102/104 16 Astro 102/104 Like for Jupiter, Saturn's interior structure is inferred from models & extrapolation of data from upper cloud layers. 17 3 Saturn's Magnetic Field Saturn: Internal vs. External Heat Sources • Examine the energy balance (heat in vs. heat out). • For Saturn: Outgoing ≈ 1.7 × Incoming. • The ratio of outgoing to incoming is about the same as it is for Jupiter. But because Saturn only receives about 25% as much sunlight as Jupiter, the heat source is unlikely to be related to the solar input. • Saturn must have a strong internal heat source: – Saturn too small to generate much heat from planetary contraction. – Primordial and radioactive heat not sufficient. – Heat released from interior phase changes! Helium condenses at high altitudes and rains down (differentiation). Atmosphere depleted in He. • Internal heat has a strong influence on atmospheric circulation. Astro 102/104 18 Satellites • • • • Saturn's metallic hydrogen interior and fast rotation generate substantial magnetic field. • Saturn has less metallic H than Jupiter so the field is weaker. • But Saturn's field is still about 1000x Earth's field. • Saturn's field is aligned with its spin axis, which supports the idea that the field is generated in the deep interior. Astro 102/104 19 Rings Huygens discovered Saturn's large satellite Titan (2575 km radius) in 1655. – Titan has a thick and complex atmosphere! 17 other smaller rocky/icy moons found between 1671 and 1990. Many more tiny outer moons discovered since 2000. More details in Lecture 30... • Saturn's rings are made of millions of housesized chunks of "dirty ice", all sharing similar orbits. • Origin? Unknown! More detail in Lecture 31... Astro 102/104 20 Astro 102/104 22 4 Summary • Saturn is a gas giant planet composed almost entirely of H and He in solar abundances. • Saturn's atmosphere has belts, zones, & storms, but smaller ll andd att lower l contrast t t than th Jupiter's. J it ' • Saturn's interior consists of liquid and metallic hydrogen, at enormous temperatures and pressures. • Saturn has ~50 known satellites, including one with a thick, organic-rich atmosphere (Titan). • Saturn has a complex and immense ring system. Power for the instruments on Cassini comes from a small nuclear reactor (~ 72 lbs of 238Pu) The Cassini Mission to Saturn Astro 102/104 23 Astro 102/104 24 Next Lecture... • Uranus: – General properties. – Atmosphere. – Interior. – Origin and evolution. • Reading: 7.1 (Uranus), 11.1. Astro 102/104 25 5