Lecture #27: Saturn Saturn: General properties. Atmosphere. Interior. Origin and evolution. Reading: Chapters 7.1 (Saturn) and 11.1. The Main Point 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. Astro 102/104 1 Astro 102/104 2 The Jovian Planets Astro 102/104 6 Basic Properties of Saturn 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 26 kg = 94 M E ; Radius: 60,270 km = 9.4 R E Density = 0.7 g/cm 3 (mostly H, He, small "rocky" core?) No "surface" like the terrestrial planets: only clouds visible. Gravity = 10.5 m/sec 2 (1.07 times Earth's). Cloud-top temperature: 95 K (-178 C). Atmosphere: Mostly H, He (plus CH 4, NH 3, H 2 O, NH 4 HS,...) Moons: 60+ known as of 08 (many discovered since 2000!) Rings! Billions of icy moonlets all orbiting together. Astro 102/104 7 1
Observations 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. Features in Saturn's atmosphere are much more subtle than in Jupiter's. Space Missions 3 robotic spacecraft have flown past Saturn. Cassini started orbiting Saturn in 2004. Mission Dates Goals and Results Pioneer 11 1979 First Saturn flyby; studied magnetic field, clouds. Voyager 1 1980 Flyby; high resolution imaging and other studies. Voyager 2 1981 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 9 Features in Saturn's Atmosphere Zones, belts, and storms like Jupiter. But smaller, lower contrasts, and shorter lifetimes. Atmospheric Circulation Saturn has stronger equatorial winds than Jupiter. Wind patterns & "shear" different than Jupiter's. Astro 102/104 10 Astro 102/104 max > 900 mph! 11 2
Atmospheric Composition Determined by spectroscopy from telescopes and spacecraft. Similar to Jupiter, but less H 2 O, more C H. Name Symbol % Volume The composition Hydrogen H 2 94 of Saturn is also Helium He 6 very close to the Methane CH 4 8 x 10-4 Ammonia NH 3 2 x 10-4 Deuterated Hydrogen HD 5 x 10-5 Deuterated Methane CH 3D } composition of the Sun 2 x 10-5 Ethane C 2H 6 5 x 10-6 Minor trace gases Phosphine PH 3 1 x 10-6 Acetylene C 2H 2 2 x 10-8 H 2 O? Propane C 3H 8 1 x 10-10 Astro 102/104 12 Atmospheric Structure Thicker haze layer may explain lower contrast of cloud features on Saturn. T (K) Astro 102/104 13 200 100 0-100 Saturn's Interior Estimated using same methods as Jupiter... Shape of Saturn particularly revealing: Saturn is very oblate (a flattened sphere). Saturn's Interior Hydrogen and helium at immense pressures and temperatures. Astro 102/104 16 Like for Jupiter, Saturn's interior structure is inferred from models & extrapolation of data from Astro 102/104 upper cloud layers. 17 3
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 Saturn's Magnetic Field 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 Satellites 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... Rings Saturn's rings are made of millions of housesized chunks of "dirty ice", all sharing similar orbits. Origin? Unknown! Astro 102/104 20 More detail in Lecture 31... Astro 102/104 22 4
Summary The Cassini Mission to Saturn Power for the instruments on Cassini comes from a small nuclear reactor (~ 72 lbs of 238 Pu) Astro 102/104 23 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 and at lower contrast t than Jupiter's. 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. Astro 102/104 24 Uranus: General properties. Atmosphere. Interior. Origin and evolution. Next Lecture... Reading: 7.1 (Uranus), 11.1. Astro 102/104 25 5