Jupiter And Saturn

Jupiter and Saturn12Guiding Questions31. Why is the best month to see Jupiter different from one yearto the next?2. Why are there important differences between the atmospheresof Jupiter and Saturn?3. What is going on in Jupiter’s Great Red Spot?4. What is the nature of the multicolored clouds of Jupiter andSaturn?5. What does the chemical composition of Jupiter’s atmosphereimply about the planet’s origin?6. How do astronomers know about the deep interiors of Jupiterand Saturn?7. How do Jupiter and Saturn generate their intense magneticfields?8. Why would it be dangerous for humans to visit certain parts ofthe space around Jupiter?9. How was it discovered that Saturn has rings?10.Are Saturn’s rings actually solid bands that encircle the planet?11. How uniform and smooth are Saturn’s rings?412.How do Saturn’s satellites affect the character of its rings?Unlike the terrestrial planets, Jupiter andSaturn exhibit differential rotationJupiter and Saturn are the most massiveplanets in the solar system Jupiter and Saturn areboth much larger thanEarth Each is composed of 71%hydrogen, 24% helium,and 5% all other elementsby mass Both planets have ahigher percentage ofheavy elements than doesthe Sun Jupiter and Saturn bothrotate so rapidly that theplanets are noticeablyflattened561

Atmospheres The visible “surfaces” of Jupiterand Saturn are actually the topsof their cloudsSpacecraft images show remarkableactivity in the clouds of Jupiter and Saturn The rapid rotation of the planetstwists the clouds into dark beltsand light zones that run parallelto the equator The outer layers of both planets’atmospheres show differentialrotation– The equatorial regions rotateslightly faster than the polarregions For both Jupiter and Saturn, thepolar rotation rate is nearly the7same as the internal rotation rate8Storms in Saturn’s atmosphere seem tobe shorter-livedStorms Both Jupiter and Saturnemit more energy thanthey receive from theSun Presumably both planetsare still cooling The colored ovals visiblein the Jovianatmosphere representgigantic storms Some, such as the GreatRed Spot, are quitestable and persist formany years9The internal heat of Jupiter and Saturnhas a major effect on the planets’atmospheres – where does this energycome from?10Jupiter and Saturn’s Atmospheres There are presumed to be three cloud layers in theatmospheres of Jupiter and Saturn The reasons for the distinctive colors of thesedifferent layers are not yet known The cloud layers in Saturn’s atmosphere are spreadout over a greater range of altitude than those ofJupiter, giving Saturn a more washed-out appearance Saturn’s atmosphere contains less helium thanJupiter’s atmosphere This lower abundance may be the result of heliumraining downward into the planet Helium “rainfall” may also account for Saturn’ssurprisingly strong heat output11122

The oblateness of Jupiter and Saturn revealstheir rocky cores13Metallic hydrogen inside Jupiter and Saturnendows the planets with strong magnetic fields Jupiter and Saturn have strong magnetic fields created bycurrents in the metallic hydrogen layer Jupiter’s huge magnetosphere contains a vast current sheetof electrically charged particles Saturn’s magnetic field and magnetosphere are much lessextensive than Jupiter’s1514Jupiter and Saturn have extensive magnetospheres The Jovian magnetosphere encloses a low-density plasma of chargedparticles The magnetosphere exists in a delicate balance between pressures fromthe plasma and from the solar wind16 When this balance is disturbed, the size of the magnetospherefluctuates drasticallyEarth-based observations reveal three major broadrings encircling SaturnSynchrotron RadiationCharged particles in the densest portions of Jupiter’smagnetosphere emit synchrotron radiation at radiowavelengths Jupiter probably has a rockycore several times more massivethan the Earth The core is surrounded by alayer of liquid “ices” (water,ammonia, methane, andassociated compounds) On top of this is a layer ofhelium and liquid metallichydrogen and an outermost layercomposed primarily of ordinaryhydrogen and helium Saturn’s internal structure issimilar to that of Jupiter, but itscore makes up a larger fractionof its volume and its liquidmetallic hydrogen mantle isshallower than that of Jupiter17183

Saturn’s rings are composed of numerous icyfragments, while Jupiter’s rings are made ofsmall rocky particles Saturn is circled by a system of thin, broad rings lying in the plane ofthe planet’s equator This system is tilted away from the plane of Saturn’s orbit, whichcauses the rings to be seen at various angles by an Earth-based19observer over the course of a Saturnian year Most of its rings exist inside the Roche limit of Saturn, wheredisruptive tidal forces are stronger than the gravitational forcesattracting the ring particles to each other Each of Saturn’s major rings is composed of a great many narrowringlets21 The principal rings of Saturn are composed ofnumerous particles of ice and ice-coated rock rangingin size from a few micrometers to about 10 m Jupiter’s faint rings are composed of a relativelysmall amount of small, dark, rocky particles thatreflect very little light20The faint F ring, which is just outside the A ring, is kept22narrow by the gravitational pull of shepherd satellitesSaturn’s inner satellites affect the appearanceand structure of its rings23244

2526Guiding Questions1. What is special about the orbits of Jupiter’s Galilean satellites?2. Are all the Galilean satellites made of rocky material, like theEarth’s moon?3. What could account for differences between the inner andouter Galilean satellites?4. Why does Io have active volcanoes? How does Io’s volcanicactivity differ from that on Earth?5. How does Io act like an electric generator?6. What is the evidence that Europa has an ocean beneath itssurface?7. What is unusual about the magnetic fields of Ganymede andCallisto?8. How is it possible for Saturn’s moon Titan to have anatmosphere?9. Why do some of Jupiter’s moons orbit in the “wrong” direction?10. What kinds of geologic activity are seen on Saturn’s mediumsized satellites?Jupiter and Saturn’s Moons27282930Jupiter’s Galilean satellites are easilyseen with Earth-based telescopes The four Galileansatellites orbit Jupiterin the plane of itsequator All are in synchronousrotation The orbital periods ofthe three innermostGalilean satellites, Io,Europa, and Ganymede,are in the ratio 1:2:45

The Galilean satellites formed like a solarsystem in miniature The two innermost Galilean satellites, Io and Europa, haveroughly the same size and density as our Moon They are composed principally of rocky material The two outermost Galilean satellites, Ganymede andCallisto, are roughly the size of Mercury Lower in density than either the Moon or Mercury, they are31made of roughly equal parts ice and rockThe Galilean satellites probably formed in a similarfashion to our solar system but on a smaller scale32Io is covered with colorful sulfur compounds ejectedfrom active volcanoes3334Tidal Heating The energy to heat Io’sinterior and produce thesatellite’s volcanicactivity comes fromtidal forces that flexthe satellite This tidal flexing isaided by the 1:2:4 ratioof orbital periods amongthe inner three Galileansatellites35366

3738Jupiter’s magnetic field makes electric currentsflow through Io The Io torus is a ring ofelectrically chargedparticles circlingJupiter at the distanceof Io’s orbit Interactions betweenthis ring and Jupiter’smagnetic field producestrong radio emissions Io may also have amagnetic field of its39own40Europa is covered with a smooth layer ofice that may cover a worldwide ocean While composed primarily ofrock, Europa is covered witha smooth layer of water ice The surface has hardly anycraters, indicating ageologically active history As for Io, tidal heating isresponsible for Europa’sinternal heat Minerals dissolved in thisocean may explain Europa’sinduced magnetic field41427

Other indications are a worldwide network of long cracks andice rafts that indicate a subsurface layer of liquid water orsoft ice4344Liquid water may also lie beneath the crateredsurfaces of Ganymede and Callisto4546Ganymede Ganymede is highlydifferentiated, andprobably has ametallic core It has a surprisinglystrong magneticfield and amagnetosphere ofits own While there is atpresent little tidalheating ofGanymede, it mayhave been heated inthis fashion in thepast An induced magneticfield suggests thatit, too, has a layer ofliquid water beneath47the surface Two types of terrain are found on the icysurface of Ganymede:– areas of dark, ancient, heavily cratered surface– regions of heavily grooved, lighter-colored,younger terrain488

Titan has a thick, opaque atmosphere richin methane, nitrogen, and hydrocarbons Callisto has a heavily cratered crust of water ice The surface shows little sign of geologic activity, because there wasnever any significant tidal heating of Callisto However, some unknown processes have erased the smallest craters andblanketed the surface with a dark, dusty substance Magnetic field data seem to suggest that Callisto has a shallow49subsurface ocean The largest Saturniansatellite, Titan, is aterrestrial world with adense nitrogenatmosphere A variety of hydrocarbonsare produced there by theinteraction of sunlightwith methane These compounds form anaerosol layer in Titan’satmosphere and possiblycover some of its surfacewith lakes of ethane 50Jupiter has dozens of small satellites thathave different origins As of early 2004, Jupiter has atotal of 63 known satellites In addition to the Galileansatellites, Jupiter has foursmall inner satellites that lieinside Io’s orbit Like the Galilean satellites,these orbit in the plane ofJupiter’s equator The remaining satellites aresmall and move in much largerorbits that are noticeablyinclined to the plane ofJupiter’s equator Many of these orbit in thedirection opposite to Jupiter’srotation5152The icy surfaces of Saturn’s six moderate-sizedmoons provide clues to their histories As of early 2004, Saturn hasa total of 31 known satellites In addition to Titan, sixmoderate-sized moons circleSaturn in regular orbits:Mimas, Enceladus, Tethys,Dione, Rhea, and Iapetus They are probably composedlargely of ice, but theirsurface features andhistories vary significantly The other, smaller moonsinclude shepherd satellitesthat control the shapes ofSaturn’s rings and capturedasteroids in large retrograde53orbits549

Uranus, Neptune and Pluto55565758Guiding Questions1. How did Uranus and Neptune come to be discovered?2. What gives Uranus its distinctive greenish-bluecolor?3. Why are the clouds on Neptune so much more visiblethan those on Uranus?4. Are Uranus and Neptune merely smaller versions ofJupiter and Saturn?5. What is so unusual about the magnetic fields ofUranus and Neptune?6. Why are the rings of Uranus and Neptune so difficultto see?7. Do the moons of Uranus show any signs of geologicactivity?8. What makes Neptune’s moon Triton unique in thesolar system?599. Are there other planets beyond Pluto?Uranus was discovered by chance, but Neptune’sexistence was predicted by applying Newtonianmechanics Uranus recognized as a planet in 1781 byWilliam Herschel Neptune’s position calculated in mid1840’s because of slight deviations inUranus’ orbit Credit shared by Le Verrier and Adams6010