Jovian Planets

Lecture 13Jovian PlanetsJiong Qiu, MSU Physics Department

Opening Q: Jovian planets are farther away from theSun than terrestrial planets. Given all we knowabout terrestrial planets, what do you expect Jovianplanets are like? MotionMass and compositionAtmosphereEnergyGeology (?)Magnetic fieldsSatellites and rings

Guiding Questions1.2.3.4.5.6.What is going on in Jupiter and Saturn’s belts andzones and the Great Red Spot?What is the composition of Jovian atmospheres? Howdoes it compare with terrestrial atmospheres?What is the nature of the clouds of Jupiter and Saturn?How do astronomers know about the deep interiors ofJupiter and Saturn?How do Jupiter and Saturn generate their intensemagnetic fields?Are Saturn’s rings actually solid bands that encirclethe planet? How uniform and smooth are Saturn’srings?

13.1 Introduction: the outer Jovian planets are gas giants.the Jovian (outer) planets Being far, they are slow orbiters: a3 P2Being far, they are cold: T 55 - 165 KRetain large amounts of light elements (similar to the Sun)More reflective than most terrestrial planets: A 0.4 - 0.6Have global magnetic fields, rings, and satellites.

fast and differential rotationmassive, large, low density, oblateatmosphere dominated by H2 and He

fast and differential rotationmassive, large, low density, oblateatmosphere dominated by H2 and He

13.2 Motionso Jovian planets are best seen at oppositions about once ayear: Jupiter is 50” and Saturn is 20”.o They exhibit fast (10 -16 hrs) differential rotation: it takesshorter time for equatorial regions to finish one round aboutthe axis than for polar regions (similar to the Sun’sdifferential rotation) -- they’re fluid.rigid rotation and differential rotationEx.1: Sun’s differential rotation

14.3 Dynamic atmospheresThe visible “surfaces” of Jovian planets are the cloud tops.Jupiter and Saturn have characteristic observable featuresdifferent from other planets: light-colored zones, darkcolored belts, and spots and ovals. The red/white/browncolors indicate clouds at different altitudes with differentcompositions -- nitrogen compounds and water vapor.

The fast rotation twists theclouds into light zonesand dark belts parallel tothe equator, indicative ofconvection patterns.Jupiter’s poles

The dynamic atmosphere of Jupiter"Various patterns of motion are apparent all across Jupiter at the cloudtop level seen here.The Great Red Spot shows its counterclockwise rotation, and the uneven distribution of itshigh haze is obvious. To the east (right) of the Red Spot, oval storms, like ball bearings, rollover and pass each other. Horizontal bands adjacent to each other move at different rates.Strings of small storms rotate around northern-hemisphere ovals. The large grayish-blue "hotspots' at the northern edge of the white Equatorial Zone change over the course of time asthey march eastward across the planet. Ovals in the north rotate counter to those in the south.Small, very bright features appear quickly and randomly in turbulent regions, candidates forlightning storms.” Image Credit: NASA/JPL/University of Arizona

Jupiter’s clouds in visible and infrared light.Q: what are the origins of the visible and infrared light?Jupiter’s dark belts are bright in infrared, indicative of lowlying warmer clouds. White zones are dark in infrared,indicating cool high-altitude clouds.

Storms on Jupiter and SaturnGigantic storms are present inthe Jovian atmosphere andvisible as colored ovals. TheGreat Red Spot, located ina white zone, is such a stormcenter persisting for centuries.Short-lived storms,“white spots”, areoccasionally seen onSaturn.

Atmosphere structure and composition on Jupiter and SaturnDifferences:o Saturn’s cloud layersare more spread out andcooler.o Composition of Jupiter’satmosphere is verysimilar to that of the Sun:75% H, 24% He, 1%others (by mass).o Saturn’s atmosphere hasQ: how is the atmospherea serious He deficiency,composition determined?perhaps due to heliumrains.Similarities:o Three cloud layers on the top with distinctive colors.o Temperature decreases with altitudeo Atmosphere consists nearly entirely of H & He.

The atmosphere dynamics are driven by the internalenergy of Jovian planets. Jovian planets emit more energy(infrared) than they receive from the Sun (visible).Ex.3: If Jupiter emitted the same amount of energy itreceives from the Sun, its surface temperature would be107K; if Jupiter emitted twice the incoming energy, what’sits temperature?Stefan - Boltzmann Law : F1 σT14 , F2 σT 2 444F2 σT 2T2 44F1 σT1T11/ 4 'T2F2 & )T1 % F1 ( F '1/ 4 T 2 & 2 ) T1 (2)1/ 4 107 K 127 K% F1 (

(optional) Ex. 4: Jupiter is still under gravitationalcontraction, the Kelvin-Helmholtz contraction, to turngravitational energy (U) into internal heat.23M GU 5 RWith decreasing R, U is converted to internal heat.Kelvin-Helmholtz contraction was thought to provide to maintain sunshine. It provides energy for protoenergystars before fusion ignition.

Discoveries of Jupiter’s atmosphere by Galileo Probe: first observations ofammonia clouds. lightening charges inJupiter’s atmosphere more heavy elements thanin the Sun missing water abundant noble gasLightening on JupiterGalileo was launched in 1989.The Probe was released toJupiter’s upper atmosphere in1995, and plunged intoJupiter’s dense atmospherein 2003.

13.4 Internal structureOblateness of Jupiter and Saturn reveals their rocky cores,surrounded by liquid ices, metallic liquid hydrogen, andhydrogen and helium gases.o Oblateness is produced by fastrotation (pulling mass outward)against gravity (pulling massinward).dense core strong gravity small oblateness.o Complicated models of massdistribution are built to fit theobserved oblateness.o Saturn’s internal structure issimilar to that of Jupiter, butwith a larger rocky core (bypercentage) and shallowerliquid metallic hydrogen mantle.Q: means to find out the interior structure of a planet?

13.5 MagnetosphereJupiter and Saturn have strong global magnetic fields forandhave extensivemagnetospheresrapidJupiterrotationof Saturnliquid metallichydrogen(how is thiscompared with the Earth?)Jovian magnetosphere isfilled with plasmas, somefrom the volcanoes on Iocaught by Jupiter’smagnetic fields.Io plasma torusJupiter’s magnetosphere islarger than the Moon(apparent angular size) andits size fluctuates with thebalance between plasmapressure and solar wind.

Ex.5: what is plasma? How does plasma behave inmagnetic fields?When electron is freed from the orbitaround the nucleus, the atom isionized. Ionized gases, which areglobally neutral, are plasmas.Ex.6: Synchrotron radiation by chargedelectrons spiraling around magnetic fieldlines.photonsSynchrotron radiation is non-thermal radiation by very highspeed (close to the speed of light) electrons, different fromthermal radiation (e.g., blackbody radiation).

Radio JupiterCredit: I. de Pater (UC Berkeley)NRAO, AUI, NSFEx.7: Radio Jupiter recorded atVLA, New Mexico. The radiowaves mapped in this false-colorimage are produced by energeticelectrons trapped within Jupiter'sintense magnetic field. The radioemitting region extends far beyondJupiter's cloud tops and surroundsJupiter. While it glows strongly atradio wavelengths, Jupiter'sradiation belt is invisible in themore familiar optical and infraredviews which show the Jovian cloudtops and atmospheric features inreflected sunlight.NB: this type of radio emission is a non-thermal emission, different fromblackbody continuum radiation or line emission.

Review Q: what do we learn from spectroscopy? thermal continuum (blackbody radiation): temperatureof the opaque radiator; can NOT tell the material of theradiator. spectral lines: composition, temperature, density,abundance, line-of-sight motion of a gas Non-thermal synchrotron radiation: particle properties(number and speed) and magnetic field. reflective spectrum: surface properties (composition,texture etc.).

Trapped plasmas emitsynchrotron radiationat radio wavelengths.The strongest emission isat the distance of theorbit of Io: the Io plasmatorus.Leaking particles produceauroras.Auroras are seen in polarregions and torus isalong the equator.Auroras in Jupiter and Saturn’s polar regionsSaturn’s magnetosphere is less strong with much fewerparticles.

13.6 RingsObservations on Earth reveal three broad rings encircling Saturn.Saturn is circled by a system of thin, broad rings -- mostfamous being A, B, and C rings -- lying in the plane of theplanet’s equator, with gaps in between.

On the sunlit side,B ring is bright, ormost reflective.Cassini division isdark.On the far side, B ring isdark. Cassini division isbright.Cassini division is NOT anempty gap!

Saturn’smajesticringsSaturn is circled by asystem of thin, broad rings-- most famous being A, B,and C rings -- lying in theplane of the planet’sequator.from Cassini Huygens. http://saturn.jpl.nasa.gov/home/index.cfm

Color variations in Saturn’s rings: colors can tell compositions.False color image from Voyager.

fine structures of Saturn’s ringsfrom Cassini Huygens. http://saturn.jpl.nasa.gov/home/index.cfm

Still more work by gravity: Saturn’s inner satellites affect, bygravitational force, the appearance and structure of itsrings.The arrangement of Saturn’s rings

Formation ofSaturn’s F ring bythe two shepherdspushing particlesinto the F ring.The Pan satellite pushesparticles outside the EnckeGap.

Saturn’s rings are composed of numerous icyfaintarefragments, while Jupiter’s rings Jupiter’sare madeofringssmallrocky particlescomposed of a relativelyJupiter’s Main RingData from the Galileospacecraft show thatrings were created bymeteoroid impacts onsmall nearby moons.Credit: M. Belton (NOAO), J.Burns (Cornell) et al., GalileoProject, JPL, NASAsmall amount of small,dark, rocky particles thatreflect very little lightSolar eclipse by Jupiter seen by GalileoSmall dust particles in Jupiter'satmosphere and the rings, can be seen byreflected sunlight.

Saturn’s ring system is made of a great number ofringlets each consisting of ring particles, or highlyreflective ice-coated rocks, orbiting around Saturn: inner rings revolve faster than outer rings, followingKepler’s third law: a3 P2Radio observations determine the size of ringparticles to be 1 cm to 5 m.Rings have different compositions.There are ring particles in the gaps as well.Arrangements of rings by combined gravitationalforces.

Ex.9: why rings exist as a collection of ring particles otherthan a whole piece of ring or a sizable satellite?Far sidewould be torn apartA whole piece of ring close to a planetby the tidal force winning over the self-gravity of the ringpiece (or satellite). The critical distance is the Roche limit.hold togethertorn aparttidal force MPRS /d3self-gravity MS /RS2dR RP(ρP/ρS)1/3Rochelimit depends on the gravity of the planet and satellite.Sunlit side

o The Roche limit is where the tidal force of a planetbalances the self-gravity of a satellite orbiting theplanet. Within the Roche limit, a satellite is torn apart.o If a planet and a moon have identical densities, thenthe Roche limit is 2.446 times the radius of the planet.o The Roche limits for some planets are:Earth - 18, 470 kmJupiter - 175,000 kmSaturn - 147,000 kmUranus - 62,000 kmNeptune - 59,000 kmo The Roche limit for the Sun is about 0.01 AU.

Roche limit: the point of no Sci102/lectures/moonsandrings.htm

Ex.10: On July 7, 1992, Comet Shoemaker-Levy 9 brokeapart in 21 pieces due to tidal forces when it made a closeapproach of Jupiter which was within the Roche limit.Find all interesting images at http://www2.jpl.nasa.gov/sl9/sl9.html

Rings in the solar system within the Roche limit