1 The Solar System 2

1Observing the Moon, Planets & StarsOsher Spring 2009, SCUMeeting 5More on Moon & SaturnMay 6, 20097:30-9:30 pmKenna 109The Solar System2All planetslie in theeclipticplane(exceptpluto)“Dr. Bill” Pezzagliawww.clifford.org/ drbillhttp://www.clifford.org/drbill/osherA.4 What about Pluto ?Hubble Space Telescope: Our best image of PlutoPluto and the New Horizons MissionTable 6-1, p.1361

Elements in the Solar systemDiscovery of Planets around other stars!Jovian Planets (Gas Giants)Jupiter: Atmosphere 1The outer planets aremuch bigger than theterrestrial ones, and aregigantic low-density gasballs unlike the highdensity rocky terrestrialplanets.49,528 km(3.8 DE)51,118 km(4.0 DE) Horizontal stripes– Zones “light”– Belts: “dark”, lower level Great Red Spot– large storm Smaller white spots120,536 km(9.4 DE)142,984 km(11 DE)12,757 km(1 DE)– smaller storms With spacecraft, lotsof other cloudfeatures2

Jupiter: Atmosphere 2Jupiter: Belts and Zones Differential Rotation Alternating light/darkstripes in atmosphere Convection – warm airrises from below, coolsoff, then sinks (sameas Earth’s troposphere) Rapid rotationstretches theseconvection cells acrossthe planet– Observe rotation period by trackingatmospheric features 9h 50m 28s at equator(fastest rotation of all planets in SS) 9h 55m 41s at poles– Solid bodies must have same rotationeverywhere evidence that Jupiter is not a solid bodyJupiter: Storms Great Red SpotGreat Red Spot high pressure system.Air flows outward, is deflected to the left by theCoriolis effect, and rotates counterclockwise.– about size of Earth– At largest, has beenseen at 3x Earth size– Rotates ccw– more than 300 yrs old White ovals– smaller, less long-lived– also rotate ccwGreat Red Spot was first seen in 1664!Jupiter: Internal Structure Core 7,000 km thick– 8x Earth mass, smaller size very dense (also high T,P)“rocky” Fe, Si, O, but indifferent forms than we’reused to Liquid ices 7,000 km thick He & liquid metallic H 45,000 km thick– electrons free to move Gaseous H and He 12,000 km thickSaturn: History / Early Observations Visible without telescope 1610 – Galileo first sees rings 1665 – Huygens discoversTitanDensity 0.69 g/cm3 (lowest of planetsin SS)– slightly smaller size– significantly smaller mass13 inch telescope from .html– cloud layers in top 100km ofthis layer3

Saturn: Atmosphere 1Saturn: Atmosphere 2 Similar basic atmstructure and colors asJupiter– Belts and zones– Storms No long-lived analog togreat red spot several analogs to whitespots– Many other cloudfeaturesJupiterSaturn storm imagesThe oblateness of Jupiter andSaturn reveals their rocky cores Jupiter probably has a rocky coreseveral times more massive thanthe Earth The core is surrounded by a layerof liquid “ices” (water, ammonia,methane, and associatedcompounds) On top of this is a layer of heliumand liquid metallic hydrogen andan outermost layer composedprimarily of ordinary hydrogen andhelium Saturn’s internal structure issimilar to that of Jupiter, but itscore makes up a larger fraction ofits volume and its liquid metallichydrogen mantle is shallower thanthat of JupiterSaturn’s Rings 1610: Galileo looks at Saturn,notices it is not circular. Bulges on sides disappear in1612, reappear in 1613 1655: With a bettertelescope,Huygens discovers ring, tiltedby 27 Huygens explains why the rings disappear. They arevery thin, and when seen edge twice a saturn year,they vanish!4

Ring-Plane CrossingCassini Division Last crossing 1995-61675 Cassini observes the“division” in the rings,separating the brighter“B” ring from the fainterouter “A” ring Next crossing 2008-9 Rings estimated to beonly 30 meters thick!Earth-based observations reveal threebroad rings encircling SaturnEncke Division (1838)A more subtle division isfound in the “A” ringRings are not solid! 1857 Maxwell argues cannot be solid (centrifugal forces would tear itapart) 1895 Keeler measures Doppler velocities, each “ringlet” has differentspeed Rings consist of many “moonlets”, each orbiting the central planet. 80% reflectivity implies they are icy particles1970 Voyager Probes From scattering of radiosignals through the rings,estimate average particles are10 cm in size Some are as small as 1 cm,some as big as 5 meters. In comparison the ring particlesof the other planets are:– Jupiter’s are small dustparticles smaller than 1mm– Uranus particles areperhaps 1 meter in size– Neptune size unknown(similar to Uranus?)5

Source of Saturn’s Rings The ice in Saturn’s rings would make a moon about 100 km across.Where had all that ice come from? Leading hypothesis: the rings were originally a moon that was pulledapart by Saturn’s tides.Roche Limit (2.4 Saturn Radii) The moon’s own gravity tends to hold it together. Tidal forces from planet tend to pull it apart Rings are temporary, they will dissipate unless refreshed The “Roche Limit” is the critical distance where the twobalance Rings get “dirty” over time, less reflective Saturn’s moons are (mostly) outside of Roche Limit Saturn’s rings must be very young (100 million years?) Saturn’s rings are (mostly) inside the Roche limit.Saturn’s Rings: Natural ColorGaps in Saturn’s RingsGaps in the rings are caused by resonances with the satellitesExample: Mimas causes the Cassini DivisionMimas makes onerevolution in 23hours. A ringparticle in TheCassini Divisionmakes onerevolution in 11 ½hours, or tworevolutions in oneMimas period.This is resonance.What is “resonantmotion”?When you push someone ina swing, each time theycome back, you give a newpush to keep them going.This is resonant motion.Each time a ring particlecomes between Saturn andMimas, it gets a pull fromMimas, causing its orbit tobecome eccentric. Thisincreases the likelihood thatit will collide with anotherparticle and be destroyed.6

Encke DivisionF Ring from Cassini probe close approach, July 3, 2004Probably created not byresonance, but by thesmall moon “Pan”which shares the orbit.Saturn’s F-ring from CassiniF Ring Shepherd MoonsThese two moons keep the narrow F ring in place.Cassini Saturn Orbit Insertion (SOI)VGRJune 30, 2004, 7:30 - 9:15 PDTEncke division in A ring; 320 km wide7

Jupiter’s Galilean satellites areeasily seen with Earth-basedtelescopesJupiter & Saturn The four Galileansatellites orbit Jupiter inthe plane of its equator All are in synchronousrotation The orbital periods of thethree innermost Galileansatellites, Io, Europa, andGanymede, are in theratio 1:2:4Part 3:The MoonsRough Draft 11/28/2006The Galilean MoonsIo is covered with colorful sulfur compoundsejected from active volcanoesIoActive volcanoes werefound on Io in 1979by the VoyagerSpacecraft.They were studied ingreater detail by theGalileo spacecraft,which orbitedJupiter for about10 years.8

IoHow can a smallbody like Ioremain hotenough insideto producesuch vigorousvolcanic activity?EuropaEuropaEuropa has no impactcraters. It’s icy surfaceshows an intricate networkof crossing cracks, similarto cracks in the Arcticice pack on Earth.There is very little verticalrelief (no mountains ordeep valleys).Europa’s surface is veryyoung.Europa is covered with a smooth layerof ice that may cover a worldwideocean While composed primarily ofrock, Europa is covered with asmooth 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 field9

Europa is heated by gravitational effects ofGanymede and Jupiter (like Io), and therefore ithas an interior layer (“mantle”) of liquid water.Liquid water may also lie beneath thecratered surfaces of Ganymede andCallistoGanymede Ganymede is highlydifferentiated, andprobably has ametallic core It has a surprisinglystrong magnetic fieldand a magnetosphereof its own While there is atpresent little tidalheating of Ganymede,it may have beenheated in this fashionin the past An induced magneticfield suggests that it,too, has a layer ofliquid water beneaththe surface Two types of terrain are found on the icy surfaceof Ganymede:– areas of dark, ancient, heavily cratered surface– regions of heavily grooved, lighter-colored, youngerterrain10

Callisto has a heavily cratered crust of water ice The surface shows little sign of geologic activity, because there was neverany 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 shallow subsurfaceoceanEnceladus (Moon of Saturn)Geysers of erupting liquid water announced in March 2006,discovered by the Cassini spacecraftTitan: Largest Moon in theSolar SystemOrbital period aroundSaturn is 16 daysThick atmosphere mademainly of N2 (nitrogen)with a small amountof CH4 (methane),which is a hydrocarbon.This is probably the source of the “E” ringMany otherhydrocarbons alsopresent.Titan is being explored by the Cassini spacecraftas it orbits Saturn.Huygens Probe Jettisoned, December 25, 2004The Huygens probe was detached from Cassiniand entered Titan’s atmosphere in January 2005.It descended through the atmosphere, makingscientific measurements, and landed on Titan’ssurface, where it continued to make measurementsfor a few hours.11

Huygens’ Descent into Titan’s AtmosphereJanuary 14, 2005Huygens finds flow channels on TitanImages from the descentProbe cameras Hydrocarbon dust formsin atmosphere Dust settles on land(H2O ice) Methane (CH4) rains onTitan, washinghydrocarbon dust intostreams and lakesTitan’s Atmosphere and SurfaceTitan’s Surface Liquid methanemay emergefrom springs,formingchannels as itflows downhill Titanlandscapefrom Huygenslander,January 14,2005 Roundedboulders inforegroundare about 4 15 cm across. The bouldersare probablyfrozen H2O.12

Source of Saturn’s Rings The moon’s own gravity tends to hold it together. Tidal forces from planet tend to pull it apart The “Roche Limit” is the critical distance where the two balance Saturn’s moons are (mostly) outside of Roche Limit Saturn’s rings are (mostly) inside the Roche limit. Roche Limit (2.4 Saturn .