!"#$%%&''()*+,-')./0'1)(234(' ' '&5'''''''&6777' '&8'''''''&6$77' Lecture 22 Giant Planets '79'''''''6&777' '&:'''''''&$$77' '$6'''''''7&777' '&:'''''''6$777' '$;'''''''66$77' Rings '$$'''''''$7777' '&9'''''''$8777' '&:'''''''8$$77' '&:'''''''87$77' '&$'''''''$&777' '$5'''''''76$77' '&8'''''''76777' '$9'''''''8&777' '&&'''''''&$777' '&$'''''''66777' '&8'''''''6&$77' '$9'''''''8&$77' '&5'''''''88777' '$5'''''''88$77' '&%'''''''&7$77' '7:'''''''$&$77' '$;'''''''7$$77' '&6'''''''6$$77' '$;''''''8$777' '$;''''''78777' '&5'''''''86$77' '&6'''''''86777' '&7'''''''&&$77' '&5''''''''68777' '&6''''''''&&777' '&:'''''''&8$77' '&6'''''''$6$77' '&;'''''''$8$77' '$;'''''''7&$77' '$8''''''''67$77' '&&'''''''68$77' '$9'''''''78$77' '$;'''''''77$77' '&8'''''''$$$77' '$9'''''''&8777' '$5'''''''$7$77' ''&7''''''$6777' The planets – ring systems None of the terrestrial planets (or Pluto) have ring systems but all of the Jovian planets do Although recently Pluto suggested to form dust rings sporadically when KBOs collide with its moons (Stern 2006) And there’s a large amount of space debris in orbit around the Earth… They are interesting because: •! circumplanetary disk processes similar to circumstellar disk processes •! test of planetary system dynamics •! finding satellites •! origin and evolution Planetary rings – Jovian rings overview Jupiter: rings discovered in Uranus: 11 rings 1979 by Voyager 1; comprised discovered in 1977; very of dust <10 µm in diameter; narrow, eccentric and Main=122,800 km (30 km inclined; shepherded by thick); Halo=Extends from satellites; dark, carbon main ring to Jupiter; particles up to m in size; Gossamer= >129,000 km; debris from smaller satellites; self-gravitational effects affected by magnetic forces Saturn: rings discovered in 1610 Neptune: several by Galileo; but Huygens in 1651 narrow rings; interpreted as rings; Cassini in including Adams ring 1675 discovered the first gap in made up of the rings; Voyager found incomplete arcs and composed of ice few µm to 10s of clumps from satellite m; evidence for shepherding interactions moons; A-G rings; braided rings !"#$%&'()*+",'%(*-'()*./012#0* •! !"#$%&23*1"*.%14&(5*162*"162&* &'()*0/012#07* 8! 6%,2*92:2&*$%&;<=20* 8! %&2*0#%==2&*'(*2>12(1* 8! 6%,2*3%&?2&*$%&;<=20* •! @6/*16'0*'0*0"5*:2*%&2*("1*04&2A* •! B162&*4(0"=,23*#/012&'207* 8! C&%(40D*&'()0*%&2*2<<2(1&'<* %(3*0=')61=/*;=123*9&"#*'10* 2E4%1"&'%=*$=%(2A* 8! F2$14(2*6%0*$%&;%=*&'()0A* SATURN - the ring system!12.4 Saturn’s Spectacular Ring System Saturn has an extraordinarily large and complex ring system, which was visible even to the first telescopes The first planet to have its rings detected was Saturn. Saturn’s rings were seen by Galileo in 1614 but disappeared soon afterwards. The rings appeared to have a variety of forms when first detected: Galileo did not fully understand what he was seeing Huygens’ Explanation Christiaan Huygens was the first person to explain the rings (and their disappearance) when in 1659 he worked out that Saturn must be surrounded by a thin flat ring that does not touch the planet. The appearance and disappearance of the rings was due the diferent viewing geometries as seen from Earth. These images were taken from the Hubble Space Telescope during a four-year period, from 1996 to 2000 (left to right), as Saturn moved along one seventh of its 29-year journey around the Sun. As viewed from near the Earth, Saturn’s rings open up from just past edge-on to nearly fully open as it moves through its seasons, from autumn towards winter in its northern hemisphere. Cassini Division between the A and B rings the first feature to be discovered Edge-on view of Saturn’s rings Titan and it’s shadow These images were taken from the Hubble Space Telescope during a four-year period, from 1996 to 2000 (left to right), as Saturn moved along one seventh of its 29-year journey around the Sun. As viewed from near the Earth, Saturn’s rings open up from just past edge-on to nearly fully open as it moves through its seasons, from autumn towards winter in its northern hemisphere. Titan 4 satellites When the Earth is in the plane of Saturn’s rings, an observer on the Earth views the rings edge on. Because the rings are so thin, they are then barely visible. Saturn’s largest satellite, Titan, is seen just above the rings (left); it is enveloped in a dark brown haze and casts a dark shadow on Saturn’s clouds. Four other moons are clustered near the other edge of Saturn’s rings (right), appearing bright white because their surfaces are covered with water ice. The rings are thin and very flat: 10 to 100 meters thick A and B are dense and bright C and D are faint and sparse F is a narrow ring at the edge of the system " # $ % & ! Two major gaps: Cassini and Encke 12.4 Saturn’sTwo major gaps: Spectacular Cassini and Encke Ring 12.4 Saturn’sSystem Spectacular Ring Overview of the System ring system Overview of the ring system B ring is the brightest B ring is the brightest Cassini Division has a few faint rings The narrow, mysterious F-Ring at the outer edge of the A-ring !"#$%&'()(*+$%&( ,-".*/%0 12'$*3,4 1567+8,4 19*/9,4 : SATURN Cassini image of the F-ring !##$%&'("#) !" Composition and Structure !"#$%& •! !"#$%&'(#%"%)&(*&+",,"(#%& (*&%-.,,&/.01',2%&(0& -((#,2)%&(0+"1#$& ',(%2&)(&)32"0&/,.#2)& –!%"42&(*&/.01',2&0.#$2%& *0(-&$0."#&(*&%.#5&)(& 3(6%27%"425&+(6,520%& Ring particles range in size from fractions of a millimeter to tens of meters Composition: Water ice—similar to snowballs Why rings? Too close to planet for moon to form—tidal forces would tear it apart !"#$%&'()$*+$,-./0'$ •! 1*.$)*2&3$ •! !"*/)-'3)$*+$&'3&4&3/-2$0&'()$ 5! '-00*6$(-7)$ 5! ."#8$3&9#0$&'$:0&(".'#))$;$.0-')7-0#'<8$ •! =-'8$&'3&4&3/-2$7-0><2#)$ 5! ,&?#)@$:*/23#0)$.*$3/).$ 5! %#A#<>4#$BCD$&<#$E)'*6:-22)F$ 5! G*22&)&*')$H##7$0&'($."&'$ !"#$%& •! '()*+,-%&./,,/0&1-2,-)3%&,(0%&& –! "##-)&2()*+,-%&)-4/,4-&.(%5-)& 56(#&56/%-&.()56-)&/75& –! )"#$&#/5&)/5(*#$8&)(56-)& "#9"4"97(,&://#,-5%&)-4/,4"#$&& •! ".&)"#$&2()*+,-%&0"9-,;&%2(+-9&<& :/4-&"#9-2-#9-#5,;& •! ".&2()*+,-%&()-&+,/%-&<&$)(4"5(*/#(,,;& "#5-)(+5& •! ://#%&+,-()&$(2%&"#&)"#$%& Maxwell’s Essay James Clerk Maxwell won the Adams Prize Essay in 1856 for his work on the stability of Saturn’s rings. He showed that the rings could not be solid but had to be composed of particles. Rings not solid - stresses would tear them apart Saturn’s rings are a truly alien environment, consisting of many small bodies in orbit around the planet. Artist’s conception of the rings Old ring model Ring Vertical Vrel <<< 15km/s Structure 15km/s “Classical” ring model New ring model A more modern, densely packed ring model 30m thick Gentle collisions & weak gravity between particles give the rings the quality of a viscous fluid The sizes of the particles have been determined by observing the scattering of radio waves of various wavelengths There are also other dynamical features that give information on the particle sizes - or the mass in the rings - such as the wavelength of various waves seen in the ring Particles in Saturn’s rings Scattering of Radio waves 0.94, 3.6, 13 cm WhenWhen the the Cassini Cassini spacecraft passed passed behind behind thethe rings rings of of Saturn, Saturn, it sentsent three three simultaneous simultaneous radioradio signals, signals, atat 0.94,0.94, 3.63.6 and and 13 13 centimeter centimeter wavelength,wavelength, throughthrough thethe ringsrings to to Earth. Earth. The The observedobserved changes changes of eacheach signal signal as as the the spacecraft spacecraft movedmoved behind behind thethe rings providedprovided a aprofi profile leof ofthe the distribution of ring material as a function of distance distribution from Saturn, of orring an materialoptical depth as a profifunctionle. of distance from Saturn, or an optical depth profile. The image shown here was constructed from these profiles, depicting the observed ring structure at about 10 kilometers in Theresolution. image shown Color is here used was to present constructed information from aboutthese the profi presenceles, depicting or absences the observedof small ring ring particles structure in different at about regions 10 kilometers in resolution.based on the Color measured is used effects to present of the threeinformation radio signals. about Purple the presence color indicates or absences regions of where small there ring is particles a lack of in particles different of regions basedsize lesson thethan measured 5 centimeters. effects Green of theand threeblue shades radio signals.indicate regionsPurple wherecolor indicatesthere are particles regions smaller where thanthere 5 iscentimeters a lack of particles of sizeand less 1 centimeter, than 5 centimeters. respectively. Green The saturated and blue broad shades white indicate band isregions the densest where region there of are the particlesB ring, which smaller blocked than two5 centimeters of andthe 1 three centimeter, radio signals. respectively. From other The evidence saturated in the broad radio white observations, band is theall ring densest regions region appear of tothe be B populated ring, which by a blocked broad two of range of particle sizes that extend to several meters across. the three radio signals. From other evidence in the radio observations, all ring regions appear to be populated by a broad range of particle sizes that extend to several meters across. KEY QUESTION Why do these particles that are gently colliding not stick together and form a satellite? The Roche limit A large satellite (top) that moves well within Tidal forces a planet’s Roche limit (dashed curve) will be torn apart by the tidal force of the planet’s gravity.