PLANETARY RINGSINTRODUCTIONSOURCES OF INFORMATIONPLANETARY SPACECRAFTEARTH-BASED OBSERVATIONSNUMERICAL STUDIESOVERVIEW OF RING STRUCTUREJUPITERURANUS and NEPTUNESATURNRING PROCESSESDENSE RINGS: INTERNAL PROCESSESEXTERNAL CAUSES OF RING STRUCTURERING ORIGINSPROSPECTS FOR THE FUTURECLBE001-ESS2E November 10, 2006 21:56100-C 25-C 50-C 75-C C+M50-C+MC+Y50-C+YM+Y50-M+Y100-M 25-M 50-M 75-M 100-Y 25-Y 50-Y 75-Y 100-K 25-K 25-19-19 50-K 50-40-40 75-K 75-64-64CHAPTER27Planetary RingsCarolyn C. PorcoSpace Science InstituteBoulder, ColoradoDouglas P. HamiltonUniversity of MarylandCollege Park, Maryland1. Introduction 5. Ring Origins2. Sources of Information 6. Prospects for the Future3. Overview of Ring Structure Bibliography4. Ring Processes1. IntroductionPlanetary rings are those strikingly flat and circular ap-pendages embracing all the giant planets in the outer SolarSystem: Jupiter, Saturn, Uranus, and Neptune. Like theircousins, the spiral galaxies, they are formed of many bod-ies, independently orbiting in a central gravitational field.Rings also share many characteristics with, and offer in-valuable insights into, flattened systems of gas and collid-ing debris that ultimately form solar systems. Ring systemsare accessible laboratories capable of providing clues aboutprocesses important in these circumstellar disks, structuresotherwise removed from us by nearly insurmountable dis-tances in space and time. Like circumstellar disks, ringshave evolved to a state of equilibrium where their randommotionsperpendiculartotheplaneareverysmallcomparedto their orbital motions. In Saturn’s main rings (Fig. 1), forexample, orbital speeds are tens of km/sec while variouslines of evidence indicate random motions as small as a fewmillimeters per second. The ratio of vertical to horizontaldimensions of the rings is consequently extreme: one partin a million or less, like a huge sheet of paper spread acrossa football field.Rings, in general, find themselves in the Roche zone oftheir mother planet, that region within which the tidal ef-fects of the planet’s gravity field prevent ring particles, vary-ing in size from micron-sized powder to objects as big ashouses, from coalescing under their own gravity into largerbodies. Rings are arranged around planets in strikingly dif-ferent ways despite the similar underlying physical pro-cesses that govern them. Gravitational tugs from satellitesaccount for some of the structure of densely-packed mas-sive rings [see Solar System Dynamics: Regular andChaotic Motion], while nongravitational effects, includ-ing solar radiation pressure and electromagnetic forces,dominate the dynamics of the fainter and more diffuse dustyrings. Spacecraft flybys of all of the giant planets and, morerecently, orbiters at Jupiter and Saturn, have revolutionizedour understanding of planetary rings. New rings have beendiscovered and many old puzzles have been resolved. Otherproblems, however, stubbornly persist and, as always, newquestions have been raised. Despite significant advancesover the past decade, it is still the case that most ring struc-ture remains unexplained.2. Sources of Information2.1 Planetary SpacecraftWhile rings have been observed from the surface of theEarth ever since Galileo Galilei discovered two curiousblobs near Saturn in 1610, the study of planetary rings didnot emerge as the rich field of scientific investigation it istoday until the Voyager spacecraft made their historic toursEncyclopedia of the Solar System 2eC2007 by Academic Press. All rights of reproduction in any form reserved. 503CLBE001-ESS2E November 10, 2006 21:56504Encyclopedia of the Solar SystemFIGURE 1 Saturn and it main ring system in near natural coloras seen from Voyager. From bottom, the satellites Rhea, Dione,and Tethys are visible against the darkness of space, with Mimasjust above them on Saturn’s bright limb. Shadowing abounds inthis image: black dots cast by Mimas and Tethys are visible onSaturn’s disk, the planet blocks light from getting to the rings atlower right, and the foreground rings paint a dark band on theplanet’s cloudtops. From the outside are the bright A and B ringsseparated by the Cassini Division. The narrow Encke Gap in theouter A ring is also visible, as is the dark C ring near set theplanet.of the outer Solar System in the 1980s. Not even the twoPioneer spacecraft, the first human artifacts to pass throughthe realms of Jupiter and Saturn in the mid to late 1970s,hinted at the enormous array of phenomena to be foundwithin these systems.Voyager 1 arrived first at Jupiter in March 1979, followedby Voyager 2 four months later. After its encounter with Sat-urn in November 1980, Voyager 1 was placed on a trajectorythat took it out of the Solar System; Voyager 2 encounteredSaturn in August 1981 and then sailed on to reach Uranusin January 1986, and Neptune, its last planetary target, inAugust 1989. Each spacecraft was equipped with a suite ofinstruments collectively capable of covering a wide rangeof wavelength and resolution. Tens of thousands of imagesof planetary ring systems in the outer Solar System wereacquired by the Voyager cameras at geometries and reso-lutions impossible to obtain from the ground. Also, occul-tations of bright stars by the rings were observed from thespacecraft, and occultations by the rings of the spacecrafttelemetry radio signals were observed from the Earth; bothproduced maps of the radial architecture of the rings at spa-tial scales of ∼100 m. In addition to these remote-sensingobservations, local (or in situ) measurements were made ofcharged particles, plasma waves, and, indirectly, impacts ofmicron-sized meteoroids as each spacecraft flew throughthe ring regions of each planet. These data sets contributedin varying degrees to the picture that ultimately emerged ofthe unique character and environment of the ring systemssurrounding the giant planets.The Galileo spacecraft, launched in 1989, became thefirst artificial satellite of Jupiter in December 1995 and re-mained in orbit until September 2003 when, fuel runningout and instruments ailing, it was directed to crash into thegiant planet. Images of the Jovian ring system are few buthave improved resolution and image quality significantlyover those obtained by Voyager. Galileo resolved one ofthree separate ring components imaged by Voyager—theGossamer Ring—into two distinct structures and clarifiedthe intimate relationship between these components andthe nearby orbiting