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Home , Flyby (spaceflight), Magnetosphere of Saturn, Voyager 1, Voyager 2

Voyager 1 Encounter with the Saturnian System Author(s): E. C. Stone and E. D. Miner Source: Science, New Series, Vol. 212, No. 4491 (Apr. 10, 1981), pp. 159-163 Published by: American Association for the Advancement of Science Stable URL: http://www.jstor.org/stable/1685660 . Accessed: 04/02/2014 18:59

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This content downloaded from 131.215.71.79 on Tue, 4 Feb 2014 18:59:21 PM All use subject to JSTOR Terms and Conditions was complicated by several factors. Sat- urn's greater distance necessitated a fac- tor of 3 reduction in the rate of data transmission (44,800 bits per second at compared to 115,200 bits per sec- Reports ond at ). Furthermore, Saturn's satellites and rings provided twice as many objects to be studied at Saturn as at Jupiter, and the close approaches to Encounter with the Saturnian System these objects all occurred within a 24- hour period, compared to nearly 72 Abstract. An overview of the Voyager I encounter with Saturn is presented, hours at Jupiter. including a brief discussion of the flight, trajectory, science plan formulation, and Scientific studies included investiga- highlights of the results described in the subsequent reports. tion of the , satellites, and of Saturn and of the at- The Voyager 1 encounter with Saturn studies of Sat- mospheres of Saturn and . Studies marked the successful accomplishment urn's atmosphere; and (iv) of the planetary atmosphere focused on of the third step in the NASA Voyager passage through the ring plane at 's dynamics, composition, structure, and program to explore the interplanetary orbit in order to minimize hazards from magnetospheric effects (). Stud- and planetary environments of the outer impinging ring particles. The actual Voy- ies of Titan were designed to determine (1). The Voyager spacecraft ager 1 trajectory and the planned Voyag- the diameter of the solid surface, atmo- was described in detail by Draper et al. er 2 trajectory also provide complemen- spheric temperature and pressure pro- (2). Voyager 1, launched on 5 September tary sets of satellite close-approach dis- files, and atmospheric composition and 1977, had its closest approach to Jupiter tances (Table 2). to search for an intrinsic on 5 , during a 98-day period Voyager 1 encounter activities began of Titan. The other satellites of Saturn of intensive acquisition of science data on 22 August 1980, when the spacecraft were studied to determine their size, on the Jovian system and its environs was 109,000,000 km (68,000,000 ) density, surface geology, temperature (3). was not far behind Voyag- from Saturn. Closest approach, at a dis- structure, and reflective properties. Ring er 1, with closest approach to Jupiter on tance of 126,000 km (78,000 miles) above studies included radial structure, tem- 9 July 1979 (4). With assistance from Saturn's cloud tops, occurred at 2346 perature, and particle size determina- Jupiter's gravity and a post-encounter UTC (coordinated universal time) on 12 tions and searches for azimuthal differ- trajectory correction maneuver, Voyag- November 1980. Titan closest approach ences and for a ring atmosphere. Magne- er I continued on toward Saturn. occurred before that of Saturn, at 0541 tospheric studies addressed the source of During its cruise from Jupiter to Sat- UTC on 12 November 1980. A view of Saturnian radio emissions, motions and urn, at a velocity of nearly 15 km/sec the Voyager 1 trajectory through the other variations within the magneto- (34,000 miles per hour), Voyager 1 ex- Saturnian system is shown in Figs. 1 and sphere, and interactions between the plored the and 2. Figure 3 depicts the planned comple- magnetosphere and the rings and satel- selected celestial sources of interest. In mentarity of latitude coverage by Voyag- lites. addition, each of its science instruments ers 1 and 2. Some highlights from the following was recalibrated in preparation for the Design of the spacecraft sequences detailed reports are summarized below. Saturn observations. During this 20- month cruise period, Voyager personnel developed and tested the detailed space- Table 1. Voyager science investigations. craft sequences needed to perform the Abbre- planned scientific studies during the 4- Investigation viation Principalinvestigator month encounter with the multiringed science ISS B. A. . Imaging Smith, Universityof Arizona (team leader) The Voyager 1 scientific investigations Infraredradiation IRIS R. A. Hanel, are listed in Table 1. The locations of the GoddardSpace Flight Center instruments and essential subsystem Photopolarimetry PPS A. L. Lane, hardware are shown in the 1 Jet PropulsionLaboratory Voyager Ultraviolet UVS A. L. All instruments func- spectroscopy Broadfoot, Jupiter report (3). University of SouthernCalifornia, tioned nominally with the exception of Space Sciences Institute the photopolarimeter, which developed Radio science RSS G. L. Tyler, anomalous behavior during the Jupiter StanfordUniversity (team leader) Magneticfields MAG N. F. Ness, encounter and has remained inoperable GoddardSpace Flight Center since then. Plasmaparticles PLS H. S. Bridge, The trajectory of Voyager 1 was cho- MassachusettsInstitute of Technology sen to provide (i) a close encounter with Plasmawaves PWS F. L. Scarf, Titan before Saturn closest so TRW Defense and Space Systems Group approach radio PRA J. W. that Planetary astronomy Warwick, atmospheric , high-reso- Radiophysics,Inc. lution imaging, Titan magnetic field stud- Low-energy'charged particles LECP S. M. Krimigis, ies, and solar and magnetospheric , wake studies could be accommodated; Applied Physics Laboratory CRS R. E. (ii) optimum geometry for radio occulta- Cosmic-rayparticles Vogt, CaliforniaInstitute of Technology tion studies of the rings; (iii) radio and SCIENCE,VOL. 212, 10 APRIL 1981 0036-8075/81/0410-0159$01.00/0Copyright ? 1981AAAS 159

This content downloaded from 131.215.71.79 on Tue, 4 Feb 2014 18:59:21 PM All use subject to JSTOR Terms and Conditions All of these studies benefited from the emissions from molecular marked by a few narrow features that prior ground-basedobservations of Sat- were also seen near the sunlit edges of correspond to orbital resonances with urn and from the Saturn re- the planet at very low latitudes. satellites 1980S1 and 1980S3. There is sults (5). The bulk of Saturn's atmosphere is evidence for satellite-driven density Saturn's atmosphere. One principal hydrogen. accounts for only waves in the region of the Encke divi- objective of the Voyager mission was a about 11 percent of the mass of the sion. Radio transmission through the comparative study of the dynamics of atmosphereabove the clouds of Saturn rings indicated that the effective diame- the atmospheres of Saturn and Jupiter. compared to 19 percent in the same ter of A ringparticles is 10 m. The small Because Saturnis colder, the cloud lay- regionon Jupiter.This differenceis con- satellite 1980S28is just beyond the outer ers are deeper in the atmospherethan at sistent with a gravitationalseparation of edge of the A ring. Jupiter and are blander in appearance helium and hydrogen in Saturn's interi- The A and B ringsare separatedby the (5). Alternating dark belts and or, which could generate the excess en- Cassini division, which contains five zones are visible in the clouds of Saturn ergy radiatedby Saturnover that which broad rings that have additional struc- and extend to much higherlatitudes than it receives from the . , am- ture. The effective particle diameter on Jupiter. monia,ethane, acetylene, and phosphine from radio transmissionis about 8 m. The velocities of the clouds have been are also detected in the atmosphere, al- The middle, or B, ring is strikingly measuredwith respect to the motion of thoughthere was relativelylittle gaseous different,consisting of numerousnarrow the interior of Saturn, as indicated ammonia due to the low atmospheric ringlets with no apparentlarge-scale or- by the 10 hour 39.4 minute rotational temperature. Temperatures decrease der. It is possible that the ringlets are period of the magnetic field. Eastward from - 150K in the upperatmosphere to formedby the action of a largenumber of wind in Saturn's near-equatorial a minimumof - 85 K at a pressure of moonlets embeddedwithin the ring. The clouds of up to 480 m/sec (1100 miles per 100 mbarand increase to - 160 K at 1.4 B ring region is also characterized by hour) are four times the highest bars. Temperatures in the southern sporadic radial markings or "spokes," measured in Jupiter's clouds. These hemisphere are warmer, apparently a perhaps the result of levitation of small winds, which are not stronglycorrelated seasonal effect. particlesabove the ringplane. The possi- with the belt and zone boundaries, de- Rings. Since first seen by in ble importanceof electrostatic charging crease smoothly to nearly zero at lati- 1610, Saturn's primary distinguishing effects in spoke formationand dynamics tudes near 40?N and 40?S. characteristic has been its rings. The is suggested by the detection at radio The nighttimeface of Saturnis illumi- distinct, broad A, B, and C rings, which wavelengths of electrostatic discharges nated by ringlight, degradingthe thresh- are easily observable from , were from the rings. These discharges are old of visual detection of lightning or found to be of distinctive character.The loosely correlated with Saturn's rota- aurorascompared to that at Jupiter. Ul- outermostring, A, is probablyclosest to tion. travioletauroras were, however, detect- the pre-Voyager concept of Saturn's The C ring, which is just inside the B ed in a ring near 80?S. Auroralike rings. Its rather uniform appearance is ring, is more transparentthan the A and

Fig. I (left). Voyager 1 path throughthe Saturnsystem shown in the Th RAJECTORY plane of the spacecraft trajectory.The projected orbits of lapetus, rEARTH , and Titan are shown along with the positions of these satellites at the times (labeled)when Voyager 1 was closest to them. The averagepositions of Saturn'sbow shock and magnetopauseare also shown. Time ticks along the trajectorymark Voyager's position at 12-hourintervals. Fig. 2 (right).Voyager 1 path throughthe orbits of the innersatellites from Mimasthrough . Positionsof the satellitesare shown at the times of Voyager'sclosest approachto them. The limits in the plane of the spacecrafttrajectory of the sun and Earth "shadows" are shown. Time ticks markVoyager's positionat 1-hourintervals. 160 SCIENCE, VOL. 212

This content downloaded from 131.215.71.79 on Tue, 4 Feb 2014 18:59:21 PM All use subject to JSTOR Terms and Conditions B rings and contains a number of dense . It orbits Saturn in precisely Enceladus, suggesting that Enceladus ringlets whose locations are regular but half the orbital period of Dione, and its may be the source of particles for this evidently unrelated to orbital resonances surface may be repeatedly flexed by ring. with the larger satellites. Radio transmis- gravitational forces from Dione in a man- , like , has deep, bowl- sion through the C ring indicated an ner similar to tidal heating processes on shaped craters. A 750-km-long valley effective particle diameter of 2 m. A still and . Saturn's E ring particles suggests that some form of internal fainter D ring, also comprised of numer- are most concentrated near the orbit of stress was present at one time, perhaps ous narrow features, was found between the inner edge of the C ring and the planet. 30 -' An atmosphere of neutral hydrogen i extends 60,000 km (36,000 miles) above 20- I and below the main rings and somewhat Fig. 3. Planetocentriclatitudes I beyond the outer edge of the A ring. of Voyager 1 (actual)and Voy- ager 2 (planned)as a function I10 Water ice in the ring material is a poten- spacecraftdistance from the of spacecraftdistance from the c, tial source for the cloud of hydrogen, center of Saturn in units of _ which has an estimated number density Saturn radii (Rs = 60,330 I of 600 atoms per cubic centimeter. km). Inbound and outbound directions are with < -i Beyond the A ring are three additional idicated arrows. Note that Voyager I rings. The F ring was discovered by crossed the (ring Pioneer 11, which also disclosed a com- plane) twice, once near the ~ -20- plex distribution of material in this re- orbit of Titan and once near gion (5). Voyager 1 images showed a the orbit of Dione. Voyager 2 will cross the ring plane only -30 very narrow ring that has local concen- once. trations in some and a multicom- regions -40 ponent braided appearance. Satellites 0 DISTANCE 1980S26 and 1980S27 provide the gravi- (Rs}s tational shepherding that maintains the narrow ring of very small particles. The other rings are noted in Table 3. Table 2. Saturn satellite data. Icy satellites. The total number of known satellites of Saturn has reached Satellite Diameter Distance Distance Period Closest approach (km) 15 with the discovery of three by Voyag- (km) (km) (1Rs)* (hours) Voyager 1 Voyager 2 er 1. Characteristics of the 15 satellites 1980S28 301 137,300t 2 .2761 14.446t 219,000 287,000 are given in Table 2. All but were 1980S27 220t 139,400t 2 .310t 14.7121 300,000 247,000 observed by Voyager 1. With the excep- 1980S26 200t 141,700t 2 .349t 15.085t 270,000 107,000 1980S3 x 2 tion of Titan, which is discussed 90 40? 151,422t .510t 16.664t 121,000 147,000 sepa- 1980S1 10x401 151,472t 2 .511t 16.672t 297,000 223,000 rately, the satellites are covered with Mimas 3901 188.224 3 .120 23.139 88,440 309,990 water ice and in some cases are com- Enceladus 500t 240,192 3 .981 33.356 202,040 87,140 posed mainly of water ice. At least some Tethys 1,050t 296,563 4 .916 45.762 415,670 93,000 1980S6 -160 6 of the newly discovered satellites 378,600t i.275t 65.7381 230,000 270,000 appear Dione 6 .283 66.133 to be an indication 1,120t 379,074 161,520 502,250 nonspherical, perhaps Rhea 1,5301 527,828 83.749 108.660 73,980 645,280 of past collisions and fragmentation. Titan 5,140t 1,221,432 20).246 382.504 6,490 665,960 1980S1 and 1980S3 are in nearly the Hyperion 290t 1,502,275 24 .901 521.743 880,440 470,840 same orbit, with 1980S 1 leading by about 1,440t 3,559,400 58 .999 1901.820 2,470,000 909,070 Phoebe -160 175.422 9755.679 105?and 1980S3 closing at the time of the 10,583,200 13,537,000 1,473,000 Voyager 1 Saturn encounter. At the *Here 1 Rs is definedas 60,330 km (37,490 miles). tFrom Voyager I data in this issue; distances and periodsare for I October 1980.The remainingentries were providedby the Voyagernavigation team. present closing rate, 1980S3 will over- take 1980S1 in early 1982, but orbital dynamicists predict that they will ex- Table 3. Saturn data. change orbits without colliding (6). ring All the with the Distance Distance Period icy satellites, possible Feature Comments exception of Enceladus, are heavily cra- (km) (Rs)* (hours) the of tered, although density cratering Cloud topst 60,330 1.000 10.657 Near 100-mbar level varies considerably on several of these D ring inner edge -67,000 1.11 4.91 Extremely small optical depth bodies, suggesting either a nonuniform C ring inner edge 73,200t 1.21 5.61 distribution of impacting particles or B ring inner edge 92,2001 1.95 7.93 B outer 1.53 11.41 Inner of Cassini division subsequent surface modification. Mimas ring edge 117,500t edge A ring inner edge 121,0001 2.01 11.93 Outer edge of Cassini division has one large almost one- Encke division 133,5001 2.21 13.82 About 200 km wide third the diameter of Mimas itself, as A ring outer edge 136,2001 2.26 14.24 well as numerous smaller, bowl-shaped F ring -140,600 2.33 14.94 Three narrow components craters and narrow grooves. G ring -170,000 2.8 19.9 Seen only in forward-scattered light long, E inner 3.5 27.3 Enceladus is the most reflective of ring edge -210,000 E ring maximum -230,000 3.8 31.3 Near orbit of Enceladus Saturn's moons, perhaps a consequence E ring outer edge -300,000 5.0 46.6 of geologically recent surface-forming *Here1 Rs is definedas 60,330km (37,490miles). tDistanceat equatorfrom Saturn's center; the periodis events. No craters have been seen on the rotationrate of the planet. tFrom Collinset al. (9). 10 APRIL 1981 161

This content downloaded from 131.215.71.79 on Tue, 4 Feb 2014 18:59:21 PM All use subject to JSTOR Terms and Conditions associated with the expansion or freez- er 1 radio signals penetratedthe obscur- hour)at a distanceof 17Rs (Saturnradii; ing of the nearly pure water ice of which ing haze and revealed a solid surface I Rs = 60,330 km) from the planet. Be- Tethys is composed. 5140 km (3194 miles) in diameter. Com- yond 8 Rs the magneticfield was altered Dione is almost a twin of Tethys in bined,with Titan's mass, this means that by the presence of large-scale electric size, but has a density correspondingto a the density of Titan is nearly twice that currents flowing in an azimuthal di- 60:40 mixture of ice and rock. Exten- of water, indicating a 50:50 mix of rection. sive white, wispy regions are evidently rock and water ice. On the outboundleg of its trajectory, largefractures in the crust throughwhich The atmospheric pressure at Titan's Voyager 1 penetratedSaturn's magnetic water has escaped and formed frost. surfaceis 1.6 bars (60 percent more than tail, which has a diameterof about 80 Rs There is also evidence for extensive re- at Earth's surface) and the temperature and is relatively devoid of at surfacing of some areas, although the is - 93 K. The bulk of the atmosphereis higher latitudes. The field lines in the source of energyfor such geologic activi- ,with less than 10 percent meth- traversedregion are closed, and copious ty is not understood. ane at the surface and about 1 percent fluxes of low-energy electrons are pres- Rhea is a somewhatlarger ice and rock methane in the upper atmosphere. In ent. Higher-rigidity particles (for in- satellite that also exhibits bright, wispy additionto the known productsof meth- stance, with energies in excess regions, dense cratering, linear grooves ane chemistry, such as acetylene, ethyl- of 2 MeV) are not stably trappedin the and troughs, and evidence for resurfac- ene, and ethane, there is a measurable outer regions of the magnetosphere, ing. Its temperaturewas measuredat 98 amount of . That the however. K just before it entered the shadow of hydrogencyanide can be producedfrom The close Titan was designed to Saturn. Inside the shadow, measure- nitrogenand methane by energetic elec- permit detailed study of the Venus-like ments yielded a temperatureof about 75 tron chemistry is evidenced by ultravio- interactionbetween the corotatingmag- K for part of the surface materials and let dayglow of N2, N+, and N. netosphere and Titan. This interaction substantiallyless for the remainder. The surface temperature of Titan is results in Titan having an induced mag- The masses of Hyperion and Iapetus near the triple point of methane, so that netic field and a dipolar magnetic tail. are poorly known, so their densities are solid, liquid, or gaseous methanemay be The plasmaturbulence resulting from the uncertain;it is likely that both are com- present, depending on season and lati- interaction may generate radio waves posed mainlyof water ice. The two faces tude. The temperature decreases to a and contribute to the energetic particle of Iapetus are very different in bright- minimum of - 70 K about 40 km (25 chemistry in Titan's atmosphere. In the ness, with the leadingface only one-fifth miles) above the surface, ensuring the wake there is evidence for a plume of as reflective as the trailing face. The formationof methane clouds if there is material(ionized hydrogenand nitrogen) observed distributionof surface bright- more than I percent methane in the stripped from the top of Titan's atmo- ness is remarkablyconsistent with the lower atmosphere. At higher altitudes sphere. The Titan interaction may also modelproposed by Morrisonet al. (7) on the temperatureincreases to 160 K. contribute to the complex character of the basis of telescopic observations of Titan has no intrinsic magnetic field the region outside 18 Rs. brightnessvariations of Iapetus. and therefore does not have a liquid, Shadowing and absorption effects by Titan.As the only satellite in the solar electrically conducting core. Its atmo- Titanwere also observed in the energetic system known to have a substantialat- sphere may be a source of the neutral particle fluxes during the close Titan mosphere,Titan was an importanttarget hydrogen atoms that form a torus of flyby. The source of the energetic parti- for study. Before the Voyager 1 flyby of particles encompassingthe orbits of Ti- cles is in the outer edges of the magneto- Titan, estimates of the atmosphericpres- tan and Rhea. sphere, and an underabundanceof heli- sure at the cloud-obscuredsurface were Magnetosphere. Pioneer 11 discov- um indicates that the source materialis uncertain by a factor of 100, ranging ered that Saturn's magnetosphere was not of solar-windorigin. Energetic mo- from 20 mbarto 2 bars, and estimates of distinctly different from the magneto- lecularhydrogen is also present, presum- surface temperatureranged from 80 to spheres of Earth and Jupiter. Voyager 1 ably acceleratedfrom Saturn'sor Titan's 200 K. Pioneer 11 found the diameterof firstobserved Saturn'smagnetosphere in atmosphere. Titan to be 5680 and 5760 km in red and January1980, when bursts of radio emis- E. C. STONE blue light, respectively, but saw no sion were detected with an average re- CaliforniaInstitute of Technology, markingsin the clouds or haze (5). currenceperiod of 10 hours 39.4 minutes Pasadena 91125 Voyager 1 also saw a thick haze that and ascribed to rotationof the planetary E. D. MINER completely obscured Titan's surface. A magnetic field (8). The emission occurs Jet PropulsionLaboratory, somewhatbrighter southern hemisphere when there is a specific orientationwith CaliforniaInstitute of Technology, may be an atmospheric seasonal effect respect to the sun and is furthermodulat- Pasadena 91109 (Titan and Saturn entered a 71/2-year- ed with a 2.7-day period, the latter sug- long southern autumn early in 1980). A gestingthat Dione interactsstrongly with Referencesand Notes dark hood was observed over Titan's the magnetosphere.Other discrete, low- I. For an extensive discussion of the spacecraft, north The resolu- emissions that other missiondesign criteria, trajectory selection, and polar region. highest frequency suggest the scientificinvestigations, see Space Sci. Rev. tion images, and nearly concurrentultra- satellites may be involved in generation 21, 75 (1977); ibid., p. 235. of the sun's of radio emissions. 2. R. F. Draper, W. I. Purdy, G. Cunningham, violet measurements appar- "The outer planet Marinerspacecraft," paper ent brightnessas Voyager 1 passed into Although there was no detectable to- No. 75-1155,presented at the AIAA/AGUCon- ferenceon the Explorationof the OuterPlanets, Titan's shadow, indicated several de- rus of ultraviolet-emittingions, as dis- St. Louis, Mo., 1975. tached haze layers up to 500 km above covered at Jupiter, there is a disk of 3. VoyagerI Jupiterresults are outlinedin Science 204, 945 (1979); Nature (London) 280, 725 the more opaque atmospherichaze. The plasma (hydrogenand possibly ) (1979); Geophys. Res. Lett. 7, 1 (1980). haze to be due to almost out to Titan's orbit. 4. Voyager2 Jupiterresults are outlinedin Science upper layers appear extending 206, 925 (1979). absorption of ultraviolet light by mole- The plasma is in nearly full corotation 5. Pioneer 11 Saturnresults are reportedin ibid. a 207, 400 (1980); J. Geophys. Res. 85, 5651 cules; the lower visible haze contains with Saturn's magnetosphere, with (1980). particlesabout 1 pm in diameter.Voyag- speed of - 150km/sec (330,000miles per 6. B. A. Smith et al., in preparation. 162 SCIENCE, VOL. 212

This content downloaded from 131.215.71.79 on Tue, 4 Feb 2014 18:59:21 PM All use subject to JSTOR Terms and Conditions 7. D. Morrison,T. J. Jones, D. P. Cruikshank,R. Voyager 1 would have been impossible.Special fromthe sun (9.5 versus 5.2 AU). Visual- E. Murphy,Icarus 24, 157 (1975). thanks are due to J. Diner for her efforts as 8. M. L. Kaiser, M. D. Desch, J. W. Warwick,J. SaturnScience Coordinatorand for her contri- ly, Saturn's atmospherediffers from Ju- B. Pearce, Science 209, 1238(1980). butions to this overview paper. The Voyager piter's in that it has lower contrast and 9. S. A. Collins et al., Nature (London) 288, 439 Programis one of the programsof the Planetary (1980). Division of NASA's Office of Space Science. fewer conspicuous features (Fig. 1). 10. Withoutthe dedicatedand enthusiasticefforts of The Voyager Project is managed by the Jet Winds are about four times on a large numberof Voyager Project personnel, PropulsionLaboratory of the CaliforniaInsti- stronger manyof whomspent untold amounts of personal tute of Technologyunder NASA contractNAS Saturn,reaching two-thirds the speed of time during evenings and weekends to meet 7-100. deadlines in a crowded sequence preparation sound near the equator (Fig. 2). Its east- schedule, the successful Saturn encounter of 9 February1981 ward and westward (zonal) jets are two to four times wider and, unlike Jupiter's jets, bear little relation to its overall bandedcloud structure.The externalpa- Encounter with Saturn: Voyager 1 Imaging Science Results rameters that govern both atmospheres are generally similar; only a few are Abstract. As Voyager I flew through the Saturn system it returned photographs substantiallydifferent. By comparingthe revealing many new and surprising characteristics of this complicated community of dynamics of Jupiter and Saturn's atmo- bodies. Saturn's atmosphere has numerous, low-contrast, discrete cloud features sphereswe hope to better understandthe and a pattern of circulation significantly different from that of Jupiter. Titan is relative importance of the parameters shrouded in a haze layer that varies in thickness and appearance. Among the icy that influence atmospheric characteris- satellites there is considerable variety in density, albedo, and surface morphology tics. and substantial evidence for endogenic surface modification. Trends in density and Figure 2 shows measuredzonal veloc- crater characteristics are quite unlike those of the Galilean satellites. Small inner ity as a function of planetographiclati- satellites, three of which were discovered in Voyager images, interact gravitationally tude. The measurementswere made with with one another and with the ring particles in ways not observed elsewhere in the the AMOS interactivecomputing system solar system.'Saturn's broad A, B, and C rings contain hundreds of "ringlets," and at Jet PropulsionLaboratory (5) and the in the densest portion of the B ring there are numerous nonaxisymmetric features. MCIDAS system at the University of The narrow F ring has three components which, in at least one instance, are kinked Wisconsin (6); similar results were ob- and crisscrossed. Two rings are observed beyond the F ring, and material is seen tained at University College London. In between the C ring and the planet. generatingthe Saturnwind profileof Fig. 2 both systems employed user-identified Saturn is one of the five that tion, the images revealed many new phe- cloud features. Feature position was were known to the ancients. The modern nomena in Saturn's atmosphere. In all, measured in two frames separatedby a exploration of the system of rings and several tens of billions of imaging bits known time interval, and the displace- satellites about this planet began in July were returned in the few days around ment interpretedas a wind vector. Al- 1610with Galileo's initial telescopic ob- closest approachto Saturn-more infor- though the possibility of misidentifying servations (1, 2). Many additional dis- mation than was obtained in the entire wave phase velocities as mass motions coveries were made in the 17th century, previoushistory of humanexploration of (wind vectors) clearly exists, for Jupiter includingHuygens' realizationthat Sat- this system. This has enabled the great- there is no differencebetween motions of urn was encircled by rings and his dis- est leap forwardsince the 17thcentury in features 50 km in size and those 100 covery of its largest satellite, Titan. The knowledge about the Saturn system. times larger (7). Since atmospheric astronomerCassini discovered Iapetus, Saturn's atmosphere. Imaging Saturn waves tend to be highly dispersive Rhea, Dione, and Tethys and first ob- is more difficult than imaging Jupiter, (phase speed tends to depend on wave- served the division in the rings that is both because of the lower intrinsic con- length), this agreementindicates that ac- named after him. He also correctly con- trastof features and because of the lower tual mass motions are being observed. cluded that the leading hemisphere of light levels at Saturn's greater distance For Saturnwe base this assertion on far Iapetus is much darker than the trailing hemisphere. Three centuries later, the unmanned U.S. spacecraftVoyager 1 examinedthe Saturn system at close range with an imaging system (3) that is a direct de- scendantof the early telescopes of Gali- Fig. 1. Colorimage of Saturn's leo, , and Cassini. During its northernhemisphere. The low contrast is when closest approachto Saturnin November evident one considers that the part of the 1980, Voyager 1 made observations of planet in the center of the im- the planet and its rings and satellites. age has morefeatures than any Some of the satellites were observed other region on Saturn. The with resolutions approaching 1 km, an dark North EquatorialBelt is of almost three orders of located near 20?N, the more improvement active North Temperate Belt magnitude over ground-basedobserva- at 40?N.The rings obscure the tions and two orders of magnitudeover equator and the region south- Pioneer 11 images (4). Voyager recorded ward. Latitudes are planeto- surface detail for the first time on eight graphicin all figures. satellites. It discovered several new sat- ellites and revealed hundredsof compo- nents of the ring system, perhaps six of which were known previously. In addi- SCIENCE, VOL. 212, 10 APRIL 1981 0036-8075/81/0410-0163$02.00/0Copyright ? 1981AAAS 163

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