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Foreword

In late su mm e r o f 1977 , the United Sta tes launched two unma nn ed Voyage r spacecra ft o nan extensive reconna issa nce of the outer pla nets, a decade- lo ng odyssey th a t co uld ta ke th emto 3 pla nets a nd as ma ny as 18 planeta ry sate llites. Th e first enco unte r was with the gia ntJov ia n pla neta ry syste m, 645 million kilo meters (400 millio n miles) away. Passing by Ju p ite rand its co mplex sa tellite system in 1979, the Voyage r spacec ra ft have co llected an d retu rned toEarth a n eno rm ous amount o f da ta and in fo rma tio n that may prove to be a keysto ne in unde rs tandin g our so la r syste m.

Th is publica tion provid es an early look a t the Jov ia n plane ta ry syste m an d co nta ins asel ec ted sa mpl e from the mo re than 30,000 images co llec ted during this phase o f th e Voyage rmiss io n . Whil e Voyager ac hieved an impress ive record of acco mplishm ents, full rea liza tio n o fth e scien tifi c va lue of thi s program mu st awa it the re ma inin g Voyage r enco unte rs with Satu rnan d pe rh aps U ra nu s, and a detailed analysis o f th e data fro m a ll the spacec raft in vestiga tions.

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R OBERT A . F ROSC II . Ac/millislr(lfOrNaJioflol Aeronautics alld Space Adminislrarioll

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Introduction

In March 1979 Voyager I swept past Jupiter, photographin g both th e gia nt planet and five of its moons. Fo urmonths later, a comrClnion r c c c m Voyager 2. made asimilar encounter. Now , with Jupiter reced in g behind them,both spacec raft are headed toward th e outer reaches of ou rsolar system. In November 1980, Voyager I will fly past Saturn . Voyager 2, traveling at slower speeds, will reach thesa me way station in August 1981. Beyond there. the itinerary is less certain. In January 1986, eight yea rs after itsdeparture from Earth, Voyager 2 may sail within range ofUranus, taking closeup pictures of that di stant planet for thefirst time. Long after they have exhausted their fuel sup pliesand their radios have fall en silent, both spacecraft will continue their traverse through space and beyond our so lar system, on an endless journey.

Preliminary results of he Voyager encounters with Ju piterare presented in this boo kl e t. As you examine the pictures,you wiJl be participating in a revo lutionary journey of exploration. Living in a society where many accomplishmentsand products are billed as "ex traordinary," "s tupendous.""o nce in a lifetime," or "unique ," we sometimes lose ou r perspective. Conditioned to hype rbole, we r.,1il to recogn ize thosead vances that are truly exceptional. We need a historian'svantage point to id entify the events that can literally changethe course of civilization. So it is that every student o f history recognizes the im po rtance of the Rena issance. anextraordinary time when man loo ked outward, re ach ingbeyond the traditions of the past to study his pl ace in the

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natural world. The results were ap parent in art, architecture,and literature, in new philosophic and gove rnm en tal systems, and in the staggering sc ientific re vo lution exemplifiedby Galileo's first examination of the heavens with a telesco pe, and in his stubborn suppo rt of the here tica l asser tionthat the Earth was not the center of the solar system.

Historians writing a hundred or two hundred years fromnow may well look on the latter part of the twentieth centuryas another turning point in civilization. For the first time, weexplored beyond Earth - first the Moon , then the neigh boring planeli, and finally the outermost planets, the very fringeof our so la r system .

How will the historian evaluate this period of expioration? First, perhaps, he will describe the Apo llo program asa visionary example of great cooperative ventures that canbe accomplished by many individuals, private companies,and government i_nstitutions. He will describe the subsequent space ventures that weave a fabric of cooperation andgoodwill between natio ns.

He will point ou t the technologica l advances incorporated in unmanned spacecraft,sophisticated robots ableto control their own activities and solve their own problems.He wiU mention the revolution in microelectronics - th e artof fabricatin g complex electrical control circuits so small th eeye cannot perceive them, a revolution accelerated by therequirement to conserve weight and generate performancein interplanetary spacecraft. He will point to the introdu c-

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Gafileo orbiler alld probe mi !>'siO l 10 1upiler il l 1985 will expand UpOIl /hcVoyager illl'e.wigfllio ll s of he 10 Liall J ' S l e l l l .

tion of new products, pa rticula rly in areas of communication, medical trea tm ent, and energy conversio n.Turning his auention to the environment. the historian

will a lm os t surely suggest that the first widespread rea lization of the fra gile naturdl ba lances on Earth ca me al a timewhen we we re first ab le to see our Ea rlh in its enti re ty. T heimpact of a picture of Ea rth from deep space , a luminouslyblue globe sur ro unded by darkness , ha s probably heen morepersuasive than lengthy trea tises desc ri bing the complex waysin which our sys tem of rocks, pla nts, ani mals, water, and ai ris interrelated. On a more practica l level, the histo rian willpoint to the new understanding of our terrestrial environ-ment. The composi tion and structure of oth er planetaryatmospheres - on Venus, Ma rs, and Jupiter- provid eim por tan t clues to what may happen in ou r ow n atmosphere, es pecia lly if we disrupt the chemical composition.SLudy or the primiLi ve cru sLor the Moon. Ma rs. and Mercury permits us to reconstruct the first bill ion years of Ea rthhistory, a time when chemi ca l ele ments were being concen trated in activity ultimately lead in g to the formation ofimpor tant ore deposits . Unm anned spacecraft missions tothe Sun increase our unders tanding of hat most fundame ntal of all energy sou rces, paving the way for the efficient conve rsion of sola r energy into many practica l applications, andreleasing us from dependence on ever-decreas ing reservesof foss il fu el s. Spact::craft l: irding the Ea rth study the upperatmospheric processes that play major roles in contro llingOllr weather. These same spacecraft look down on Eart h.

A solar eleclric propul.\ioll . 'fwCt'craft would ejeci all ills/rulllenl ed probeIOwaI'd I/allcy's comel il/ 1986 (lnd COil/il1l1c 01/ 10 ( ' n d e I ' ( J I I wilh{[I IO /her come/. Tempel} .aid ing us with increasingly accurate forecasts ofweather a ndcrop producLi viLY.Looking beyond matters of technology and the environmen t, th e h is torian ma y cite the latter part of th e twen tieth ce nt ury as a time of ex plosive exploration. com parableto the 15th and 16th ce ntury ex ploration of the Earth 's oceansand the distant lands that bounded them. In a sense, exp loration- whether it is physical or intellectual- provides itsow n rewards. The United States has a lways been a nationth at moves forward , pushing back the frontiers of the West,pushing back the frontiers ofsocial and econom ic develop-ment, a nd now pushing hack the frontiers of space . It isarguable that this spirit of ex pl ora tion is indispensab le to avigorous society, a nd that any socie ty that ceases LO ex plo re,to inquire, and to s trive is only a few years from decline.

And so Lhc hisLorian may recall the ea rly days o r luna rex ploration, the Apo llo project , the land ing of unma nnedVi king spacecra ft on Ma rs, and the encounters of Voyage rspacecrart with Jupiter and Saturn as th e first steps in a sustained program of space ex ploration - a program th at isprorou ndly changing man's perspecLve or himself. or LheEa rLh, a nd or the larger cosmos beyond.

THOMA S A. M UTe ) l , Associate Administratorfor Space ScienceNational Aeronautics and Space Administration

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.Jupiter's atmosphere is undergoing constantchange, presenting an ever-shift ing face toobservers. The Great Red Spot hasundergone three major periods of activityin the las t 15 years. These images of Jupiter:taken by Voyager I (top) and Voyager 2(bottom) almost four months aparl, shmvthat cloud movement in the Jovianatmosphere is not uniform because windspeeds vary at different lat itudes. Forexample, the white ova ls which appearbelow the Great Red Spot dramaticallysh ifted between January and May, the timeinterval between these two pictures. Thebright "longue" extending upward frollllhcGreat Red Spot interacted with a thin ,bright cloud above illhal had traveled twicearound Jupiter in four months. Eddy pat-terns to the left of the Great Reel Spot ,which have been observed since 1975 ,appear to be breaking ur .

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The Great Red Spot on Jupiter is a tremendous atmospheric storm, twice the size ofEarth, that has been observed for centuries. The Great Red Spot rotates counterclockwisewith one revolution every six days. Wind currents on the top flow east to west. andCurrents on the bottom flow west to east. This Voyager I picture shows the complex flowand turbulent patterns that result from the Great Red Spot's interactions with theseflows . The large white oval is a simi lar, but smaller. storm center that has ex istedfor about 40 years.

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A comparison of the Voyager 2 photograph above with the preceding Voyager Iphotograph shows several distinct changes in the Jovian atmosphere around the GreatRed Spot. The white oval beneath the Great Red Spot in the first picture has movedfarther around Jupiter, and a different white oval has appeared under the Grer.tRed Spot in the Voyager 2 picture taken four months later The disturbed cloudregions around the Great Red Spot have noticeably changed, and the white zone westof the Great Red Spot has narrowed.

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High-speed wind currents in the midlatitudes of Jupiter are shown in this highresolution Voyager I photograph. The paleorange line running diagonally to the upperright is the high-speed north temperatecurrent with a wind speed of about 120meters per second (260 miles per hour) , overtwice as fast as severe hurricane winds onEarth. Toward the top of the picture, aweaker jet of approximately 30 meters persecond (65 miles per hour) is characterizedby wave patterns and cloud features thatrotate in a clockwise manner.

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The large brown-colored oval appearing inthis Voyager I picture was selected as onc ofthe targets to be photographed near closestapproach to Jupiter because it is probablyan opening in the upper cloud deck thatexposes deeper, war mer cloud levels. Brownovals (wh ich ca n also be seen in thephotographs above and on the next page)are common features in Jupiter's northernlatitudes and have an average lifet ime of oneto two years.

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The infrared image of Jupiter on the le ft was ta ken fro m Earth a nd shows hea tradiatin g fro m deep holes in Jupiter's clo uds. Bright areas in th e image are higherte mperatu re reg ions than the da rk areas an d co rres pond to pa rts of the atmosphere thatare relati ve ly free o f obscuring clo uds. T he Grea t Red Spo t ap pea rs on the le flli lllb, o redge of the planet, as a da rk a rea encircl ed by a bright ring, ind ica ti ng that the Spo t iscoo ler than the sur rounding region. Th e infrare d image was reco rded by th e 20D-inchHale telescope on Moun t Palomar in Cali fornia. The Voyager I picture o n the right wasa lso taken the sam e day, about one hour a fter th e in fra red image.

1/1 0 /79 535 million km (332 m

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3/ 4 / 79 12 million km (750.000 mil

The first evidence of a ring around Jupiter isseen in this photograph taken by Voyage r I.Thi s photograph was part of a sequeneeplanned to sea rch for such rings arou ndJupiter. The multiple image of the extremelythin, faint ring appears as a broad lightband crossing the ce nter of the picture. Thismultiple image and the elongated, wavymotion of the background stars are due tothe II -minute, 12-second exposure and thevery slow natural oscillation of thespaeeeraft . The ring, which is in Jupiter'sequatorial plane, is invisible from Earthbecause orits thinness and transparency andbecause of Jupite r's brightness. Th e blackdots in the picture a rc calibration points inthe camera.

Because of Voyager I's discovery of a ring arollnd J upi ter. Voyager 2 was programmed totake additional pi ctures of the ring. The three Voyager 2 images on the next page showJu piter's ring in pro gressively higher resol ution. The pictures we re take n when Jup iterwas eclipsed by the Sun, and th e ring appears unusually bright because of the forwardsca ttering of su nligh t by sma ll ring particles. In the top four-picture mosa ic, the arms ofthe ring curving toward the spacecraft (on the near side of the planet) a re cut off by theplanet's shadow. Scientists est imate that the distance from the Jovian cloud tops to theouter edge of the ring is 55 ,000 kilometers (35,000 miles) . In the center picture, which iscomposed of six images, there is evidence ofstruc tu re within the ring, but the spacec raftmotion during these long exposures obscured the highest resolu tion detail. However, thereis speculation that the ring width , est imated at 6000 kilometers (4000 miles), contai nsmore than o ne ring. The bottom photograph is an enlargemen t of the isolated le ft framein the first picture and revea ls a density grad ient of ve ry sma ll par ticles extending inwa rdfrom the ring. The thickness of the ring has been es timated at less than one kil ome ter(0.6 mile) a lthough the ring appears about 30 kilomete rs ( 19 miles) thick in the image.due to camera motion and finite reso lution. Compos ition of th e low-albedo (da rk )particles is not known, but particle s ize probably ranges from microscopic to at most afew meters in diameter. Jf collected together to form a single body. the total mass of theJovia n rings would form an object with a d iame ter less than twice that of tiny Amalthea.

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Jupiter and two of its planet-sized satellites, 10 at left and Europa at right, are visiblein this Voyager I pictu re. Jup iter's four largest satellites- la, Europa, Ganymede a ndCa llisto - were discovered in 1610 by Ga li leo Galilei. The two outer Galilean sate lli tesare Ganymede and Ca llisto, not shown in this picture. All four satell ites probablyformed about four billion years ago but th eir surfaces va ry in (\ge Iremenrlollsly. 10 andEuropa have younger, more ac tive surfaces than Ganymede and Ca llisto. Like ou rMoon , the sa tellites keep the sa me face toward Jupiter. In this picture, the sides of thesatellites that a lways face away from the planet are visible.

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Amalthea, Jupiter's innerm ost satellite,was discovered in 1892. It is so small andclose to Jupiter that it is extremely difficultto observe from Earth. Amalthea's surfaceis dark and red, quite unlike any of theGalilean satellites . The three Voyager Ipictures on the left and the one Voyager 2pi cture above (seen against the disk ofJupiter) reveal a sma ll, elonga ted object,about 265 kilometers (165 miles) long and150 kilometers (90 miles) in diameter.Amalthea keeps its long ax is pointed towardJupiter as it orbits around the planet eve ry12 hours. Amalthea was obs erved end -on inthe Voyager 2 picture, which has beencomputer-processed to enhance the image.

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10, Jupiter's innermost Galilean satellite. disp lays great diversity in color andbrightness. This Voyager I fourpicture mosaic shows lo's complex coloration of redorange, black , and white regions, and the two major topographic features: volcanicregions, the most prominent of which is the "hoofprint" (volcanic deposition feature)in the ccnterr igh t , and the intcrvolcanic plains that are relatively featureless. lo's vividcoloring is probably due to its composition of sulfur-rich materials that have beenbrought to the surface by volcanic activity.

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The bright area at the upper right inthis Voyager I picture of 10 appears to be acaldera (co llapsed volcano) that is ventingclouds of gases. The clouds may condenseto form extremely fine particles that sca tterlight and appear blue. Because th e infraredspectrometer discovered sulfur dioxide on10, scientists believe this gas may be themain component of th e clouds. Sulfurdioxide clouds would ra pidly freeze andsnow back to the sur face. It is also possiblethat dark areas in the floors of the ca lderasare pools of enc rusted liquid su lfur.

Ev idence of erosion in lo 's southern polarregion is visible in this Voyager I highresolution image. The picture has been computer-enhanced to bring out su rface detailwhile suppressing bright markings. Adepressed segment of the crust, bounded byfaults, is seen near the terminator in th eupper right portion of the image. At thelower center are complicated scarps (slopes)and port ions of isolated elevated terrain thatgeologists interpret as " islands" left beh indas the scarps eroded. Scientists speculatethat sulfur dioxide (as a subsur face liquid)may be a determinant in the creation ofthese features.

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lo 's surface, less than ten million years old, is quite young compared to the otherGalilean satellites and to other terrestrial bodies, such as Mercury and the Moon. Thesurface is composed of large amounts of sulfur and sulfur dioxide frost, both of whichaccount for 1110st of the surface color. This picture was taken by Voyager I.

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The first ac tive volcanic erupt ions otherthan on Ea rth were discovered on 10. T hesevolcanoes arc extremely ex plos ive withej ec tion ve loc iti es of more tha n onekilometer per second (2200 mi les perhour) , which is mo rc vio le nt tha n Elna.Vesuv ius, o r Krakatoa o n Earlh. In the to ppicture, the plume visible on th e ri ght edgeextends more than 100 kilometers (60miles) above th e surface. Th e same volcanois shown in the lowe r pic ture. photog raphedone hou r and 52 minutes ea rl ier. Bothpictures were taken by Voyage r I. On thepreceding page, the material deposi ted bythe volcano ca n be seen as a wh ite r ingnear the center of 10.

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Of the e ight active volcanoes discove red o n 10 by Voyager I. six or the sevcn volcanoessighted by Voyager 2 were st ill act ivc. The giant volca no observcd by Voyage r l overthe "hoofprint" reg ion (see page 18) had beco me inac tivc . Scientists. the refo re. believethat the satellite is undergoing continllous volcan ic ac tivity. mak ing 10'5 surfhce themost ac tive in the solar systc m. Thi s Voyager 2 photograp h, which shows th ree ac ti vcvo lca noes, wa s one of the last of an extcnsive sequence of "vo lcano watch" pictll resplanned as a resu lt o f Voyager I's vo lca no di scovery. Th e blac k dots arc ca librationpoints o n the camera.

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7/9/79 240,000 km (150,000 m,)

Europa's surface is probably a thin ice crustove rlying water or soner ice (slush) about100 kilomelers (60 miles) thick that cove rs asilicate interior. Th e tectonic processes o nEuropa's surface create patterns that a rcdras tically different from the fault systems seen onGanymede's surface, where pieces of thecrust have moved relative to eac h other. OnEuropa, the cr ust evidently fractu res, but thepieces remain roughly in their o riginal pos it ion.This Voyager 2 picture is composed ofthree images.

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Long linear fractures or faults whichcrisscross Europa's surface in variousdirections are over 1000 kilometers (600miles) long in some places. Large fracturesare 200 to 300 kilometers (125 to 185 miles)wide, wider than the crust is thick. Alsovisible are somewhat darker mottled regionsthat appear to have a slightly pittedappearance. No large craters (more than fivekilometers in diameter) are identifiable inthis Voyager 2 picture, indicating that thissatellite has a very young surface relative toGanymede and Callisto, although perhapsnot as young as lo's surface. Scientistsbelieve that the surface is a thin ice crustoverlying water or softer ice and that thefracture systems are breaks in the crust.Resurfacing processes, such as theproduction of fresh ice or snow along thecracks and cold glacier-like flows , haveprobably removed evidenceor impact events (cratering). Europa,therefore, appears to have many propertiessimilar to Ganymede and 10.

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7/ 9/ 79 240.000 kin (150.000 ml)

Complex narrow ridges, seen as curved br ight streaks 5 to 10 ki lometers (3 to 6 miles)wid e a nd typica ll y 100 kilometers (60 m il es) long, characterize the surface topography ofthis view of Eu ropa. The dark bands a lso visible in this Voyage r 2 pho to are 20 to 40kilometers ( 12 to 25 miles) wide and up to thousands of kilometers long. The fract ures onthe icy su rface are fi lled with material from beneath. probably asa result or in terna l tida lflex ing which continually hea ts the thin outer ice crust A few fea tu res are suggestive ofdegraded imp act craters.

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7/7 /79 12 milli on km (750,000 ml)

The dark, cratered, circular feature in this Voyager 2 photograph is about 3200kilometers (2000 miles) in diameter and is on the side of Ganymede opposite to thatshown in the previous picture. This region is apparently the largest piece of ancient.heavily cratered crust left on Ganymede. The light branching bands are ridged andgrooved terrain which are younger than the more heavily cratered dark regions.Despite the dramatic surface appearance, Ganymede is relatively devoid of topographic relief due to the consequences of glacier-like "creep" in the icy crust.

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Several different types of terrain common to Ganymede's surnice are visib le in thisVoyager 2 picture. The bo unclary of the largest region of dark ancient terrain (also shownin the previous photo) can be seen to the righ t. revea ling the ligh t linear features that maybe the remain s of shock rings from an ancient impact. The broad light reg ions are thetypica l grooved structu res contained with in the light reg ions on Ganymede. On the lowerleft is another example of what might be evidence of large-sca le latera l fau lting in thecru st; the band appears to be offset by a linear feature perpendicular to it. These are th efirst clear exam ples of lateral faulting seen o n a ny planet ot her than Earth.

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This color reconstruction of part of Ganymede's northern hemisphere, taken by Voyager2, encompasses an area about 1300 kilometers (800 miles) across. It shows part of a dark,densely cratered region that contains numerous craters, many with central peaks.The la rge bright circular features have little relief and are probab ly the remnants ofold , la rge craters that have been annealed by the flow of icy material near thesurface. The gradually curving lines that press through the dark region suggest thepresence of a large impact basin to the southwest, wh ich has been oblitera ted by th esubsequent forma tio n of younger grooved terrain .

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A broad, north-south strip of groovedterrain on Ganymede, onset by a traversingfault in the upper part of the picture, isshown in this Voyager I photograph . Thereare several other perpendicular fhuh linesfarther down on th e fault. Within the majorlight stripes, the more closely spaced,shallow grooves run parallel to theboundaries of the stripes. Th e large r stripedreaLures divide the cratered terrain intoisolated polygons several hundred to about1000 kilometers (600 miles) across.

The grooved terrain at higher resolutionemphasizes numerous interwoven linearfeatures in this Voyager I picture, near theterm inator on Ga nymede. This suggests anearly period in Ganymede's history whenthe crust was active and mobile , resemblingEarth's plate tectonics in some ways. Thecauses of the extreme differences in crustalevolution between CallislO and Ganymedeare under investigation. Combinations ofradioactive heating an d a greater deg ree oftidal heating for Ganymede are possibil ities.

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This mosaic of Ganymede, composed ofphotographs taken by Voyager 2, showsnumerous impact craters, many with brightray systems. The rough terrain at the lowerright is the outer portion of a large, freshimpact basin that postdates most of theother terrain. The dark patches of heavilycratered terrain (right center) are probablyancient mixtures of ice and rock formedprior 1O the grooved terrain. The large rayedcrater at the upper center is about 150kilometers (95 miles) in diameter.

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Curved troughs and ridges in this high-resolution Voyager 2 photograph ofGanymede are the distinctive characteristicsof an enormous, ancient impact basin. Thebasin itself has been eroded by latergeologic processes; only the shock ringfeatures are preserved on the ancient surface.Near the bottom of the picture these curvedmarkings are perforated with the younger,grooved terrain.

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Th e prominent concentric ring s truc tureshown in this Voyager I fou r-p ict u remosaic of Call isto is bel ieved to be a la rgeimpacl basi n, similar to Mare Orien taleo n the Moon a nd Ca lo r is Bas in on Mer-cll ry. T he b ri ght c irc lilar spot is ab o ll t 600kilo mete rs (360 mil es) across, and the Olltc rr ing is ab ollt 2600 kilometers ( 1560m iles) across. T his is the first recogn izedba s in in the Jov ian system a nd su ppor ts theassumption tha t Ca ll is to's surface is old.Th e lack of hi gh rid ges, ring mounta ins, ora large cent ra l depression suggests that theimpact ing body ca used melting. so me now.and shoc k waves, a nd that the re freez ingoccurred in time to p reserve the concentricshock ri ngs.

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Callisto is the most heavily crateredplanetary body in ou r so lar system . In thisVoyager 2 nin e- frame mosaic, a specialcompu ter filter wa s used to provide highcontrast in the surface topography. Theimpact structure visible at the upper rightedge of the sa tellite is sma ller than thelargest one found by Voyager I but mo redetail is obvious; it is es timated that 15concentric rings su rround the bright center.Many hun d reds of mod erate-s ized cra tersare also visib le, a few with brigh t raypatterns. The lim b is smooth, which isconsistent with Ca llisto's icy composition.

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This highresolution image of Ca llisto , pho tographed by Vnyager I, shows details of thelarge ring stru cture surrounding the remainsof the ancient impact basin visible on page35. The surface area shown in this image isat the right edge and slightly above the cen-ter of the pi cture on page 35. The relativelyundisturbed region on the right shows theshoulder-to-shoulder large impact craterstypical of most of Ca llisto's surface. Adecrease in crater density toward the cen terof the structure ( to the left) is evident. and iscaused by the destruction o f very o ld cra tersby the large impact that formed the ringstructure.

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The Voyager MissionTh e Voyager mission is focused on the exploration of

the Jupiter and Saturn syste ms. The a lignment of theselarge planets permits the use of a gravity-assist trajectoryin which the gravity field of Jupiter ancl Jupiter's motionthrough space may be used to hurl the spacecra ft on toSaturn. In 1977, a rare alignment (once every 176 years)of our four outer planets - Jupiter, Saturn, Uranus. andNeptune- may permit a gravity-assist trajectory to Uranusand even to Nep tune for Voyager 2.

Voyagers I and 2 began their journeys in the latesummer o f 1977, catapu lted int o space by a Titan /Centaur launch ve hicle from Cape Canaveral, Florida.With them went the hopes and drea ms of thousands ofpeop le who had worked to create them and their mission.

The Voyager spacecraft are unique in many respects.Since their journeys are taking them far from the Sun. thcVoyagers are nuclear powered rather than so la r powered.Th e Voyagers are the fastest man-made objects ever tohave le ft Earth. In fewer than ten hours, they had crossedthe Moon's orbit. Thi s com pa res to about three days foran Apollo flight and one day for the Mariner and Vikingspacec raft. Their launches marked the end of an era inspace trave l - th e end of the planned use of Ti tan /Cen taur launch ve hicl es. With the advent of the SpaceShuttle in the 1980s, future spacecra ft will be launchedfrom the Shuttle Orbiter.

Voyager 1 was launched 16 days after its siste r sh ip,but because of a different trajectory, it arri ved a t Jupiterfour month s ahead of Voyager 2. Both spacecra ft spen tmore than nine months cross ing the asteroid belt, a vastrin g o f space debris circling the Sun between the o rbit s ofMars and Jupiter. During their 16- and 20-month journeys to Jupiter, the spacecra ft tes ted and cal ibrated all oftheir instruments, exercised the ir scan platfo rm s, an dmea sure d particles and Acids in interplane tary space. Asthe spacecraft neared the planet , the ca meras showed thedramatic visible cha nges that had taken place in Lhe fiveyears si nce Jupiter had been photographed by Pioneer II.And for the firs t time, we gut a duse luuk at sOlJle u f Ju piter's moons: Amalthea. 10, Europa , Ganymede. an dCall isto.

Targeted for the closes t look at 10, Voyager I fiew themore haza rdous course. passing between Jupiter ancl fo,where the radiation environment is the most intense. Voyager 2's flight path gave Ju p iter and its intense ra dia tion amu ch wider berth. Unlike Voyager I, which encou nteredthe nve inn e rmost sa tellites as it was le av ing Ju piter. Voyager 2 enco untered the sate lli tes as it was ap proach in g the

Ma rch 5, 1979. Voyager I IInique jlighl pmh allowed sciCli/iSIS 10 .1'Il{{{ r al(-lose nmge 5 of J l l p i l e r : 13 knOll'll .wlle/liles. Each is sholl'l1 (1/ il .\' doseslpoilll 10 rhe Imjeclm:r li f Vi?ra!;er I\ , oll/houlld jtiglll ( / ) r { ~ l ' from JupilaClosesr approach 11m' 2S0,OOO kilo/Helers ( / 74.000 miles) from JI/pifCl:

.Jul y 9, 1979, V(!I'liKeI' 2 \- dosesl IIpprom'h II ! JI/piler IIUS 645,(X}(1 kilolllelers(400.000 Illites) from Ihe ploll('r. 11ml!;(,/, 2 ('!!('()IIIII('red Ihe .mll'lIiles Oi l ilsil/bol/lld j O l l m ( ~ I /() Jllpilt'l: \l'hich e/ilih/ed Ihi' ( , ( ' m l i 10 pholOf,mph Iheopposilt' sides oI "!e sa/e/liles.

planet, thus p rov iding closeup photograp hy of oppositesides o f the sa te llites.

Arriving at Jupiter from slightly diffe rent angles, bothspac ecraft measured the large. doughnut-shaped ring ofcharged sul fur and oxygen ions, called a lOr us, enc ircling

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HIGH-FIELD

LOW-FIELDMAGNETOMETEIl (2)

RADIOISOTOPETHERMOELECTRIC GENERATOR (3)

HIGH-GAIN ANTENNA(3.7-merer diameter)

PLANETARY RADIO ASTRONOMY ANDPLASMA WAVE ANTENNA (2)

J I ( ~ r a g ( ' / " JjJocecmji alld sc iell l i j ic il1Sfmmel1lS.the planet at about the orbit of 10. Then, both spacecraftdisappeared behind Jupiter, out of view of Earth and Sun.for about two hours. During this time, measurements weretaken on the planet's dark side. Each spacecraft took over15 ,000 photographs of Jupiter and its satellites.

From the moment of launch, the Voyager spacecrafthave been monitored by a worldwide tracking system ofnine giant antennas strategically located around the worldin California, Spain , and Australia to ensure constant radiocontact with the spacecraft as the Earth rotates. Radiocontact with Voyagers I and 2 has not been instantaneous,however. When Voyager I flew past Jupiter, radio signa lsbetween Earth an d the spacecraft took 37 minutes: whenVoyager 2 a rri ved, the sig nals took 52 minutes because bythen the planet wa s farther from Earth.

The pictures in this book were taken hy a shutteredte levision-type camcra. Each p icture is co mp osed o r640 ,000 dots , which were converted into hinary numbersbefore being radioed to Earth. \Vhen the sig nals reachedEarth, they were reconverted by computer into dots andreassembled into the original image. Mos t or the co lorpictures are composed of three images, each one takenthrough a difH:rent color niter: blue, orange, or green. The

38

U L r R A V I O L l SPEcrROMETER

COSMIC RAY

INFRAREDSPECTROMETERAND RADIOMETER

PI-IOTOPOLARIMETER

LOW-ENERGYCHAnGED PARTICLE

oprlCAL CA LIBRATIONTARGET

images were combined a nd the origina l color wa s reconstructed by computer. The computer el iminated many ofthe imperfections that crept into the images. and enhancedsome of the images by emphasizing different colors.

Designed to provide a broad spectrum of scient incinvestigations at Jupiter, the science instruments investigated atmospheres, satellites, and magnetosphe res. Thescientific investigations for the Voyager mi ssion and theirJovian encounter objectives are shown in the table onpage 40.

After their closest approaches to Jupiter, both spacecraft fired their thrusters, retargeting for their next goal,the Saturn system. Scientists will still be studying the wea lthof new information about Jupiter when Voyager I reachesSaturn in November 1980, and Voyager 2 follows in August1981. After Voyager I encounters Saturn, Voyagcr 2 ma ybe retargeted to Oy paSl Uranus in 1986. Upon completion of their plane tary missions, both spacecraft will searchfor the outer limit of the solar wind, that boundary somewhere in our part of the Milky Way where the influence ofthe Sun gives way to other stars of the galaxy. Voyagers Iand 2 will continue to study interstellar space unti l thespacecraft signals can no longer be received.

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Scientific Highlights

Some orthe most important information gathered byVoyagers I and 2 on the Jovia n system is presented pictorially in this book an d is supp lemented here with briefsummaries orthe major discove ri es, observations, an dtheories.

JupiterThe atmosphere of Jupiter is colorful , with cloud

bands of alternating colors. A major characteristic of theatmosphere is the appearance of regularly spaced fea tures. Around the northern edge of the equator, a train ofplumes is observed, which has bright centers represe ntative of cumulus convection similar to that seen on Earth.At both northern and sou thern latitudes, cloud spots areobserved spaced almost all the way around the planet ,suggestive of wave interactions. Th e cloud st ructures inthe northern and southern hemispheres are distinctly di fferent. However, the velocit ies between the bright zonesand dark belts appear to be symmetric about the equator,and stable over many decades. This suggests that suchlong-lived and stable reatures may be controlled by theatmosphere far beneath the visible clouds. The Great RedSpot possesses the sa me meteorological properties ofinternal structure and counterclockwise rotation as thesmaller white spots. The color of the Great Red Spot mayindicate that it extends deep into the Jovian atmosphere.Cloud-top lightning bolts, similar to those on Earth, havealso been found in the Jov ian atmosphere. At the polarregions, auroras have been observed. A very thin ring ofmaterial less than one kilometer (0.6 mile) in thicknessand about 6000 kilometers (4000 miles) i.n radial extent hasbeen observed circling the planet about 55 ,000 kilometers(35,000 miles) above the cloud tops.Amalthea

Amalthea is an elongated, irregularly shaped satel liteor reddtsh color. It is 265 kilometers (165 milcs) long andISO kilometers (90 miles) wide. Just like the large Galileansatellites, Amailhea is in synchronous rotation, with its longaxis always oriented toward Jupiter. At least one significantcolor variation has been detected on its surface.To

Eight active volcanoes have bee n detected on 10, withsome plumes ex tending up to 320 kilometers (200 miles)

above the surface. Over the four-month interval between theVoyager 1and 2 encounters, the active volcanism appears tohavc continucd. Scvcn ofthc volcanoes were photographedby Voyager 2, an d six were still erupting.

The relative smoothness of la 's surface and its volcanicactivity suggest that it has the youngest surface of Jupiter'smoons. Its surface is composed of large amounts of sulfurand sulfur dioxide frost , which account for the primarily ye l-low-orange surface color. Th e volcanoes seem to eject a sufficient amount of sulfur dioxide to form a doughnut-shapedring (torus) of ionized ~ " ' U l f u r and oxygen atoms arollnd Jupiternear lo's orbit. The Jovian magnetic field lines that go throughthe torus allow particles to precipitate into the pola r regionsof Jupitel; resulting in intense ultmviolet and visible auroras.Europa

Europa, the brightest of Jupiter 's (Jalilean satellites.may have a surface of thin ice crust overlying water or sanerice. with large-scale fra cture and ridge sys tem s appearing inthe crust. Europa has a density about three times that ofwater. suggesting it is a mixture of silicate rock and somewater. Very few impact craters are visible on the surfllce.implying a continual resurfacing process. perhaps by theproduct io n of fresh ice or snow along cracks a nd coldglacier-like flows.

GanymedeGanymede, largest of Jupiter's 13 satell ites. has bright

"young" ray craters; light. linear stripes resembling the outerrings of a very large, ancient impact basin; grooved terrainwith many faults; and regions of dark , heavily cratered terrain. Among the Galilean satellites, Ganymede probably hasthe greatest variety ofgeologic processes recorded on its surface and may be the best example for studying the evolutionof Jupiter's inner satellites. Imbedded within Jupiter's magnetosphere, Ganymede is subjected to the influences of thecorotating charged-particle plasma and an interaction mayexist with this plasma. No atmosphe re has been de tec ted.Callisto

The icy, dirt-laden surrace of Callisto appea rs to bevery ancient an d heavily cratered. The large concentric ringsindicate the rema i.l1s ofseveral enormous impact basins. created by huge meteors craShing into the surface. and sinceerased by the flow of the crust. Callisto's density (less thantwice that of water) is very close to that of Ganymede , ye t

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there is tittle or no evidence of the crustal motion and internal activity that is visible on Ganymede.

magne tic field , and rotates around Jupiter. T he Ga lilean satellites are loca ted in the inner regions of th e magnetospherewhere they are subjected to intense radiation bombardment.It appears that 10 is a source of the su lfur and oxygen ionswhich fill the magnetosphere. Another magnetosphericinteraction is the e lect rica l connec tion between 10 and Jupiter along the magnetic field lines that leave Jup iter and inter sect 10. This magnetic flux tube was exa mined by Voyager Iand a flow of about five million amperes of current wasmeasured, whi ch was cons id erably more than anticipated.Voyager also discovered a new low-freq uency radio emissioncoming from Jupi te r, which is possibly associated with the10 torus.

The MagnetospherePerhaps the largest structure in the so lar system is the

magnetosphere of Jupiter. This is the region of space whichis filled with Jupiter's magnetic field and is bounded by theinteraction of that magneti c field with the so lar wind, wh ichis the Sun's outward flow of charged par ticles. T he plasmaof e lect rica lly charged particles that exists in the magnetosp here is flattened into a large disk more than 4.8 millionkil ometers (3 million miles) in diameter, is coupled to the

Scientific investigations of the Voyager missionInvestigation

Imaging science

Infrared radiation

Photopolarimetry

Radio science

Ultraviol et spectrosco py

Magnetic fields

Plasma particles

Plasma waves

Planetary radio astronomy

Low-energy charged particles

Cosmic ray particles

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Typical Jovian encounter objectivesH g - e s ~ l u t i o n rec,?nnaissance over large phase angles; atmosphericdynamics; geologiC stru ctu re of sate llites

Atmosp her ic compos ition, thermal structure and dynamics; sa tel litesurface composition and thermal properties

Atmospheric aerosols: satellite surface texture and sodium cloud

Atmospheric and ionospheric structure, constituen ts, and dynamics

Upper atmospheric composition and structure; auroral p rocesses;dis tribution of ions and neutral atoms in the Jovian system

Planetary magnetic Ac Id: magnetospheric structure; 10 flux tube currents

Magnetosp heric ion and elec tron dist ri butio n; solar wind interaction withJu piter: ions from satellites

Plasm.a ~ I e c t r o n densities: wave-part icl e interactions: low-frequency waveemISSions

Pola rizatio n and spectra of radio freq uency emissions; 10 radio modulationprocess : plasma den sities

Distribut io n, composi tion, and flow of energetic ions and electrons:satellite-energe tic particle in teract ions

Distribution, compos ition. a nd now ofh igh-e nergy trapped nuclei;energetic elcclron spectra

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End of Document