Voyager ep 191


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Voyager ep 191

  1. 1. THE VOYAGER FLIGHTS TO JUPITER AND SATURN s the giant outer planets-Jupiter, Saturn, Uranus, and Neptune - perlorm their ancient, stately dance with the Sun, they whirl into a pattern each 175 years that allows a spacecraft launched from Earth to Jupi- ter to visit them all. Each planets gravity bends the spacecrafts flight path toward the next and increases the velocity. In the 1960s, engineers at NASAs Jet Propul- sion Laboratory proposed a mission for 1977 that would take advantage of the next such opportunity. The mission became ~~~~m~ known as the Grand Tour. The Grand Tour, however, would require an entirely new kind of space- craft, a design with capabilities far beyond the simpler machines that had reached the Moon, Venus, and Mars; it would have to sur- vive the intense radiation at Jupiter and operate almost flawlessly for more than a decade. The.. new craft would need the decision-making capability to detect and react to a variety of internal problems, since command times from Earth, to provide a solution, would stretch to hours during the long flight. That long-life requirement alone was staggering-far longer than for any other space machine those engi- neers had designed.
  2. 2. But a less ambitious mission, a mini-GrandTour to Jupiter and Saturn, was voted by Congress in 1972. A flight project began in May of that year, named Mariner Jupiter/Saturn,later to be renamed Voyager. To preserve the concept of a Grand Tour andachieve its scientific goals at both Jupiter andSaturn, engineers at JPL designed and built two advanced Mariner-class spacecraft, the most complex unmanned machines ever designed and built at the Laboratory. The two spacecraft would be launched in1977 -during the Grand Tour launch opportu-nity Both would reach Jupiter in 1979. Theleading spacecraft would arrive at Saturn in late1980, and the second in the summer of 1981. Weighing 825 kilograms (1,820 pounds) each,they would carry a mix of 11 instruments(essentially those planned for the Grand Tour)to thoroughly probe the planets and theirmagnetic environments, the rings of Saturn, thefleets of satellites escorting the planets, and theinterplanetary medium. 1)0"" c;.. ~_....--- 4,~ The date of each photogra ph and the dis- tance of the spacecraft from the planet or satellite are included with each picture.
  3. 3. Two identical Voyager spacecraft have completed their initial inves- tigations of I upiter and Saturn."Their ships are swift a s a bird or a thought." HOMER Each Voyager car- ries a record con- taining sounds and pictures of Earth: messages to any who might find them. For sale by the Superintendent of Documents U.S. Government Printing Office Washington, D.C. 20402 1
  4. 4. FOREWORD Earth is a world called 10 We no longer are only that is very much alive, "riders on the Earth Voyagers 1 and 2 are the a moon with active vol- together," but inhabitantsdistant eyes and ears canoes. The other three of a solar system whoseof an outward-bound moons, though less dra- other worlds we have seencivilization. matic than 10, are individ- and whose music we have The swiftest and most ually fascinating and, heard.complex robotic exten- together with Jupiter, form The wondrous discover-sions of human intelli- a system that parallels ies chronicled in this bookgence ever built, they have that of the Sun and the are but a hint of thosejourneyed more than a inner planets. And Jupiter awaiting us. We willbillion miles into space itself is an astonishingly return to Jupiter with thesince their launch in 1977. dynamic planet with vio- Galileo spacecraft, thisAnd by the end of the cen- lent storms and swirling time to enter its atmo-tury they will have tra- eddies. We later saw the sphere. We will map theversed billions of miles majestic rings of Saturn unseen surface of Venus,more to the edges of true and that planets many and initiate the explora-interstellar space, the moons, and discovered tion of the comets andpoint where the void that one-Titan-has a asteroids.between the stars begins. thick atmosphere contain- Even as we continue toThat is when their job will ing hydrocarbon mole- explore the solar system,end, because we will no cules and, probably, we will be charting newlonger be able to track methane clouds and rain. paths among the stars.them. Such is the pace of our The Infrared Astronomical Voyager 1, having com- discoveries that we have Satellite, followed first bypleted its planetary explo- rewritten the astronomy the Space Telescope andration tasks, is now textbooks several times then the Gamma Raysampling the ocean-like during the past decade. Observatory, will be ourtides and currents of the This new wealth of new eyes and ears as wesolar wind and searching information has enhanced move from the neighbor-for the limits of our Suns our understanding of the hood of the planets to thatinfluence. Voyager 2 is on Earth by giving us a of the stars and galaxies.course to Uranus and to vastly broader context in We have truly only justdistant Neptune for close which to view our planet. begun our journey ofencounters in 1986 and 1989 discovery.respectively. At long last, we are ------- *- -...:::;::::::-- James M. Beggsbeginning to know other *~--=:. - -- _ ... - - =:.. .lI- NASA Administratorheavenly bodies as inti- July 1982matelyas we know our -~r~;--Earth. And we are begin-ning to feel at home in the -----:::~Universe. Before Voyager 1 sent - ""back the first close-up pic- --~tures of Jupiters Galileanmoons, scientists had beenexpecting views of fourworlds preserved in coldstorage for aeons. But wefound, incredibly, that ahalf-billion miles fromThree huge antennas around the worldtrack th e Voyagers. This 64-meter (210-foot) giant is located in th e Californiadesert. ... 3
  5. 5. THE FLIGHTS TO JUPITER knew that a color motion Galilei first saw that Jupiter of exciting new findings to the picture of Jupiters turbulent was circled by its own cluster hundreds of reporters who ugust 20, 1977, was a clouds, to be made from several of satellites. jammed the Voyager news roomA steamy Florida day on the Titan-Centaurlaunch pad at Cape Canaveral hundred still photos, would yield new information on the complex atmosphere of Jupiter. THE FIRST JUPITER ENCOUNTER at JPL. Easily the most exciting was the discovery of the first extra-Air Force Station. At 10:29 a.m ., Thus the new year of 1979 terrestrial volcanoes. At leastVoyager 2 lifted into the sky on brought the first close-up, high- eight or nine volcanoes on the Fthe opening leg of its trip to the resolution viewing of Jupiter. rom the beginning of the reddish satellite 10 had eruptedouter reaches of the solar sys- Voyager 1 made its closest first Jupiter encounter, before the spacecraft cameras.tem. (The first spacecraft approach to Jupiter on March 5 scientists were stunned No one had ever seen an activelaunched was called Voyager 2, and continued its exploration of by the discoveries contained in volcano anywhere beyondbecause the second Voyager to the Jupiter system into early the data. Each day, as the Voyag- launched would overtake it April. By the time the first ers poured millions of bits of One photo of 10 showed ain flight and become Voyager 1.) encounter had ended, Voyager 1 data to Earth around the clock, plume-shaped object standing Within 10 hours, Voyager 2 had taken 16,500 photos, and science teams reported a bountypassed the Moon. By November our knowledge of Jupiter hadit had passed Mars orbit. In changed as profoundly as it didearly December Voyager 2 was 369 years before, when Galileosailing into the edge of the aster-oid belt, a region of interplane-tary reefs and shoals out beyondMars. Voyager 1 followed its twinfrom the Florida launch padtwo weeks later, on September5, 1977. It followed the sameitinerary as Voyager 2, overtak-ing it just after entering theasteroid belt in mid-December. A year after entering theasteroid belt, Voyager 1 wasdrawing near to Jupiter. Its cam-eras began sending photos toEarth at a range of 50 millionkilometers (31 million miles) Callistofrom Jupiter, pictures that soonsurpassed the best takenthrough Earth-based telescopes.The resolution improved, untilfeatures only a few kilometersin diameter appeared. And sci-entists converged on JPL for thefirst encounter. One hundred fifty scientists,members of the 11 scienceteams, were exultant. Notonly would their instrumentsquickly provide a more detailedstudy of Jupiter than had beencompiled during the {;enturiesthe planet had been observed,but they knew they stood per-sonally on the brink of excitingscientific discoveries. Membersof the photo team, for example,
  6. 6. Voyager 1 took the first single-frame photo of Earth and Moon, and an early picture of huge, multicolored Jupiter. 12 / 10 / 78 83.9 million km (52 million mil9 / 18 / 77 11 .66 million km (7.25 million mil II tiThe heavens themselves, the planets, and this center, Observe degree, priority, and place." SHAKESPEARE A Voyager leaves Earth atop its flam- ing Titan-Centaur la unch vehicle in the summer of 1977. 5
  7. 7. off the satellites limb. A large with long lines-perhaps either Red 10 and white Europa orbit abovevolcano was erupting material cracks or ridges-on its surface. Jupiters multihued to an altitude of 250 kilometers Ganymede was peppered with clouds. Th e Great (155 miles) above the Ionian craters, interspersed with Red Spot is the largest of manysurface. Once the first volcano grooves that wound their way storm s. c>was found, discovery of others over the surface like huge dune-followed quickly. One volcano buggy tracks. The surface ofearned the nickname "the Callisto was a jumble of thou-tarantula; since it resembled sands of craters of all sizes;a large spider standing above Callisto was the most heavilythe surface on legs of ejecta. bombarded satellite ever seen. A volcano, nick- One scientist, a veteran of the named "th e of other planets, called twa, " on the J upiter sails the cosmic sea Jovian satellite 10,the discovery, "the most impor- in a vast, intense field of spouts e;ecta. Eighttant, so far, in the planetary- radiation, making it the or nine erupted asexploration program: Voyager 1 fl ew strongest emitter of radio noise past. In the motion picture of Jupi- of all the planets in our solar 3 / 4/79 499,000 km (310.000 milter mentioned earlier, scientists system. Radio astronomers hadcould see, for the first time, theactivity in Jupiters giganticGreat Red Spot. The feature hadbeen observed from Earth forabout 300 years, but never insuch detail. The Great Red Spotis a counterclockwise-rotatingstorm system. Its outer edgemakes a complete circuit onceevery six days, while at thecenter almost no motion canbe seen. d Jupiter had a ring, la va lakes coverA similar to those of Sat- urn and Uranus. While no one really expected to see a th e landscape and continually resur- fac e it, so that any impact craters have disappeared.ring of dust or ice at Jupiter, asingle opportunity to look forone had been thoughtfully pro-grammed into Voyager lssequences. At the moment Voy-ager 1 dived across Jupitersequatorial plane on its inboundleg, its cameras shuttered, andabout 37 minutes later theimage began to build up on theTV monitors at JPL. The ringaround Jupiter was extremelyfaint, but it was there, a band offine particles that had eenundetectable from Earth. Jupiters ring and the trails of distant The surfaces of all four Gali- stars appear inlean satellites, seen in detail for this ring-discovery photo by Vo yager 1.the first time, proved to be asurprise. 10 was covered withreddish sulfur from its volca-noes. Europa appeared laced
  8. 8. 7
  9. 9. been able to determine Jupitersrotation rate from the radionoise that originated in theplanets magnetosphere. They also had found that astrange, tube-like flow of electriccurrent and energetic particlestravels along an invisible mag-netic pathway between Jupiterand 10. From Earth-based obser-vations, scientists predictedthat the tube might carry about1 million amperes of current.~ oyager 1 was targeted to fly directly through the tube to make the firstdirect measurements of itsstrength. Although the space-craft went exactly where it wastargeted, the tube was not there.The flux tube had been skewed 3 / 79 Ganymede: 3.4 million km (2 million mil 3 / 79 Callisto: 6.9 million km (4.3 million ml)from the predicted position by Gan ym ede (l eft ) surface is a record and Callisto, Jupi- of an cien tphysical forces. But the Voyager ters largest satel- bomba rdmen were sufficient to reveal lites, of wh ich Gan ym ede is thethat the tube carried about larger; Calli stosthree times the current antic i-pated-3 million amperes. Sci-entists hastened to add,however, that the voltage was solow that the current could not "Man can learn nothingbe detected by anyone standing unless he proceeds fromon 10. the known to the unknown." CLAUDE BERNARDEuropa, the Voyager looks atsm allest of the ligh ted cres-Jupiters Galilean cent of a rapidlysa tellites, h as a retrea ting Jupiter.surface that is cov-ered with ice.
  10. 10. 9
  11. 11. VOYAGER 2 AT JUPITER ridges on Europa, on closer scru- While the dust particles of the bright portion of the ring tiny, were so flat, "They looked the ring appeared to extend itself. The dust may be due to voyager l s encounter as if they had been painted on inward toward Jupiter, probably micrometeorites striking largerA drew to an end, scien- tists and engineersaltered many of the mission the satellite with a felt marker: And the largest of los volcanoes had stopped erupting. Voyager 2 took pictures of all the way to the cloud tops, the ring had a hard outer edge, as if cut from cardboard. bodies in the ring. Voyager 2s Jupiter encounter ended in September 1979, and scientists and engineers beganinstructions programmed intoVoyager 2s computers, radioing Jupiters ring on the inbound lose examination of preparing the intricate com-target and timing changes toexploit the flood of new datafrom Voyager l. leg, but more interesting were the pictures it took while behind Jupiter, looking back at the ring. Where Voyager ls pic- C Voyager photos after the encounter revealed two tiny satellites, orbiting near mand sequences for the two encounters with Saturn. Voyager 2s encounter began the outer edge of the ring andin late April, two weeks after tures were faint, the ring now herding the particles in a tightVoyager 1 turned from looking stood out sharp and bright in boundary. The source of theover its shoulder at the receding the newest photos, telling scien- rings dust probably lies withinJupiter and settled down for the tists instantly that the rings par-long cruise to Saturn. ticles scattered sunlight forward Voyager 2 made its closest more efficiently than they scat-approach to Jupiter on July 9, tered it backward, and therefore1979, and the planet provided were tiny, dust-like data on its tumultuous (Large particles backscatterweather. The swirling cloud more efficiently.)patterns recorded by Voyager 1had changed and continuedto change as Voyager 2 ap-proached. Those cracks or The ice-crusted surface of Europa, laced with lines that show littleVoyager 2 looksback at Jupiter tosee the ring spar·kling in sunlight.The ring is brighterwhen lookingtoward the Sun. 11
  12. 12. THE TUPITER PICTURES~ uPiter and two of its largest satellites, 10 and Europa, are captured by Voyagers cam-eras (A) . 10 is the red satellite atthe left. Many Jovian storm sys-tems are visible, including theGreat Red Spot, the white ovalsbelow it, and alternating darkand light cloud bands calledbelts and zones. The strongestwinds-up to 400 kilometers(250 miles) an hour-are foundat boundaries between belts andzones. Between opposing jetstreams the clouds are turbu-lent. The area immediatelybelow the wide, bright, whitezone is an example of that tur-bulence. Wind shear stretchesconvective features into longplumes; one can be seen nearthe center of the photo. Holesin the clouds permit observa-tion of deeper layers of theatmosphere. One such area is abluish region north of the equa-tor and left of the white cloudplume. The blue regions arestrong emitters of infraredradiation coming from highertemperatures deep in the atmo-sphere. (B) 10 (left) and Europapass before the planets GreatRed Spot. The rim of the GreatRed Spot circulates counter-clockwise in six days, whilethere is almost no circulation atthe center. The long brightregion between two white ovals(across center of picture)extends higher into Jupitersatmosphere and is thereforecolder than the surroundingclouds. Almost all the whiteovals exhibit the same counter-clockwise motion as the GreatRed Spot, which has beenobserved continuously forabout 300 years; formation ofthe white ovals was firstobserved about 40 years ago.
  13. 13. A dramatic view of Jupiters Great Red Spot and the sur- rounding area (C) shows cloud details as small as 160 kilome- ters (100 miles) . The turbulent cloud pattern to the left of the Great Red Spot is a region of extraordinarily complex and variable winds. It has the appearance of liquids, such as oil and water, that do not mix. As relatively slow winds above the Red Spot flow past it, the smooth flow pattern is severely disturbed, creating the tumbled and twisted appearance. Some material from the region is also drawn into the Great Red Spot. Scientists dont understand why the cloud colors remain unmixed. As wind blows through the region between the Great Red Spot and the white oval, a turbulent pattern forms to the left. A lighter portion at the top of the Great Red Spot shows that some turbulent material is drawn into the Red Spot itself. Smaller amounts appear to interact similarly with the white oval. South polar regions at the bottom are darker than at mid-latitudes, probably because cloud circulation pat- terns there are different, and solar illumination is more oblique. The abundance of dis- tinctive cloud patterns leads sci- entists to compare some aspects of Jupiters winds with currents in the oceans of Earth.North Polar RegIOn - - - - - - - / ~North Temperate zone----)/~~~~~~~~~~~~~~~"North Temperate Belt tLI==============~"North Tropical Zone - -I ,North Equatorial Belt - - -1 - - - - - - - - - - - - - - - - - - - - 1Equatorial ZoneEquatorial Band - - - - 1-----------------1South Equatorial Belt - - {========::::::::~5::=====#Great Red Spot _ _ _ _i-==============_~ /South Tropical Zone - ~ ~~~~~~~~~~~~~~~1South Temperate Belt -:::~.::o:o~----- ~ ~---~ / 13
  14. 14. ~ uPiters Great Red Spot dominates a close-up Voy- ager photo (A) that hasbeen compared to an abstractpainting. The Great Red Spot,about 25,000 kilometers (16,000miles) on its long axis, wouldcover three Earths. The GreatRed Spot is higher than the sur-rounding clouds. The whiteoval immediately below theGreat Red Spot is a similar, photo (B) shows a startlingcounterclockwise-rotating amount of color differencestorm. Scientists do not know below and to the left of thewhat causes the color differ- Great Red Spot, which is par-ences, but they believe the dis- tially obscured at the top bytinctive color of the Great Red high, white, ammonia cirrusSpot may be due to minor con- clouds. Because hurricane-likestituents that come from deeper winds extend in a columnwithin the atmosphere. The above and below the Great RedGreat Red Spots center has the Spot, the spot seems to act like apuffy appearance generally asso- stationary object, producingciated with little or no wind eddies in adjacent clouds simi-and cloud motion, while its lar to eddies in a stream causedouter rim shows the stream- by an obstruction.lined shapes of 360-kilometer-(225-mile-) per-hour winds. Thewhite ovals inner spiral struc-ture, seen in this photo for thefirst time, is also the result ofcounterclockwise winds. Thewhite zone at left, at the samelatitude as the Great Red Spot,disappears almost entirely as itmoves past, then is regeneratedto the east. The normal-color 15
  15. 15. T he Jovian ring sparkles satellites near its outer edge; as three separate color bands, ring; therefore the planets limb as Voyager looks back- those satellites were also discov- because color pictures are failed to register. The ring has a ward at the dark side of ered by Voyager. The bright por- assembled on Earth from black- diffuse halo of fine particlesthe planet (A). The upper por- tion of the ring is about 5,200 and-white photos taken through inside the two segments. Thetion, the closer segment, disap- kilometers (3,200 miles) wide. color filters, and the spacecraft rings extreme faintness dictatedpears where it enters Jupiters Jupiters bright limb shows had moved between each long camera exposures.shadow. The ring, mainly photo. Computers on Earth reg-extremely fine dust particles, istered the pictures to show theapparently comes from two tiny
  16. 16. UPiter,s ring has a sharpl outer edge (B) , patrolled by two tiny satellites, whilethe inner edge fades gradually.A faint star trail (C) shinesthrough the rings inner halo.The halo is an area of extremelydiffuse ring material extendinginward to the top of Jupitersatmosphere, indicating that thering is continually losing mate-rial. The spots are reseau marks,engraved on the face of thecameras vidicon tube to allowscientists to reconstruct the pic-tures geometry precisely. Thebottom photo (D) is a differentcomputer processing of therings used to bring out slightlydifferent details. Gaps in thering are an artifact of joiningseveral photos together. 17
  17. 17. A
  18. 18. is the first body beyond physically associated with the 1 B 0 Earth where scientists have plume named Loki, next to it. seen volcanic activity in The lakes temperature is in progress. The heart-shaped excess of 200 0 K (-99°F) , about deposit (A) from the volcanic 80 0 K (144°F) higher than the plume named Pele changed to temperature of the surrounding an oval shape between encoun- surface. Voyager 1 looked almost ters. Pele hurled ejecta 280 kilo- directly up at los south polar meters (175 miles) above los region (B) as it passed beneath. surface. Total width of the A variety of terrain types can be plume was 1,000 kilometers seen, including isolated moun- (620 miles). Sulfur dioxide gas tains up to 10 kilometers (6.2 has been identified venting miles) high. No impact craters from the volcanoes. The dark, are visible on los surface, since U-shaped object (upper left) is constant eruptions continually probably a lava lake with an resurface the satellite with sul- encrusted surface that may be fur and silicates. A close-up photo (C) shows two volcanic calderas and associated lava flows on 10. 10 is the most geo- logically active body known in Eoo the solar system. It has a diame-<0~ ter of 3,632 kilometers (2,257 E-"oo miles), similar to that of EuropaoN and the Moon."cE0~",-to~00U1as~i<!~~" 19
  19. 19. ~ oyager 2 monitored los volcanic activity continu- ously for six hours. One of those photos (A) shows two blue volcanic plumes standingabout 100 kilometers (62 miles)above the surface. los activity isa result of extreme tidal pump-ing: The satellites ellipticalorbit around Jupiter, caused byresonance with Europa andGanymede, creates differencesin Jupiters gravitational pull on10 (at different locations in losorbit), which flex los surfaceand the region just beneath.The result is mechanical heat-ing that melts almost the entirebody. A volcano (B) spews mate-rial at a velocity of about 3,300kilometers (2,000 miles) perhour. Mount Etna, one ofEarths most explosive volca-noes, ejects material at onlyabout 65 kilometers (40 miles)an hour. The brightness in thisphoto has been enhanced, whilecolor has been preserved. Lokiappears in a false-color image(C), in which ultraviolet light,sensitive to the smallest ejectaparticles, has been processed inblue to show the symmetricalstructure of the plume. Eightor nine volcanoes were eruptingas Voyager 1 flew past; atleast six were still eruptingwhen Voyager 2 arrived fourmonths later.
  20. 20. 21
  21. 21. uropa also provided sci-E entists with puzzling phenomena. The ice-coated satellite (A) is remark-ably smooth, and shows fewcraters. The surface consists pri-marily of uniformly bright ter-rain, mainly water ice, crossedby linear markings with no top-ographic relief, except for lightlines such as the chainlikeridges (B), which are only a fewhundred meters high. Theycould result from cracking ofthe surface and extrusion offresh ice from below. Scientistsbelieve tidal forces similar tothose on 10 are responsible forthose processes, although tidalheating is not sufficient to causeactive volcanoes as on 10. A sim-ilar light-colored ridge meandersacross Europas surface (C).Europas surface ice could be upto 100 kilometers (62 miles)thick; the rest appears to berocky and metallic. Europasdiameter is 3,126 kilometers (l ,942 miles).
  22. 22. T he surface of Ganymede is one of great diversity, indicating severalperiods of geologic activity.Many dark areas (A) are heavilycratered and probably date fromthe time of an early bombard-ment more than 4 billion yearsago. Other regions are clearly aproduct of intense internal geo-logic activity, which happenedover the next few hundredmillion years. The latter regionsgenerally have high albedo andconsist of many parallel lines ofmountains and valleys (B) inter-sected by what appear to befaultlike discontinuities. Craterssurrounded by bright ejectablankets are probably muchyounger than the dark craters.Though its surface is muchdarker than Europas, Gany-mede is half water ice and halfrock, resulting in a densityabout two-thirds that of Europa.Scientists had thought Titanwas the largest satellite in thesolar system, but Voyager mea-sured Ganymede and later mea-sured Titan, and found thatGanymede is larger-the largestsatellite in the solar system.Ganymedes diameter is 5,276kilometers (3,278 miles) ;Titans diameter is 5,150 kilo-meters (3,200 miles) . Gany-medes surface is a frozen recordof its tectonic history-the geo-logic processes by which halfof the ancient, darker surfacewas replaced by the younger,brighter grooved terrain. 23
  23. 23. malthea is darker and AB smaller than any Gali- lean satellite, and orbits closer to Jupiter than 10 does. The tiny satellite has an irregu- lar shape (C, D, and E). One Voyager scientist said he doubted "if one astronomer in 100 had ever seen Amalthea." Amalthea was the first interme- diate-size planetary object pho- tographed in detail. It is about 270 by 165 by 150 kilometers (168 by 103 by 93 miles), 10 times larger than the Martian satellite Phobos. Its long axis always points to the center of Jupiter. The red color is real and has been computer enhanced to make it brighter; it reflects only about 5 percent of the sunlight that strikes it. Amalthea is deeply embedded in Jupiters intense radiation field, which undoubtedly alters surface materials.Eg0.o~~c~EN least 10 ridges with radii extend- L he outermost Galilean enough to bear the weight. satellite is Callisto. More recent bombardment is ing up to 1,500 kilometers (930 Although similar in size evident in craters that are sur- miles) from the center. The fea- and composition to Ganymede, rounded by bright rays, which ture formed when an object col- its surface is strikingly different. are fresh ice overlaid on the lided with Callisto, fracturing Obvious at first glance are the older, darker surface. Also and melting the surface, which many craters on Callistos sur- obvious (A) is a huge, ancient quickly refroze again. Since face (B), which indicate that the impact structure, a multiringed Callisto is half water ice, the surface must date back to the feature that is, in some respects, basin was unable to hold its late torrential bombardment similar to large impact basins on shape, slowly slumping back period some four billion years the surfaces of the Moon and until only shadows of the sur- ago. There is no evidence for Mercury. However, there is no rounding rings remain. Callisto any further geologic activity, central basin, just a bright, cir- is the second largest Galilean once Callistos surface froze. cular patch surrounded by at satellite, with a diameter of Because the impact craters 4,820 kilometers (2,995 miles) , formed in an icy surface, many and is larger than Mercury and of the larger ones have sagged Pluto. back nearly to their original sur- face level, since ice is not strong 25
  24. 24. THE FIRST SATURN The photos from Voyager 1 vations from Earth, and in their from Earth observations andENCOUNTER also revealed other baffling phe- photographs they discovered three by Voyager. Two that were nomena-dark features that new satellites. Some were discovered in Voyager images he results of the two resembled spokes in the bright small-only 100 kilometers appeared to shepherd the nar-T Jupiter encounters had a cautionary effect onplanning for the Voyagers B-ring. The spokes appeared to rotate around Saturn with the ring. Here again was something across or less. One appeared to orbit Saturn at the same dis- tance as a large known satellite, Dione. Two others orbited the row F-ring. Two more, discov- ered from Earth, had appeared to share the same orbit. Inspec-encounters at Saturn. Astrono- that defied quick explanation. tion of Voyager photos, how-mers had observed Jupiter The speed at which one object planet just beyond the edge of ever, showed that the satellitesextensively from Earth, but orbits another depends on its the narrow F-ring, which dwells orbits are about 50 kilometerswere nevertheless surprised by distance from the primary body. outside the well-known A-, B-, (31 miles) apart; a little calcula-the Voyager discoveries. Their It moves rapidly if it is near; and C-rings. tion yielded the astonishing pre-Earth-based observations of ever more slowly at successively diction that, as the two satellites greater distances. While some Voyager 1 closed in on ASaturn had been less complete, approached each other in Janu-because of Saturns greater dis- spokes appeared to follow that Saturn, more satellites ary 1982, they would tradetance and because less time had set of physical laws, others were seen, until a total orbits and continue on theirbeen spent at the telescopes. looked as if they kept their radi- of six had been found - three way, to resume their game ofTherefore the scientists were a al form as they circled Saturn.little wary of predicting what In support of the Voyagerthey would find when the missions, astronomers hadVoyagers arrived at the ringed increased their telescopic obser-planet. As the summer of 1980 con-tinued, Voyager 1 began itsencounter with Saturn. Theplanet appeared almost feature-less in early photos. It was sur-rounded by the three classicrings that had always beenstudied from Earth: the outerA-ring, the middle and brightestB-ring, and the gossamer, inner-most C-ring. (Other faint ringshad been, and would be, discov-ered from Earth, from PioneerSaturn, and by the Voyagers.They would be called 0-, E-, F-,and G-rings.) But it appeared, inearly photos, as if there werenot just three rings, but scores,then hundreds, and finallythousands of thin ringlets. Itwould turn out that they werenot individual rings separatedby gaps; some of the variationswere caused by the gravitationalattraction of nearby satellites,pulling millions of particles intomotion, spiraling outwardacross the rings like waves in anocean. The causes of other vari-ations are still unknown; a veryfew may be due to tiny satellitesembedded within the rings. Sat-urns rings are dynamic, chang-ing with every passing day.
  25. 25. An early Voyager picture of Saturn shows about the same amount of detail as a good Earth-based tele- scopic photograph. "Physics is experience, arranged in economical order." ERNST MACHSaturn appears toglow in this false-color rendition of aphoto takenthrough ultra-violet, green, andviolet filters. Two tiny satellites shepherd Saturns Poring. The Poring is multistranded and kinked in some places. 27
  26. 26. musical chairs the next time amino acids, essential ingredi- Saturn s escort of satellites is appar·they approached. ents of life. Titans low tempera- ent as Voyager A day before Voyager 1 swept ture, however-about 95°K draws near. The Saturn system pro·by Saturn, it flew within 4,000 (-288°F)-inhibits the com- vided a cascade ofkilometers (2,500 miles) of the plex organic chemistry that discoveries. [>huge satellite Titan and passed could lead to formation of life.directly behind it, making whatscientists had predicted would O nce beyond Titan, Voy-be extremely important observa-tions. Titan was shrouded ager 1 flew past Saturn Saturns rings,by a thick, opaque haze that and briefly disappeared beautiful throughcompletely obscured its surface behind it. En route to Earth the any Earth-based radio signals penetrated Saturns telescope, revealfrom the cameras. But the new structure ininfrared instrument and atmosphere and passed through this Voyager photo.the spacecraft radio probed the the rings. Measurements of theatmosphere to measure the way the atmosphere altered thediameter of the satellite and signals, and the rings scatteredthe thickness, temperature, and them, would tell much aboutcomposition of its atmosphere. the atmosphere and help deter- Titans atmospheric pressure, mine the sizes of particles thatVoyager found, is greater by make up the rings.60 percent than Earths. Inaddition to the methane thathad been detected from Earth,Voyager 1 found the atmo-spheres major constituent isnitrogen, which is not detecta-ble from Earth. The discovery isimportant because nitrogenappears to be rare except onEarth, and it is absolutely neces-sary for biological activity, asnitrogen is one of the primaryconstituents of living matter. he methane could pos-T sibly play much the same role on Titans surface aswater plays on Earth -as a gas, aliquid, and a solid. Rivers andlakes of methane may reflect asmoggy, orange sky above icymountain ranges. Clouds maydrop methane rain or snow.Voyagers instruments showedthat continuing organic chemis-try converts some of the meth- Titan, Saturnsane to ethane, acetyl ne, largest satellite, hasethylene, propane, methyl acet- a dense atmosphere of nitrogen andylene, and (when combined methane, and iswith the nitrogen) hydrogen swathed in a thick, orange photo·cyanide and other nitrogen- chemical haze.bearing compounds. The hydrogen cyanide is anespecially important molecule,since it is a building block of
  27. 27. 29
  28. 28. he radio experiment probe for galactic cosmic rays, T indicated the A-rings average particles are about eight meters (25 feet) in the edge of the solar system, and the beginning of interstel- lar space. diameter, while those in the C- ring are about two meters (six Saturn shines feet). The radio signal transmit- through its rings. ted by Voyager through the B- Voyager took this picture as it ring was too weak to permit a streaked away similar analysis of that ring, but from the planet scientists believe that particles after closest approach. there are larger than in the A-ring. They hasten to add, however, that they believe that all the rings are seeded with much smaller particles; the radio signals pass right by the smaller bits of ice without "Not fare wen, being affected by them. But fare forward, voyagers." Voyager 1 swept past its targets and took a new course T.S. ELIOT upward from the plane in which the planets orbit the Sun, outward toward the edge of the solar system. Its cameras and its ultraviolet and infrared instruments were turned off, but other experiments still<J Rings of Saturn, with spokes, ring- lets, gaps, and waves, hold clues to the origin and evolution of the solar system. Saturns rings show a wealth of detail to Voyagers cam· eras as the space· craft dra ws closer. 31
  29. 29. THE SECOND LOOK One of Voyager 2s most lOO-meter (330-foot) slice of water ice, and is the most reflec-AT SATURN important experiments involved the rings. It will take a decade tive body in the entire solar sys- an instrument called a photo- to process and analyze the data. tem. Iapetus, with one dark and ack on Earth, mean- polarimeter, which measures Voyager 2 photographed and one bright side, was a strangeB while, scientists pon- dered the same kinds ofunexpected results they had light intensity. As Voyager 2 passed above Saturn, a distant star named Delta Scorpii appeared to move behind the measured all the satellites that were then known-their num- ber had swelled to 17. At the end of the encounter, scientists object that would require spe- cial analysis. Distant Phoebe, photographed several weeks after Voyager 2s closestencountered at Jupiter. So, inthe few months that remained rings. By measuring the starlight had detailed· data on all of approach to Saturn, was a mav-before Voyager 2s arrival at Sat- as it passed through the rings, them. Further study of Voyager erick. It is almost certainly aurn, the Voyager teams under- the photo polarimeter detected pictures produced still more captured asteroid, a relativetook to restructure a major changes in the starlights inten- satellites, bringing the total to newcomer to Saturns fleet ofportion of the encounter se- sity as it was altered by changes more than 20. celestial escorts.quences for Voyager 2s 11 in the thickness of the rings. As Voyager 2 flew behind Sat-science instruments. Quick analysis of the data aturns satellites are differ- urn and out of the view of sci- For this encounter, emphasiswould shift from Saturn andTitan to the rings and to other showed that the rings structure was far different from what it appeared to be in the photos. No region was totally empty of S ent from those that orbit other planets. Some are composed of 30 to 40 percent entists and engineers back on Earth, a problem developed thatsatellites. The unexpected rock covered with ice; Encela-appearance of the rings dictated ring particles. The members of dus appeared to be almost puremore time to their study, and the photo polarimeter team havedetailed study of the satellites 800,000 samples, each one awas a high-priority scientificobjective. The summer of 1981 began,and with it the encounter ofVoyager 2 with Saturn. By now,profiting from their earlier expe-rience, the scientists were ableto set camera exposures at moreexact levels, to cope with thelow light levels and the generalblandness of Saturn. On thisapproach Saturn presented alter-nating dark and bright bands ofclouds and high-speed jetstreams. Swirling cloud patterns,which were smaller versions ofthe large and intense stormsseen on Jupiter, were also visiblethrough Saturns haze layer. Voyager 2s cameras zeroed inon the rings, and scientistssearched for small satellites inthe rings that might cause themultiringed appearance. Thosemoonlets, some scientistsbelieved, might sweeI1 up mate-rial in the rings, creatih g gaps.Voyager 2 would soar closerto the rings, and the improvedresolution of the picturesshould show structure as smallas one kilometer (0.6 mile)in diameter.
  30. 30. Huge stonns and high-speed winds howl through Saturns atmo- sphere. Winds were clocked by Voyager at J,700 kilometers (1,100 miles) an hour. One huge stonn, above the storm is shaped like a great a 530-kilometer- 6, swirls in Saturns (330-mile-) per- upper clouds. lust hour jet stream. "I have seen starry archipelagoes! and islands Whose raving skies are opened to the voyager ... " ARTHUR RIMBAUDTwo satellites orbitfar above the stormclouds of Saturnin a false-colorVoyager picture. <Xl 33
  31. 31. could endanger the remaining observations of this encounter and perhaps degrade the encounters with Uranus and Neptune years hence. While A departure photo Voyager 2 was behind the shows Saturn in fa lse color. The planet, its scan platform, which rings can iust be carries its cameras and its seen , and their infrared, ultraviolet, and photo- sh adow fa lls across the planet. metric instruments, suddenly jammed in one direction of movement. The problem became apparent as soon as Voy- ager reappeared: Photos of black space appeared on the monitors at JPL. ngineers on the project E This false·color succeeded in moving photo of Saturn s B- and C-rings the platform within a reveals fine deta ils few days, to get important pho- and subtle color diffe rences. tos of Saturn and of Phoebe, the outermost of the satellites, then worked for weeks more with a duplicate scan platform at JPL to understand the problem. They determined that a worn shaft was the cause, but careful use of the platform should not prevent Photopolarimeter a successful Uranus encounter results show that in January 1986. even the na rrow F-ring consists of many sm aller strands.Eoooo"e~c: "It ended ...gE Half of Iapetus is With his body changed to light, dark and th e other half is brigh t. This A star that burns forever in that sky."§. is the satellites northern AZTEC-THE FLIGHT OF QUETZALCOATL""" hemisphere.
  32. 32. 35
  33. 33. AB
  34. 34. THE SATURN PICTURES B-ring. The C- or crepe ring is visible (B) inside the B-ring. aturn has been known for Three of Saturns satellites (A)S hundreds of years as lithe ringed planet: Althoughgenerally similar to Jupiter, it (top to bottom), Tethys, Dione, and Rhea, are visible. Tethys casts its shadow on Saturns clouds. Spokelike features in thehas marked differences. Becauseit is twice as far from the Sun, rings are at the extreme upperSaturn is much colder than left. In the shadow cast by theJupiter-the amount of sunlight rings (C) on the bland, almostreaching it is only one-fourth of featureless planet, sunlight canthat reaching Jupiter. Saturn (A) be seen streaming through thehas much less intrinsic color Cassini Division and anotherthan Jupiter. One storm system, gap at the inner edge of thewith bright, white clouds, is B-ring. The shadow cast by thereadily visible (B) just below a C-ring is not as dark. Two icyband of white clouds. Saturn is satellites, Tethys and Dione,clearly visible through the wide orbit the planet. As Voyager flewgap in the rings called the Cas- away from Saturn, its camerasini Division. The blue color on took (D), which shows thethe limb, which is artificially northern illuminated side of theenhanced, is caused by the scat- rings; the spokes were darkertering of sunlight in the upper than the rest of the rings duringpart of the atmosphere. Saturn approach, but appear lighterwould look like (A) to an here, indicating the spoke mate-observer aboard the spacecraft. rial is composed of fine grainsThe clouds we can see on that forward-scatter sunlight.Saturn are mostly frozen crys- The shadow of the planet fallstals of ammonia ice. The on the rings, and the ringapparent difference seen here shadow on the planet. Saturnsbetween the clouds in the bright crescent can be seennorthern and southern hemi- through the rings, indicatingspheres may be a seasonal effect, that the rings are opticallysince it is spring in Saturns relatively thin. The rings arenorthern hemisphere. The wide thought to be composed pri-gap in the rings separates the marily of water ice.outer A-ring from the brighter 37
  35. 35. alse-color imaging helpsF scientists see features that cannot be seen as well intrue color. Green, violet, andultraviolet filters were used totake (A) , while (B) was taken ingreen and violet only. They areof the same region in Saturnsnorthern hemisphere, and showthree large storms. The threestorms would appear brown innormal color and are called"brown spots: Ultraviolet image(A) shows that the storms havecenters brighter than surround-ing regions, indicating a high-altitude haze layer above. Thelargest of the spots is about5,000 kilometers (3,100 miles)on its long axis and has an anti-cyclonic (counterclockwise)motion that reaches speeds of30 meters per second (67 milesan hour) . There is a distinct dif-ference in the hazes and cloudson opposite sides of the wavy jetstream above the three spots.There is also a large differencein atmospheric temperaturesbetween the two opposite sides.The jet stream has winds of upto 160 meters per second (330miles per hour). The bottomphoto shows puffy convectiveclouds in the region farthernorth of the jet stream. Theyare generally organized into achevronlike pattern (whichpoints to the left) because ofthe different wind speeds.
  36. 36. c The northern and southern hemispheres of Saturn are seen in these two photos. A region near Saturns north pole (C) shows three circular storms; two are near 72 degrees north latitude. Some scientists suggest that atmospheric circulation extends deep into Saturn, per- haps right down to the core and extending, from north to south, clear through to the other hemi- sphere. There are striking simi- larities between Saturns northern and southern hemi- spheres. Eastward winds reach a maximum speed of almost 1/50 kilometers (1)00 miles) an hour near the equator, and fall off almost identically with increasing latitude, both north and south. A storm rages in Sat- urns southern hemisphere (D), similar to Jupiters Great Red" Spot, but only 3,000 kilometers"" (1,850 miles) in diameter. Although its lifetime is not known, the storm was observed0 by both Voyagers, nine months apart. The source of energy for all the winds and storms on Sat- urn may be generated internally. Infrared measurements of atmo- spheric temperatures indicate that Saturn, like Jupiter, emits about twice as much energy as it absorbs from the Sun. How- ever, in the case of Jupiter, the excess energy is believed to be a remnant of the high tempera- tures associated with its for- mation 4.6 billion years ago. Saturn, on the other hand, must have long since lost most of its original energy; it is possible that, at the lower tempera~res of Saturns interior, the helium separates from the hydrogenEc and sinks toward the center of~ the planet, releasing gravita-E~ tional energy; which keeps the~ planet warm.cgE"0~~ 39
  37. 37. aturns three main rings much less reflective in orange Cring show surprising color However, closer examinationS exhibit very subtle color differences, which havebeen enhanced in (A). Al- light. There are also large-scale color differences between the A-ring, which appears to be differences between narrow adjacent ringlets. In particular, tiny eccentric ringlets found in showed that there are no major gaps in the B-ring. The ring is continuous, and the "ringlets"though the main compqnent rather uniform, and the B-ring, the Cring and the Cassini Divi- are waves, most of which are setof the rings is thought to be which is highly structured, sion appear yellower in false- up by gravitational resonancewater ice, the color differences implying that mixing of mate- color images than adjacent ring with several nearby satellites.probably indicate slight differ- rial over geologic time is material. From this distance,ences in chemical composition incomplete. Similarly, higher- large features in the B-ring, atand particle sizes. In particular, resolution color photos of the first believed to be individualthe Cassini Division and Cring, ringlets, appear to have a scalewhich are more transparent of several hundred kilometers.than the A- and B-rings, are also
  38. 38. he closer Voyager cameT to Saturns rings, the more details appeared.What seemed to be ringlets,however, were later determinedto be alternating areas of thickerand thinner material caused bygravitational effects from nearbysatellites. The areas (B) in theouter B-ring are further dividedby fine-scale structures that are15-kilometer- (nine-mile-) widewave features that move acrossthe rings and are also caused bythe gravitational interactions.The fine structures were foundto be noncircular, as is the outeredge of the B-ring. Comparisonof photos of the outer edge ofthe B-ring reveals it is elliptical,as would be expected from grav-itational interaction with thesatellite Mimas. Saturn is at thecenter of the ellipse, rather thanat one focus. The difference inradial distance from the centerof Saturn may be as great as 140kilometers (87 miles). Specialprocessing brings out radialstructure (C) in the rings whileignoring differences in bright-ness. Thus the normally darkerand more transparent Gringand Cassini Division appear tobe as bright as the normally bril-liant A- and B-rings. (Dark spotsare engraved on the camerasvidicon tube and are not ringfeatures.)A-Ring Gapr-:=t:-<::::---------F-Ring rro:P""""~------ A-Ring -=~~~- Cassini Division { - -M.........,.-+~,---- B-RingC-Ring --:----:1- 41
  39. 39. dial features in theR exist between spokes and elec- rings that initially trostatic discharges observed by reminded scientists of the planetary radio-astronomyspokes on a wheel are apparent experiment. If the dischargesin the B-ring. Spoke features (A) originate in the rings, as sus-are more apparent on the morn- pected, they might be linkeding side of rings, so scientists either with formation or decayspeculate that their formation of the spokes. The spokes (B)may be related to recent emer- are not continuous over brightgence of the ring material from portions of the rings, butSaturns shadow. Spokes appear are clearly seen against theto form radially over a distance darker areas.of 10,000 kilometers (6,200miles), while older spokes areskewed because of their normalorbital motion that causes ringparticles closer to the planet tomove faster than those fartheraway. The spokes are composedof tiny particles, since theyappeared darker in back-scattered sunlight as Voyagerapproached, and brighter in for-ward-scattered light as Voyagerdeparted. If such small particleswere electrostatically charged,they could be levitated abovethe main B-ring. A link may
  40. 40. aturns rings appear dra- tion of fine material in the underside and topside photosS matically different when viewed from the unillumi-nated side (C and D), when B-ring absorbs blue light, result- ing in a reddish hue on its underside. A similar phenome- of rings greatly aids in the anal- ysis of ring thicknesses and particle sizes.the sun is shining on the oppo- non on Earth is a sunset, wheresite face. The more transparent high-altitude, fine dust particlesC-ring and Cassini Division are absorb blue light and give thebright, because the sunlight can sky its red color. The narrow,penetrate them, while the nor- outer F-ring, discovered in 1979mally brighter B-ring, which is by Pioneer 11, lies just outsidemuch denser, is quite dark. the A-ring and is comparativelyThus the rings brightness in brighter when viewed fromthese photos indicates the the underside. Comparison ofamount of sunlight transmittedthrough them. It is apparentthat the inner half of the B-ringtransmits more light, and istherefore optically thinner, thanthe outer half. A large popula- 43
  41. 41. he changing geometryT along Voyagers flight path provided a varietyof striking views of the rings.The C-ring and inner B-ring (A)show subtle differences in falsecolor. Large apparent color dif-ferences between the C- andB-rings, as well as slight differ-ences within the C-ring, proba-bly indicate particle-sizedifferences or slight chemical-composition differences. Thestriking photo (B) shows the F-ring composed of three strands;two appear braided. The braid-ing is thought to occur in localareas of the F-ring when theinner of the two small shep-herding satellites, which is in aneccentric orbit, comes closest tothe ring material. The braids areconstantly changing, since thesatellites and the ring are con-stantly moving relative to eachother. Just as Voyager 2 sweptacross the ring plane, the cam-era captured this oblique view(C) of the rings, in which sev-eral spokes appear as bright,horizontal streaks above thedarker B-ring.
  42. 42. o ~ N imas, one of SaturnsM icy satellites, revealed a huge crater (B),nearly centered on the leadinghemisphere. The crater is solarge, about 130 kilometers (81miles) in diameter, that had theimpact that caused it beenmuch larger, it could have splitthe 392-kilometer- (244-mile-)diameter satellite apart. Thecentral icy peak of the craterrises almost 10 kilometers (6.2miles) above the floor, higherthan Mount Everest. The southpolar region of Mimas (A) rec-ords the history of heavy mete-oritic bombardment. The den-sity of small craters on Mimas isabout the same as that for theuplands of the Moon. Evidenceof surface fracturing is apparentin several deep canyons thatwander across the face. It is pos-sible the markings originated atthe time of the crater-causingimpact. Mimas, like most of Sat-urns satellites, is believed to becomposed mainly of water ice,and to contain between 20 per-cent and 50 percent rock. 45
  43. 43. saturns a satellites exhibit wide variety of sizes and A shapes. Satellite 1980S26,the outer F-ring shepherd (A), is 110 by 90 by 70 kilometers (68by 56 by 43 miles). The F-ringcan be seen near it, the faintline just above and to the left ofthe satellite. The shadow of therings is in the lower right cor-ner. Iapetus (B) is the second-most-remote Saturnian satellite.Scientists knew Iapetus has onedark and one bright hemi-sphere, but Voyager providedthe first detailed photographs ofits surface. Dark material in thisnorth-polar view reflects lessthan 5 percent of the sunlightstriking it. The brighter portionreflects about 50 percent. Darkmaterial is centered on Iapetusleading hemisphere. That ledsome scientists to suggest thatthe dark material, perhaps fromPhoebe, was swept out of orbit.The presence of dark materialin the floors of some craters onthe trailing hemisphere, how-ever, leads others to prefer a E 0 0 0model where the dark material 0 on !:=.comes up from Iapetus interior. E " cSince Iapetus is only 15 percent ~ Edenser than water, it could con- ~tain low-density, carbon-bearing comatter such as methane, which on ~could give rise to the darker <Xlmaterial. Its diameter is 1,460 ckilometers (907 miles). Hyper-ion (C), one of the most bat-tered bodies ever seen, is 410 by260 by 220 kilometers (254 by162 by 137 miles), probably theremains of a larger satellite thatwas destroyed by collision.Hyperion is relatively dark,reflecting only about 30 percentof the sunlight. If Hyperion isan icy body, then its dark color E 0might be the result of contami- 0 0. 0 0nation by material from else- ~ Ewhere, as has been suggested " 0 0 0for Iapetus. 0 ~ co <T N <Xl
  44. 44. E hoebe (D), outermost of Saturn in about 550 days. Like Saturns satellites, moves the irregular outer satellites of in a retrograde orbit Jupiter, it is thought to be a cap- almost 13 million kilometers tured asteroid, rather than an away, more than three times as ice ball that formed near Saturn. far from Saturn as Iapetus, its If it is an asteroid, Voyagers nearest neighbor. Phoebe is photos are the first ever taken of spherical (the photo was greatly such an object. enhanced) and is the darkest Saturnian satellite. Unlike all others, the 220-kilometer- (137- mile-) diameter object orbits Saturn in the ecliptic plane, rather than in Saturns equato- rial plane. Phoebe is the only Saturnian satellite that is known not to keep the same face always toward Saturn. It rotates in about nine hours and orbits everal small, unnamedS the leading and trailing coorbi- coorbital is the largest of the tiny satellites have been discov- Saturnian satellites were tals, respectively, called 1980S1 unnamed satellites. The next ered in Voyager pictures since captured in a composite and 1980S3. They are (leading) pair (top and bottom) are the second encounter, bringingfamily portrait. (£, left to right) 220 by 200 by 160 kilometers 1980S25 and 1980S13. They are the total to more than 20.1980S28 [1980 (year), S (137 by 124 by 99 miles) and called the Tethys Trojans, since(Saturn), 28 (sighting)] orbits (trailing) 140 by 120 by 100 kil- they share Tethys orbit, aboutSaturn just outside the A-ring. It ometers (87 by 75 by 62 miles). 60 degrees behind and 60is 40 by 20 kilometers (25 by 12 As the trailing satellite overtakes degrees ahead. Similarly, 1980S6miles) and helps to confine the the leading, they exchange (far right) moves in the sameA-rings outer edge. The next orbits without colliding, circling orbit as Dione, about 60 degreespair (top and bottom) are Saturn in a four-year pattern of ahead. 1980S6 is 36 by 32 by 301980S26 and 1980S27, the F-ring musical chairs. The leading kilometers (22 by 20 by 19shepherds. They are 110 by 90 miles). The Tethys Trojans areby 70 kilometers (68 by 56 by only slightly smaller. At least43 miles), and 140 by 100 by 80 four, and possibly several more,kilometers (87 by 62 by 50miles), respectively. The top sat-ellite is the outer shepherd. Thenext pair (top and bottom) are 47
  45. 45. BE nceladus is the most geo- logically evolved Satur- nian satellite and has ayounger surface with a widediversity of terrain types. Encel-adus would look about like (A)to an observer on the spacecraft,while (B) has been strongly con-trast-stretched to bring out sur-face detail. At least five distinctterrain types have been identi-fied . Even the most heavily cra-tered areas of the surface aremuch more lightly crateredthan the other icy satellites, andareas to the right show no cra-ters, down to the limit of resolu-tion (2 kilometers or l.2 miles) ,indicating that Enceladus sur-face was changed during the last1 billion years. Two large cratersnear the terminator appear tohave formed in a relatively softsurface; the upper crater appearsto overlie the lower. On eitheredge of the ridged plains, aseries of truncated craters implythat several episodes of meltingof the surface occurred after theend of the intense crateringperiod. Linear markings in thesouthern hemisphere are recti-linear fault lines associated withmovement of the crust. Curvedlines appear to be a complexsystem of ridges, similar to thegrooved terrain on Ganylnede,with more than 1 kilometer(3,200 feet) of relief. Enceladushas had a complex geologic his-tory during which the surfacehas been replaced in multiple 0;stages. " N OJ
  46. 46. AUge, globe-girdling canyon and a jumble of impact craters mark Tethys surface. A portion of the canyon (A), stretching over three-quarters of the circumfer- ence, loops from horizon to horizon. The canyon, named Ithaca Chasma, is about 2,500 kilometers (1,550 miles) long, has an average width of about 100 kilometers (62 miles), and a depth of 3 to 5 kilometers (l.8 to 3.1 miles). If Tethys were once a ball of liquid water cov- ered with a thin, solid crust, freezing of the interior would have produced surface expan- sion comparable to the area of the chasm, but not necessarily as a single, circumferential can- yon. A huge crater dominates the photo at lower left (B). It is 400 kilometers (249 miles) in diameter, more than one third Tethys 1,060-kilometer (660- mile) diameter. Creep or viscous flow in the outer layers of the body has probably allowed the crater to recover almost to the original shape of Tethys surface. The crater compares with the large one on Mimas. Calcula- tions have shown that, assum- ing similar thermal gradients and compositions for the outer layers of both satellites, the great B crater on Tethys could flatten over geologic time, while that on Mimas could retain its origi- nal shape. Relative positions of the crater and Ithaca Chasma suggest a connection. The high- est resolution photo of Tethys (C), with two kilometers (l.2 miles) per line pair resolution, shows a region of heavily cra- tered ancient terrain. In addi-E tion, Tethys, like Dione, has00"-v one region on its trailing face"-~ that is relatively uncratered,000 probably the result of floodingc:i~ of the area by material erupting from the interior.~<0c::<0 49