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    [Illustration: Cover: A cylindrical projection of Jupiter’s
    atmosphere was made from ten color images taken by Voyager 1 during
    a single ten-hour rotation of the planet.]

    [Illustration: A computer-generated mosaic of Voyager 1 pictures
    showing Jupiter from directly below the south pole. This view shows
    features as far north as 20 degrees latitude. The black area at the
    Pole results from missing information.]




                                Voyager
                               Encounters
                                Jupiter


                               July 1979


                                  NASA
                        National Aeronautics and
                          Space Administration




                                CONTENTS


  Foreword                                                              3
  Introduction                                                          4
  Images of Jupiter and Its Satellites                                  6
  The Voyager Mission                                                  37
  Scientific Highlights                                                39
      Jupiter                                                          39
      Amalthea                                                         39
      Io                                                               39
      Europa                                                           39
      Ganymede                                                         39
      Callisto                                                         39
      The Magnetosphere                                                40
      Scientific investigations of the Voyager mission                 40


    [Illustration: _A Titan/Centaur rocket served as the launch vehicle
    for Voyager and was the last planned use of this type of launch
    vehicle prior to the era of the Space Transportation System (Shuttle
    Orbiter)._]




                                Foreword


In late summer of 1977, the United States launched two unmanned Voyager
spacecraft on an extensive reconnaissance of the outer planets, a
decade-long odyssey that could take them to 3 planets and as many as 18
planetary satellites. The first encounter was with the giant Jovian
planetary system, 645 million kilometers (400 million miles) away.
Passing by Jupiter and its complex satellite system in 1979, the Voyager
spacecraft have collected and returned to Earth an enormous amount of
data and information that may prove to be a keystone in understanding
our solar system.

This publication provides an early look at the Jovian planetary system
and contains a selected sample from the more than 30,000 images
collected during this phase of the Voyager mission. While Voyager
achieved an impressive record of accomplishments, full realization of
the scientific value of this program must await the remaining Voyager
encounters with Saturn and perhaps Uranus, and a detailed analysis of
the data from all the spacecraft investigations.

                                       Robert A. Frosch, _Administrator_
                         _National Aeronautics and Space Administration_




                              Introduction


In March 1979 Voyager 1 swept past Jupiter, photographing both the giant
planet and five of its moons. Four months later, a companion spacecraft,
Voyager 2, made a similar encounter. Now, with Jupiter receding behind
them, both spacecraft are headed toward the outer reaches of our solar
system. In November 1980, Voyager 1 will fly past Saturn. Voyager 2,
traveling at slower speeds, will reach the same way station in August
1981. Beyond there, the itinerary is less certain. In January 1986,
eight years after its departure from Earth, Voyager 2 may sail within
range of Uranus, taking closeup pictures of that distant planet for the
first time. Long after they have exhausted their fuel supplies and their
radios have fallen silent, both spacecraft will continue their traverse
through space and beyond our solar system, on an endless journey.

    [Illustration: _An Apollo 12 astronaut retrieves Surveyor 3 hardware
    for Earth laboratory analysis after 30 months exposure on lunar
    surface._]

    [Illustration: _Viking Lander 2 surveys the boulder-strewn Utopian
    Plain and reddish sky of Mars._]

Preliminary results of the Voyager encounters with Jupiter are presented
in this booklet. As you examine the pictures, you will be participating
in a revolutionary journey of exploration. Living in a society where
many accomplishments and products are billed as “extraordinary,”
“stupendous,” “once in a lifetime,” or “unique,” we sometimes lose our
perspective. Conditioned to hyperbole, we fail to recognize those
advances that are truly exceptional. We need a historian’s vantage point
to identify the events that can literally change the course of
civilization. So it is that every student of history recognizes the
importance of the Renaissance, an extraordinary time when man looked
outward, reaching beyond the traditions of the past to study his place
in the natural world. The results were apparent in art, architecture,
and literature, in new philosophic and governmental systems, and in the
staggering scientific revolution exemplified by Galileo’s first
examination of the heavens with a telescope, and in his stubborn support
of the heretical assertion that the Earth was _not_ the center of the
solar system.

Historians writing a hundred or two hundred years from now may well look
on the latter part of the twentieth century as another turning point in
civilization. For the first time, we explored beyond Earth—first the
Moon, then the neighboring planets, and finally the outermost planets,
the very fringe of our solar system.

How will the historian evaluate this period of exploration? First,
perhaps, he will describe the Apollo program as a visionary example of
great cooperative ventures that can be accomplished by many individuals,
private companies, and government institutions. He will describe the
subsequent space ventures that weave a fabric of cooperation and
goodwill between nations.

He will point out the technological advances incorporated in unmanned
spacecraft, sophisticated robots able to control their own activities
and solve their own problems. He will mention the revolution in
microelectronics—the art of fabricating complex electrical control
circuits so small the eye cannot perceive them, a revolution accelerated
by the requirement to conserve weight and generate performance in
interplanetary spacecraft. He will point to the introduction of new
products, particularly in areas of communication, medical treatment, and
energy conversion.

    [Illustration: _Galileo orbiter and probe mission to Jupiter in 1985
    will expand upon the Voyager investigations of the Jovian system._]

    [Illustration: _A solar electric propulsion spacecraft would eject
    an instrumented probe toward Halley’s comet in 1986 and continue on
    to rendezvous with another comet, Tempel 2._]

Turning his attention to the environment, the historian will almost
surely suggest that the first widespread realization of the fragile
natural balances on Earth came at a time when we were first able to see
our Earth in its entirety. The impact of a picture of Earth from deep
space, a luminously blue globe surrounded by darkness, has probably been
more persuasive than lengthy treatises describing the complex ways in
which our system of rocks, plants, animals, water, and air is
interrelated. On a more practical level, the historian will point to the
new understanding of our terrestrial environment. The composition and
structure of other planetary atmospheres—on Venus, Mars, and
Jupiter—provide important clues to what may happen in our own
atmosphere, especially if we disrupt the chemical composition. Study of
the primitive crusts of the Moon, Mars, and Mercury permits us to
reconstruct the first billion years of Earth history, a time when
chemical elements were being concentrated in activity ultimately leading
to the formation of important ore deposits. Unmanned spacecraft missions
to the Sun increase our understanding of that most fundamental of all
energy sources, paving the way for the efficient conversion of solar
energy into many practical applications, and releasing us from
dependence on ever-decreasing reserves of fossil fuels. Spacecraft
circling the Earth study the upper atmospheric processes that play major
roles in controlling our weather. These same spacecraft look down on
Earth, aiding us with increasingly accurate forecasts of weather and
crop productivity.

Looking beyond matters of technology and the environment, the historian
may cite the latter part of the twentieth century as a time of explosive
exploration, comparable to the 15th and 16th century exploration of the
Earth’s oceans and the distant lands that bounded them. In a sense,
exploration—whether it is physical or intellectual—provides its own
rewards. The United States has always been a nation that moves forward,
pushing back the frontiers of the West, pushing back the frontiers of
social and economic development, and now pushing back the frontiers of
space. It is arguable that this spirit of exploration is indispensable
to a vigorous society, and that any society that ceases to explore, to
inquire, and to strive is only a few years from decline.

And so the historian may recall the early days of lunar exploration, the
Apollo project, the landing of unmanned Viking spacecraft on Mars, and
the encounters of Voyager spacecraft with Jupiter and Saturn as the
first steps in a sustained program of space exploration—a program that
is profoundly changing man’s perspective of himself, of the Earth, and
of the larger cosmos beyond.

            Thomas A. Mutch, _Associate Administrator for Space Science_
                         _National Aeronautics and Space Administration_




                  Images of Jupiter and Its Satellites


  The date of each photograph and the distance of the spacecraft from
  the planet or satellite are included with each picture.

    [Illustration: 2/5/79    28.4 million km    (17.6 million mi)

    Jupiter is the largest planet in our solar system, with a diameter
    11 times that of Earth. Jupiter rotates very quickly, making one
    full rotation in just under ten hours. Composed primarily of
    hydrogen and helium, Jupiter’s colorfully banded atmosphere displays
    complex patterns highlighted by the Great Red Spot, a large,
    circulating atmospheric disturbance. Three of Jupiter’s 13 known
    satellites are also visible in this Voyager 1 photograph. The
    innermost large satellite, Io, can be seen in front of Jupiter and
    is distinguished by its bright, orange surface. To the right of
    Jupiter is Europa, also very bright but with fainter surface
    markings. Callisto is barely visible beneath Jupiter. These
    satellites orbit Jupiter in the equatorial plane and appear in their
    present position because Voyager is above the plane.]

    [Illustration: Jupiter’s atmosphere is undergoing constant change,
    presenting an ever-shifting face to observers. The Great Red Spot
    has undergone three major periods of activity in the last 15 years.
    These images of Jupiter, taken by Voyager 1 (top) and Voyager 2
    (bottom) almost four months apart, show that cloud movement in the
    Jovian atmosphere is not uniform because wind speeds vary at
    different latitudes. For example, the white ovals which appear below
    the Great Red Spot dramatically shifted between January and May, the
    time interval between these two pictures. The bright “tongue”
    extending upward from the Great Red Spot interacted with a thin,
    bright cloud above it that had traveled twice around Jupiter in four
    months. Eddy patterns to the left of the Great Red Spot, which have
    been observed since 1975, appear to be breaking up.]

    [Illustration: 1/24/79    40 million km    (25 million mi)]

    [Illustration: 5/9/79    46.3 million km    (28.7 million mi)]

    [Illustration: 2/25/79    9.2 million km    (5.7 million mi)

    The Great Red Spot on Jupiter is a tremendous atmospheric storm,
    twice the size of Earth, that has been observed for centuries. The
    Great Red Spot rotates counterclockwise with one revolution every
    six days. Wind currents on the top flow east to west, and currents
    on the bottom flow west to east. This Voyager 1 picture shows the
    complex flow and turbulent patterns that result from the Great Red
    Spot’s interactions with these flows. The large white oval is a
    similar, but smaller, storm center that has existed for about 40
    years.]

    [Illustration: 7/3/79    6 million km    (3.72 million mi)

    A comparison of the Voyager 2 photograph above with the preceding
    Voyager 1 photograph shows several distinct changes in the Jovian
    atmosphere around the Great Red Spot. The white oval beneath the
    Great Red Spot in the first picture has moved farther around
    Jupiter, and a different white oval has appeared under the Great Red
    Spot in the Voyager 2 picture taken four months later. The disturbed
    cloud regions around the Great Red Spot have noticeably changed, and
    the white zone west of the Great Red Spot has narrowed.]

    [Illustration: 3/2/79    4 million km    (2.5 million mi)

    High-speed wind currents in the mid-latitudes of Jupiter are shown
    in this high-resolution Voyager 1 photograph. The pale orange line
    running diagonally to the upper right is the high-speed north
    temperate current with a wind speed of about 120 meters per second
    (260 miles per hour), over twice as fast as severe hurricane winds
    on Earth. Toward the top of the picture, a weaker jet of
    approximately 30 meters per second (65 miles per hour) is
    characterized by wave patterns and cloud features that rotate in a
    clockwise manner.]

    [Illustration: 3/2/79    4 million km    (2.5 million mi)

    The large brown-colored oval appearing in this Voyager 1 picture was
    selected as one of the targets to be photographed near closest
    approach to Jupiter because it is probably an opening in the upper
    cloud deck that exposes deeper, warmer cloud levels. Brown ovals
    (which can also be seen in the preceding and following photographs)
    are common features in Jupiter’s northern latitudes and have an
    average lifetime of one to two years.]

    [Illustration: 6/28/79    10.3 million km    (6.4 million mi)

    Jupiter’s Equatorial Zone is the broad, orange band that traverses
    the center of this Voyager 2 picture. This zone is characterized by
    the wispy clouds along its northern edge. The brown oval was
    observed by Voyager 1 four months earlier, illustrating the
    stability of this type of feature in the Jovian atmosphere. In
    contrast, the turbulent region in the lower right of the picture,
    which lies just to the left of the Great Red Spot, shows features
    that are relatively short lived. With the exception of the cooler
    Great Red Spot, as colors range from white to orange to brown, we
    are generally looking at deeper and warmer layers in the Jovian
    atmosphere.]

    [Illustration: This infrared image of Jupiter was taken from Earth
    and shows heat radiating from deep holes in Jupiter’s clouds. Bright
    areas in the image are higher temperature regions than the dark
    areas and correspond to parts of the atmosphere that are relatively
    free of obscuring clouds. The Great Red Spot appears on the left
    limb, or edge of the planet, as a dark area encircled by a bright
    ring, indicating that the Spot is cooler than the surrounding
    region. The infrared image was recorded by the 200-inch Hale
    telescope on Mount Palomar in California.]

    [Illustration: 1/10/79    535,000 km    (332 million mi)

    This Voyager 1 picture was also taken the same day, about one hour
    after the infrared image.]

    [Illustration: 3/5/79    515,000 km    (320,000 mi)

    The largest aurora ever observed, nearly 29,000 kilometers (18,000
    miles) long, appears in this Voyager 1 photograph, taken on the dark
    side of Jupiter six hours after closest encounter. The auroral
    lights are brighter than any northern lights seen on Earth.
    Jupiter’s north pole is approximately midway along the auroral arc.
    This timed exposure of the aurora also shows what appear to be
    lightning storms several thousand kilometers below the aurora. The
    strength of the lightning bolts is comparable to that of superbolts
    seen near cloud tops above Earth. Lightning had been suspected to
    exist on Jupiter, but at lower levels in the atmosphere.]

    [Illustration: 3/4/79    1.2 million km    (750,000 mi)

    The first evidence of a ring around Jupiter is seen in this
    photograph taken by Voyager 1. This photograph was part of a
    sequence planned to search for such rings around Jupiter. The
    multiple image of the extremely thin, faint ring appears as a broad
    light band crossing the center of the picture. This multiple image
    and the elongated, wavy motion of the background stars are due to
    the 11-minute, 12-second exposure and the very slow natural
    oscillation of the spacecraft. The ring, which is in Jupiter’s
    equatorial plane, is invisible from Earth because of its thinness
    and transparency and because of Jupiter’s brightness. The black dots
    in the picture are calibration points in the camera.]

    [Illustration: Because of Voyager 1’s discovery of a ring around
    Jupiter, Voyager 2 was programmed to take additional pictures of the
    ring. These three Voyager 2 images show Jupiter’s ring in
    progressively higher resolution. The pictures were taken when
    Jupiter was eclipsed by the Sun, and the ring appears unusually
    bright because of the forward scattering of sunlight by small ring
    particles.]

    [Illustration: 7/10/79    1.45 million km    (900,000 mi)

    In this four-picture mosaic, the arms of the ring curving toward the
    spacecraft (on the near side of the planet) are cut off by the
    planet’s shadow. Scientists estimate that the distance from the
    Jovian cloud tops to the outer edge of the ring is 55,000 kilometers
    (35,000 miles).]

    [Illustration: 7/10/79    1.55 million km    (961,000 mi)

    In this picture, which is composed of six images, there is evidence
    of structure within the ring, but the spacecraft motion during these
    long exposures obscured the highest resolution detail. However,
    there is speculation that the ring width, estimated at 6000
    kilometers (4000 miles), contains more than one ring.]

    [Illustration: 7/10/79    1.45 million km    (900,000 mi)

    This photograph is an enlargement of the isolated left frame in the
    first picture and reveals a density gradient of very small particles
    extending inward from the ring. The thickness of the ring has been
    estimated at less than one kilometer (0.6 mile) although the ring
    appears about 30 kilometers (19 miles) thick in the image, due to
    camera motion and finite resolution. Composition of the low-albedo
    (dark) particles is not known, but particle size probably ranges
    from microscopic to at most a few meters in diameter. If collected
    together to form a single body, the total mass of the Jovian rings
    would form an object with a diameter less than twice that of tiny
    Amalthea.]

    [Illustration: 2/13/79    20 million km    (12.4 million mi)

    Jupiter and two of its planet-sized satellites, Io at left and
    Europa at right, are visible in this Voyager 1 picture. Jupiter’s
    four largest satellites—Io, Europa, Ganymede and Callisto—were
    discovered in 1610 by Galileo Galilei. The two outer Galilean
    satellites are Ganymede and Callisto, not shown in this picture. All
    four satellites probably formed about four billion years ago but
    their surfaces vary in age tremendously. Io and Europa have younger,
    more active surfaces than Ganymede and Callisto. Like our Moon, the
    satellites keep the same face toward Jupiter. In this picture, the
    sides of the satellites that always face away from the planet are
    visible.]

    [Illustration: Amalthea, Jupiter’s innermost satellite, was
    discovered in 1892. It is so small and close to Jupiter that it is
    extremely difficult to observe from Earth. Amalthea’s surface is
    dark and red, quite unlike any of the Galilean satellites. The three
    Voyager 1 pictures and the one Voyager 2 picture following (seen
    against the disk of Jupiter) reveal a small, elongated object, about
    265 kilometers (165 miles) long and 150 kilometers (90 miles) in
    diameter. Amalthea keeps its long axis pointed toward Jupiter as it
    orbits around the planet every 12 hours.]

    [Illustration: 3/4/79    1.25 million km    (780,000 mi)]

    [Illustration: 3/4/79    695,000 km    (430,000 mi)]

    [Illustration: 3/5/79    425,000 km    (264,000 mi)]

    [Illustration: 7/9/79    560,000 km    (350,000 mi)

    Amalthea was observed end-on in the Voyager 2 picture, which has
    been computer-processed to enhance the image.]

    [Illustration: 3/4/79    377,000 km    (234,000 mi)

    Io, Jupiter’s innermost Galilean satellite, displays great diversity
    in color and brightness. This Voyager 1 four-picture mosaic shows
    Io’s complex coloration of red-orange, black, and white regions, and
    the two major topographic features: volcanic regions, the most
    prominent of which is the “hoofprint” (volcanic deposition feature)
    in the center-right, and the intervolcanic plains that are
    relatively featureless. Io’s vivid coloring is probably due to its
    composition of sulfur-rich materials that have been brought to the
    surface by volcanic activity.]

    [Illustration: 3/5/79    129,600 km    (80,500 mi)

    The bright area at the upper right in this Voyager 1 picture of Io
    appears to be a caldera (collapsed volcano) that is venting clouds
    of gases. The clouds may condense to form extremely fine particles
    that scatter light and appear blue. Because the infrared
    spectrometer discovered sulfur dioxide on Io, scientists believe
    this gas may be the main component of the clouds. Sulfur dioxide
    clouds would rapidly freeze and snow back to the surface. It is also
    possible that dark areas in the floors of the calderas are pools of
    encrusted liquid sulfur.]

    [Illustration: 3/5/79    66,000 km    (41,000 mi)

    Evidence of erosion in Io’s southern polar region is visible in this
    Voyager 1 high-resolution image. The picture has been
    computer-enhanced to bring out surface detail while suppressing
    bright markings. A depressed segment of the crust, bounded by
    faults, is seen near the terminator in the upper right portion of
    the image. At the lower center are complicated scarps (slopes) and
    portions of isolated elevated terrain that geologists interpret as
    “islands” left behind as the scarps eroded. Scientists speculate
    that sulfur dioxide (as a subsurface liquid) may be a determinant in
    the creation of these features.]

    [Illustration: 3/4/79    862,000 km    (540,000 mi)

    Io’s surface, less than ten million years old, is quite young
    compared to the other Galilean satellites and to other terrestrial
    bodies, such as Mercury and the Moon. The surface is composed of
    large amounts of sulfur and sulfur dioxide frost, both of which
    account for most of the surface color. This picture was taken by
    Voyager 1. Material deposited by the volcano (see following
    pictures) can be seen as a white ring near the center of Io.]

    [Illustration: The first active volcanic eruptions other than on
    Earth were discovered on Io. These volcanoes are extremely explosive
    with ejection velocities of more than one kilometer per second (2200
    miles per hour), which is more violent than Etna, Vesuvius, or
    Krakatoa on Earth. Both pictures below were taken by Voyager 1.]

    [Illustration: 3/4/79    450,000 km    (280,000 mi)

    In this picture, the plume visible on the right edge extends more
    than 100 kilometers (60 miles) above the surface.]

    [Illustration: 3/4/79    499,000 km    (310,000 mi)

    The same volcano is shown in this picture, photographed one hour and
    52 minutes earlier.]

    [Illustration: 3/4/79    490,000 km    (304,000 mi)

    Special color reconstruction by means of ultraviolet, blue, green,
    and orange filters allowed scientists to study the amount of gas and
    dust and the size of the dust particles that erupted from the
    volcano on Io shown in this Voyager 1 image. The region that is
    brighter in the ultraviolet (blue area) is about 210 kilometers (130
    miles) high, over twice the height of the denser, bright yellow
    core. The vent area is visible on page 18 as a dark ring in the
    upper left region of Io.]

    [Illustration: 7/10/79    1.2 million km    (750,000 mi)

    Of the eight active volcanoes discovered on Io by Voyager 1, six of
    the seven volcanoes sighted by Voyager 2 were still active. The
    giant volcano observed by Voyager 1 over the “hoofprint” region (see
    page 18) had become inactive. Scientists, therefore, believe that
    the satellite is undergoing continuous volcanic activity, making
    Io’s surface the most active in the solar system. This Voyager 2
    photograph, which shows three active volcanoes, was one of the last
    of an extensive sequence of “volcano watch” pictures planned as a
    result of Voyager 1’s volcano discovery. The black dots are
    calibration points on the camera.]

    [Illustration: 7/8/79    1.2 million km    (750,000 mi)

    Europa, approximately the same size and density as our Moon, is the
    brightest Galilean satellite. The surface displays a complex array
    of streaks, indicating that the crust has been fractured. In
    contrast to its icy neighbors Ganymede and Callisto, Europa has very
    few impact craters. The relative absence of features and low
    topography indicate that the crust is young and probably warm a few
    kilometers below the surface. The warmth is probably due to a
    combination of radioactive and tidal heating. The tidal heating
    within Europa is estimated to be ten percent that of the stronger
    tidal heating effect within Io. The regions that appear blue in this
    Voyager 2 image are actually white.]

    [Illustration: 7/9/79    240,000 km    (150,000 mi)

    Europa’s surface is probably a thin ice crust overlying water or
    softer ice (slush) about 100 kilometers (60 miles) thick that covers
    a silicate interior. The tectonic processes on Europa’s surface
    create patterns that are drastically different from the fault
    systems seen on Ganymede’s surface, where pieces of the crust have
    moved relative to each other. On Europa, the crust evidently
    fractures, but the pieces remain roughly in their original position.
    This Voyager 2 picture is composed of three images.]

    [Illustration: 7/9/79    240,000 km    (150,000 mi)

    Long linear fractures or faults which crisscross Europa’s surface in
    various directions are over 1000 kilometers (600 miles) long in some
    places. Large fractures are 200 to 300 kilometers (125 to 185 miles)
    wide, wider than the crust is thick. Also visible are somewhat
    darker mottled regions that appear to have a slightly pitted
    appearance. No large craters (more than five kilometers in diameter)
    are identifiable in this Voyager 2 picture, indicating that this
    satellite has a very young surface relative to Ganymede and
    Callisto, although perhaps not as young as Io’s surface. Scientists
    believe that the surface is a thin ice crust overlying water or
    softer ice and that the fracture systems are breaks in the crust.
    Resurfacing processes, such as the production of fresh ice or snow
    along the cracks and cold glacier-like flows, have probably removed
    evidence of impact events (cratering). Europa, therefore, appears to
    have many properties similar to Ganymede and Io.]

    [Illustration: 7/9/79    240,000 km    (150,000 mi)

    Complex narrow ridges, seen as curved bright streaks 5 to 10
    kilometers (3 to 6 miles) wide and typically 100 kilometers (60
    miles) long, characterize the surface topography of this view of
    Europa. The dark bands also visible in this Voyager 2 photo are 20
    to 40 kilometers (12 to 25 miles) wide and up to thousands of
    kilometers long. The fractures on the icy surface are filled with
    material from beneath, probably as a result of internal tidal
    flexing which continually heats the thin outer ice crust. A few
    features are suggestive of degraded impact craters.]

    [Illustration: 3/4/79    2.6 million km    (1.6 million mi)

    Ganymede, Jupiter’s largest satellite, is about one and one-half
    times the size of our Moon but only about half as dense and is
    composed of about 50 percent water or ice and the rest rock. The
    bright surface of Ganymede is a complex montage of ancient,
    relatively dark and cratered terrain, grooved terrain that resulted
    from a dramatic history of tectonic movement in the icy crust, and
    bright young ray craters that expose fresh ice. This photograph was
    taken by Voyager 1.]

    [Illustration: 7/7/79    1.2 million km    (750,000 mi)

    The dark, cratered, circular feature in this Voyager 2 photograph is
    about 3200 kilometers (2000 miles) in diameter and is on the side of
    Ganymede opposite to that shown 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 and grooved
    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.]

    [Illustration: 7/8/79    312,000 km    (194,000 mi)

    Several different types of terrain common to Ganymede’s surface are
    visible in this Voyager 2 picture. The boundary of the largest
    region of dark ancient terrain (also shown in the previous photo)
    can be seen to the right, revealing the light linear features that
    may be the remains of shock rings from an ancient impact. The broad
    light regions are the typical grooved structures contained within
    the light regions on Ganymede. On the lower left is another example
    of what might be evidence of large-scale lateral faulting in the
    crust; the band appears to be offset by a linear feature
    perpendicular to it. These are the first clear examples of lateral
    faulting seen on any planet other than Earth.]

    [Illustration: 7/8/79    313,000 km    (194,500 mi)

    This color reconstruction of part of Ganymede’s northern hemisphere,
    taken by Voyager 2, 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 large bright
    circular features have little relief and are probably the remnants
    of old, large craters that have been annealed by the flow of icy
    material near the surface. The gradually curving lines that press
    through the dark region suggest the presence of a large impact basin
    to the southwest, which has been obliterated by the subsequent
    formation of younger grooved terrain.]

    [Illustration: 3/5/79    165,000 km    (103,000 mi)

    A broad, north-south strip of grooved terrain on Ganymede, offset by
    a traversing fault in the upper part of the picture, is shown in
    this Voyager 1 photograph. There are several other perpendicular
    fault lines farther down on the fault. Within the major light
    stripes, the more closely spaced, shallow grooves run parallel to
    the boundaries of the stripes. The larger striped features divide
    the cratered terrain into isolated polygons several hundred to about
    1000 kilometers (600 miles) across.]

    [Illustration: 3/5/79    145,000 km    (90,000 mi)

    The grooved terrain at higher resolution emphasizes numerous
    interwoven linear features in this Voyager 1 picture, near the
    terminator on Ganymede. This suggests an early period in Ganymede’s
    history when the crust was active and mobile, resembling Earth’s
    plate tectonics in some ways. The causes of the extreme differences
    in crustal evolution between Callisto and Ganymede are under
    investigation. Combinations of radioactive heating and a greater
    degree of tidal heating for Ganymede are possibilities.]

    [Illustration: 7/9/79    100,000 km    (62,000 ml)

    This mosaic of Ganymede, composed of photographs taken by Voyager 2,
    shows numerous impact craters, many with bright ray systems. The
    rough terrain at the lower right is the outer portion of a large,
    fresh impact basin that postdates most of the other terrain. The
    dark patches of heavily cratered terrain (right center) are probably
    ancient mixtures of ice and rock formed prior to the grooved
    terrain. The large rayed crater at the upper center is about 150
    kilometers (95 miles) in diameter.]

    [Illustration: 7/8/79    85,000 km    (53,000 mi)

    Curved troughs and ridges in this high-resolution Voyager 2
    photograph of Ganymede are the distinctive characteristics of an
    enormous, ancient impact basin. The basin itself has been eroded by
    later geologic processes; only the shock ring features are preserved
    on the ancient surface. Near the bottom of the picture these curved
    markings are perforated with the younger, grooved terrain.]

    [Illustration: 7/7/79    2.3 million km    (1.4 million mi)

    Callisto, only slightly smaller than Ganymede, has the lowest
    density of all the Galilean satellites, implying that it has large
    amounts of water in its bulk composition. Its surface is darker than
    the other Galilean satellites, although it is still twice as bright
    as our Moon. This Voyager 2 image shows Callisto to have the most
    heavily cratered and, therefore, the oldest surface of the Galilean
    satellites, probably dating back to the period of heavy meteoritic
    bombardment ending about four billion years ago.]

    [Illustration: 3/6/79    350,000 km    (217,000 mi)

    The prominent concentric ring structure shown in this Voyager 1
    four-picture mosaic of Callisto is believed to be a large impact
    basin, similar to Mare Orientale on the Moon and Caloris Basin on
    Mercury. The bright circular spot is about 600 kilometers (360
    miles) across, and the outer ring is about 2600 kilometers (1560
    miles) across. This is the first recognized basin in the Jovian
    system and supports the assumption that Callisto’s surface is old.
    The lack of high ridges, ring mountains, or a large central
    depression suggests that the impacting body caused melting, some
    flow, and shock waves, and that the refreezing occurred in time to
    preserve the concentric shock rings.]

    [Illustration: 7/7/79    390,000 km    (245,000 mi)

    Callisto is the most heavily cratered planetary body in our solar
    system. In this Voyager 2 nine-frame mosaic, a special computer
    filter was used to provide high contrast in the surface topography.
    The impact structure visible at the upper right edge of the
    satellite is smaller than the largest one found by Voyager 1 but
    more detail is obvious; it is estimated that 15 concentric rings
    surround the bright center. Many hundreds of moderate-sized craters
    are also visible, a few with bright ray patterns. The limb is
    smooth, which is consistent with Callisto’s icy composition.]

    [Illustration: 3/6/79    200,000 km    (125,000 mi)

    This high-resolution image of Callisto, photographed by Voyager 1,
    shows details of the large ring structure surrounding the remains of
    the ancient impact basin visible on page 35. The surface area shown
    in this image is at the right edge and slightly above the center of
    the picture on page 35. The relatively undisturbed region on the
    right shows the shoulder-to-shoulder large impact craters typical of
    most of Callisto’s surface. A decrease in crater density toward the
    center of the structure (to the left) is evident, and is caused by
    the destruction of very old craters by the large impact that formed
    the ring]




                          The Voyager Mission


The Voyager mission is focused on the exploration of the Jupiter and
Saturn systems. The alignment of these large planets permits the use of
a gravity-assist trajectory in which the gravity field of Jupiter and
Jupiter’s motion through space may be used to hurl the spacecraft on to
Saturn. In 1977, a rare alignment (once every 176 years) of our four
outer planets—Jupiter, Saturn, Uranus, and Neptune—may permit a
gravity-assist trajectory to Uranus and even to Neptune for Voyager 2.

Voyagers 1 and 2 began their journeys in the late summer of 1977,
catapulted into space by a Titan/Centaur launch vehicle from Cape
Canaveral, Florida. With them went the hopes and dreams of thousands of
people 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, the Voyagers are nuclear powered
rather than solar powered. The Voyagers are the fastest man-made objects
ever to have left Earth. In fewer than ten hours, they had crossed the
Moon’s orbit. This compares to about three days for an Apollo flight and
one day for the Mariner and Viking spacecraft. Their launches marked the
end of an era in space travel—the end of the planned use of
Titan/Centaur launch vehicles. With the advent of the Space Shuttle in
the 1980s, future spacecraft will be launched from the Shuttle Orbiter.

Voyager 1 was launched 16 days after its sister ship, but because of a
different trajectory, it arrived at Jupiter four months ahead of Voyager
2. Both spacecraft spent more than nine months crossing the asteroid
belt, a vast ring of space debris circling the Sun between the orbits of
Mars and Jupiter. During their 16- and 20-month journeys to Jupiter, the
spacecraft tested and calibrated all of their instruments, exercised
their scan platforms, and measured particles and fields in
interplanetary space. As the spacecraft neared the planet, the cameras
showed the dramatic visible changes that had taken place in the five
years since Jupiter had been photographed by Pioneer 11. And for the
first time, we got a close look at some of Jupiter’s moons: Amalthea,
Io, Europa, Ganymede, and Callisto.

Targeted for the closest look at Io, Voyager 1 flew the more hazardous
course, passing between Jupiter and Io, where the radiation environment
is the most intense. Voyager 2’s flight path gave Jupiter and its
intense radiation a much wider berth. Unlike Voyager 1, which
encountered the five innermost satellites as it was leaving Jupiter,
Voyager 2 encountered the satellites as it was approaching the planet,
thus providing closeup photography of opposite sides of the satellites.

    [Illustration: March 5, 1979. _Voyager 1’s unique flight path
    allowed scientists to study at close range 5 of Jupiter’s 13 known
    satellites. Each is shown at its closest point to the trajectory of
    Voyager 1’s outbound flight away from Jupiter. Closest approach was
    280,000 kilometers (174,000 miles) from Jupiter._]

    [Illustration: July 9, 1979. _Voyager 2’s closest approach to
    Jupiter was 645,000 kilometers (400,000 miles) from the planet.
    Voyager 2 encountered the satellites on its inbound journey to
    Jupiter, which enabled the spacecraft to photograph the opposite
    sides of the satellites._]

Arriving at Jupiter from slightly different angles, both spacecraft
measured the large, doughnut-shaped ring of charged sulfur and oxygen
ions, called a torus, encircling the planet at about the orbit of Io.
Then, both spacecraft disappeared behind Jupiter, out of view of Earth
and Sun, for about two hours. During this time, measurements were taken
on the planet’s dark side. Each spacecraft took over 15,000 photographs
of Jupiter and its satellites.

    [Illustration: _Voyager spacecraft and scientific instruments._]

From the moment of launch, the Voyager spacecraft have been monitored by
a worldwide tracking system of nine giant antennas strategically located
around the world in California, Spain, and Australia to ensure constant
radio contact with the spacecraft as the Earth rotates. Radio contact
with Voyagers 1 and 2 has not been instantaneous, however. When Voyager
1 flew past Jupiter, radio signals between Earth and the spacecraft took
37 minutes; when Voyager 2 arrived, the signals took 52 minutes because
by then the planet was farther from Earth.

The pictures in this book were taken by a shuttered television-type
camera. Each picture is composed of 640,000 dots, which were converted
into binary numbers before being radioed to Earth. When the signals
reached Earth, they were reconverted by computer into dots and
reassembled into the original image. Most of the color pictures are
composed of three images, each one taken through a different color
filter: blue, orange, or green. The images were combined and the
original color was reconstructed by computer. The computer eliminated
many of the imperfections that crept into the images, and enhanced some
of the images by emphasizing different colors.

Designed to provide a broad spectrum of scientific investigations at
Jupiter, the science instruments investigated atmospheres, satellites,
and magnetospheres. The scientific investigations for the Voyager
mission and their Jovian encounter objectives are shown in the table on
page 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 wealth of new information about
Jupiter when Voyager 1 reaches Saturn in November 1980, and Voyager 2
follows in August 1981. After Voyager 1 encounters Saturn, Voyager 2 may
be retargeted to fly past Uranus in 1986. Upon completion of their
planetary missions, both spacecraft will search for the outer limit of
the solar wind, that boundary somewhere in our part of the Milky Way
where the influence of the Sun gives way to other stars of the galaxy.
Voyagers 1 and 2 will continue to study interstellar space until the
spacecraft signals can no longer be received.




                         Scientific Highlights


Some of the most important information gathered by Voyagers 1 and 2 on
the Jovian system is presented pictorially in this book and is
supplemented here with brief summaries of the major discoveries,
observations, and theories.


Jupiter

The atmosphere of Jupiter is colorful, with cloud bands of alternating
colors. A major characteristic of the atmosphere is the appearance of
regularly spaced features. Around the northern edge of the equator, a
train of plumes is observed, which has bright centers representative of
cumulus convection similar to that seen on Earth. At both northern and
southern latitudes, cloud spots are observed spaced almost all the way
around the planet, suggestive of wave interactions. The cloud structures
in the northern and southern hemispheres are distinctly different.
However, the velocities between the bright zones and dark belts appear
to be symmetric about the equator, and stable over many decades. This
suggests that such long-lived and stable features may be controlled by
the atmosphere far beneath the visible clouds. The Great Red Spot
possesses the same meteorological properties of internal structure and
counterclockwise rotation as the smaller white spots. The color of the
Great Red Spot may indicate that it extends deep into the Jovian
atmosphere. Cloud-top lightning bolts, similar to those on Earth, have
also been found in the Jovian atmosphere. At the polar regions, auroras
have been observed. A very thin ring of material less than one kilometer
(0.6 mile) in thickness and about 6000 kilometers (4000 miles) in radial
extent has been observed circling the planet about 55,000 kilometers
(35,000 miles) above the cloud tops.


Amalthea

Amalthea is an elongated, irregularly shaped satellite of reddish color.
It is 265 kilometers (165 miles) long and 150 kilometers (90 miles)
wide. Just like the large Galilean satellites, Amalthea is in
synchronous rotation, with its long axis always oriented toward Jupiter.
At least one significant color variation has been detected on its
surface.


Io

Eight active volcanoes have been detected on Io, with some plumes
extending up to 320 kilometers (200 miles) above the surface. Over the
four-month interval between the Voyager 1 and 2 encounters, the active
volcanism appears to have continued. Seven of the volcanoes were
photographed by Voyager 2, and six were still erupting.

The relative smoothness of Io’s surface and its volcanic activity
suggest that it has the youngest surface of Jupiter’s moons. Its surface
is composed of large amounts of sulfur and sulfur dioxide frost, which
account for the primarily yellow-orange surface color. The volcanoes
seem to eject a sufficient amount of sulfur dioxide to form a
doughnut-shaped ring (torus) of ionized sulfur and oxygen atoms around
Jupiter near Io’s orbit. The Jovian magnetic field lines that go through
the torus allow particles to precipitate into the polar regions of
Jupiter, resulting in intense ultraviolet and visible auroras.


Europa

Europa, the brightest of Jupiter’s Galilean satellites, may have a
surface of thin ice crust overlying water or softer ice, with
large-scale fracture and ridge systems appearing in the crust. Europa
has a density about three times that of water, suggesting it is a
mixture of silicate rock and some water. Very few impact craters are
visible on the surface, implying a continual resurfacing process,
perhaps by the production of fresh ice or snow along cracks and cold
glacier-like flows.


Ganymede

Ganymede, largest of Jupiter’s 13 satellites, has bright “young” ray
craters; light, linear stripes resembling the outer rings of a very
large, ancient impact basin; grooved terrain with many faults; and
regions of dark, heavily cratered terrain. Among the Galilean
satellites, Ganymede probably has the greatest variety of geologic
processes recorded on its surface and may be the best example for
studying the evolution of Jupiter’s inner satellites. Imbedded within
Jupiter’s magnetosphere, Ganymede is subjected to the influences of the
corotating charged-particle plasma and an interaction may exist with
this plasma. No atmosphere has been detected.


Callisto

The icy, dirt-laden surface of Callisto appears to be very ancient and
heavily cratered. The large concentric rings indicate the remains of
several enormous impact basins, created by huge meteors crashing into
the surface, and since erased by the flow of the crust. Callisto’s
density (less than twice that of water) is very close to that of
Ganymede, yet there is little or no evidence of the crustal motion and
internal activity that is visible on Ganymede.


The Magnetosphere

Perhaps the largest structure in the solar system is the magnetosphere
of Jupiter. This is the region of space which is filled with Jupiter’s
magnetic field and is bounded by the interaction of that magnetic field
with the solar wind, which is the Sun’s outward flow of charged
particles. The plasma of electrically charged particles that exists in
the magnetosphere is flattened into a large disk more than 4.8 million
kilometers (3 million miles) in diameter, is coupled to the magnetic
field, and rotates around Jupiter. The Galilean satellites are located
in the inner regions of the magnetosphere where they are subjected to
intense radiation bombardment. It appears that Io is a source of the
sulfur and oxygen ions which fill the magnetosphere. Another
magnetospheric interaction is the electrical connection between Io and
Jupiter along the magnetic field lines that leave Jupiter and intersect
Io. This magnetic flux tube was examined by Voyager 1 and a flow of
about five million amperes of current was measured, which was
considerably more than anticipated. Voyager also discovered a new
low-frequency radio emission coming from Jupiter, which is possibly
associated with the Io torus.


Scientific investigations of the Voyager mission

 Investigation                  Typical Jovian encounter objectives

 Imaging science                High-resolution reconnaissance over large
                                phase angles; atmospheric dynamics;
                                geologic structure of satellites
 Infrared radiation             Atmospheric composition, thermal
                                structure and dynamics; satellite surface
                                composition and thermal properties
 Photopolarimetry               Atmospheric aerosols; satellite surface
                                texture and sodium cloud
 Radio science                  Atmospheric and ionospheric structure,
                                constituents, and dynamics
 Ultraviolet spectroscopy       Upper atmospheric composition and
                                structure; auroral processes;
                                distribution of ions and neutral atoms in
                                the Jovian system
 Magnetic fields                Planetary magnetic field; magnetospheric
                                structure; Io flux tube currents
 Plasma particles               Magnetospheric ion and electron
                                distribution; solar wind interaction with
                                Jupiter; ions from satellites
 Plasma waves                   Plasma electron densities; wave-particle
                                interactions; low-frequency wave emissions
 Planetary radio astronomy      Polarization and spectra of radio
                                frequency emissions; Io radio modulation
                                process; plasma densities
 Low-energy charged particles   Distribution, composition, and flow of
                                energetic ions and electrons;
                                satellite-energetic particle interactions
 Cosmic ray particles           Distribution, composition, and flow of
                                high-energy trapped nuclei; energetic
                                electron spectra

    [Illustration: A computer-generated mosaic of Voyager 1 pictures
    showing Jupiter from directly above the north pole. This view shows
    features to about 20 degrees south latitude. The black area at the
    pole results from missing information.]


    [Illustration: NASA]

  National Aeronautics and Space Administration

  Jet Propulsion Laboratory
  California Institute of Technology
  Pasadena, California

JPL 400-24 7/79




                          Transcriber’s Notes


—Retained publication information from the printed edition: this eBook
  is public-domain in the country of publication.

—Silently corrected a few palpable typos.

—Moved captions nearer the relevant images; tweaked image references
  within captions accordingly.

—Added a Table of Contents.

—In the text versions only, text in italics is delimited by
  _underscores_.