THE SECOND ENCOUNTER: MORE SURPRISES FROM THE “LAND” OF THE GIANT
Approaching Jupiter
At the beginning of July, the dry summer heat had returned to Pasadena,
and so had the press. The scientists had come days or weeks earlier to
look at data being transmitted from the second Voyager as it approached
Jupiter and its satellites. The mood at JPL seemed quieter than it had
been in March for Voyager 1, although the press room would once again be
deluged with observers on the day of encounter. This would be our second
good, close look at the Jovian system, but it was to be no summer rerun.
Voyager 2 would have a different view of each world, and, in addition,
both Io and Jupiter had undergone changes, as though to ensure that no
one would become bored and fall asleep in front of a TV monitor. In a
sense, this encounter was to be another first look at Jupiter and its
satellites, with a view of each object that was quite different from
what had been seen before.
Changes in Jupiter’s cloud formations became noticeable long before
July. After a gap of six weeks following the first flyby, Voyager 2’s
observatory phase began on April 24, 1979, seventy-six days before its
July 9 encounter with Jupiter. During this time, the spacecraft’s
ultraviolet and fields and particles instruments studied the Jovian
system and its interaction with the solar wind. Between April 24 and May
27, Voyager 2’s imaging system concentrated on the motions in Jupiter’s
atmosphere, creating another approach time-lapse “movie.” From May 27 to
29 photographs were taken in a more rapid sequence, showing the planet
during five 10-hour rotations. From these studies it was apparent that
Garry Hunt’s prediction had been right—the weather _had_ changed by
July. A month before the encounter JPL’s Voyager Bulletin—Mission Status
Report announced that “Jupiter is sporting quite a different face than
it did just four months ago. The bright ‘tongue’ extending upward from
the Red Spot is interacting with a thin, bright cloud above it that has
traveled twice around Jupiter in four months.” The turbulent region west
of the Great Red Spot had begun to break up and separate from the Red
Spot. The white ovals south of the Red Spot had drifted to the east
(about 0.35 degrees a day), while the Red Spot itself had drifted west
(about 0.26 degrees a day). The white zone seen just south of the Red
Spot by Voyager 1 had become very narrow—like a thin white line just
barely outlining the bottom of the spot. The Red Spot had also changed:
It had become a more uniform orange-red, perhaps reverting to the color
seen by Pioneers 10 and 11. The brown spots that had been seen in the
north temperate region at the same longitude as the Red Spot were now on
the other side of the planet. A dark brown spot not present during the
Voyager 1 flyby had developed along the northern edge of the brown
equatorial region on the Red Spot side of the planet. Some of the white
markings that seemed to have protruded into the equatorial region at the
time of the first flyby were missing in the Voyager 2 photographs.
As Voyager 2 entered the far encounter period on May 29, all instruments
on the spacecraft (except for the photopolarimeter) seemed to be in good
shape for encounter. As was the case with Voyager 1, the polarization
wheel on Voyager 2’s photopolarimeter was stuck, so the instrument was
able to obtain only color photometry measurements.
[Illustration: In early June, as Voyager 2 carried out its
observatory phase, additional changes in Jupiter’s face began to be
apparent. These two images, taken from a distance of 24 million
kilometers, have a resolution of about 500 kilometers.]
[Illustration: The Great Red Spot and the white oval south of it are
seen to be followed on the west by regions of chaotic and turbulent
clouds. This is not the same white oval that was near the Red Spot
in March; the differential rotation of the planet carried a
different oval close to the Red Spot during the intervening three
months. [P-21713C]]
[Illustration: Io is visible to the right of the planet, and the
shadow of Ganymede falls on the colored clouds of Jupiter’s
equatorial belt. [P-21714C]]
[Illustration: The Voyager 2 trajectory was complementary to that of
Voyager 1. This time, the satellites were encountered before
Jupiter, revealing their other hemispheres. As shown in this
drawing, the spacecraft flew by first Callisto, then Ganymede, then
Europa. The ten-hour Io volcano watch took place immediately after
closest approach to Jupiter. [260-533A]]
Voyager 2 trajectory
View normal to Jupiter equator
Sun occultation
Earth occultation
Launch date = 8/20/77
Jupiter arrival date = 7/9/79
Periapsis
Satellite closest approach
Amalthea
Europa
Io
Ganymede
Callisto
[Illustration: These two faces of Jupiter were photographed by
Voyager 2 on May 9 at a distance of 46 million kilometers from the
planet. Voyager scientists began to detect significant changes in
the cloud patterns since the Voyager 1 encounter two months earlier.
[260-507]]
[Illustration: Jupiter.]
[Illustration: Jupiter.]
[Illustration: The weather is changing over one of the northern
hemisphere brown ovals in this picture taken July 6. The brown ovals
are regions in which breaks in the upper layer of ammonia clouds
reveal darker clouds below. A high, white cloud is seen moving over
the darker cloud, providing an indication of the structure of the
cloud layers. Thin white clouds are also seen within the dark cloud.
At right, blue areas, free of high clouds, are seen. [P-21753C]]
Although Voyager 2’s radio receiver still could not track a
Doppler-shifted radio signal from Earth (the problem is that it “hears a
monotone,” explained Deputy Project Manager Esker K. Davis), the Deep
Space Network engineers had learned to work with the spacecraft,
determining what frequency the spacecraft would listen to at any
particular time. They had discovered that some of the “housekeeping”
telemetry signals from the receiver were sensitive to the match between
the incoming frequency and the receiver frequency. By monitoring these
signals, they could detect a frequency drift in time to correct the
transmission, thus keeping the system in tune in spite of slow changes
in the receiver. The system was slow and demanding but effective; all
the necessary command sequences were successfully loaded into the
computer, and communications during the encounter were entirely
successful.
The timing offset experienced by Voyager 1 as a result of Jupiter’s
intense radiation environment was not expected to be a problem on
Voyager 2 for two reasons: Even at closest approach, Voyager 2 would
still be more than twice as far from Jupiter as Voyager 1 had been, and
the Voyager 2 computer was programmed to resynchronize the spacecraft’s
timing systems automatically every hour. In this way, even if the
radiation environment proved to be much higher than anticipated, the
image smear that might occur from a timing offset would be prevented.
[Illustration: Complex activity in the southern hemisphere of
Jupiter continued during the Voyager 2 encounter, although changes
had occurred in the region of the Great Red Spot. A white oval,
different from the one observed in a similar position at the time of
the Voyager 1 encounter, was situated south of the Red Spot. The
region of white clouds extended from east of the Red Spot and around
its northern boundary, preventing small cloud vortices from circling
the feature. The disturbed region west of the Red Spot had also
changed since the equivalent Voyager 1 image. The picture was taken
on July 3 from a distance of 6 million kilometers. [P-21742C]]
As a result of the discoveries made by Voyager 1, the project scientists
decided to modify some of Voyager 2’s preplanned sequences. As early as
April 1, the painstaking job of constructing new computer commands
began. A ten-hour Io Volcano Watch was added to the spacecraft’s
program, taking advantage of the fact that shortly after closest
approach to Jupiter, the spacecraft would remain within about 1 million
kilometers of Io for a long period, keeping nearly the same face in
view. Provisions were also made to take extensive ultraviolet
measurements of the emission from the glowing torus surrounding Jupiter
near the orbit of Io. Further studies would be made of the dark side of
Jupiter to search for lightning and auroral activity, and there was also
the hope that the plasma wave instrument would be able to detect
lightning whistlers (radio signals created by lightning bolts) as the
Voyager 1 instrument had done. A high priority was given to observations
of the newly discovered ring, which had not been in the original Voyager
2 sequence at all. The spacecraft would cross the ring plane twice,
photographing the ring during both the inbound and the outbound
passages.
As was originally planned, Voyager 2 would make its closest approaches
to Callisto, Ganymede, Europa, and Amalthea before encounter with
Jupiter. Because of the difference in the trajectories of the two
spacecraft, Voyager 2 would see the faces of Callisto and Ganymede not
seen by Voyager 1. The most important difference, however, was that the
second Voyager would fly much closer to Europa than Voyager 1 did,
giving scientists their first good look at the mysterious streaks
scratched on the surface of that bright golden world. Voyager 2 would
also have a closer flyby of Ganymede, giving us a second chance to
examine its strange “snowmobile tracks.” The major loss, of course, was
Io, which would be seen from Voyager 2 only at distances of a million
kilometers or more.
[Illustration: Io appeared in front of Jupiter as seen by Voyager on
June 25, at a range of 12 million kilometers. At a resolution of
about 200 kilometers, the bright and dark spots on the satellite are
just beginning to be resolved, but it was not possible to determine
if any eruptions were still in progress. [P-21719C]]
The Encounter
Wednesday, July 4.
(_Range to Jupiter, 5.3 million kilometers; range to Earth, 921 million
kilometers_). While most of the nation celebrated Independence Day with
picnics, sports events, and fireworks, the scientists and engineers at
JPL were working around the clock. Voyager 2 had already entered
Jupiter’s territory, crossing the bow shock for the first time on July 2
at a distance of 99 R_J from Jupiter, indicating that the magnetosphere
had expanded in the interval between the two encounters. At about noon
on July 3, the spacecraft encountered the magnetopause, but on July 4,
the data from the particles and fields instruments were ambiguous.
Apparently the magnetosphere was pulsating in response to changing
pressures, and the spacecraft was playing tag with the rapidly shifting
boundaries of the bow shock and the magnetopause.
As the low energy charged particle instrument began to measure particles
coming from inside the Jovian magnetosphere, it became apparent that
some important changes had taken place since Voyager 1’s encounter. From
the composition of the particles, it appeared that they were largely of
solar origin, unlike the heavy concentrations of ions of sulfur and
oxygen seen by Voyager 1. Scientists began to speculate that the Io
volcanoes, which presumably eject sulfur and oxygen into the
magnetosphere, might have declined in activity. In the evening, the
first images of Io at a resolution high enough to allow the volcanic
plumes to be seen would be beamed back to Earth.
ENCOUNTER DISTANCES FOR VOYAGER 2
Object Range to Center at Closest Best Image Resolution
Approach (kilometers) (km per line pair)
Jupiter 722 000 15
Amalthea 558 000 10
Io 1 130 000 20
Europa 206 000 4
Ganymede 62 000 1
Callisto 215 000 4
[Illustration: Voyager scientists anxiously awaited the first views
of Io that would show whether the volcanic eruptions seen in March
were still active. This picture was taken on July 4, at a range of
4.7 million kilometers, about the same as that of the volcano
discovery picture on March 8. One large plume is clearly visible,
rising nearly 200 kilometers above the surface. At the time of
release of this picture on July 6, the scientists wrote, “The
volcano apparently has been erupting since it was observed by
Voyager 1 in March. This suggests that the volcanoes on Io probably
are in continuous eruption.” [P-21738B/W]]
Thursday, July 5.
(_Range to Jupiter, 4.4 million kilometers_). The press room at Von
Karman Auditorium opened and the members of the press, most of them
veterans of the first encounter, arrived at JPL. Meanwhile, the
spacecraft continued to measure fluctuations in the magnetospheric
boundary. By noon, JPL had reported at least eleven crossings of the bow
shock as the solar wind flirted with Jupiter’s magnetosphere. Apparently
the solar wind was much more variable in July than it had been in March.
At times the bow shock seemed to be thicker than that experienced by
Voyager 1; one Voyager 2 crossing took ten minutes, whereas the longest
Voyager 1 crossing was only one minute long. Even though the processes
affecting the magnetosphere seemed more complex, the magnetosphere was
less compressed; when Voyager 2 actually entered the magnetosphere at a
distance of 62 R_J, it was much farther from Jupiter than Voyager 1 had
been at its final crossing (47 R_J).
Photos obtained the day before from over 4 million kilometers showed
that at least one of Io’s volcanoes was still active. A total of eight
ongoing eruptions had been seen by Voyager 1, and scientists were
anxious to see how many of these were still erupting four months later.
VOYAGER 2 BOW SHOCK (S) AND MAGNETOPAUSE (M) CROSSINGS
Boundary Day Distance (R_J)
_Inbound_
S 7/02 99 (multiple)
S 7/02 97
S 7/03 87
M 7/04 72 (multiple)
M 7/05 71
S 7/05 69
S 7/05 67
M 7/05 62
_Outbound_
M 7/23 169
M 7/23 173
M 7/24 174
M 7/24 175
M 7/24 176
M 7/24 177
M 7/25 184
M 7/25 185
M 7/27 213
M 7/31 253
M 8/01 258
M 8/01 262 (multiple)
M 8/03 279 (multiple)
S 8/03 283 (multiple)
[Illustration: Although Voyager 2 did not come as close to Io as had
Voyager 1, some changes in the surface during the four months
between encounters were so large that they could still be easily
seen. These two pictures of the region of the volcano Pele were
taken in early March and early July, respectively. The most dramatic
change was the filling in of the indentation in the ejecta ring,
turning the hoofprint into a symmetric oval. The oval is about 1000
by 700 kilometers in outermost dimension, and the area that changed
amounts to more than 10 000 square kilometers. [260-687AC]]
[Illustration: Volcano Pele on Io.]
[Illustration: Volcano Pele on Io.]
While attention at JPL focused on the unfolding drama of the Jupiter
encounter, many members of the world’s press seemed more interested in
the fate of Skylab, which was nearing its death plunge into the Earth’s
atmosphere. Launched in 1973, Skylab had been one of NASA’s more
successful projects. Three crews of astronauts had visited it, carrying
out intensive studies of the Sun and breaking one record after another
for the duration of manned space flight. Since the departure of the
final group of three astronauts in 1974, Skylab had been sinking
gradually lower as a result of friction with the extreme upper
atmosphere of Earth. During the past year, higher temperatures in the
atmosphere had increased this drag, and now the end was near. With a
strange fascination, the world watched the end of this old spacecraft,
almost seeming to forget the spectacular new results being transmitted
from Jupiter. To the frustration of the Voyager team and the press
“camped out” in Von Karman Auditorium for the second encounter, the
exaggerated stories of a possible Skylab disaster took precedence over
Voyager news. Ultimately, Skylab fell over the Indian Ocean and
Australia on Wednesday, July 11, just as the major findings of Voyager 2
were being released.
Friday, July 6.
(_Range to Jupiter, 3.5 million kilometers_). With the first satellite
encounter still two days away, Voyager 2 continued to make a variety of
measurements of Jupiter and all the Galilean satellites. As the distance
to Io decreased, it was possible to see detailed surface features as
well as to look for the volcanic plumes at the edge of the disk,
silhouetted against black space. By the end of the day, the Great Red
Spot loomed so large that six imaging frames (a 2 × 3 mosaic) were
required to encompass it and its immediate surroundings.
At the first formal press conference of the Voyager 2 encounter, Project
Scientist Ed Stone reviewed the progress of the mission. Because the
ailing spacecraft receiver was working so well, Ray Heacock, Voyager
Project Manager, announced that the major trajectory correction maneuver
at Jupiter had been rescheduled to take place only two hours after
closest approach. Since the geometry was especially favorable at this
time, the 76-minute rocket burn could put the spacecraft on its planned
route to Saturn with a minimum expenditure of fuel, thereby preserving
the option of sending the spacecraft on to Uranus.
The Io torus was under observation, both directly by the ultraviolet
spectrometer, and indirectly by the charged particle instruments. The
LECP instrument had begun to pick up sulfur ions, but at lower energies
and lower concentrations than those recorded during the first encounter.
In the ultraviolet, glows could be seen both from the torus and from
aurorae in the polar regions of Jupiter.
Photographs of Io showed that the heart-shaped feature surrounding Pele
(P₁), Io’s largest volcano, had changed shape. The indentation of the
heart had disappeared, making the heart into an oval. Apparently a new
deposit of volcanic ejecta had blanketed the surface, altering its
color. Perhaps an earlier obstruction in the volcanic vent, or the shape
of the vent itself, had caused the area surrounding Pele to look
heart-shaped. In any case, whatever had caused the indentation was now
gone. At the same time, new photos failed to show a plume above Pele,
and there was speculation that changes in this eruption might be related
to the altered population of charged particles in the magnetosphere.
[Illustration: The Voyager 2 pictures of Callisto looked remarkably
similar to those obtained of the other side of the satellite by
Voyager 1. Seen from a distance of 2.3 million kilometers, the large
craters (100 kilometers or more across) appear as light spots. No
new major impact features such as Valhalla, discovered by Voyager 1,
are visible on the hemisphere seen by Voyager 2. [P-21740C]]
Saturday, July 7.
(_Range to Jupiter, 2.6 million kilometers_). As the spacecraft rapidly
closed on Callisto, better and better photographs were taken of the
previously unseen hemisphere. As with the Voyager 1 observations,
however, the main impression was one of heavy cratering, unrelieved by
other geologic structures. Meanwhile, the coverage of Io had improved as
the satellite rotated to the point at which a census of the volcanic
eruptions seen in the first encounter began to emerge.
At the 11:00 a.m. press conference, Larry Soderblom announced that four
of the volcanoes discovered by Voyager 1 had been looked at again by
Voyager 2, and three of them—Prometheus (P₂), Loki (P₃), and Marduk
(P₇)—were still active. However, there was no trace of volcanic activity
coming from Pele, the source of the largest plume seen by Voyager 1. P₁
was either greatly subdued or had turned off completely.
Dr. Soderblom also announced that Voyager 2 images had detected another
giant ring structure on Callisto, bringing the total to three, and there
were probably more. This particular ring feature was estimated to be
about 1500 kilometers across. It was also noted that although Callisto
generally seemed to be saturated with shoulder-to-shoulder craters, the
crater density near the ring structures seemed to be lower.
Saturday was a fairly quiet time for the scientists but not for the
spacecraft or the spacecraft team. “We blocked out about 7½ hours,”
explained Michael Devirian, Ground Data Systems Development,
Integration, and Test Director, “in which we could send it a set of
commands and re-send it if necessary to make sure all close-encounter
commands were received by Voyager 2 until all the commands got through.
The whole thing went perfectly the first time.” So everything was “go”
for close encounter. The near encounter phase began at 6:36 p.m. PDT.
[Illustration: A new face of Ganymede was revealed to Voyager 2.
This image was taken July 7 from a distance of 1.2 million
kilometers and clearly shows the large dark area Regio Galileo, as
well as much of the lighter grooved terrain discovered by Voyager 1.
The bright spots are impact craters. This image also shows what
appear to be polar caps, extending down to about latitude 45° in
both the northern and southern hemispheres. [260-670]]
Sunday, July 8.
(_Range to Jupiter, 1.5 million kilometers_). At 2:30 a.m. the first
long-exposure sequence of ring pictures was taken, and at 3:00 a.m. the
intensive period of the Callisto encounter began. Eighty high-resolution
images were obtained of the satellite, centered around closest approach
(215 000 kilometers) at 6:13 a.m. Incoming photos showed some features
that looked like double-walled craters, but no more giant ring
structures were seen. It appeared that there was an asymmetry in the
distribution of large impact features over Callisto’s surface. “Callisto
may turn out to be the most heavily cratered body in the solar system,”
Torrence Johnson remarked. Garry Hunt was to add later on, “There’s just
not room for another crater on that body—it’s totally full.”
At the press conference, Brad Smith confirmed the earlier finding that
Io’s volcano Pele was quite dead—at least for now. Although P₄ had not
yet been looked at, all other volcanoes discovered by Voyager 1 were
still active, but no new plumes had been found. However, new ultraviolet
images of P₂ (Loki) suggested that the eruption had increased in size.
(In a later report, the imaging team announced that P₂ had increased in
height to 175 kilometers and had changed to a two-column plume.) The new
photographs of Jupiter’s ring showed it to be quite narrow and
ribbonlike, Dr. Smith announced. The artist’s drawing (released during
the Voyager 1 encounter), intended to show the outer edge of the ring,
turned out to be a good representation of the actual ring, Dr. Smith
said.
There seemed to be less high-speed sulfur and oxygen inside Jupiter’s
magnetosphere than there had been during the Voyager 1 encounter, George
Gloeckler announced. Voyager 2’s low energy charged particle instrument
was finding substantial amounts of carbon, silicon, magnesium, and other
elements of solar origin, but the Io-associated elements were almost
depleted. The ultraviolet instrument had found as much glowing sulfur in
Io’s torus as before, but less of it seemed to be raised to energies
high enough to leave the torus and be detected elsewhere in the
magnetosphere.
There were other indications of Jupiter’s changing weather. In a Voyager
report Sunday evening, Garry Hunt remarked, “One very exciting
observation came the other day which caused major excitement down in the
imaging area. We actually saw a white cloud starting to intrude across a
dark barge [large brownish oval-shaped feature in Jupiter’s northern
hemisphere]. Atmospheric scientists get very excited by that because
this is showing us how the colors layer themselves up—that white cloud
is clearly above the dark brown. We’re desperately trying to understand
the relationship of colors on Jupiter.”
[Illustration: The first close-up views of Europa were both exciting
and perplexing to Voyager scientists. The best Voyager 1 resolution
had been only about 30 kilometers, but the Voyager 2 trajectory
permitted a much closer flyby. These pictures, taken on July 9 at a
range of 240 000 kilometers, have a resolution of about 5
kilometers. The bright icy crust of Europa is covered with a
spectacular series of dark streaks, giving the satellite a cracked
appearance. In a few cases, narrower light lines run down the
centers of the dark streaks, which are typically a few tens of
kilometers in width. Very few, if any, impact craters are visible on
Europa. [P-21760C and P-21764C]]
[Illustration: Europa]
Monday, July 9.
(_Range to Jupiter at encounter, 722 000 kilometers_). Encounter day!
And not just one encounter, but a whole sequence: Ganymede, Europa,
Amalthea, Jupiter, and Io. By early Sunday evening, a wealth of new data
on Ganymede was pouring in. Not only was this a side of the satellite
not seen before, but Voyager 2 would pass closer to Ganymede than had
Voyager 1. Encounter took place at 1:06 a.m., at a range of 62 000
kilometers. Between 9:00 p.m. Sunday night and 1:30 a.m. Monday morning
a total of 217 photos, plus infrared and ultraviolet spectra, were
scheduled. Sixty-nine photos were sent back in real time; others were
recorded for playback later.
As the Ganymede pictures appeared on the TV screens, they revealed a
world of tremendous variety. Some regions were heavily cratered:
“Ganymede looks like Mercury or the highlands of the moon,” one Voyager
scientist remarked. Other parts of the surface, however, showed very
different features: long, parallel mountain ridges that looked like
grooves made with a giant’s rake; narrow, segmented lines; white ejecta
blankets from impacts that looked like a dazzling, snow-covered
landscape. Some of the pictures suggested cracking and slipping of
Ganymede’s crust, while others showed what appeared to be remnants of
ancient terrain unaffected by subsequent intense geologic activity. Many
of the highest resolution frames were not seen at this time; they were
recorded on the spacecraft for playback later.
Starting at about 8 a.m., Earth began receiving the first closeup views
of Europa. Europa “could be the most exciting satellite in the whole
Jovian system,” said Larry Soderblom, “because it’s sort of the
transition body between the solid silicate body, Io, and the ice balls,
Ganymede and Callisto.” The icy crust looked as though it “had been
ruptured all over—as though it was in pieces—just as though it had been
broken in place and left there.” At 11:43 a.m., closest approach took
place at a range of 206 000 kilometers. By this time the scientists were
dazzled by what they had seen; some were calling Europa the most bizarre
of all the Galilean satellites. In the Imaging Team viewing area, David
Morrison compared Europa’s surface to “a cracked egg,” and Gene
Shoemaker said, “It looks like sea ice to me.” When someone commented
that the canal-like streaks were reminiscent of Mars, Torrence Johnson
replied, “It looks like some pictures of Mars I’ve seen, but only on the
walls of Lowell Observatory.” Another quipped, “Where is Percival
Lowell, now that we need him!”
There were to be two press conferences: one to present spacecraft and
scientific results and one to celebrate the second successful flyby and
to talk of new goals—Saturn, and perhaps Uranus.
At the first conference, Ed Stone began by discussing the radiation
Voyager was experiencing. One of Jupiter’s surprises was that the
radiation environment was greater than had been anticipated, and this
caused problems with the radio receiver. The receiver frequency was
shifting “more rapidly than we had anticipated,” said Ray Heacock, “and
we have not been able to keep an uplink continuously with the
spacecraft.” The solution was to keep sending up commands at different
frequencies until a frequency the spacecraft would accept was found.
Just how bad were the radiation levels? Ed Stone commented, “The
penetrating radiation at a given distance is more intense now at this
distance than it was when Voyager 1 flew by.” From a preliminary
analysis it seemed that, overall, Voyager 2 would still be subjected to
lower radiation levels than Voyager 1 had been, but to higher levels
than had been expected. In addition, Voyager 2’s radio receiver was much
more sensitive. The higher than expected radiation intensity also led
Voyager scientists to have the ultraviolet spectrometer shut off, since
that instrument was also quite sensitive to the radiation.
The fourth member of the Galilean satellites had finally been seen, and
Larry Soderblom happily introduced Europa. “Well, some few months ago,
before the Voyager 1 encounter, we thought we had some idea of what
planets were like—at least the planets in the inner solar system: Mars,
Mercury, the Moon, the Earth. And we’ve discovered many times over in
the last couple of months how narrow our vision really was. Included in
the Jovian collection of satellites are the oldest (Callisto), the
youngest (Io), the darkest (Amalthea), the brightest (Europa), the
reddest (Amalthea and Io), the whitest (Europa), the most active (Io),
and the least active (Callisto). Today we found the flattest (Europa).”
In spite of the appearance of a cracked or broken surface, Europa showed
no topography at all. Toward the sunset line, where the low angle of
illumination should reveal even low relief, “the surface disappears—as
if it were the surface of a billiard ball.” It seemed clear that Europa
has much less relief than the other two icy satellites, Ganymede and
Callisto. But why can’t Europa’s surface support relief? Perhaps Europa
has a thick ice mantle—on the order of 100 kilometers. If Europa is
affected by tidal heating, then such an ice mantle might be “sort of
soft and slushy” rather than rigid as are the crusts of Ganymede and
Callisto. “The fact that the surface of Europa cannot support relief of
any substantial amount suggests that the surface must be soft.” But, Dr.
Soderblom added, there does not appear to be much lateral motion or
rotation causing the surface markings—they don’t seem to be
offset—rather, “it’s as if Europa had been cracked, broken, by some
process which crushed it like an eggshell and just left the pieces
sitting there. Expansion and contraction of ice and water are a good way
to crunch up the surface.”
[Illustration: Regio Galileo is the largest remnant of the ancient,
heavily cratered crust of Ganymede. This Voyager 2 color
reconstruction was made from pictures taken at a range of 310 000
kilometers; the scene is about 1300 kilometers across. Numerous
craters, many with central peaks, are visible. 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
near-surface material. The closely spaced, arcuate linear features
are analogous to features on Callisto, such as the “ripple” marks
surrounding the ancient impact feature Valhalla. [P-21761C]]
[Illustration: At high resolution, the grooved terrain on Ganymede
shows a wonderful complexity. Surface features as small as 1
kilometer across can be seen in this mosaic of Voyager 2 images
taken July 9. The grooves are basically long, parallel mountain
ridges, 10 to 15 kilometers from crest to crest—about the same scale
as the Appalachian mountains in the Eastern United States. The
numerous impact craters superposed on the mountain ridges indicate
that they are old—probably formed several billion years ago.
[260-637]]
The other side of Ganymede presented quite a different face from the one
Voyager 1 had seen. Here were the dark ancient cratered terrains, the
shoulder-to-shoulder craters reminiscent of Callisto, and there was a
huge circular feature on Ganymede looking like the remnant of a
Callisto-style ringed basin, preserved in the ancient, dark terrain. The
very large dark feature revealed by Voyager in the northern hemisphere
which bears these impact scars was later named “Regio Galileo,” for the
discoverer of the Galilean satellites. It was seen in the low-resolution
Pioneer 10 picture of Ganymede taken in 1973, but its nature was not
understood. It is so large it has even been glimpsed on occasions of
exceptionally stable “seeing” with ground-based telescopes.
3:29 p.m. PDT—Jupiter Encounter! In the press room half a dozen cameras
clicked in unison as the universal clock declared the Voyager 2 had made
its closest approach to Jupiter—650 000 kilometers from the cloud tops,
zipping by at about 73 000 kilometers per hour—neither as close nor as
fast as Voyager 1. By the time of the special press conference at 4:30
p.m., everyone at JPL was in a party mood. Thomas A. Mutch, who had
replaced Noel Hinners as NASA Associate Administrator for Space Science,
Robert Parks, and Rodney Mills were the speakers.
The Jovian system is a place of “incredible beauty and mystery. Jupiter
has been a nice place to go by, but we wouldn’t want to stop there—we’re
going on to Saturn,” Rod Mills explained, and Bob Parks agreed.
Tim Mutch had a different perspective. “Although we have just heard
Jupiter somewhat downgraded in favor of Saturn, nonetheless what we have
been witnessing, first in March and now, in July, is a truly
revolutionary journey of exploration. We have gone beyond the familiar
part of the solar system to objects that are so exotic that their very
existence, at least as far as I’m concerned, was something I’d accepted
intellectually, but didn’t really accept in an immediate sense. We’re
starting out in our own space program on a new stage of space
exploration—on our own long journeys beyond the solar system to distant
lands. We never like to think, or rather, it’s statistically unlikely,
that we’re at a turning point in history. But if you look back at
history books, such events are clearly read into the record. And I
submit to you that when the history books are written a hundred years
from now, two hundred years from now, the historians are going to cite
this particular period of exploration as a turning point in our
cultural, our scientific, our intellectual development.”
Although everyone was already celebrating another successful mission,
the encounter was far from over. Data continued to come in; there was
still the ten-hour Io Volcano Watch, which had begun at 4:31 p.m.; there
were more observations of Jupiter, including scheduled ring observations
and dark side searches for aurorae and lightning bolts. There was a lot
of work and excitement yet to come. Jupiter had another surprise in
store for Voyager 2.
[Illustration: During the 10-hour Io volcano watch on July 9, the
spacecraft kept nearly the same face of Io in view. Most of the
surface was turned away from the Sun, however, and only a thin
crescent could be seen, shrinking as the observations continued.
These four frames were all photographed with identical exposures
from a range of about 1 million kilometers. These images show
Amirani (P₅) and Maui (P₆) on the west edge, brightening as the Sun
illuminates them more nearly from behind. [260-677]]
[Illustration: Io volcanoes.]
[Illustration: Masubi (P₈) is faintly visible in the crescent (above
and below).]
[Illustration: Io volcanoes.]
[Illustration: Loki (P₂) rises 250 kilometers above the surface,
catching the morning sunlight on the east edge of Io.]
Tuesday, July 10.
(_Range to Jupiter, 1.4 million kilometers_). The Io watch continued
through the night. As time passed, the satellite rotated in the same
direction as the motion of the spacecraft, keeping nearly the same side
in view. Because of this, a few volcanoes could be closely watched, but
most would be missed entirely. During the sequence, the illuminated
crescent steadily shrank, until at the end, volcanic plumes could be
seen on both edges, one illuminated by the setting Sun, the other
shining in the dawn light.
At the 11 a.m. press conference, Esker Davis announced that engineers
had lost contact with the spacecraft radio receiver Monday evening
(probably due to Jupiter’s radiation) and had to “chase it around most
of the night,” sending commands at various frequencies until they locked
on to the frequency the spacecraft would accept. The major trajectory
correction maneuver, begun at about the same time contact with the
receiver was lost, was successful. The 76-minute thruster firing, done
at periapsis instead of two weeks after encounter, enabled the
spacecraft to get a bigger “boost” from Jupiter than was originally
planned, amounting to a fuel saving of about 10 kilograms of hydrazine,
enough to preserve the option of going on from Saturn to Uranus.
Andrew Ingersoll discussed some results of the analysis of the Jovian
atmosphere. “At first, Voyager seemed to do nothing but emphasize the
chaos, not the order.” But, with the help of ground-based observations,
Reta Beebe found that there is a “regular alternation of eastward and
westward jets” underlying the seemingly chaotic visible features. “The
turbulence we see in the visible clouds seems to be a minor side show,
or a process without much energy or mass compared to the very great
energy and mass that might be moving around in the deep atmosphere.”
“We’re continuing to operate in our panic mode to try to get pictures to
the press,” Brad Smith said as he introduced new photographs of the
satellites. In earlier photos, Ganymede had seemed to have two different
kinds of terrain—an ancient, cratered, Callisto-like surface, and the
stranger, grooved terrain—terrains that might be representative of two
very different types of major episodes in Ganymede’s history. The most
recent images showed a much more confused picture, with several
additional types of surface geology.
At the daily project science briefing, another interpretation was being
discussed. Lyle Broadfoot reported that new measurements of the position
of the ultraviolet aurora demonstrated that it was caused by charged
particles from the Io torus, not from the outer parts of the
magnetosphere. Apparently these plasma particles arise in the volcanic
eruptions, are trapped for a time in the torus, and then fall into the
polar regions of Jupiter, where they excite auroral emissions. A
terrestrial aurora, in contrast, is caused by particles that originate
in the solar wind. Jim Sullivan of the plasma investigation estimated
that about two tons of material each second are fed from Io into the
plasma torus. This plasma, driven by the rotation of the Jovian magnetic
field, appears to be able to supply the million-million watts of power
radiated in the ultraviolet.
By 5:00 p.m. the excitement had died down; many of the scientists had
parties to attend that evening, and some members of the press were
planning parties of their own. The schedule of spacecraft activities
also seemed to have slowed. There were dark-side observations planned to
search for lightning and aurorae. There would be a few more ring
pictures—not too much to see on the monitors that night ... or so many
people thought. But a few people were waiting around, perhaps to catch a
glimpse of lightning or auroral activity, or to wait for another look at
Jupiter’s faint ring.
Between 5:52 and 6:16 p.m., six long-exposure, wide-angle photographs of
the dark side of Jupiter had been scheduled to search for aurorae and
lightning. The spacecraft was 1 450 000 kilometers from Jupiter and
about two degrees below the equatorial plane.
Shortly after 6 p.m., the first of these ring photographs appeared on
the TV monitors with unexpected brilliance. Taken in orange and violet
light, the images showed the outline of Jupiter and, protruding from it,
two narrow lines—one reaching all the way to Jupiter’s limb, the other
broken off, apparently hidden by the shadow of the giant planet. Seen
from the new perspective of the shadow of Jupiter, the tenuous rings
were remarkably clear. A sudden renewal of excitement surged through the
devotees remaining in the press room. About 6:15, Brad Smith came down
to join the press to watch the remainder of this series of pictures come
in. “Hey Brad, are you going to burn out the camera with the ring?”
someone joked. “Well, the rings do forward scatter nicely, don’t they?”
Dr. Smith replied. As the wide-angle pictures were followed by
narrow-angle views, more and more detail became apparent. For the first
time, a definite width for the ring could be seen, and there was even a
hint of additional material inside the main ring. All in all, Voyager
had provided one more splendid series of pictures before it took off for
Saturn.
[Illustration: From a vantage point 2.5 degrees above the ring
plane, Voyager 2 was able for the first time to determine the width
of Jupiter’s ring. This picture shows that the ring is ribbon-like
and only a few thousand kilometers wide, quite unlike the broad
rings of Saturn. [P-21757B/W]]
Wednesday, July 11.
JPL Public Information Officer Frank Bristow opened the 10 a.m. press
conference with an announcement: “We’ll have the report from the Imaging
Team including the tremendous pictures that we received here last night
of the Jupiter ring that excited the entire team.”
Brad Smith showed the ring pictures. “As many of you who were here last
night know, we got some rather nice pictures of the ring of Jupiter.
It’s as though Voyager 2 was fearful that we might be becoming just a
little bit apathetic after this series of marvelous discoveries and felt
that it had to dazzle us one more time before it left for Saturn. The
rings appear very much brighter than we had expected them to be.” The
outer ring is about 6500 kilometers wide. There is material inside the
ring. There is a rather sharp outer boundary and a somewhat diffuse
inner region. “And it is now our belief that the material in the ring
goes all the way down to the surface of Jupiter.” There is a very narrow
relatively bright outer ring and an extremely faint inner ring that goes
all the way down to Jupiter’s cloud tops.
Larry Soderblom summarized the satellite data: With respect to the
Galilean satellites, “We’re in a relatively high state of ignorance.”
The Io Volcano Watch images seemed to indicate that plume P₂ was now the
highest volcano on Io, since P₁ seemed to have become quiet. Io may be
the easiest Galilean satellite to try to understand, because we can
actually see the geological processes that are shaping the planet. Io’s
“twin,” Europa, seems to be where “our highest state of ignorance” lies.
“The faint bright streaks which show some relief are evidently different
from the diffuse dark bands which don’t seem to show topography, but the
similarity of these forms [that both the light and dark markings are of
planetary scale] suggests that they must be related.”
Ed Stone speculated about the other two Galilean satellites. Ganymede
and Callisto are essentially identical in size, mass, and probably
composition. By examining them, we can perhaps learn what happens when
bodies with very similar chemistry have different “life histories” and
different surface properties (there are indications that Ganymede’s
crust may not have been as rigid as Callisto’s). Going further, he added
that Callisto and Mercury, the least dense and the most dense,
respectively, of the terrestrial-style planets, although totally
different in composition and density, seem to have similar surfaces and
similar histories. What would have happened to Mercury if it had been
made of ice, water, and rock as Callisto is? Would it have evolved as
Callisto did?
[Illustration: One of the most spectacular of the Voyager 2 images
was obtained from inside the shadow of Jupiter. Looking back toward
the planet and the rings with its wide-angle camera, the spacecraft
took these photos on July 10 from a distance of 1.5 million
kilometers. The ribbon-like nature of the rings is clearly shown.
The planet is outlined by sunlight scattered from a haze layer high
in the atmosphere. On each side, the arms of the ring curving back
toward the spacecraft are cut off by the planet’s shadow as they
approach the brightly outlined disk. [P-21774B/W]]
[Illustration: The rings of Jupiter proved to be unexpectedly bright
when seen with the Sun nearly behind them. Strong forward scattering
of sunlight is characteristic of small particles. These two views
were obtained by Voyager 2 on July 10 from a perspective inside the
shadow of Jupiter. The distance of the spacecraft from the rings was
about 1.5 million kilometers. Although the resolution has been
degraded by camera motion during the time exposures, these images
reveal that the rings have some radial structure. [260-610B/W and
260-674]]
[Illustration: Rings of Jupiter.]
HIGHLIGHTS OF THE VOYAGER 2 SCIENTIFIC FINDINGS[3]
Atmosphere
The main atmospheric jet streams were present during both Voyager
encounters, with some changes in velocity.
The Great Red Spot, the white ovals, and the smaller white spots at
41°S, appear to be meteorologically similar.
The formation of a structure east of the Great Red Spot created a
barrier to the flow of small spots which earlier were circulating about
the Great Red Spot.
The ethane to acetylene abundance ratio in the upper atmosphere appears
to be larger in the polar regions than at lower latitudes and appears to
be 1.7 times higher on Voyager 2 than on Voyager 1.
An ultraviolet map of Jupiter shows the distribution of absorbing haze.
The polar regions are surprisingly dark, suggesting that the absorbing
material must be at high altitudes.
Equatorial ultraviolet emissions indicate planet-wide precipitation of
charged particles into the atmosphere from the magnetosphere.
The high-latitude ultraviolet auroral activity is due to charged
particles that originate in the Io torus.
Satellites and Ring System
The ring consists of a bright, narrow segment surrounded by a broader,
dimmer segment, with a total width of about 5800 kilometers.
The interior of the ring is filled with much fainter material that may
extend down to the top of the atmosphere.
Images of Amalthea in silhouette against Jupiter indicate that the
satellite may be faceted or diamond shaped.
Volcanic activity on Io changed somewhat, with six of the plumes
observed by Voyager 1 still erupting.
The largest Voyager 1 plume (Pele) had ceased, while the dimensions of
another plume (Loki) had increased by 50 percent.
Several large-scale changes in Io’s appearance had occurred, consistent
with surface deposition rates calculated for the large eruptions.
Europa is remarkably smooth with very few craters. The surface consists
primarily of uniformly bright terrain crossed by linear markings and
very low ridges.
There are four basic terrain types on Ganymede, including younger,
smooth terrain and a rugged impact basin first observed by Voyager 2.
Callisto’s entire surface is densely cratered and is likely to be
several billion years old.
Equatorial surface temperatures on the Galilean satellites range from 80
K (night) to 155 K (the subsolar point on Callisto).
Magnetosphere
The outer region of the magnetosphere contains a hot plasma consisting
primarily of hydrogen, oxygen, and sulfur ions.
The hot plasma generally flows in the corotation direction out to the
boundary of the magnetosphere.
Beyond about 160 R_J, the hot plasma streams nearly antisunward.
Outbound the spacecraft experienced multiple magnetopause crossings
between 204 R_J and 215 R_J.
The abundance of oxygen and sulfur relative to helium at high energy
increases with decreasing distance from Jupiter.
Measurements of high energy oxygen suggest that these nuclei are
diffusing inward toward Jupiter.
The ultraviolet emission from the Io plasma torus was twice as bright as
four months earlier and the temperature had decreased by 30 percent to
60 000 K.
The low-frequency (kilometric) radio emissions from Jupiter have a
strong latitude dependence and often contain narrowband emissions that
drift to lower or higher frequencies with time.
A complex magnetospheric interaction with Ganymede was observed in the
magnetic field, plasma, and energetic particles up to about 200 000
kilometers from the satellite.
[3]Adapted from a summary prepared by E. C. Stone and A. L. Lane for the
Voyager 2 Thirty-Day Report.
[Illustration: A new inner satellite of Jupiter, provisionally
designated 1979J1, was discovered by David Jewitt and Ed Danielson
of Caltech in these Voyager 2 ring photographs.]
[Illustration: In a 15-second exposure with the wide-angle camera,
the edge-on ring shows as a faint line, and the satellite is the dot
indicated by the arrow. [260-807]]
[Illustration: In a narrow angle 96-second exposure, the motion of
the satellite can be seen. Again, the faint band is the ring,
blurred by camera motion, and the arrow indicates the streak due to
the satellite. A star streak is located above and to the left of the
satellite; note that the length and angle of the two trails are
different, owing to satellite motion. [P-22172]]
A New Satellite
One of the most fascinating discoveries of Voyager 2 was not recognized
at first. Graduate student David Jewitt of the California Institute of
Technology, working with Imaging Team member Ed Danielson, began a
detailed analysis of all the ring photos in late summer. In early
October he determined that a short streak on a photo taken July 8,
previously presumed to be an image of a star trailed by the time
exposure, did not correspond to any known star position. Perhaps this
was a new satellite! Additional sleuthing turned up a second image of
the same part of the ring that also showed the anomalous object,
together with trails due to known stars. The differing angles and
lengths of the trails of the object and the stars confirmed that this
was indeed a 14th satellite of Jupiter. Following the guidelines of the
International Astronomical Union, it was designated 1979J1, pending
later assignment of a mythological name. The proposed name is Adrastea,
a nymph who nursed the infant Zeus in Greek legend.
The newly discovered satellite orbits Jupiter at a distance of 58 000
kilometers above the equatorial cloud tops, placing it just at the outer
edge of the ring and much closer to the planet than is Amalthea,
previously thought to be the innermost satellite. It travels at 30
kilometers per second (nearly 70 000 miles per hour), circling Jupiter
in just seven hours and eight minutes. From its brightness, scientists
guessed that it might be 30-40 kilometers in diameter.
The proximity of Adrastea to the ring suggests a relationship between
the two. When the discovery was announced to the press in mid-October,
it was speculated that the ring material might originate on the
satellite, perhaps eroded away by the energetic charged particles in the
inner Jovian magnetosphere. Once again, Voyager had added to our
perspective on planetary processes, suggesting that undiscovered but
similar small satellites might also be associated with the rings of
Saturn and Uranus.
Voyager 2 had certainly added a few years’ of data of its own to Voyager
1’s “ten years’ worth of data.” It had given a different view of the
Jovian system, helping to solve some of the mystery surrounding Jupiter
and its satellites, and creating new mysteries. As Voyager 2 sped out
away from Jupiter, riding along the giant planet’s huge magnetotail,
attention turned to Saturn: What would Pioneer 11, the Pathfinder,
discover in September 1979? What would the Voyagers learn in November
1980 and August 1981? Would all go well? Would Voyager 2 fly on to
Uranus?
There was also a yearning to examine more closely, with the Galileo
Project, what had been unknown for so long, yet had become so familiar
in only a few months’ time—the little dark, red “potato” Amalthea, the
volcano-covered world Io, the mysterious “cracked billiard ball” Europa,
cratered and groovy Ganymede, ancient Callisto, and the king of the
planets itself, a colorful, banded world of stable climate and
ever-changing weather patterns.
[Illustration: A fifteenth satellite of Jupiter was discovered in
the spring of 1980 by Steven Synnott of JPL. It was first seen on
this Voyager 1 image taken March 5, 1979, in which the
75-kilometer-diameter satellite shows as a dark oval against the
planet. Also visible is the shadow of the satellite, designated
1979J2. This satellite orbits between Io and Amalthea with a period
of 16 hours and 11 minutes. [P-22580B/W]]
[Illustration: The Jupiter seen by the Voyager cameras is a
cloud-belted world of rapid jet streams and complex cloud forms.
Prominent in this Voyager 1 image, taken February 5 at a range of
28.4 million kilometers, is the alternating structure of light zones
and dark belts, and the Great Red Spot and numerous smaller spots.
Also easily visible are the two inner Galilean satellites, Io and
Europa. The resolution in this picture is 500 kilometers, about five
times better than can be obtained from Earth-based telescopes.
Callisto can be faintly seen at the lower left. [P-21083C]]