THE FIRST ENCOUNTER
The Giant Is Full of Surprises
The Voyager 1 encounter took place at 4:42 a.m. PST, March 5, 1979.
About six hours before, while the spacecraft continued to hurtle on
toward Jupiter, overflow crowds had poured out of Beckman Auditorium on
the campus of the California Institute of Technology. There had been a
symposium entitled Jupiter and the Mind of Man. More than one face that
evening had turned to look toward the Pasadena night, for there,
glittering against the fabric of the sky, was the “star” of the show—a
planet so huge that the Earth would be but a blemish on its Great Red
Spot. One might have tried to imagine Voyager 1, large by spacecraft
standards, rapidly closing in on Jupiter—a pesky, investigative
“mosquito” buzzing around the Jovian system.
Meanwhile, at JPL, pictures taken by Voyager 1 were flashing back every
48 seconds, pictures that were already revealing more and more of
Jupiter’s atmosphere and would soon disclose four new worlds—the
Galilean satellites. The encounter was almost at hand.
4:42 a.m.—Voyager 1 had made its closest approach to Jupiter 37 minutes
earlier. Only now were the signals that had been sent out from the
spacecraft at 4:05 a.m. reaching the Earth.
But the moment of closest encounter did not have the same impact a
landing would have had. Although the champagne would flow later for
those who had worked so long and hard on this successful mission, there
was none now. In the press room there were just four coffeepots working
overtime to help keep the press alert. In the science areas, focus had
shifted to the satellite encounters, which would stretch over the next
24 hours; meanwhile, a few persons tried to catch a little sleep at
their desks before the first closeups of Io came in. The instant of
encounter came ... and went ... with no screams, no New Year’s
noisemakers. All the excitement of the mission lay both behind ... and
ahead.
Thursday, February 22.
The encounter activity was kicked off in Washington, D.C., by a press
conference at NASA Headquarters. After introductory statements by NASA
and JPL officials, some of the scientific results from the observatory
and far encounter phases were presented.
Both the plasma wave and the planetary radio astronomy instruments had
detected very-low-frequency radio emission that generally increased in
intensity whenever either the north or south magnetic pole of Jupiter
tipped toward the spacecraft. The plasma wave experiment had also
detected radiation that seemed to come from a region either near or
beyond the orbit of Io—perhaps even as far as the outer magnetosphere.
In addition, Frederick L. Scarf, Principal Investigator for the plasma
wave instrument, released a recording of the ion sound waves created by
5 to 10 kilovolt energy protons traveling upstream from Jupiter.
James Warwick, Principal Investigator of the radio astronomy
investigation, enthusiastically reported the detection of a striking new
low-frequency radiation from Jupiter, at wavelengths of tens of
kilometers. Such radio waves cannot penetrate the ionosphere of the
Earth, and thus had never been detected before. Because of their huge
scale, these bursts probably did not originate at Jupiter itself, but in
magnetospheric regions above the planet—perhaps in association with the
high-density plasma torus associated with Io. Warwick commented that
“only from the magnificent perspective of the Voyager as it approaches
Jupiter have we been able to get this complete picture.”
[Illustration: The Voyager 1 encounter with Jupiter took place
during a little more than 48 hours, from the inbound to the outbound
crossing of the orbit of Callisto. This figure shows the spacecraft
path as it would be seen from above the north pole of Jupiter.
Closest approach to Jupiter was 350 000 kilometers. The close flybys
of Io, Ganymede, and Callisto all took place as the spacecraft was
outbound.]
Voyager 1 trajectory
Launch date = 9/5/77
Jupiter arrival date = 3/5/79
Satellite closest approach
Sun occultation
Earth occultation
Io
Ganymede
Periapsis
Callisto
Amalthea
Europa
Another unexpected result was announced by Lyle Broadfoot, Principal
Investigator for the ultraviolet spectrometer investigation. The
scientists had expected to find very weak ultraviolet emissions on the
sunlit side of Jupiter, caused by sunlight being scattered from hydrogen
and helium in Jupiter’s upper atmosphere. “Instead we are seeing a
spectacular auroral display. There are two features of the emission—the
auroral emission which comes from the planet and a second type of
emission which appears to come from a radiating torus or shell around
the planet at the orbit of Io. The spectral content of these two
radiating sources is distinctly different. What we find is that the
auroral emission from Jupiter’s atmosphere is so strong that it
completely dominates the emission spectrum even on the sunlit side of
the atmosphere.”
“Not since Mariner 4 carried its TV camera to Mars fifteen years ago
have we been less prepared—have we been less certain of what we are
about to see over the next two weeks,” said Bradford Smith, Imaging Team
Leader. He mentioned the time-lapse “rotation movie,” in which the
colorful planet spun through ten full Jupiter days; tiny images of the
satellites passed across Jupiter’s face as though being whipped along by
the rotation of the giant. A week or so earlier, when this film had been
shown for the first time to the full Imaging Team, it provided an
occasion for good-humored rivalry between planet people and satellite
people, with jokes about the satellites getting in the way of the
important studies of Jupiter. For the next few days, the imaging focus
remained on Jupiter; it shifted to Io, Ganymede, and Callisto, as each
was passed in turn after closest approach to the planet.
Tuesday, February 27.
(_Range to Jupiter, 7.1 million kilometers_). At a distance of 660
million kilometers from Earth, within 90 Jovian radii (R_J) of Jupiter,
Voyager 1 was prepared to begin the encounter with the planet’s
magnetosphere. On the previous day the spacecraft had crossed the point,
at 100 R_J, at which Pioneers 10 and 11 had found the bow shock, the
first indication of the magnetospheric boundary. The start of Voyager’s
plunge into the Jovian magnetosphere was overdue, and scientists
anxiously watched the data from the particles and fields instruments,
looking for the first indication of disordered magnetic fields and
altered particle densities that would mark the bow shock. Apparently,
higher solar wind pressure, associated with increased solar activity
since 1974, had compressed the magnetosphere, but no one could predict
how strong this compression might be.
ENCOUNTER DISTANCES FOR VOYAGER 1
Object Range to Center at Closest Best Image Resolution
Approach (kilometers) (km per line pair)
Jupiter 349 000 8
Amalthea 417 000 8
Io 21 000 1
Europa 734 000 33
Ganymede 115 000 2
Callisto 126 000 2
For the first time since its discovery, Lyle Broadfoot and his UVS
colleagues suggested a probable identification for the unexpected
ultraviolet emission near the orbit of Io. The most likely candidate was
sulfur atoms with two electrons removed (S III), at an inferred
temperature perhaps as high as 200 000 K. An additional indication of
sulfur came from Mike Krimigis, who reported that the low-energy charged
particles instrument had detected bursts of sulfur ions streaming away
from Jupiter that had apparently escaped from the inner magnetosphere.
No explanations were offered, however, for the presence of large amounts
of this element.
At JPL, a press room had been opened in Von Karman Auditorium to
accommodate the hundred or so reporters expected to arrive. To keep all
the interested people informed of Voyager progress, frequent television
reports were beamed over closed-circuit TV throughout JPL. From an
in-lab television studio called the Blue Room, JPL scientist Al Hibbs,
who had played a similar role during the Viking Mission to Mars,
provided hourly reports and interviewed members of the Voyager teams. As
the pressure for constant commentary and instant analysis increased,
Garry Hunt of the Imaging Team was also called on to host activities in
the Blue Room, where his British accent added an additional touch of
class to the operation.
As the encounter progressed, the JPL television reports reached a wider
audience. In the Los Angeles area, KCET Public Television began a
nightly “Jupiter Watch” program, with Dr. Hibbs as host. During the
encounter days, service was extended to interested public television
stations throughout the nation. In this way, tens of thousands of
persons were able to experience the thrill of discovery, seeing the
closeup pictures of Jupiter and its satellites at the same moment as the
scientists at JPL saw them, and listening to the excited and frequently
awestruck commentary as the first tentative interpretations were
attempted. Unfortunately, the commercial television networks did not
make use of this opportunity, and the greatest coverage available to
most of the country was a 90-second commentary on the nightly network
news.
Wednesday, February 28.
(_Range to Jupiter, 5.9 million kilometers_). At 6:33 a.m., at a range
of 86 R_J, Voyager 1 finally reached Jupiter’s bow shock. But by 12:28
p.m. the solar wind had pushed the magnetosphere back toward Jupiter,
and Voyager was once more outside, back in the solar wind. Not until
March 2, at a distance of less than 45 R_J, would the spacecraft enter
the magnetosphere for the final time.
At 11 a.m. the first daily briefing to the press was given. “After
nearly two months of atmospheric imaging and perhaps a week or two of
satellite viewing, [we’re] happily bewildered,” said Brad Smith. The
Jovian atmosphere is “where our greatest state of confusion seems to
exist right at the moment, although over the next several days we may
find that some of our smirking geology friends will find themselves in a
similar state. I think, for the most part, we have to say that the
existing atmospheric circulation models have all been shot to hell by
Voyager. Although these models can still explain some of the coarse
zonal flow patterns, they fail entirely in explaining the detailed
behavior that Voyager is now revealing.” It was thought, from Pioneer
results, that Jupiter’s atmosphere showed primarily horizontal or zonal
flow near the equatorial region, but that the zonal flow pattern broke
down at high latitudes. But Voyager found that “zonal flow exists
poleward as far as we can see.”
Smith also showed a time-lapse movie of Jupiter assembled from images
obtained during the month of January. Once each rotation, approximately
every ten hours, a color picture had been taken. Viewed consecutively,
these frames displayed the complex cloud motions on a single hemisphere
of Jupiter, as they would be seen from a fixed point above the equator
of the planet. The film revealed that clouds move around the Great Red
Spot in a period of about six days, at speeds of perhaps 100 meters per
second. The Great Red Spot, as well as many of the smaller spots that
dot the planet, appeared to be rotating anticyclonically. Anticyclonic
motion is characteristic of high-pressure regions, unlike terrestrial
storms. Smith noted that “Jupiter is far more complex in its atmospheric
motions than we had ever imagined. We are seeing a much more complicated
flow of cyclonic and anticyclonic vorticity, circulation. We see
currents which flow along and seem to interact with an obstacle and turn
around and flow back.” There is a Jovian jet stream that is “moving
along at well over 100 meters per second. Several of these curious
little dark features that appear to be small brown spots near Jupiter’s
north temperate region have been seen to overtake one another and gobble
each other up. And then they occasionally spit out a piece here and
there as they move along.”
Thursday, March 1.
(_Range to Jupiter, 4.8 million kilometers_). At 5 a.m., at a distance
of 71 R_J, Voyager crossed the bow shock for the third time, catching up
with the contracting magnetosphere of the planet. About noon, at 66 R_J,
the spacecraft finally reached the boundary of the magnetosphere, called
the magnetopause. Herbert Bridge, the plasma instrument Principal
Investigator, noted that the solar wind pressure as monitored by Voyager
2, still between the Sun and Jupiter, had been for several days from two
to five times greater than its level during the Pioneer 10 and 11
encounters. Presumably, this high pressure was the cause of the
compressed state of the magnetosphere. However, in the previous few
hours the solar pressure had dropped, so Bridge anticipated that the
Jovian magnetosphere might soon inflate and expand outward.
[Illustration: The southern hemisphere of Jupiter presents a
tremendous diversity of atmospheric structure and motion. The Great
Red Spot rotates counterclockwise in about six days; above and below
it high-speed jet streams flow to the right and the left, while a
complex, dynamic cloud pattern develops in its wake. This picture
was taken on February 25, when Voyager 1 was 9 million kilometers
from the planet. [P-21151C]]
Fred Scarf intrigued the press with a tape of the sounds made by
high-energy protons coming upstream from Jupiter. The plasma wave
instrument had recorded the noise of the protons, mixed with the noise
of the spacecraft itself, producing sound effects that sounded somewhat
like a mixture of singing whales, a Nor’easter, and the Daytona 500.
At the press briefing, interest in the imaging results began to shift
from Jupiter toward the satellites. Pictures of each of the four big
Galilean moons revealed bright and dark features as small as about 200
kilometers across. Unfortunately, this resolution is not enough to be
diagnostic—the spots cannot be interpreted in terms of recognizable
geological features, such as mountains or craters. Today one could only
speculate, but tomorrow or the next day the answers would begin to come
in. Deputy Imaging Team Leader Larry Soderblom conveyed his excitement
through a metaphor that would be repeated many times during the next
week: “We’re beginning a stage in this mission which represents, I
think, one of the most exciting points in man’s scientific exploration
of the solar system—in the next few days, we’ll explore four new
worlds,” seeing in a few days’ time what it took us centuries to learn
about other worlds in our solar system. In terms of our experience with
the exploration of Mars, “it is about 1700 AD this morning, tomorrow it
will be about 1800, and it will be about 1976 [the year of Viking] by
Tuesday evening.”
Friday, March 2.
(_Range to Jupiter, 3.6 million kilometers_). Early in the morning, a
twelvefold increase in solar wind pressure caused another contraction of
the magnetosphere, which was behaving like a spring, compressing in
response to outside forces. As the magnetopause boundary moved rapidly
inward, it crossed the spacecraft at 59 R_J from the planet. An hour
later the bow shock also flashed past, and Voyager was once more in the
interplanetary medium. By noon reinflation of the magnetosphere began
again; Voyager crossed the bow shock for the fifth and final time at 55
R_J, followed by three more magnetopause crossings, as the
magnetospheric boundary flopped in and out between 45 R_J and 50 R_J.
[Illustration: A few days before encounter, the Voyager images of
the larger Galilean satellites, Callisto and Ganymede, were
beginning to show distinctive surfaces with many bright spots. These
two pictures were taken on March 5 at a range of 8 million
kilometers; the resolution is about 100 kilometers. Although
extremely tantalizing, these images were uninterpretable, because
the spots could not be associated with any recognizable geological
features, such as mountains or craters. Like a naked-eye view of the
Moon, these pictures seemed to reveal more than was actually
meaningful. [P-21188C and P-21150C]]
[Illustration: Ganymede]
Some Project officials began to worry about the contracted state of the
magnetosphere. The radiation “hardening” of Voyager was carried out to
protect against the energetic particle fluxes observed by Pioneers 10
and 11. Under the new conditions, would the particles in the inner
magnetosphere be more concentrated and perhaps increase the radiation
levels beyond the design limits? Scientists asked how long the
compression might last and speculated about how much energy might be
pumped into the Jovian radiation belts, but only time could provide the
answers.
VOYAGER 1 BOW SHOCK (S) AND MAGNETOPAUSE (M) CROSSINGS
Boundary Day Distance (R_J)
_Inbound_
S 2/28 86
S 2/28 82
S 3/01 72
M 3/01 67
M 3/02 59
S 3/02 58
S 3/02 56
M 3/03 47
_Outbound_
M 3/13 158
M 3/13 163
M 3/13 165
S 3/16 199
S 3/18 227
S 3/18 227
S 3/19 240
S 3/20 256
S 3/20 258
Accurate recording of the X-band data being sent from Voyager at 115
thousand bits per second required fairly clear weather at the tracking
sites. The three Deep Space Network (DSN) stations in California, Spain,
and Australia are located at normally dry sites. However, early in the
morning, heavy rain at the Australian site interfered with reception of
the Voyager signal for fourteen minutes. Fortunately, the loss occurred
when the DSN tracking of the spacecraft was being switched from
Goldstone, California, to the Australian station. Mission control was
able to extend Goldstone coverage for several minutes so that only about
three minutes’ worth of data was lost completely.
A more positive announcement was made at the press conference that
morning by Donald Shemansky of the Ultraviolet Spectroscopy Team: the
discovery of a high-energy torus of doubly ionized sulfur (S III)
circling Jupiter in the region of Io’s orbit. “We were surprised out of
our chairs to see a spectacularly bright emission in the 650-1100
angstrom region, immediately implying that we were looking at a plasma
that had to be at a temperature of about 100 000 degrees.” The
scientists estimated that the density of this torus must be at least 500
ions per cubic centimeter, and that the power needed to keep this plasma
at such a high temperature must be in the neighborhood of 500 billion
watts. As Al Hibbs mentioned on “Jupiter Watch” that night, 500 billion
watts of power is the total amount of installed generating capacity of
the United States.
Saturday, March 3.
(_Range to Jupiter, 2.5 million kilometers_). Early in the morning the
final crossing of the magnetopause took place at 47 R_J, and Voyager
finally joined the Jovian system. At the end of the day, the rapidly
moving spacecraft crossed the orbit of Callisto, but that satellite
itself was on the far side of the planet. Not until the outbound
crossing of its orbit on March 6 would closeup views of Callisto be
obtained.
During the preceding night, a severe summer thunderstorm in Australia
again caused a loss of data. A line of storms over the tracking station
blocked the high-rate, X-band science data for three hours and twenty
minutes. Attempts were made to save a part of the data by commanding the
spacecraft to slow its transmission rate, rather like speaking slowly to
a partially deaf listener. But by the time the craft received the signal
and responded, the storms had intensified, and no signal could get
through. Closeups of the Great Red Spot and an extended series of
observations of the glowing sodium cloud around Io were lost.
A black-and-white movie assembled from photos obtained in January and
February was shown at the press conference. The movie was taken from
observatory phase pictures photographed in blue light. From these
pictures, the imaging team “put together this so-called ‘blue movie’,”
said Brad Smith, introducing the film. The film showed changes taking
place in Jupiter’s atmosphere over a period of about seventy Jovian
days. Near the equator there were “bright plumes floating by—at high
speed the plumes seem to wave around, something like a flag waving in
the breeze.” Farther north, along the edge of the north temperate zone,
one could see one of the dark ovals—“a rather fuzzy one would move up,
catch up with the one just ahead of it, get stuck to the outside and
roll around on it for a while, then get ejected a little later.” Dr.
Smith also showed new closeup views of the region around the Red Spot,
showing not only the anticyclonic features but filamentary
“spaghettilike” material which was rotating in a cyclonic direction,
indicating a low-pressure region. “The filamentary material still seems
to be rather a mystery—very difficult to see the details of the motion.”
But the photographs were already showing what seemed to be stream lines
in the white ovals. “In appearance the white ovals seem to resemble the
Red Spot. The stream lines are at least suggestive of divergent flow,
that is, material in each of these spots which is upwelling in these
areas and then moving out tends to go around in a counterclockwise
anticyclonic motion but may, at the same time, be slowly diverging
outward.”
[Illustration: At a resolution of about 100 kilometers a planetary
surface just begins to reveal its personality. Io was photographed
against the disk of Jupiter on February 26, from a distance of about
8 million kilometers. Such early pictures whetted the appetites of
the Voyager scientists at JPL as they anxiously speculated about
what they would find in closer views of the surface of this
remarkable satellite. [P-21185 B/W]]
The large white ovals are about forty years old. Dr. Smith explained how
they formed: Between 1939 and 1940, “where those three white spots exist
right now, was a rather bright band similar to the north temperate zone
we see on Jupiter right now. In that time period of a year or so, three
darkish spots formed. A dark cloud spread out at each one of those three
locations and just kept spreading longitudinally until the white
material condensed between them.” In the end, everything was dark except
for the three ovals, which have persisted ever since.
Torrence Johnson of the Imaging Team commented on a photo of Io in which
features resembling circular crater-type structures seemed to be
visible. “Whatever they are, [those circular features] are approaching
the size of the things that we would call basins if they were impact
structures on other planets. We don’t really know whether they’re impact
structures. They have some characteristics that look reminiscent of
impact structures. They could be endogenic—volcanic in origin—or
internally generated in some other way.” Johnson also showed another
photograph of Io, this one taken against black sky, showing a
“strikingly different face” looking, perhaps, like someone’s nightmare,
glaring back at the intruder from Earth. One huge feature—a “bullseye”
or “hoof print” on Io—appeared to be approximately 1000 kilometers long.
No one had ever seen such a feature, and the imaging scientists could
only speculate about its significance.
[Illustration: Small-scale structures in the jet streams of
Jupiter’s north tropical zone reveal details of atmospheric
circulation. The small dark oval near the right edge of the zone may
offer a glimpse deep into Jupiter’s atmosphere. Between the
regularly spaced dark ovals near the bottom of the frame are more
small-scale features that are being studied for their roles in
Jovian atmospheric activity. The blue-gray regions along the shear
line between the equatorial zone and the north equatorial belt also
appear to be windows into the deeper regions of the atmosphere. This
photo was taken February 19 by Voyager 1 from a distance of 14
million kilometers. [P-21160C]]
A few days earlier, someone had posted in the Imaging Team area a quote
from a 1975 review paper on the Jovian satellites by David Morrison and
Joseph Burns. The section on Io began, “Io is one of the most intriguing
objects in the solar system.” This statement seemed more and more
appropriate as Voyager images improved. Johnson referred to this day as
equivalent to the “late 1960s” in our study of the Jovian satellites.
“We can see much more clearly than ever before, but still not clearly
enough to provide understanding of what we are seeing.”
Sunday, March 4.
(_Range to Jupiter, 1.2 million kilometers_). At 4:37 a.m., the near
encounter phase began: Voyager 1 was almost there! In the press room,
someone taped a fortune cookie message to the Voyager TV monitor: “There
is a prospect of a thrilling time ahead for you.”
Pulled by the powerful gravity of Jupiter, the spacecraft was now on a
curved path through the inner Jovian system. At 2 p.m. PST, it crossed
the orbit of Ganymede, and later in the afternoon it passed within less
than 2 million kilometers of Europa, providing Voyager 1’s closest look
at this satellite. During the afternoon and evening, a number of views
were obtained of Amalthea, the small inner satellite, at a range of less
than 500 000 kilometers. At 8 p.m. the orbit of Europa was crossed, and
increasing attention was drawn to the coming encounter with Io. At about
7 p.m. a full-frame color sequence of Io was received with a resolution
of 16 kilometers. During the night, as Imaging Team members scratched
their heads trying to prepare a press release caption to interpret the
peculiar structures seen, the JPL Image Processing Lab rushed to prepare
a color version for release the next day.
[Illustration: The Great Red Spot became more and more spectacular
as Voyager 1 approached, with each day revealing new and intricate
detail in the clouds. This view was obtained on March 1 at a
distance of 5 million kilometers; the smallest features that can be
made out are about 100 kilometers across. To the west of the Great
Red Spot is a region of great turbulence, and to the south is one of
the three white ovals. [P-21182C]]
[Illustration: Large brown ovals in the northern hemisphere of
Jupiter are apparently regions in which an opening in the upper,
ammonia clouds reveals darker regions below. This oval, about the
same length as the diameter of the Earth, was at latitude 15°N.
Features of this sort are not rare on Jupiter and have an average
lifetime of one to two years. Above the feature is the pale orange
north temperate belt, bounded on the south by the high-speed north
temperate current, with winds of 120 meters per second. The range to
Jupiter at the time this photograph was obtained on March 2 was 4
million kilometers, with the smallest resolvable features being 75
kilometers across. [P-21194C]]
[Illustration: The best Voyager 1 photos of Europa were obtained on
March 4 from a distance of about 2 million kilometers. This view of
the hemisphere centered at about 300° longitude has a resolution of
about 40 kilometers. Most of the surface is bland and highly
reflective, being composed almost entirely of water-ice. No craters
resulting from meteoric impacts can be seen. The most striking
visual features, which set Europa off from the other satellites, are
the dark streaks, as much as several thousand kilometers long, that
cross the surface. Just barely visible to Voyager 1, these streaks
were dramatically apparent to Voyager 2 in its closer flyby in July.
[P-21208C]]
Once again, tracking station problems interrupted the smooth flow of
data from the spacecraft. Failure at Madrid to reestablish the correct
receiver frequency after a spacecraft maneuver resulted in the loss of
53 minutes of irreplaceable data; an additional eleven minutes were lost
an hour later. As the first problem was being corrected, the tracking
antenna became misaligned, resulting in a noisy data return. Meanwhile,
only a few of the watchers at JPL mourned the lost data, so exciting
were the other new results that kept pouring in.
“For the highlights of this morning Dr. Soderblom will come up and show
some beautiful satellite pictures,” Brad Smith announced at the daily
press briefing. Larry Soderblom’s excitement was hard to contain. “Today
is probably going to turn out to be one of the most memorable days in
our exploration program. For the planetary geologists it’s truly
Christmas Eve. We see tonight the beginning of the exploration of four
new worlds. We’re racing through time and space at an incredible
rate—the rate at which we are learning things is awe-inspiring in
itself.” Callisto was still too far away to see well “but the things
we’re seeing on the closer three satellites have really got us charged
in anticipation.” Ganymede—“What can I say? Loops and swirls and
incredible patches that are difficult now to hazard a guess about.”
Io—an eerie-looking red, orange, and yellow world—“this one we’ve got
all figured out. [Laughter from the press and applause.] It is covered
with thin candy shells of anything from sulfates and sulfur and salts to
all kinds of strange things.”
The low energy charged particle instrument had discovered high-speed
sulfur in the outer Jovian magnetosphere, ten times as far from the
planet as the sulfur torus around Io. The high-speed sulfur, which
whizzed by Voyager 1 at 8000 kilometers per second, was first detected
as the spacecraft crossed the magnetopause and entered the
magnetosphere. Apparently this sulfur had picked up speed as it moved
outward from Io’s torus, but the mechanism for this acceleration was not
known. Roby Vogt reported that the cosmic ray instrument was detecting
two distinct groups of atoms closer in to the planet. One group was
apparently of solar composition (derived from the solar wind), and the
other showed enhanced oxygen as well as sulfur. The plasma instrument
had also begun to measure sulfur in the same form (S III) as that
detected by the UVS in the Io torus. Where, the scientists asked, could
the oxygen and sulfur be coming from? Could Io be the source of these
atoms?
At one point, as questions from the press became too detailed, asking
for “instant analyses” of the data, Dr. Smith was prompted to make the
following statement: “I want to say something about what’s going on
right now in the imaging area because we get a lot of pointed questions.
We have a remarkably good system that is getting extremely good
photographs. Not only are the cameras working well, but Jupiter and the
satellites are cooperating by showing a lot of truly remarkable detail.
And it may sound unprofessional, but a lot of the people up in the
Imaging Team area are just standing around with their mouths hanging
open watching the pictures come in, and you don’t like to tear yourself
away to go and start looking at numbers on a printout. We will do that,
but in the meantime we’re just caught up in the excitement of what’s
going on.”
While the press briefing was underway, between 11:38 and 11:50 a.m., a
special experiment was being tried on board the Voyager. As the
spacecraft passed through the plane of the equator of Jupiter, it aimed
its narrow-angle camera to a point in space halfway between the cloud
tops and the orbit of Amalthea and took a single, eleven-minute
exposure. The purpose: to search for a possible faint ring around
Jupiter, which, if present, could best be seen by sighting along the
plane of the equator. The image appeared briefly on the TV monitors,
clearly showing something—a strange band of light—streaking across the
center of the frame. Up in the imaging area, analyses began at once to
determine if the strange band were really the sought-for ring, but it
would not be until March 7 that the identification would be confirmed
and the discovery announced.
[Illustration: The “bullseye” of Io was first photographed at a
range of 2.8 million kilometers on March 3, and this color version
was released on March 4. At the time, Imaging Team scientists wrote
that “The large heart-shaped feature with a dark spot near its
center could be Io’s equivalent of an impact basin such as Mare
Orientale on the Moon. Its outer dimensions are about 800 by 1000
kilometers. Subsequent high-resolution coverage should reveal
whether the small dark spots are impact craters, or perhaps
something more exotic such as volcanoes. The reddish color of Io has
been attributed to sulfur in the salts which are believed by some to
make up the surface of Io.” It would be another week before this
feature, later named “Pele” for the Hawaiian volcano goddess, would
be recognized as an active, erupting volcano. [P-21187C]]
Monday, March 5.
(_Encounter Day. Minimum range to Jupiter, 780 000 kilometers; speed of
spacecraft, almost 100 000 kilometers per hour_). Many celebrities,
including the Governor of California, spent the night at JPL to witness
the historic occasion. In Washington, D.C., a special TV monitor was set
up in the White House for the President and his family.
As late Sunday night eased into the early morning of encounter, closeup
images of Jupiter, looking more like abstract art than like planetary
science, flashed across the TV screens, and verbal images far less wild
than the visuals from Jupiter were heard from commentators and from
members of the press. “Are you sure Van Gogh didn’t paint that?” “That’s
not Jupiter; it looks like a closeup of a salad.” “They’re not showing
us Jupiter, that’s some medical school anatomy slide.”
Shortly before closest approach to Jupiter, Voyager began its intensive
observations of Io. Much of this information, taken while the Australian
station was tracking the spacecraft, was recorded on Voyager’s onboard
tape-recorder for playback later that day. But even before the results
of that imaging were known, Larry Soderblom was calling Io “one of the
most spectacular bodies in the solar system.” As more and more vivid
photos of Io appeared on the monitors, members of the Imaging Team in
the Blue Room buzzed with excitement. “This is incredible.” “The element
of surprise is coming up in every one of these frames.” “I knew it would
be wild from what we saw on approach but to anticipate anything like
this would have required some sort of heavenly perspective. I think this
is incredible.”
At 7:35 a.m. Voyager was scheduled to pass through the flux tube of Io,
the region in which tremendous electric currents were calculated to be
flowing back and forth between the satellite and Jupiter. Norm Ness
suggested, after examining magnetometer data, that Voyager skirted the
edge of the flux tube, and that the current in the tube was about one
million amps. As the flux tube results were received, champagne bottles
began to pop in the particles and fields science offices, in celebration
of the successful passage through the inner magnetosphere. Meanwhile, at
7:47 a.m., closest approach to Io occurred, at a range of only 22 000
kilometers. Voyager was 25 000 times closer to this satellite than were
the watchers on Earth.
At 8:14 a.m., while still within 30 000 kilometers of Io, the spacecraft
passed out of sight behind the edge of Jupiter. All scientific data for
the next two hours and six minutes were stored on the onboard tape
recorder for later transmission to Earth. Meanwhile, the radio
communication signal was used to probe the atmosphere of Jupiter,
yielding a profile of electron density in the ionosphere and of the gas
pressure and temperature in the upper atmosphere. While out of sight
from Earth, at 9:07 a.m., Voyager plunged into the shadow of Jupiter. As
the Sun set on the spacecraft, the ultraviolet instrument used the
absorption of sunlight to determine the composition and temperature of
the upper atmosphere. In the darkness, the infrared IRIS measured the
night-side temperatures of the planet, and long-exposure images were
taken to search for aurora, lightning, and fireballs in the Jovian
atmosphere. At 10:20 a.m., Voyager reappeared from behind Jupiter and
radio contact was restored; at 11:24 a.m., it emerged from shadow into
sunlight, speeding on toward encounter with Ganymede.
At 8 a.m. a special press conference was held to mark the successful
Jupiter flyby. Noel Hinners, Associate Administrator for Space Science
and the highest ranking NASA official present, congratulated all those
who had made the Voyager Mission a success. The encounter was the
“culmination of a fantastic amount of dedication and effort. The result
is a spectacular feat of technology and a beginning of a new era of
science for the solar system. Just watching the data come in has been
fantastic. I had a fear that things on the satellites were going to look
like the lunar highlands. Nature wins again. If we’re going to see
exploration of this nature occurring in the 1980s and 1990s we must
continue to expound the results of what we’re finding here, the role of
exploration in the history of our country, what it means to us as a
vigorous national society.”
As time passed, it became apparent that Voyager 1 had been affected by
Jupiter’s radiation environment. The basic timing—the main clock on the
spacecraft—had slowed down. First it slowed by 6.3 seconds, but by March
6 it was found to have slowed a total of eight seconds. In addition, the
two central computers apparently got out of synchronization both with
themselves and with the flight data subsystem. On March 6 it was
reported that the spacecraft cameras were shuttering one frametime (48
seconds) early; this was partly offset by the eight-second spacecraft
“masterclock” slow-down resulting in images (according to our clocks)
being photographed forty seconds early. This timing error resulted in
the camera taking some pictures while the scan platform was moving,
causing some blurred images. A number of the highest resolution images
of Io and Ganymede were seriously degraded by this malfunction.
[Illustration: When the first color close-up of Io was released,
Imaging Team Leader Brad Smith said that he had seen “better looking
pizzas”; hence this view, taken March 4 at a range of about 860 000
kilometers, became known as “the pizza picture.” The circular
feature in the center (the piece of pepperoni) was later revealed to
be the active volcano Prometheus, but at the time of its release
this lovely but bizarre picture baffled scientists and press alike.
[P-21457C]]
At the regular 11 a.m. press briefing, Brad Smith glowed. “We’re all
recovering from what I would call the most exciting, the most
fascinating, what may ultimately prove to be the most scientifically
rewarding mission in the unmanned space program. The Io pictures this
morning were truly spectacular and the atmosphere up in the imaging area
was punctuated by whoops of joy or amazement or both.” The new color
photo of Io taken the night before was released, showing strange surface
features in tones of yellow, orange, and white. The image defied
description; the Imaging Team used terms like “grotesque,” “diseased,”
“gross,” “bizarre.” Smith introduced the picture with the comment, “Io
looks better than a lot of pizzas I’ve seen.” Larry Soderblom added,
“Well, you may recall [that we] told you yesterday that when we flew by
we’d figure all this out. I hope you didn’t believe it.”
One thing was certain: There were no impact craters on Io. Unless the
satellites of Jupiter had somehow been shielded from the meteoric
impacts that cratered objects such as the Moon, Mars, and Mercury, the
absence of craters must indicate the presence of erosion or of internal
processes that destroy or cover up craters. Io did not look like a dead
planet. Imaging Team member Hal Masursky, looking at the “pizza”
picture, estimated that the surface of Io must be no more than 100
million years old—that is, some agent must have erased impact craters
during the last 100 million years. This interpretation depended on how
often cratering impacts occur on Io. No one could be sure that there had
been any interplanetary debris in the Jovian system to impact the
surfaces of the satellites. Perhaps none of them would be cratered. The
forthcoming flybys of Ganymede and Callisto would soon provide this
information.
[Illustration: The giant volcanic feature Pele, about 1000
kilometers across, mystified Voyager scientists when this picture of
Io was taken on March 5 from a distance of about 400 000 kilometers.
The brilliant colors, the strange shapes of the surface deposits,
and the absence of impact craters all testified that Io was unlike
any world previously encountered in the exploration of the solar
system. [P-21226C]]
[Illustration: As Voyager 1 approached Io, the images of the surface
became more and more spectacular. On the morning of March 5, at a
range of 130 000 kilometers, this picture was taken centered near
longitude 320° and latitude 10°S. The width of the picture is about
1000 kilometers (the distance from the Mexican border to the
northern edge of California). There are no impact craters,
signifying a geologically young surface, and the dark center with
radiating red flows indicates recent volcanic activity of some sort.
[P-21277C]]
The close flyby of Ganymede took place at 6:53 p.m., at a range of 115
000 kilometers. During the preceding four hours, photos revealed a
surface covered with impact craters. Watching these photos and supplying
commentary for the television listeners, David Morrison remarked, “While
I’m delighted to see craters, it’s just the opposite of what I would
have expected. I was telling everyone a few days ago that I thought Io
would have plenty of craters and that Ganymede, because of the ice
surface, simply would not be able to hold large craters over geological
time. So this is fascinating and this is confusing—both what has
happened on Io to erase the craters and why Ganymede’s surface is strong
enough to preserve them.”
Just before closest approach, at 6:35 p.m., the ultraviolet instrument
watched as a bright star passed behind Ganymede and reemerged ten
minutes later. No dimming that could be attributed to an atmosphere was
seen; when the data were analyzed later, scientists set an upper limit
for any gas on this satellite at one-billionth of the atmospheric
pressure at the surface of the Earth.
As encounter day drew to a close, celebrations took place all over JPL.
For many, however, the excitement was tempered by exhaustion. After 48
hours of intense activity, sleep was imperative for some. But the close
approach to Callisto was still to come, as was an examination of the
data already received.
[Illustration: After its close flyby of Io, Voyager 1 headed for
Ganymede, the largest of the Galilean satellites. This global view
of Ganymede, taken on March 4 at a range of 2.6 million kilometers,
shows features as small as 50 kilometers across. At the time,
Voyager scientists speculated that the numerous white spots were
impact craters, surrounded in some cases by icy ejecta blankets
splashed onto the surrounding surface. However, many narrow white
streaks, especially those in the lower left quadrant, promised new
and exciting geological features on this satellite. [P-21207C]]
Tuesday, March 6.
Voyager was now receding from Jupiter, accelerated on a new
trajectory—one that would speed it on toward its November 1980 encounter
with Saturn. But first came the encounter with Callisto, with closest
approach at 9:50 a.m., at a range of 126 000 kilometers. The satellite
was littered with craters and there appeared one huge “bullseye” pattern
that might have been the result of an impact. As the photos of Callisto
came in, Garry Hunt described the scene in the imaging area: “The
activity around the monitors now is quite incredible with people caught
breathless by the pictures coming in. Every satellite we’ve seen has
been a different world.” Asked how he felt about the images of Jupiter’s
atmosphere, he replied, “I’m absolutely delighted with what I’ve seen,
and I’m delighted Voyager 2 is not far behind since I’m convinced that
we’ll see yet another face of Jupiter by then. The weather will have
changed by July.”
At the morning press briefing, the wealth of new data began to be
revealed. “If Jupiter had ever posed for Monet, it would probably have
turned out like this,” said Brad Smith as he introduced some enhanced
(exaggerated) color images that had been specially processed to show
more detail in the Great Red Spot region. Indeed, these photographs did
look like paintings with Jupiter displaying some of its best abstract
art.
[Illustration: As Voyager 1 approached Ganymede on March 5, many
strange new surface features became visible. In this frame, taken
from a distance of 250 000 kilometers and showing features as small
as 5 kilometers across, three distinct types of terrain are seen:
polygons of old dark surface, extensive areas of lighter, younger
material, and brilliant white ejecta patterns (probably water-ice)
around fresh craters. The bright rays in the upper part of the
picture are 300-500 kilometers long; at the bottom are several
craters with only faint, muted ejecta patterns. [P-21262C]]
[Illustration: Voyager 1 found that the surface of Ganymede was
geologically very complex. This frame, taken March 5 from a range of
250 000 kilometers, shows a region about 1000 kilometers across
centered near longitude 0° and latitude 20°S. The surface displays
many craters, some (probably the younger ones) with bright ray
systems. Bright grooved bands traverse the surface in various
directions. One of these bands, running in a north-south direction
in the lower left of the picture, is offset along a white line that
may represent a fault. Ganymede is the only Galilean satellite to
show indications of such lateral offsets in the crust. [P-21266]]
The first picture of Amalthea was shown, revealing an elongated, dark,
reddish object about 265 kilometers long. Smith reported: “There is
actually some structure that one can see—a crater—a couple of bright
features for which we have no interpretation and some other evidence of
cratering. It doesn’t look like much, but after all, Amalthea has never
been seen as anything more than a point of light from the Earth, and, in
fact, there are very few people that have even seen it as a point of
light. I doubt that more than one astronomer out of a hundred has
actually seen Amalthea.”
Larry Soderblom introduced new photographs of the satellites. Io still
showed no craters, even at high resolution. The craters that “should”
have been on Io were on Ganymede and Callisto. Ganymede not only had
craters, it had fault lines as well. “There is transverse motion along
these faults. Things get offset, apparently, for hundreds of kilometers.
So it’s the first time we’ve seen major kinds of transverse motion on
the surface of another planet.” Ganymede, in effect, had shown evidence
of having its own icy version of the San Andreas Fault.
[Illustration: Amalthea was thought to be the innermost satellite of
Jupiter until Voyager 2 discovered tiny Adrastea (1979J1). The
Voyager 1 camera revealed Amalthea as an irregular dark reddish
object with dimensions of 270 × 160 kilometers. These three images
have resolutions of (b) 25 kilometers; (c) 13 kilometers; (d) 8
kilometers. [260-503]]
[Illustration: Observation geometry for Amalthea]
[Illustration: (b)]
[Illustration: (c)]
[Illustration: (d)]
Although Callisto was heavily covered with craters up to 200 kilometers
in diameter, Dr. Soderblom commented on the absence of larger impact
basins. Perhaps there was one in the huge bullseye feature; however, it
was not a “standard” looking basin like those on the Moon. Callisto was
“extremely smooth and free of any relief. The structure [the bullseye],
if impact caused, shows no relief; the limb does not show any relief;
maybe it’s possible that Callisto cannot support relief.” By the next
day, the geologists on the Imaging Team were becoming more convinced
that the Callisto “bullseye” was the frozen remnant of an enormous
impact into Callisto’s surface. Since Callisto is composed in large part
of water and has an icy crust, the team speculated that any raised
features created by the impact would eventually “slide” back into the
surface, “and the ripple marks from the shock wave caused by the impact
were frozen” into the surface.
[Illustration: Callisto was the last of the Galilean satellites to
be studied by Voyager 1. In this photo, taken March 5 from a
distance of 1.2 million kilometers, with a resolution of about 25
kilometers, the extensive cratering of the surface began to be
apparent. Near the upper left edge is the large impact basin
Valhalla; the numerous light spots are craters 100 kilometers or
more in diameter. This is the same side of Callisto that was
photographed at higher resolution during the Voyager 1 flyby of the
satellite a day later. [P-21284C]]
Lyle Broadfoot announced that the ultraviolet spectrometer had detected
very strong auroral emission in Jupiter’s north and south polar regions.
The aurora seemed to be caused primarily by the excitation of molecular
hydrogen, although some atomic hydrogen was also detected. Auroral
emissions from helium atoms were not detected.
[Illustration: The largest ancient impact basin on Callisto is
called Valhalla. The central light area is about 600 kilometers
across. Surrounding it is a set of concentric low ridges, looking
like frozen ripple marks, extending about 1500 kilometers from the
center. This picture was taken by Voyager 1 on March 6 at a range of
350 000 kilometers. [P-21287C]]
The IRIS infrared measurements required more computer processing than
other Voyager data, and therefore they were not available until a day or
two after the observations were made. However, Rudy Hanel already had
two new results to report. First, the Great Red Spot was about 3° C
cooler than its surroundings, and this cooling extended many kilometers
above the clouds, into the thin upper atmosphere. Second, the thermal
emission from Io was peculiar, with an unexpected shape to the spectrum.
Tentatively, Hanel suggested there might be some hot spots on the
surface of the satellite.
Jupiter has been full of surprises, but the excitement was far from
over. Major discoveries were yet to be made.
[Illustration: Voyager 1 discovered the rings of Jupiter on March 4
in a single eleven-minute exposure with the narrow-angle camera.
Spacecraft motion during the time exposure streaked the picture, as
can be seen from the hairpin-like images of the stars. (The star
field was unusually rich, since it happened to include the Beehive
star cluster in Cancer.) The ring image itself is a multiple
exposure, with six separate images side by side. The ring does not
extend out of the right side of the picture, indicating that this
image captured the outer edge of the ring, about halfway between the
cloud tops and the orbit of Amalthea. [P-21258]]
[Illustration: Camera with fields of view]
Wednesday, March 7.
At the press briefing, Brad Smith made a spectacular announcement: “This
morning I would like to add yet another important discovery to be
claimed by this outstanding mission—that of a thin flat ring of
particles surrounding Jupiter. Thus Jupiter now joins Saturn and Uranus
to become the third planet of our solar system known to possess a
planetary ring system and leaves Neptune as the only member of the group
of giant planets without a known ring. The discovery of the ring was
unexpected in that the current theory which treats long-term stability
of planetary rings would not predict the existence of such a ring around
Jupiter. The single Voyager camera image which recorded the ring was
planned by the Voyager Imaging Team several years ago, not really with
any great expectation of a positive result, but more for the purpose of
providing a degree of completeness to Voyager’s survey of the entire
Jupiter system. The observation, as planned, involved looking off to the
right of the limb of Jupiter in the planet’s equatorial plane at the
exact moment that the spacecraft would be crossing the equatorial plane.
The image actually was taken at 16 hours and 52 minutes before encounter
from a distance of about 1.2 million kilometers. Exposure time was 11.2
minutes. We weren’t certain of the exact moment that we would cross the
equatorial plane, so we planned to open our shutter and leave it open as
we went through.”
[Illustration: Since the Voyager 1 ring photograph was taken exactly
edge-on, it was not possible to determine the width of the ring. In
this artist’s conception, the ring is drawn as if it were
ribbon-like, with very little width, quite unlike the broad flat
rings of Saturn. Voyager 2 later showed this to have been a lucky
guess. [P-21259]]
The image showed six exposures of the ring, together with streaked
trails of background stars. Smith reported that the thickness of the
ring was less than 30 kilometers, and that it extended to a point 128
000 kilometers from the center of Jupiter, or 57 000 kilometers above
the clouds. In response to this discovery, scientists were eagerly
planning to alter the Voyager 2 encounter sequence to try to obtain
additional information on the ring.
The newspapers carried stories of the ring discovery on Thursday, March
8. One story was seen by two University of Hawaii astronomers, Eric
Becklin and Gareth Wynn-Williams, who were observing at Mauna Kea
Observatory, at an altitude of 4200 meters on the Big Island of Hawaii.
Within two days they had succeeded in detecting the rings by their
reflected sunlight, at an infrared wavelength of 2.2 micrometers,
providing a rapid confirmation of the Voyager discovery.
As the plasma measurements from the Io flyby were analyzed, an
additional clue to the origin of sulfur and oxygen was revealed. Herb
Bridge reported the detection of sulfur dioxide (SO₂), the simplest
molecule composed of these two atoms.
The high-resolution tape-recorded pictures of Io baffled imaging
scientists. A number of features looked like volcanic flows; together
with the absence of impact craters, these features indicated a
geologically active planet. A central point of discussion was the recent
theoretical work on Io by Stanton Peale of the University of California
and two NASA scientists, Pat Cassen and Ray Reynolds. These authors had
just published a paper in the March 2 issue of _Science_ showing how
tidal heating from Jupiter’s gravity could melt the interior of Io. They
wrote that “widespread and recurrent surface volcanism might occur.” It
began to look as if the prediction had been correct.
Thursday, March 8.
The last Voyager 1 press briefing was held. Each speaker was allotted
only a few minutes, prompting Larry Soderblom to preface his remarks by
trying to explain how difficult it was to describe four new planets—the
Galilean satellites—in the time allowed. “Torrence [Johnson] was sitting
with me last night, puzzling. He said, ‘You know, Larry, it’s sort of
like imagining we’d flown into the solar system the day before
yesterday, and said, “There’s a thing we’ll call Mercury, and there’s
the Moon, and there’s Earth, and there’s Mars. Now let’s explain them in
ten minutes”.’” There was Callisto, with the highest density of craters
of any Galilean satellite—the oldest of the Galilean surfaces—featuring
a huge “bullseye” that is “the largest single contiguous feature seen so
far in the solar system.” There was Ganymede, cratered, but also overrun
with fault lines that looked, according to one person, like “tire tracks
in the desert,” showing a surface that “had laterally slid—faulted and
sheared and sheared again—twisted and torn apart.” There was Io, the
most bizarre—the one that scientists had thought would be most
lunarlike—showing a surface that had apparently been “cooked and steamed
and fumed out leaving deposits all over the surface much like you might
see around a fumarole at Yellowstone National Park. The fact that these
things exhibit such youth makes it likely that the planet is still
volcanically active.” There was Europa, with huge linear features unlike
those of the other three Galileans—Europa the mystery satellite, waiting
for the probing eyes of Voyager 2 to survey it in early July.
Ed Stone summed it all up: “I think we have had almost a decade’s worth
of discovery in this two-week period, and I think that all of the people
who have been talking to you feel the same saturation of new information
which has occurred. And in fact, we will probably be studying it in
great detail for at least five years.”
Over the next day or so the press packed up and went home. The TV
monitors showed the spacecraft’s parting glance at a crescent Jupiter,
only hinting at the vastness of space Voyager 1 would travel until its
encounter with Saturn in the autumn of 1980. Maybe now there would be a
relative calm that would allow the scientists to begin analyzing that
“decade’s worth of data.” But things were not to be calm just yet.
Fire and Brimstone
At about 5 a.m. on March 8, Voyager had taken a historic picture.
Looking back at a crescent Io from a distance of 4.5 million kilometers,
the camera had been used to obtain a long-exposure view for the
spacecraft navigation team—one that showed the satellite against the
field of background stars. During the day, Linda Morabito, an optical
navigation engineer, began to work with this picture on her
computer-controlled image display. She noted what appeared to be a
crescent cloud, extending beyond the edge of Io. But Io has no
atmosphere, so a cloud rising hundreds of kilometers above the surface
did not seem to make sense.
The next day, working with her colleagues, Morabito eliminated all
possibilities for the new feature on Io except the obvious—a cloud. If
it were a cloud, it must be the result of an ongoing volcanic eruption
of incredible violence. The picture was shown to members of the Imaging
Team, who agreed with the identification. But it was Friday and Brad
Smith and Larry Soderblom, along with most of the other team members,
had left for the weekend to try to get some rest. The picture would have
to wait two more days.
HIGHLIGHTS OF THE VOYAGER 1 SCIENTIFIC FINDINGS[2]
Atmosphere
Uniform wind speeds for cloud features with widely different size
scales, suggesting that mass motion and not wave motion is being
observed.
Rapid formation and spreading of bright cloud material, perhaps the
result of disturbances that trigger convective activity.
A pattern of east-west winds in the polar regions, previously thought to
have been dominated by convective upwelling and downwelling.
Anticyclonic motion of material associated with the Great Red Spot, with
a rotational period of about six days.
Interactions of smaller spots with the Great Red Spot and with each
other.
Auroral emissions in the polar regions, both in the ultraviolet (which
were not present during the 1973 Pioneer encounter) and in the visible.
Cloud-top lightning bolts, similar to terrestrial superbolts, and
meteoritic fireballs.
A temperature inversion layer in the stratosphere and a temperature of
160 K at the level at which the atmospheric pressure is 0.01 bar.
Very strong ultraviolet emission from the disk, indicating a
thermospheric temperature of more than 1000 K.
A hot (1100 K) upper ionosphere on the dayside that was not observed by
Pioneer 10, suggesting there may be large temporal or spatial changes.
An atmospheric composition with volume fraction of helium of 0.11 ±
0.03.
A substantially colder atmosphere above the Great Red Spot than in the
surrounding regions.
Satellites and Rings
At least eight currently active volcanoes on Io, probably the result of
tidal heating of the interior of the satellite, with plumes extending up
to 250 kilometers above the surface.
A large hot spot on Io near the volcano Loki that is about 150 K warmer
than the surrounding surface.
Numerous intersecting, linear features on Europa, possibly due to
crustal cracking.
Two distinct types of terrain, cratered and grooved, on Ganymede,
suggesting that the entire ice-rich crust was once under tension.
An ancient, heavily cratered crust on Callisto, with vestigial rings of
enormous impact basins since erased by flow of the ice-laden crust.
The elliptical shape of Amalthea (270 × 160 kilometers).
A faint ring of material about Jupiter, with an outer edge of 128 000
kilometers from the center of the planet.
Magnetosphere
An electrical current of more than a million amperes flowing in the
magnetic flux tube linking Jupiter and Io.
Very strong ultraviolet emissions from ionized sulfur and oxygen in the
Io plasma torus, indicating a hot (hundred thousand degree) plasma that
evidently was not present at the time of Pioneer 10 encounter.
Plasma electron densities exceeding 4500 per cubic centimeter in some
regions of the Io plasma torus.
A cold, corotating plasma inside 6 R_J with ions of sulfur, oxygen, and
sulfur dioxide.
High-energy trapped particles inside 6 R_J with significantly enhanced
abundances of oxygen, sodium, and sulfur.
Hot plasma near the magnetopause predominantly composed of protons,
oxygen, and sulfur.
Jovian radio emission at kilometer wavelengths, which may be generated
by plasma oscillations in the Io plasma torus.
Corotating plasma flows unexpectedly far from Jupiter in the dayside
outer magnetosphere.
Evidence suggesting a transition from closed magnetic field lines to a
Jovian magnetotail at about 25 R_J from Jupiter.
Whistler emission interpreted as lightning whistlers from the Jovian
atmosphere.
[1]Adapted from a paper by E. C. Stone and A. L. Lane in the Voyager 1
Thirty-Day Report.
[2]Adapted from a summary prepared by E. C. Stone and A. L. Lane for the
Voyager 1 Thirty-Day Report.
[Illustration: The dramatic discovery of active volcanoes on Io was
made by Linda Morabito and her colleagues from this navigation
picture, taken March 8 at a range from Io of 4.5 million kilometers.
On the bright edge, the immense plume of volcanic ash from Pele (P₁)
rises nearly 300 kilometers above the surface. At the terminator,
the border between day and night on Io, a second smaller cloud from
the volcano Loki (P₂) catches the sunlight. These two
eruptions—captured on this single discovery photograph—are much
larger than the largest terrestrial volcanic eruption known.
[P-21306 B/W]]
[Illustration: Once the existence of giant volcanic eruptions on Io
was recognized, a reexamination of the Voyager 1 encounter pictures
revealed many more plumes. These two views of Prometheus (P₃) were
found by Joseph Veverka and Robert Strom on March 12 when they
reproduced earlier pictures.]
[Illustration: The plume is silhouetted against the black space,
although it is also possible to see dark “feet” where the falling
material reaches the surface. [P-21295]]
[Illustration: The complex jets of material are clearly seen as dark
streaks against the light background of the surface of Io. The plume
itself rose more than 100 kilometers above Io’s surface. [P-21294]]
Meanwhile, new information about Jupiter was released to the public. A
long-exposure (three minutes and twelve seconds) image of the dark side
of the planet, taken with the wide-angle camera while in the shadow of
the planet, caught Jupiter showing off some Jovian “fireworks.” A long,
broad, white streak across the picture was a visible aurora, the largest
aurora ever seen—almost 29 000 kilometers long. In addition, nineteen
smaller bright splotches, looking insignificant by comparison, were in
reality “superbolts” of lightning. Since huge electrical discharges such
as lightning can, under the right circumstances, power chemical
reactions that form complex organic molecules, the discovery of
lightning on Jupiter could have profound implications. Was
“lightning-inspired” organic synthesis going on in Jupiter’s atmosphere?
No one knew.
Returning to JPL on Sunday night, Brad Smith got his first look at the
Morabito picture of the volcanic cloud. Early Monday morning, other
Imaging Team scientists saw it. As soon as the JPL computers were
operating, Joseph Veverka and Robert Strom began working with the two
interactive TV terminals to look for evidence in other pictures of
ongoing eruptions. Faint clouds or plumes would not show up in normally
processed pictures, but could be brought out easily with the
computer-controlled displays. By midmorning, several additional volcanic
plumes had been found.
[Illustration: Linda Morabito shows the discovery photo of the
volcanic eruptions on Io. [P-21718]]
[Illustration: The dark side of Jupiter revealed many surprising
phenomena to Voyager 1. A wide-angle view, taken on March 5, led to
the discovery of a double auroral arc at north-polar latitudes and
numerous flashes of lightning illuminating the clouds during this
3-minute, 12-second exposure, taken at a range of half a million
kilometers. [260-460]]
Meanwhile, on March 11, John Pearl of the IRIS team had independently
drawn the conclusion that volcanic activity must be taking place on Io.
He and Rudy Hanel found evidence of strongly enhanced thermal emission
from parts of the satellite. The most prominent was a source nearly 200°
C hotter than its surroundings. On March 12, Pearl brought his new
results to the Imaging Team, and sure enough, the hot spot was located
near one of the volcanic plumes! A month later, continuing analysis of
IRIS spectra yielded identification of sulfur dioxide (SO₂) gas over
this same erupting volcano. At last a source had been located for the
enigmatic sulfur and oxygen ions in the magnetosphere.
The volcanoes provided a thread with which to weave together the
disparate data on Io. A few months earlier there had been a report of a
sudden brightening of Io in the infrared; now it seemed plausible that
thermal emission from an eruption was the source. The Voyager
ultraviolet experimenters had been worrying over the source of the
intense sulfur emissions they had seen and had been disturbed by the
changes in the gas clouds around Io since the Pioneer 10 and 11 flybys;
now a variable source for these gas clouds was identified. In addition,
the craterfree surface and bizarre features seen in the Voyager images
could be recognized as the product of violent explosive eruptions on Io.
It appeared that Peale, Cassen, and Reynolds had found, in their
theoretical calculations, the key to the most geologically active body
ever encountered in the solar system.
News of the discovery was released to the press on Monday, March 12.
During the next few days, a total of eight gigantic eruptions were
located in the Voyager pictures of Io. Within a few weeks, scientists
all over the world were thinking with renewed energy about this
incredible satellite.
With four new planet-sized satellites now photographed, there was a
sudden requirement for maps and for names to be assigned to the newly
discovered features. The maps were produced from Voyager images at the
Astrogeology Branch of the U.S. Geological Survey at Flagstaff, Arizona.
The names, proposed by a group of scientists headed by Voyager Imaging
Team members Tobias Owen and Hal Masursky, were given official approval
by the International Astronomical Union in August. For a time, a dual
nomenclature persisted for the erupting volcanoes on Io. The eruption
plumes were given numbers, P₁, P₂, etc., while the volcanic features
were given names taken from the mythology of fire and volcano legends.
Thus the “hoof print” of Io was called Pele, for the Hawaiian volcano
goddess, and the 280-kilometer-high plume associated with it was called
P₁. By the time of the Voyager 2 encounter, scientists had prepared maps
on which to plot their new discoveries.
While the Voyager scientists fanned out across the world to share their
findings with colleagues, attention at JPL turned to Voyager 2. In
response to the discoveries of the first encounter, changes were
required in the sequencing of scientific observations for July. Voyager
2, still troubled by a faulty receiver, might require more coddling from
the spacecraft team than had its sister spacecraft, now safely on the
way to Saturn.
[Illustration: The particles of the rings of Jupiter are stronger
reflectors of red light than of blue, as can be seen in this view,
assembled from two images taken in orange and violet light. Since
the images were registered on the rings, not the planet, the bright
bands of colors along the edge of Jupiter are an artifact of
misalignment. [P-21779]]