CHAPTER 6. The First Encounter

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.”

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 (RJ) 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 RJ, 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 RJ, 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 RJ, 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 RJ, Voyager crossed the bow shock for the third time, catching up with the contracting magnetosphere of the planet. About noon, at 66 RJ, 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.

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 RJ 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 RJ, followed by three more magnetopause crossings, as the magnetospheric boundary flopped in and out between 45 RJ and 50 RJ.

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 (RJ) 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.”

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.”

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

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.

Adapted from a paper by E. C. Stone and A. L. Lane in the Voyager 1 Thirty-Day Report.

Adapted from a summary prepared by E. C. Stone and A. L. Lane for the Voyager 1 Thirty-Day Report.

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.

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.