Fire and Brimstone

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.”

(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 RJfrom 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 RJand 50 RJ.

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]

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.

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.

(Range to Jupiter, 2.5 million kilometers). Early in the morning the final crossing of the magnetopause took place at 47 RJ, 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 littlelater.” 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.”

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 thewhite 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.

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 imagesimproved. 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.”

(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.

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]

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]

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 wasapparently 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.

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]

(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.

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.

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]

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.

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]

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 describedthe 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.

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]

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 forwhich 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.

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]

Observation geometry for Amalthea

(b)

(c)

(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 doesnot 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.

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.

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.

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]

Camera with fields of view

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 Voyagercamera 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.”

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 indetecting 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 ofScienceshowing 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.

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.

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 moredays.

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