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