Voyage to Jupiter

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.

CHAPTER 7
THE SECOND ENCOUNTER: MORE SURPRISES FROM THE “LAND” OF THE GIANT

Approaching Jupiter

At the beginning of July, the dry summer heat had returned to Pasadena, and so had the press. The scientists had come days or weeks earlier to look at data being transmitted from the second Voyager as it approached Jupiter and its satellites. The mood at JPL seemed quieter than it had been in March for Voyager 1, although the press room would once again be deluged with observers on the day of encounter. This would be our second good, close look at the Jovian system, but it was to be no summer rerun. Voyager 2 would have a different view of each world, and, in addition, both Io and Jupiter had undergone changes, as though to ensure that no one would become bored and fall asleep in front of a TV monitor. In a sense, this encounter was to be another first look at Jupiter and its satellites, with a view of each object that was quite different from what had been seen before.

Changes in Jupiter’s cloud formations became noticeable long before July. After a gap of six weeks following the first flyby, Voyager 2’s observatory phase began on April 24, 1979, seventy-six days before its July 9 encounter with Jupiter. During this time, the spacecraft’s ultraviolet and fields and particles instruments studied the Jovian system and its interaction with the solar wind. Between April 24 and May 27, Voyager 2’s imaging system concentrated on the motions in Jupiter’s atmosphere, creating another approach time-lapse “movie.” From May 27 to 29 photographs were taken in a more rapid sequence, showing the planet during five 10-hour rotations. From these studies it was apparent that Garry Hunt’s prediction had been right—the weather had changed by July. A month before the encounter JPL’s Voyager Bulletin—Mission Status Report announced that “Jupiter is sporting quite a different face than it did just four months ago. The bright ‘tongue’ extending upward from the Red Spot is interacting with a thin, bright cloud above it that has traveled twice around Jupiter in four months.” The turbulent region west of the Great Red Spot had begun to break up and separate from the Red Spot. The white ovals south of the Red Spot had drifted to the east (about 0.35 degrees a day), while the Red Spot itself had drifted west (about 0.26 degrees a day). The white zone seen just south of the Red Spot by Voyager 1 had become very narrow—like a thin white line just barely outlining the bottom of the spot. The Red Spot had also changed: It had become a more uniform orange-red, perhaps reverting to the color seen by Pioneers 10 and 11. The brown spots that had been seen in the north temperate region at the same longitude as the Red Spot were now on the other side of the planet. A dark brown spot not present during the Voyager 1 flyby had developed along the northern edge of the brown equatorial region on the Red Spot side of the planet. Some of the white markings that seemed to have protruded into the equatorial region at the time of the first flyby were missing in the Voyager 2 photographs.

As Voyager 2 entered the far encounter period on May 29, all instruments on the spacecraft (except for the photopolarimeter) seemed to be in good shape for encounter. As was the case with Voyager 1, the polarization wheel on Voyager 2’s photopolarimeter was stuck, so the instrument was able to obtain only color photometry measurements.

Voyager 2 trajectory

View normal to Jupiter equator

Sun occultation

Earth occultation

Launch date = 8/20/77

Jupiter arrival date = 7/9/79

Periapsis

Satellite closest approach

Amalthea

Europa

Io

Ganymede

Callisto

Although Voyager 2’s radio receiver still could not track a Doppler-shifted radio signal from Earth (the problem is that it “hears a monotone,” explained Deputy Project Manager Esker K. Davis), the Deep Space Network engineers had learned to work with the spacecraft, determining what frequency the spacecraft would listen to at any particular time. They had discovered that some of the “housekeeping” telemetry signals from the receiver were sensitive to the match between the incoming frequency and the receiver frequency. By monitoring these signals, they could detect a frequency drift in time to correct the transmission, thus keeping the system in tune in spite of slow changes in the receiver. The system was slow and demanding but effective; all the necessary command sequences were successfully loaded into the computer, and communications during the encounter were entirely successful.

The timing offset experienced by Voyager 1 as a result of Jupiter’s intense radiation environment was not expected to be a problem on Voyager 2 for two reasons: Even at closest approach, Voyager 2 would still be more than twice as far from Jupiter as Voyager 1 had been, and the Voyager 2 computer was programmed to resynchronize the spacecraft’s timing systems automatically every hour. In this way, even if the radiation environment proved to be much higher than anticipated, the image smear that might occur from a timing offset would be prevented.

As a result of the discoveries made by Voyager 1, the project scientists decided to modify some of Voyager 2’s preplanned sequences. As early as April 1, the painstaking job of constructing new computer commands began. A ten-hour Io Volcano Watch was added to the spacecraft’s program, taking advantage of the fact that shortly after closest approach to Jupiter, the spacecraft would remain within about 1 million kilometers of Io for a long period, keeping nearly the same face in view. Provisions were also made to take extensive ultraviolet measurements of the emission from the glowing torus surrounding Jupiter near the orbit of Io. Further studies would be made of the dark side of Jupiter to search for lightning and auroral activity, and there was also the hope that the plasma wave instrument would be able to detect lightning whistlers (radio signals created by lightning bolts) as the Voyager 1 instrument had done. A high priority was given to observations of the newly discovered ring, which had not been in the original Voyager 2 sequence at all. The spacecraft would cross the ring plane twice, photographing the ring during both the inbound and the outbound passages.

As was originally planned, Voyager 2 would make its closest approaches to Callisto, Ganymede, Europa, and Amalthea before encounter with Jupiter. Because of the difference in the trajectories of the two spacecraft, Voyager 2 would see the faces of Callisto and Ganymede not seen by Voyager 1. The most important difference, however, was that the second Voyager would fly much closer to Europa than Voyager 1 did, giving scientists their first good look at the mysterious streaks scratched on the surface of that bright golden world. Voyager 2 would also have a closer flyby of Ganymede, giving us a second chance to examine its strange “snowmobile tracks.” The major loss, of course, was Io, which would be seen from Voyager 2 only at distances of a million kilometers or more.

The Encounter

Wednesday, July 4.

(Range to Jupiter, 5.3 million kilometers; range to Earth, 921 million kilometers). While most of the nation celebrated Independence Day with picnics, sports events, and fireworks, the scientists and engineers at JPL were working around the clock. Voyager 2 had already entered Jupiter’s territory, crossing the bow shock for the first time on July 2 at a distance of 99 R_J from Jupiter, indicating that the magnetosphere had expanded in the interval between the two encounters. At about noon on July 3, the spacecraft encountered the magnetopause, but on July 4, the data from the particles and fields instruments were ambiguous. Apparently the magnetosphere was pulsating in response to changing pressures, and the spacecraft was playing tag with the rapidly shifting boundaries of the bow shock and the magnetopause.

As the low energy charged particle instrument began to measure particles coming from inside the Jovian magnetosphere, it became apparent that some important changes had taken place since Voyager 1’s encounter. From the composition of the particles, it appeared that they were largely of solar origin, unlike the heavy concentrations of ions of sulfur and oxygen seen by Voyager 1. Scientists began to speculate that the Io volcanoes, which presumably eject sulfur and oxygen into the magnetosphere, might have declined in activity. In the evening, the first images of Io at a resolution high enough to allow the volcanic plumes to be seen would be beamed back to Earth.

Thursday, July 5.

(Range to Jupiter, 4.4 million kilometers). The press room at Von Karman Auditorium opened and the members of the press, most of them veterans of the first encounter, arrived at JPL. Meanwhile, the spacecraft continued to measure fluctuations in the magnetospheric boundary. By noon, JPL had reported at least eleven crossings of the bow shock as the solar wind flirted with Jupiter’s magnetosphere. Apparently the solar wind was much more variable in July than it had been in March. At times the bow shock seemed to be thicker than that experienced by Voyager 1; one Voyager 2 crossing took ten minutes, whereas the longest Voyager 1 crossing was only one minute long. Even though the processes affecting the magnetosphere seemed more complex, the magnetosphere was less compressed; when Voyager 2 actually entered the magnetosphere at a distance of 62 R_J, it was much farther from Jupiter than Voyager 1 had been at its final crossing (47 R_J).

Photos obtained the day before from over 4 million kilometers showed that at least one of Io’s volcanoes was still active. A total of eight ongoing eruptions had been seen by Voyager 1, and scientists were anxious to see how many of these were still erupting four months later.

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.

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.

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.”

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.”