A New Satellite

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 observationsof 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]

Io volcanoes.

Masubi (P₈) is faintly visible in the crescent (above and below).

Io volcanoes.

Loki (P₂) rises 250 kilometers above the surface, catching the morning sunlight on the east edge of Io.

(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]

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]

Rings of Jupiter.

HIGHLIGHTS OF THE VOYAGER 2 SCIENTIFIC FINDINGS[3]AtmosphereThe 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 SystemThe 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).MagnetosphereThe 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 RJand 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.

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 RJand 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]

One of the most fascinating discoveries of Voyager 2 was not recognized at first. Graduate student David Jewitt of the California Institute of Technology, working with Imaging Team member Ed Danielson, began a detailed analysis of all the ring photos in late summer. In early October he determined that a shortstreak on a photo taken July 8, previously presumed to be an image of a star trailed by the time exposure, did not correspond to any known star position. Perhaps this was a new satellite! Additional sleuthing turned up a second image of the same part of the ring that also showed the anomalous object, together with trails due to known stars. The differing angles and lengths of the trails of the object and the stars confirmed that this was indeed a 14th satellite of Jupiter. Following the guidelines of the International Astronomical Union, it was designated 1979J1, pending later assignment of a mythological name. The proposed name is Adrastea, a nymph who nursed the infant Zeus in Greek legend.

The newly discovered satellite orbits Jupiter at a distance of 58 000 kilometers above the equatorial cloud tops, placing it just at the outer edge of the ring and much closer to the planet than is Amalthea, previously thought to be the innermost satellite. It travels at 30 kilometers per second (nearly 70 000 miles per hour), circling Jupiter in just seven hours and eight minutes. From its brightness, scientists guessed that it might be 30-40 kilometers in diameter.

The proximity of Adrastea to the ring suggests a relationship between the two. When the discovery was announced to the press in mid-October, it was speculated that the ring material might originate on the satellite, perhaps eroded away by the energetic charged particles in the inner Jovian magnetosphere. Once again, Voyager had added to our perspective on planetary processes, suggesting that undiscovered but similar small satellites might also be associated with the rings of Saturn and Uranus.

Voyager 2 had certainly added a few years’ of data of its own to Voyager 1’s “ten years’ worth of data.” It had given a different view of the Jovian system, helping to solve some of the mystery surrounding Jupiter and its satellites, and creating new mysteries. As Voyager 2 sped out away from Jupiter, riding along the giant planet’s huge magnetotail, attention turned to Saturn: What would Pioneer 11, the Pathfinder, discover in September 1979? What would the Voyagers learn in November 1980 and August 1981? Would all go well? Would Voyager 2 fly on to Uranus?

There was also a yearning to examine more closely, with the Galileo Project, what had been unknown for so long, yet had become so familiar in only a few months’ time—the little dark, red “potato” Amalthea, the volcano-covered world Io, the mysterious “cracked billiard ball” Europa, cratered and groovy Ganymede, ancient Callisto, and the king of the planets itself, a colorful, banded world of stable climate and ever-changing weather patterns.

A fifteenth satellite of Jupiter was discovered in the spring of 1980 by Steven Synnott of JPL. It was first seen on this Voyager 1 image taken March 5, 1979, in which the 75-kilometer-diameter satellite shows as a dark oval against the planet. Also visible is the shadow of the satellite, designated 1979J2. This satellite orbits between Io and Amalthea with a period of 16 hours and 11 minutes. [P-22580B/W]

The Jupiter seen by the Voyager cameras is a cloud-belted world of rapid jet streams and complex cloud forms. Prominent in this Voyager 1 image, taken February 5 at a range of 28.4 million kilometers, is the alternating structure of light zones and dark belts, and the Great Red Spot and numerous smaller spots. Also easily visible are the two inner Galilean satellites, Io and Europa. The resolution in this picture is 500 kilometers, about five times better than can be obtained from Earth-based telescopes. Callisto can be faintly seen at the lower left. [P-21083C]

More massive than all the other planets combined, Jupiter dominates the planetary system. The giant revealed by Voyager is a gas planet of great complexity; its atmosphere is in constant motion, driven by heat escaping from a glowing interior as well as by sunlight absorbed from above. Energetic atomic particles stream around it, caught in a magnetic field that reaches out nearly 10 million kilometers into the surrounding space, embracing the seven inner satellites. From its deep interior through its seething clouds out to its pulsating magnetosphere, Jupiter is a place where forces of incredible energy contend.

At its birth, Jupiter shone like a star. The energy released by infalling material from the solar nebula heated its interior, and the larger it grew the hotter it became. Theorists calculate that when the nebular material was finally exhausted, Jupiter had a diameter more than ten times its present one, a central temperature of about 50 000 K, and a luminosity about one percent as great as that of the Sun today.

At this early stage, Jupiter rivaled the Sun. Had it been perhaps 70 times more massive than it was, it would have continued to contract and increase in temperature, until self-sustaining nuclear reactions could ignite in its interior. If this had happened, the Sun would have been a double star, and the Earth and the other planets might not have formed. However, Jupiter did not make it as a star; after a brief flash of glory, it began to cool.

At first Jupiter continued to collapse. Within the first ten million years of its life, the planet was reduced to nearly its present size, with only a few percent additional shrinkage during the past 4.5 billion years. The luminosity also dropped as internal heat was carried to the surface by convection and radiated away to space. After a million years Jupiter emitted only one-hundred thousandth as much radiation as the Sun, and today its luminosity is only one-ten billionth of the Sun’s.

Jupiter’s internal energy, although small by stellar standards, has important effects on the planet. About 10¹⁷ watts of power, comparable to that received by Jupiter from the Sun, reach the surface from the still-luminous interior. The central temperature is still thought to be about 30 000 K, sufficient to maintain the interior in a molten state. Scientists generally agree that Jupiter is an entirely fluid planet, with no solid core whatever.

Because of its great mass, Jupiter has been undiscriminating in its composition. All gases and solids available in the early solar nebula were attracted and held by its powerful gravity. Thus it is expected that Jupiter has the same basic composition as the Sun, with both bodies preserving a sample of the original cosmic material from which the solar system formed.

Jupiter is a gas giant, composed of the same elements as the Sun and stars—primarily hydrogen and helium. Its internal structure is dominated by the properties of hydrogen, its most abundant constituent and by the high temperatures in the deep interior that remain from its luminous youth. Most of the interior is liquid: metallic hydrogen at great depths and high pressures, and normal hydrogen nearer the surface. In the upper few thousand kilometers, the hydrogen is a gas. The primary known or suspected cloud layers are, from the top down, thin hydrocarbon “smog”; ammonia; ammonium hydrosulfide; water-ice, and liquid water. [260-828]

The primary constituents of Jupiter have long been suspected to be hydrogen and helium, the two simplest and lightest atoms. However, it has proved impossible to derive accurate measurements of the abundance of these two elements from astronomical observations. On the basis of a rather simple infrared measurement, Pioneer investigators found He/H₂ = 0.14 ± 0.08. On Voyager, IRIS was able to obtain much improved infrared spectra, yielding an initial value of He/H₂ = 0.11 ± 0.3. Voyager scientists expect that further analysis will reduce the uncertainty to about ± 0.01. The ratio of 0.11 is in excellent agreement with the solar value of about 0.12, supporting the idea that Jupiter and the Sun have similar elemental compositions.

Astronomers have known for a long time that, in addition to hydrogen and helium, the compounds methane (CH₄) and ammonia (NH₃) are present in the visible atmosphere of Jupiter. In the 1970s, additional spectra in the infrared resulted in the discovery of water (H₂O), ethane (C₂H₆), germane (GeH₄), acetylene (C₂H₂), phosphine (PH₃), carbon monoxide (CO), hydrogen cyanide (HCN), and carbon dioxide (CO₂). All these are trace constituents, with two of them, ethane and acetylene, apparently formed at high altitudes by the action of sunlight on methane.

A total of approximately 100 000 infrared spectra, many of small regions on the disk, were obtained by IRIS. These spectra generally show hydrogen, helium, methane, ammonia, phosphine, ethane, and acetylene. In addition, excellent spectra were obtained in “hot spots,” regions in which breaks in the upper clouds permit radiation from deeper layers to escape. (The hot spots generally correspond to dark brown regions on photographs of the planet.) IRIS measured temperatures in the hot spots up to -13° C but no higher; apparently this temperature corresponds to the top of a deeper cloud deck. Spectral features indicative of the presence of water vapor and germane were clearly seen in the hot spots.

Further analysis of the IRIS spectra will be required to derive the abundances of the gases detected. However, even the preliminary data showed how variable Jupiter can be, especially in its upper atmosphere. The two hydrocarbons, ethane and acetylene, vary in relative abundance with latitude; there is less acetylene near the poles. In addition to this planetwide trend, smaller variations were seen from place to place and between the observations in March and July. All the variations will eventually provide information on the processes of formation, transportation, and destruction of hydrocarbons in the upper atmosphere.

Voyager did not make any direct measurements of the chemical composition of the clouds, but theorists generally agree that the uppermost clouds are ammonia cirrus, and that layers of ammonium hydrosulfide (NH₄SH) and water exist at deeper levels. All these clouds are formed in the troposphere, the layer of the atmosphere in which convection takes place. The top of the ammonia cloud deck is thought to have a pressure of about 1 atmosphere and a temperature of about -113° C.

Ammonia cirrus is white, yet Jupiter’s clouds display a spectacular range of colors. Voyager did not determine the nature of the coloring agents; they may be minor constituents—trace impurities in a sea of white clouds. Perhaps organic polymers, formed from atmospheric chemicals such as methane and ammonia that have reacted with lightning, are responsible for the oranges and yellows. The color of the Red Spot could be caused by red phosphorus (P₄). According to this theory, phosphine (PH₃) from deep in Jupiter’s atmosphere is brought to high altitudes by the upwelling of the Great Red Spot. Ultraviolet light, penetrating the upper reaches of the Red Spot, splits the phosphine molecules, and, through a series of chemical reactions, converts the phosphine into pure phosphorus. However, this theory fails to explain the existence of the smaller red spots on Jupiter; these spots are not at such high altitudes as the Great Red Spot (which is the highest and coldest of Jupiter’s visible clouds), so it is unlikely that ultraviolet light could react with any phosphine in these areas to produce red phosphorus.

Although the Voyager spacecraft never flew over the poles of Jupiter, it is possible to reconstruct from several images the View that would be seen from directly above or below the planet. Note the absence of a strong banded structure near both poles. The regular spacing of cloud features is obvious. In the Southern hemisphere, the three white ovals are 90 degrees apart in longitude, but a fourth oval at the other quadrant is missing. The irregular black areas at each pole are places for which no Voyager data exist. The resolution of the original pictures from which these polar projections were made was about 600 kilometers.

North pole. [P-21638C]

South pole. [P-21639C]

Various forms of elemental sulfur might be responsible for the riot of color we see on Jupiter. Sulfur forms polymers (S₃, S₄, S₅, S₈,) that are yellow, red, and brown, but no sulfur in any form has been detected on Jupiter. “We never promised you we were going to identify the colors on Jupiter with this mission,” one of the atmospheric scientists remarked, “but we will have a probe that is going into the atmosphere in the mid-1980s—Galileo.” Perhaps the mystery of the Jovian clouds will have to wait till then.

Temperature maps of Jupiter were obtained by IRIS in radiation arising at different levels above the clouds. Maps show temperatures at pressures of 0.8 atmosphere near the clouds, and 0.2 atmosphere near the top of the troposphere. In addition to the low temperatures over the bright zones and the higher temperatures over dark belts, there is a great deal of smaller scale structure. It is interesting that a cold area corresponding to the Great Red Spot is clearly visible even near the top of the troposphere, indicating that this feature disturbs the atmosphere to very high altitudes.

The structure of the atmosphere of Jupiter above the troposphere was investigated through the radio occultation experiment as well as by IRIS. The level in which the minimum temperature of about -173° C occurs has a pressure of 0.1 atmosphere. Above this point lies the stratosphere, in which temperatures increase with altitude as a result of sunlight absorbed by the gas or by aerosol particles resembling smog. At 70 kilometers above the ammonia clouds, the temperature is about -113° C. Above this level, the temperature stays approximately constant, although at extreme altitudes the temperature again rises in the ionosphere.

If one could “unwrap” Jupiter like a map, views such as these would be obtained. The comparison between the pictures shows the relative motions of features in Jupiter’s atmosphere. It can be seen, for example, that the Great Red Spot moved westward and the white ovals eastward during the time between the acquisition of these pictures. Regular plume patterns are equidistant around the northern edge of the equator, while a train of small spots moved eastward at approximately latitude 80° S. In addition to these relative motions, significant changes are evident in the recirculating flow east of the Great Red Spot, in the disturbed region west of the Great Red Spot, and as seen in the brightening of material spreading into the equatorial region from the more southerly latitudes. [P-21771C]

The planet as it appeared about March 1.

As it was in early July.

The Voyager pictures reveal a planet of complex atmospheric motions. Spots chase after each other, meet, whirl around, mingle, and then split up again; filamentary structures curl into spirals that open outward; feathery cloud systems reach out toward neighboring regions; cumulus clouds that look like ostrich plumes may brighten suddenly as they float toward the east; spots stream around the Red Spot or get caught up in its vortical motion—all in an incredible interplay of color, texture, and eastward and westward flows. Such changes can be noticed in the space of only a few Jovian days.

Differing characteristics of Jupiter’s meteorology are apparent in high-resolution images, such as this one taken by Voyager 1 on March 2 at a range of 4 million kilometers. The well-defined pale orange line running from southwest to northeast (north is at the top) marks the high-speed north temperate current with wind speeds of about 120 meters per second. Toward the top of the picture, a weaker jet of approximately 30 meters per second is characterized by wave patterns and cloud features which have been observed to rotate in a clockwise manner at these latitudes of about 35°N. These clouds have been observed to have lifetimes of one to two years. [P-21193C]

On a broader time scale, greater changes on the face of Jupiter can be seen. Features drift around the planet; even the large white ovals and the Great Red Spot slide along in their respective latitudes. Belts or zones intrude upon each other, resulting in one of the banded structures splitting up or seeming to squeeze together and eventually disappear. Small structures form, then die. The largest spots may slowly shrink in size, and the Red Spot itself changes its size and color.

The Jupiter of Pioneers 10 and 11 was quite unlike the planet seen by Voyager 1. At the time of the Pioneer exploration, the Great Red Spot, embedded in a huge white zone, was more uniformly colored, and pale brown bands circled the northern hemisphere. In the intervening years, the south temperate latitudes have changed completely, developing the complex turbulent clouds seen around the Red Spot by Voyager 1. Yet, even between the two Voyagers, Jupiter appeared to be undergoing a dynamic “facelift.” At a quick glance, Voyager 2 photographs showed the visage that had been familiar since early in 1979, but a closer look showed that it is not quite the same. The whiteband below the Great Red Spot, fairly broad during the first flyby, had become a thin white ribbon where it rims the southern edge of the Spot. The turbulence to the west of the Red Spot had stretched out and become “blander” than it was before. Small rotating clouds seemed to be forming out of the waves in this region. The cloud structure that had been east of the Red Spot during the Voyager 1 flyby spread out, covering the northern boundary and preventing small clouds from circling the huge red oval. The Red Spot itself also changed. Its northern boundary seemed—at least visually—to be more set off from the clouds that surround it, and the feature appeared to be more uniform in color, perhaps reverting back to the personality it had in Pioneer days.

Jupiter’s cloud patterns changed significantly in the few months between the two Voyager flybys. Most of the changes are the result of differential rotation, in which the prevailing winds at different latitudes shift long-lived features with respect to those north or south. Thus, for example, the three large white ovals shifted nearly 90 degrees in longitude, relative to the Great Red Spot, between March and July. [P-21599]

Cloud patterns from Voyager 2.

The most obvious features in the atmosphere of Jupiter, after the banded belts and zones, are the Great Red Spot and the three white ovals. These have often been described as “storms” in Jupiter’s atmosphere. The ovals are about the size of the Moon, and the Red Spot is larger than the Earth. Voyager has revealed that in many respects the white ovals, which formed in 1939, resemble their ancient red relative. All four spots are southern hemispheric anticyclonic features that exhibit counterclockwise motion; hence they are meteorologically similar. Other smaller bright elliptical and circular spots also exhibit anticyclonic motion, rotating clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. In general, these features are circled by filamentary rings that are darker than the spots they surround. Hints of interior spiral structure can be seen in some of these spots. All the elliptical features in the southern hemisphere lie to the south of the strong westward-blowing jet streams. The spots tend to become rounder the closer they are to the poles.

Along the northern edge of the equator are a number of cloud plumes, which appear to be regularly spaced all around the planet. Some of the plumes have been observed to brighten rapidly, which may be an indication of convective activity; indeed, some of the plume structures seem to resemble the convective storms that form in the Earth’s tropics. The plumes travel eastward at speeds ranging from about 100 to 150 meters per second, but they do not move as a unit.

The Great Red Spot of Jupiter is a magnificent sight, whether viewed in normal or exaggerated color. These pictures were taken by Voyager 1 at a range of about 1 million kilometers; the area shown is about 25 000 kilometers, with features visible on the originals that are as small as 30 kilometers across. The Red Spot is partly obscured on the north by a thin layer of overlying ammonia cirrus cloud. South of the Red Spot is one of the three white ovals, which are also anticyclonic vortices in the atmosphere.

This frame is in natural color. [P-21430C]

The red and blue have been greatly exaggerated in this frame to bring out fine detail in the cloud structure. [P-21431C]

The most visible cloud interactions take place in the region of the Great Red Spot. Material within the Red Spot rotates about once every six days. Infrared measurements show that the Red Spot is a region of atmospheric upwelling, which extends to very high altitudes; however, the divergent flow suggested by this upwelling seems to be very small—one bright feature was observed to circle the Red Spot for sixty days without appreciably changing its distance from the spot’s center. During the Voyager 1 flyby, spots were seen to move toward the Red Spot from the east, flow along its northern border, then either flow on to the west past the Red Spot or into the outer regions of its vortex. A spot caught on the outer edge of the Red Spot flow might break in two as it reached the eastern edge of the spot, with one piece remaining in the vortex and the other moving off to the east. Alternatively, a spot floating toward the Red Spot from the east might be pushed northward to join the eastward current flowing north of the giant red oval.

By the time of the second Voyager encounter, a ribbon of white clouds curled around the northern border of the Red Spot, blocking the motion of small spots that might otherwise have been caught up in the vortex. Spots approaching the Red Spot from the east just turned around and headed back in the direction from which they had come.

Voyager 2 captured the Red Spot region four months after Voyager 1, when some changes had taken place in the cloud circulation pattern around it. This is a mosaic of Voyager 2 frames, taken on July 6. The white oval to the south is not the same one that was present in a similar location during the Voyager 1 flyby, because of differential rotation at the two latitudes. [260-606]

In the northern hemisphere, small brown anticyclonic features speed around the planet, often colliding with one another. On collision,the spots may combine and roll around together for a while. Ultimately, part of the mass of the combined spots is ejected as a streamer, and the remaining material continues on its eastward path.

Despite all the turbulence in Jupiter’s atmosphere—this ever-changing chaotic mixture of cyclonic and anticyclonic flows, of ovals and filaments, of reds, browns, and whites—a pattern may be emerging: There is an underlying order to the seemingly random mixing of patterns we see in the Jovian atmosphere.

First, the changing weather patterns are in some sense cyclic. The fact that Jupiter may be reverting to the appearance it presented at the time of Pioneers 10 and 11 is not surprising. From Earth-based studies, astronomers have found that the face of Jupiter often goes through a major change every few years. The transition is very rapid, but the planet maintains its new “look” for some time—until the next major transition. At the time of the Pioneer flybys, “The Red Spot was prominent, but it was surrounded by intense cloud. There was no visible structure at all in the south tropical zone—it was totally bland. You couldn’t see the turbulent area to the west,” explained Garry Hunt. “And, I believe that the buildup of cloud that we’re seeing to the east of the Red Spot is the beginning of the transition that will produce the Pioneer look.”

There is even more order underlying Jupiter’s changing atmosphere. This order is revealed in part in the alternating belts and zones. It is believed that the cloud-covered zones are regions of rising air, and the belts are regions of descending air. The internal energy of Jupiter provides the power to maintain this pattern of slow vertical circulation. In addition, there are horizontal or zonal flows that are much more regular than the changing cloud patterns.

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