CHAPTER 8. Jupiter—King of the Planets

JUPITER—KING OF THE PLANETS

A Star That Failed

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.

Composition and Atmospheric Structure

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.

Cloud top-aerosols Ammonia crystals Ammonium hydrosulfide clouds Ice crystal clouds Water droplets Trace compounds Fluid molecular hydrogen Transition Zone Fluid metallic hydrogen Possible core

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.

ELEMENTS DETECTED IN THE JOVIAN MAGNETOSPHERE Element Atomic Instruments Number

Hydrogen (H) 1 UVS, Plasma, LECP, CRS Helium (He) 2 Plasma, LECP Carbon (C) 6 LECP Nitrogen (N) 7 CRS Oxygen (O) 8 UVS, Plasma, LECP, CRS Neon (Ne) 10 CRS Sodium (Na) 11 LECP, CRS Magnesium (Mg) 12 CRS Silicon (Si) 14 CRS Sulfur (S) 16 UVS, Plasma, LECP, CRS Iron (Fe) 26 CRS

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.

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.

Weather on Jupiter

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.

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 white band 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.

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 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.

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.

Order out of Chaos

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.

“At first, Voyager seemed to do nothing but emphasize the chaos, not the order in Jupiter’s atmosphere,” Andy Ingersoll stated. “There are turbulent regions in which individual little spots seem to change every Jovian rotation. And the whole texture in certain turbulent regions is unrecognizable in one earth day.” With the Voyager spacecraft, more detail could be seen than ever before; in addition, changes could be observed on a small timescale, as they happened. “It became much more of a mystery how large-scale order could exist in the face of all this small-scale chaos. But I think we are beginning to see the order underneath. What we are looking at when we observe Jupiter are minute cloud particles representing only a small fraction of the mass of the atmosphere.” The large-scale order the scientists had found was a regular alternation of eastward and westward jets. “If we take all the measurements from Earth-based observations over the last 75 years, we find that every current that has ever been seen from the Earth over 75 years is visible in one ten-hour rotation. They’re all there—they were just invisible.” The ever-changing appearance was dancing above a regular, almost stationary pattern of alternating flows, which may come from deep within Jupiter’s atmosphere. Why this alternating pattern persists remains a mystery. Even if the underlying pattern can be thought of as a sort of Jovian climate, it still does not explain the mechanics of “the minor sideshow”—the changing weather patterns. Analysis of the Voyager pictures is sure to keep planetary meteorologists busy for many years to come.

Lights in the Night Sky

Toward the end of the first encounter period, Voyager 1 flew behind Jupiter, and the spacecraft’s wide-angle camera scanned the northern hemisphere on the nightside of the planet, searching for aurorae and lightning bolts. The most impressive darkside feature found was a tremendous aurora in the north polar region. But this was not the first time Jovian aurorae had been detected. Very-high-energy auroral emissions resulting from ultraviolet glows of atomic and molecular hydrogen had been detected prior to encounter on the bright side of Jupiter by the ultraviolet spectrometer. The ultraviolet observations indicate that atmospheric temperatures in the auroral regions are at least 1000 K. In both the visible and the ultraviolet spectra, the aurorae are confined to the polar regions and result from charged magnetospheric particles striking the upper atmosphere. The ultraviolet aurorae are created when high-energy particles from the Io plasma torus spiral in toward Jupiter on magnetic field lines.

Several meteor trails were also evident in the darkside pictures of Jupiter’s atmosphere. Traveling at roughly 60 kilometers per second as they entered, these fireballs brightened quickly and seemed to survive for about 1000 kilometers before they died.

Clusters of lightning bolts—indicative of electrical storms—were also discovered on Jupiter’s nightside. This particular phenomenon does not seem to depend on latitude. The Voyager 1 photograph that captured the huge Jovian aurora also caught the electrical discharges of 19 superbolts of lightning, and Voyager 2 photographs located eight additional flashes. Radio emission (whistlers) from lightning discharges were also detected by the Voyager radio astronomy receivers and the plasma wave instrument.

Magnetic Field

Deep in the interior of Jupiter, the pressures are so great that hydrogen becomes an electrical conductor, like a metal. Currents driven by the rapid rotation of the planet are thought to flow in this metallic core. The result is a magnetic field that penetrates the space around Jupiter.

Direct measurements of the Jovian magnetic field were first made by the Pioneers, and Voyager results generally confirm the initial findings. The strength of the Jovian field is about 4000 times greater than that of the Earth. The dipolar axis is not at the center of Jupiter, but offset by about 10 000 kilometers and tipped by 11 degrees from the axis of rotation. Each time the planet spins, the field wobbles up and down, carrying with it the trapped plasma of the radiation belts. The Voyager particles and fields instruments concentrated not on the planetary magnetic field but on the processes taking place in the magnetosphere.

Wavelength (µm) H₂O CH₃D GeH₄ NH₃ Voyager 1 Voyager 2 Wave number (cm⁻¹) Voyager 1 brightness temperature (K) Voyager 2 brightness temperature (K)

The Magnetosphere

Giant Jupiter has an enormous realm—from the size of its satellite system to its tremendous aurorae and superbolts of lightning, to the huge planet-sized cloud features that surround its atmosphere. The most gargantuan Jovian feature is its magnetosphere, which envelopes the satellites and constantly changes in size, pumping in and out at the whim of the solar wind. The Pioneer and Voyager spacecraft provided four cuts through this dynamic region, showing that its borders in the upwind solar direction lie between 50 RJ and 100 RJ from Jupiter. Downwind, away from the Sun, the magnetosphere extends much farther; some scientists postulate that a magnetotail may reach as far as the orbit of Saturn.

Charged particles in the magnetosphere are subject to powerful forces. Tightly embedded in Jupiter, the magnetic field spins with a ten-hour period as the planet rotates. The particles are caught in the spinning field and accelerated to high speeds. The result is a co-rotating plasma in the magnetic equator of Jupiter, extending outward to at least 20 R_J. Beyond this distance, the flow breaks up and the magnetosphere is more unstable. Within the co-rotation region, the spinning plasma sets up a powerful electric current girdling the planet.

Charged particles can be accelerated in the magnetosphere to high energies, corresponding to speeds tens of thousands of kilometers per second. Some of these particle streams escape from the inner parts of the magnetosphere and can penetrate the magnetopause and be ejected from the Jovian system. On Voyager 1, the low energy charged particle instrument began detecting these streams of “hot” plasma on 22 January, when Voyager 1 was still 600 RJ (almost 50 million kilometers) from the planet. Voyager 2 first detected Jovian particles at an even greater distance, 800 RJ. Hydrogen and helium ions (protons and alpha particles) dominate the magnetosphere at great distances from Jupiter, but increasing amounts of sulfur and oxygen appeared as the spacecraft crossed the magnetopause. The heavier ions presumably originate from Io.

In the inner magnetosphere, the Galilean satellites have a powerful influence on the populations of fast-moving particles. During the Voyager 1 encounter, the primary effect was observed at Io, where the satellite apparently sweeps up energetic electrons. In the million-volt energy range, these particles are depleted near Io, with peaks observed both inside and outside the satellite’s orbit. Voyager 2 passed close to Ganymede, and here also major effects were seen, with the satellite apparently absorbing electrons. In the wake created by the motion of the co-rotating plasma past Ganymede, the particle populations showed large and complex variations.

Just inside the Jovian magnetosphere is the “hot spot” of the solar system: a 300-400 million degree plasma detected by Voyager 1 while it was still about 5 million kilometers from Jupiter. T. P. Armstrong commented, “Even the interior of the Sun is estimated to be less than 20 million degrees.” S. M. Krimigis added that the temperature of this plasma is “the highest yet measured anywhere in the solar system.” Fortunately for Voyager, this region of incredibly hot plasma is also one of the solar system’s best vacuums. The spacecraft was in little danger because the bow shock protects this region from the solar wind, and most of the particles in Jupiter’s magnetosphere are held in much closer to the planet.

The very hot plasma in the outer magnetosphere discovered by Voyager is thought to play an important role in establishing the size of the Jovian magnetosphere. Although the density is low, only about one charged particle per hundred cubic centimeters, this plasma actually carries a great deal of energy because of the high speed of the particles. It is this plasma pressure, rather than the magnetic field pressure, that appears to hold off the pressure of the solar wind. However, the balance between hot plasma inside the magnetopause and the solar wind outside is not very stable. The Voyager experimenters suggest that a small change in solar wind pressure can cause the boundary to become suddenly unstable. A large quantity of the hot plasma can then be lost, producing the bursts seen at large distances and permitting a sudden collapse of the outer magnetosphere. Continued injection of hot plasma from within would then reinflate the magnetosphere, which would expand like a balloon until another instability developed. Processes of this sort may be the cause of the rapidly varying magnetospheric boundaries observed by both Voyager spacecraft.

Rings of Jupiter

One of the spectacular discoveries of Voyager was the existence of a ring system of small particles circling Jupiter. Saturn and Uranus were known to have rings, but none had been seen before at Jupiter.

As revealed by the Voyager cameras, the rings extend outward from the upper atmosphere to a distance of 53 000 kilometers above the cloud tops, 1.8 R_J from the center of the planet. The main rings, however, are much narrower, spanning from 47 000 to 53 000 kilometers above Jupiter. There are two main rings, a 5000-kilometer-wide segment, and a brighter, outer 800-kilometer segment. The thickness of the rings is unknown, except that it is certainly less than 30 kilometers, and probably under 1 kilometer.

The rings of Jupiter are quite tenuous, which explains why they are invisible from Earth [although they have been detected from Earth since Voyager]. Seen face on, the brightest part of the ring blocks less than one part in ten thousand of the light passing through, making it essentially transparent. In fact, the ring does not even offer much resistance to a spacecraft; Pioneer 11 traversed the ring in 1974 with no obvious ill consequences. Apparently the individual particles that make up the ring are widely dispersed. They can be seen only when the rings are viewed nearly edge-on, or toward the Sun, where they show up well in forward scattered light. It is this extra brilliance when backlit that created the excellent photos taken by Voyager 2 from inside the shadow of Jupiter.

The individual ring particles are probably dark, rocky fragments that are very small—essentially dust grains. They move around Jupiter in individual orbits, circling the planet in 5 - 7 hours. Scientists postulate that such orbits are not stable and that the particles fall slowly in toward Jupiter. Apparently the rings are constantly renewed from some source, which may be the satellite Adrastea (J14), discovered by Voyager 2. There has also been speculation that Adrastea may influence the ring structure by sweeping particles out of the ring. At present the rings of Jupiter remain mysterious. They are clearly very different from the rings of Saturn and Uranus, and reaching an understanding of their origins and dynamics presents many challenges to planetary scientists.