SP-349/396 PIONEER ODYSSEY
 

6

Results at the New Frontiers

 

 

[95] ON ITS WAY to the historic encounter with Jupiter, Pioneer 10 completed a number of scientific experiments in the unexplored space beyond the orbit of Mars. After encounter, these experiments continued and are expected to do so for many years. They were supplemented by information from Pioneer 11, which also proceeded beyond Jupiter to explore Saturn and then the outer Solar System. By contrast with exploration of the inner Solar System, these new scientific frontiers are probed more slowly because of the vast distances involved. It may be decades before all the scientific information is analyzed and evaluated.

But already a broadening understanding emerges of the Solar System beyond Mars from the data gathered by these pioneering spacecraft. The new information describes the interplanetary medium beyond the orbit of Mars, the asteroid belt, and the environment of the Jovian system, together with more accurate physical details of Jupiter itself.

 

The Interplanetary Medium Beyond the Orbit of Mars

Theoretically the solar wind, blowing through the interplanetary medium, might be expected to expand radially from the Sun in a symmetrical

fashion so as to expand and cool adiabatically: i.e., it does not exchange heat with its surroundings. In such a theoretical model, the temperature of the solar wind would decrease with distance according to a four-thirds power law. At the distance of the Earth from the Sun, experiments showed that this law is not quite valid - the solar wind behaves somewhat differently-and it seems that nonuniformities in the solar wind arise from hot spots in the solar corona. Since the temperature of the solar corona determines the speed of the solar wind, such hot spots would be expected to give rise to solar wind streams of different speeds. Moreover, because the Sun rotates on its axis, a fast moving stream of the solar wind can catch up with a slow moving stream that starts out earlier from a cooler part of the corona.

When a fast stream catches up with a slow moving stream, it tries to penetrate it but is prevented from doing so by magnetic fields carried by the streams. These fields are carried along because the energy density of the solar wind is about 100 times the energy density of the interplanetary field.

The solar wind streams act somewhat like billiard balls, colliding and rebounding. There are steep magnetic gradients between the streams at the times of collision. It is the magnetic interface that becomes a scattering region for cosmic rays....

 


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Figure 6-1. Fast streams of solar wind catch up with slow streams and produce scattering regions that prevent low energy cosmic rays from penetrating into the Solar System.

Figure 6-1. Fast streams of solar wind catch up with slow streams and produce scattering regions that prevent low energy cosmic rays from penetrating into the Solar System. The average velocity of the solar wind changes imperceptibly to the orbit of Jupiter but, as shown in the lower diagram, the range of fluctuations is remarkably diminished at this distance. Pioneer 10 and 11 might reach the boundary where this scattering effect ends and galactic cosmic rays would be expected to increase in intensity. To eight times Earth's distance from the Sun there is no sign of approaching such a boundary.

 

[97] ....coming into the Solar System from the Galaxy, not the solar wind itself (Figure 6-1).

Because of these scattering centers, no low energy cosmic rays are able to penetrate into the inner Solar System. The question is: how far into the Solar System do the low energy galactic cosmic rays penetrate'?

It is predicted from the Pioneer observations that the scattering regions will not damp down until perhaps 20 to 30 times Earth's distance from the Sun. This distance will be reached by the Pioneer spacecraft. Pioneer 11 is moving in the same direction as the Solar System is traveling through the Galaxy in the direction of a hypothetical bow shock of the heliosphere and the interstellar medium, and Pioneer 10 is traveling in the opposite direction. The heliopause, or boundary of the heliosphere, is expected to be closer to the Sun along the trajectory of Pioneer 11.

The question is: will the spacecraft be able to return information from so far away? The survival of Pioneer to the vast distances beyond Jupiter is very important in checking on the changes to the solar wind in the outer Solar System. And, if all goes well, both Pioneers should do this, though Pioneer 11 may not survive its encounter with Saturn, particularly if it passes through the inner ring plane. Even as the RTG power supply output begins to fall due to an expected decreased output from the power-converting thermopiles, experiments can be cycled shut off and then later brought on again - to conserve power and store this power in the battery for short periods of data transmission to the limits of communications distance-possibly to 20 times Earth's distance from the Sun, i.e., some two billion miles.

The very low intrinsic magnetic field of the Pioneer spacecraft makes it possible to investigate the extremely weak interplanetary magnetic field far out into the Solar System. Additionally, because the flight of Pioneer 10 is at a time of minimum solar activity in the 11-year solar cycle, the effects of the Sun on cosmic rays are at a minimum too. Thus, these particles from the Galaxy, possibly representing material from a dense con

centration of stars at the center of the Galaxy, may be penetrating far enough into the Solar System for the Pioneers to detect them. To eight times Earth's distance from the Sun Pioneer 10 has shown that contrary to some theories there is no increase in the intensity of galactic cosmic rays consisting of particles with energies above 80 MeV. Traditional cosmic ray theory has assumed that as the solar wind is diffused at increasing distances from the Sun, the intensity of galactic cosmic rays would increase. The Pioneer measurements are five times less than the intensities expected by traditional cosmic ray modulation theory.

One speculation gaining credence today is that a proportion of these lower energy 'cosmic rays' is not in fact of galactic origin but instead consists of ions of the solar wind accelerated to higher energies within the magnetospheres of planets of the Solar System. Moreover, the results point toward an increased possibility that the heliosphere extends much farther from the Sun than once thought-even as far as 100 times Earth's distance from the Sun, i.e., far beyond the orbit of Pluto.

Pioneer 11 will follow Pioneer 10 into the outer Solar System. If the second spacecraft survives its encounter with Saturn and its rings it, too, will investigate the heliosphere and the magnetic fields in the outer Solar System. These two Pioneers have advantages over the Mariner Jupiter/Saturn spacecraft in that they are magnetically very clean and are exploring the Solar System at a time when, because solar activity is minimal, the heliopause might be expected to be closer to the Sun.

The two Pioneer spacecraft may clear up much of the present speculation about the origin of cosmic rays and the flux of light and of particles from stars in general (Figure 6-2). One of the big questions facing astronomers is whether or not stars give off as much energy in particles as they do in light. Looking from Earth at the entire sky, they find that the light energy received from stars is about equal to the energy of incoming cosmic rays. Yet, the output of the Sun is very different: it pours out much more energy in the form of light than as particles. So it could be that the high cosmic [98] ray flux from the Galaxy is some purely local effect, such as from the violent explosion of a star or a group of stars in our part of the Galaxy, or the charged particle residue from the death of very old stars formed billions of years before our own Sun, and trapped within the magnetic confines of our Galaxy, or even generated locally in planetary magnetospheres.

 


Figure 6-2. A big unanswered question is whether cosmic rays come from the stars in general (a- top) or from exploding stars (b-bottom) in the neighborhood of the Solar System. (Photo. Hale Observatories)

Figure 6-2. A big unanswered question is whether cosmic rays come from the stars in general (a- top) or from exploding stars (b-bottom) in the neighborhood of the Solar System. (Photo. Hale Observatories)

 

Pioneer experiments showed that as far out as 565 million km (350 million mi.) from the Sun, solar magnetic field strength, solar wind density, and numbers of solar high energy particles, all declined roughly as the square of distance from the Sun, as was expected. But surprisingly, as mentioned earlier, the galactic cosmic ray intensity did not increase. As it moves outward, the solar wind stream becomes less variable while its gases cool much less rapidly; the high speed streams are converted into random thermal motions of particles (see lower diagram on Figure 6-1).

The experimenters found out, too, that the uncharged hydrogen atoms of an interstellar wind -the gas between the stars-stream into the Solar System along the plane of the Earth's orbit. Pioneer also found helium atoms in space for the first time. Experimenters believe that these, too, are from interstellar space.

Pioneer 10 produced new information about the Zodiacal Light, the faint band of light along the Zodiac, believed to be an effect of sunlight reflected from particles in interplanetary space. The slight enhancement of the glow exactly opposite to the Sun in the sky - the Gegenschein - could be caused by distant particles illuminated like miniature full moons opposite the Sun, or by a stream of particles extending as a comet's tail from Earth.

The imaging photopolarimeter was turned on March 10, 1972, seven days after Pioneer 10 was launched. During the first few weeks of the mission, when the Sun angle was about 26 degrees from the spacecraft's spin axis, only that part of the sky more than 60 degrees from the Sun line could be inspected by the imaging photopolarimeter. So observations concentrated on the Gegenschein. It quickly became apparent that the Gegenschein could not be associated with Earth because, although the spacecraft had not moved much farther from the Sun, it had moved ahead along the orbit of the Earth. While the direction of the Gegenschein was directly away from the Sun as seen from the spacecraft, this direction was by this time different from the direction of the Gegenschein [99] from Earth (Figure 6-3). So the anti-solar glow was confirmed as being associated with the light reflected from particles spread around the Solar System, not from particles associated with the Earth itself. Experimenters later measured the faint glow of the Gegenschein to near the orbit of Mars, again confirming its interplanetary nature.

As Pioneer 10 moved away from the Sun, the brightness of the Gegenschein also decreased, thereby showing that it results from particles in the inner Solar System. There was, however, a decrease in the rate at which the brightness faded within the asteroid belt, which indicates that the particles responsible for the counterglow increase somewhat within the belt. But beyond the belt there is virtually no Gegenschein.

As Pioneer moved out from Earth, it became possible to start mapping the whole of the sky to look at the Zodiacal Light. Scientists found that the Zodiacal Light also decreases in brightness as the square of the distance from the Sun. The rate....

 


Figure 6-3. The Gegenschein cannot be associated with Earth because seen from Pioneer and seen from Earth it is in different directions in space.

Figure 6-3. The Gegenschein cannot be associated with Earth because seen from Pioneer and seen from Earth it is in different directions in space.

 

...of decrease slowed within the asteroid belt, indicating that particles responsible for the Zodiacal Light, although concentrated in the inner Solar System, also increase somewhat within the asteroid belt itself. But beyond three and a half times Earth's distance from the Sun, the Zodiacal Light is negligible, and experimenters were able to record the integrated starlight from the Galaxy tree of the Zodiacal Light for the first time. Since the Zodiacal Light's brightness, at 2.41 times Earth's distance from the Sun, is less than one tenth that at Earth's orbit, experimenters conclude that the asteroid belt beyond this distance does not contribute significantly to the Zodiacal Light as seen from Earth. Zodiacal Light brightness fades almost completely at a distance of 3.3 times Earth's distance from the Sun. This is where particles would have to circle the Sun in orbits having a period of half that of Jupiter. Jupiter's gravity appears to sweep the Solar System clear of such particles beyond this resonance orbit. Thus, there is virtually no Zodiacal Light nor Gegenschein beyond the asteroid belt.

 

Meteoroids and the Asteroid Belt

Pioneer 10 provided some surprises even before reaching the orbit of Mars. At one time, it was speculated that because Soviet and U.S. spacecraft encountered trouble on their way to Mars at about 175 million km (110 million mi.) from the Sun, a concentration of asteroids occurred inside the orbit of Mars, or a band of dust inside the orbit of Mars presented a hazard to spacecraft. Pioneer 10 showed the speculation to be unfounded and provided data to suggest that Mars might even be sweeping its orbit clean of particles.

The 280 million km ( 175 million mi.) wide asteroid belt did not prove to be as hazardous as some speculation had suggested prior to Pioneer 10's epic voyage. Astronomers, who had observed the large number of minor planets in the asteroid belt, had postulated that the small bodies might be colliding with each other. As a consequence over the billions of years since the formation [100] of the Solar System, these collisions might have populated the zone of the asteroids with innumerable particles, ranging in size from the major asteroids to grains of dust. Such particles in myriads could present a serious hazard to spacecraft.

By June of 1972, just before the spacecraft entered the asteroid belt, Pioneer 10's detector cells had recorded 41 puncturing impacts. These occurred at a fairly steady rate from launch in March of that year. By October, when Pioneer was half way through the belt, the counting rate remained much the same and another 42 impacts had been recorded. This rate continued relatively unchanged all the way through the belt.

But, by the middle of February 1973, Pioneer 10 had cleared the asteroid belt safely. There was no indication of myriad tiny bodies ready to pepper any spacecraft in these regions of space. Thus, from a hazardous particle point of view, there appeared to be no asteroid belt. Fine particles seem to be fairly evenly distributed between the planets. These results confirm the Zodiacal Light and Gegenschein observations, and imply that the asteroid belt is not a serious hazard to spacecraft traversing it.

Pioneer 11 confirmed these results as it also penetrated the asteroid belt safely.

Also, very small particles of interplanetary dust seem to be swept by Mars and Earth to produce a gap from 1.14 to 1.34 times Earth's distance from the Sun, while they appear to be concentrated in the vicinity of Jupiter by Jupiter's gravity. This concentration at Jupiter was detected by Pioneer 10 undergoing 300 times more impacts of the tiny dust particles as it had in any region of interplanetary space since leaving Earth. Such a concentration around Jupiter is not a hazard to spacecraft flying by the planet, but may be a hazard to orbiting spacecraft.

Pioneer 10 detected an increase in the number of meteoroid-sized particles hitting its detectors as it flew by Jupiter, suggesting a one hundred-fold increase in dust density compared with that in interplanetary space. The Pioneer 11 detector - because its design was less sensitive to small particles - detected fewer particles during its flyby. But the number of particles detected in the steeply inclined and retrograde trajectory of Pioneer 11 appears to be consistent with the gravitational collection of particles from interplanetary space by the mass of Jupiter rather than a dust zone surrounding Jupiter analogous to the rings of Saturn.

 

The Jovian System

The paths of the two Pioneer spacecraft through the Jovian system, observed by tracking them from Earth, revealed that the system Jupiter plus the satellites-is heavier than previously calculated by about twice the mass of Earth's Moon. Jupiter, itself, is about one Moon mass heavier than previously calculated-namely 317.8 Earth masses.

The close approach (to 1.62 Jupiter radii from the center of the planet) by Pioneer 11 provided additional details of the gravitational field of the giant planet and confirmed the Pioneer 10 results.

Analysis of the gravity field results from the Pioneer 11 flyby shows that Jupiter is a very symmetrical planet, almost as though it had been turned on a lathe and with no gravitational anomalies like those of the terrestrial planets. This situation is best modeled by a planet that is almost entirely liquid.

A new measurement of the diameter of Jupiter and of the planet's polar flattening was made. Jupiter is slightly more flattened than derived from the best visual observations from Earth. The diameter of the planet was measured at a pressure of 800 millibars near to the cloud tops. Its polar diameter is 135,516 km (82,967 mi.) compared with an equatorial diameter of 142,796 km (88,734 mi.). These new values were confirmed by the timing of the occultation of the spacecraft by Jupiter. Thus, Jupiter is ten times as flattened as is Earth, probably because of its fluid state and its high speed of rotation. The average density of Jupiter, calculated from its mass and its volume, is confirmed at one and one third that of water.

 


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Figure 6-4. Pioneer 10 provided specific information on the physical characteristics of the Jovian satellites.

Figure 6-4. Pioneer 10 provided specific information on the physical characteristics of the Jovian satellites.

 

The Pioneers provided new information about the physical characteristics of the large Jovian satellites (Figure 6-4). In terms of the mass of Earth's Moon (1/81 of Earth's mass), the masses of the satellites in order of distance from Jupiter are determined as: IO, 1.22; Europa, 0.67; Ganymede, 2.02; and Callisto, 1.44 lunar masses. This measurement of Io's mass is 23 percent greater than that calculated before the Pioneer odyssey. The density of the satellites decreases with increasing distance from Jupiter and was refined as the result of Pioneer 11 observations Io's density is 3.52 times that of water; Europa's, 3.28; Ganymede's, 1.95; and Callisto's, 1.63. The two inner satellites thus seem to be rocky bodies-Io's density is, indeed, greater than that of Earth's Moon while the outer satellites could consist largely of water ice to account for their low density. The four satellites each have an average daylight surface temperature of about -145° C (-230° F).

It would seem that these satellites formed in such a way that lighter elements were depleted close to Jupiter or that water did not condense on Io and Europa because of their higher temperature near to Jupiter either during the condensation from the original nebula or as a result of subsequent heating of Jupiter.

Pioneer 10 was occulted by lo and, thus, was able to probe into the satellite's atmosphere. While a spurious command prevented a spin-scan image of IO, images were obtained of Europa and Ganymede. The Ganymede picture resolves features to 400 km (240 mi.) and shows a south polar mare and a central mare, each about 800 km (480 mi.) in diameter, and a bright north polar region (Figure 6-5). These isolated dark areas may, however, be areas where frost is not being formed as fast (by upwelling from the liquid watery interior) as evaporation takes place from a surface that is essentially without an atmosphere.

 


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Figure 6-5. Ganymede appears more like Mars than the Moon or Mercury in this close-in spin-scan image from Pioneer 10.

Figure 6-5. Ganymede appears more like Mars than the Moon or Mercury in this close-in spin-scan image from Pioneer 10. Earlier eclipse observations from Earth suggested that Ganymede has a surface of high porosity, the upper half-inch of which might be composed of loose, fine-grained rock.


Figure 6-6. Europa was too far away for Pioneer to obtain a detailed image.

Figure 6-6. Europa was too far away for Pioneer to obtain a detailed image. (See also Chapter 9.)

 

Probed by radar from Earth, Ganymede appears to have a surface rougher than Mercury, Mars or the Moon. This Jovian satellite may have a surface consisting of rocky or metallic material embedded in ice. While weathered smooth on the surface, the blocks of material within the ice would present a rough surface to the radar probing since the ice is relatively transparent to radar of the wavelengths used.

A less detailed spin-scan image of Europa by Pioneer 10 shows a somewhat similar appearance to that of Ganymede, but the satellite was too far from the spacecraft to provide a satisfactory picture (Figure 6-6), although bright and dark regions can be distinguished on the image. Pioneer 11 also obtained spin-scan images of several of the Galilean satellites (see Chapter 9) which reveal surface markings.

The satellite lo appears to be quite different from the other Jovian satellites. Almost as large as the planet Mercury, lo was known to be orange in color and one of the most reflective objects in the Solar System. Dark polar caps were also seen. The phenomenal brilliance of lo may be due to an extensive crystalline layer much like salt flats in the American West. Sodium vapor emissions were detected from Earth and showed to be from a cloud of sodium vapor that extends 16,000 km (10,000 mi.) from lo's surface. This sodium may originate from the salts deposited on the surface of lo when water from its interior evaporated into space. The sodium may be removed from the surface by the impact of high energy particles trapped in the magnetosphere of Jupiter and intercepted by lo in its orbit around the giant planet.

During an occultation of Pioneer 10 by lo, the radio waves traveling from the spacecraft to Earth probed the atmosphere of lo. They showed that it has a density of some 20,000 times less than that of the Earth's, but it extends some 115 km (70 mi.) above the surface of lo. This satellite is thus one of the smallest planetary bodies known to possess an atmosphere.

An ionosphere was discovered on lo which extends 700 km (420 mi.) high above the dayside [103] of the small satellite. Io is revealed as a unique planetary body in that it possesses an ionosphere while buried in the magnetic field of its mother planet. The ionosphere of lo is affected by the magnetic field of Jupiter to produce quite different day and night aspects. It is theorized that the higher levels of the ionosphere are swept away by the magnetic field of Jupiter. So, at night, when sunlight is not affecting Io's ionosphere, the upper layers decay. This is quite different from Earth, where the lower ionosphere layers decay at night.

The ionospheric density varies from 60,000 electrons per cubic centimeter on the day side to 9,000 on the night side. The unusual density and extent suggest an unusual gas mixture, possibly of sodium, hydrogen, and nitrogen.

Pioneer 10 also found that lo is embedded in a cloud of hydrogen that extends a third of the way around its orbit. This was quite unexpected. Perhaps 161,000 km (100,000 mi.) wide and the same high, the cloud is 805,000 km (500,000 mi.) long, and resembles one-third of a doughnut moving in orbit around Jupiter at a distance of 402,000 km (250,000 mi.) from the planet. No similar hydrogen clouds were detected for the other large satellites though looked for by Pioneer 11 following the Pioneer 10 discovery.

Like the Earth, Jupiter has a bow shock wave which is produced when the high speed solar wind, carrying a magnetic field, interacts with the magnetic field of Jupiter (Figure 6-7). The solar wind is abruptly slowed down so that its effective temperature....

 


Figure 6-7. The bow shock and the magnetosphere of Jupiter are mapped by Pioneer 10 and 11 and shown to be vastly more extensive than those of the Earth.

Figure 6-7. The bow shock and the magnetosphere of Jupiter are mapped by Pioneer 10 and 11 and shown to be vastly more extensive than those of the Earth. The shock wave and the magnetosphere vary in distance from Jupiter proportionally much more than do those of Earth. Jupiter's magnetic tail extends beyond the orbit of Saturn.

 

[104] ...is increased ten times. A magnetosphere surrounds Jupiter just as one surrounds Earth. This protects the planet from the solar wind which cannot penetrate into the magnetosphere. Between the magnetosphere and the bow shock is a turbulent region - the magnetosheath -in which the solar wind is deflected around the magnetosphere: but these phenomena are experienced around Jupiter on a scale vastly greater than around Earth.

Jupiter's magnetosphere has a diameter such that, if it could be seen around Jupiter from Earth, it would be about the apparent diameter of the Moon or Sun in Earth's sky. Pioneer 10's crossings of Jupiter's bow shock show a wave that is over 26 million km (16 million mi.) "wide" in the ecliptic plane, or about 80 percent of the distance between the orbits of Earth and Venus. Jupiter's magnetic tail reaches beyond the orbit of Saturn. The Jovian system is on a truly gigantic scale by earthly standards.

Pioneer 11 discovered that the magnetosphere is blunt on the sunward side and extends at least 80 radii of Jupiter in a vertical direction above and below the planet.

Jupiter's magnetosphere rotates at several hundred thousand miles per hour, along with the planet, and consists of an inner region shaped something like a doughnut with Jupiter in the hole. Outside of the doughnut is a highly unstable outer region caused by ionized gas perhaps thrown out into space as a consequence of the planet's rapid rotation coupled with the changing pressure of the solar wind.

The magnetosphere might also be described as being spongy since it pulsates in the solar wind and often shrinks to half its size. Pioneer 10 crossed the sharply defined boundary of the magnetosphere at 6.8 million km (4.2 million mi.) from Jupiter. [hen, as the magnetosphere abruptly changed size, Pioneer's instruments again sensed leaving the magnetosphere and entering it again later, all happening while the spacecraft continued its journey toward the planet. Pioneer 10 actually crossed this constantly pulsating bow shock wave 17 times on the post-encounter trajectory away from Jupiter, as the configuration of the bow shock changed due to its interaction with changes in the solar wind.

Pioneer 11 recorded three crossings of the bow shock on the inbound trajectory and three outbound. Again, the multiple bow shock and magnetopause crossings during the Pioneer 11 flyby suggest that there is a very dynamic interaction between the solar wind and the magnetosphere.

The Pioneer results show that there are three distinct regions of the Jovian magnetosphere (Figure 6-8): an inner magnetosphere within about 20 radii of Jupiter where the magnetic field of Jupiter predominates; a middle magnetosphere from 20 to 60 Jupiter radii where the magnetic field of the planet is severely distorted by trapped energetic particles; and an outer magnetosphere, beyond 60 Jupiter radii, which exhibits significant irregularities in both the magnitude and direction of the magnetic field.

In the middle magnetosphere, ionized particles form an electric current sheet around Jupiter. In turn, this current flow produces magnetic fields which at large distances from the planet are greater than the magnetic field of Jupiter itself. While in the inner magnetosphere the magnetic field directs particles along the magnetic equator, in the middle field the particles are in control and move parallel to the rotational equator of the planet itself.

At times, when the solar wind affects the magnetosphere, the outer field collapses and accelerates low energy particles to such high velocities that they are squirted out like jets from Jupiter; a new discovery by Pioneer. These particle jets make Jupiter a second source of high energy particle radiation in the Solar System, the other source being the Sun. Pioneer 10 detected these particles 225 million km (140 million mi.) away from Jupiter, and scientists have now confirmed that the particles have been detected at Earth's orbit for several years, but before Pioneer 10 scientists did not know that the particles originated at Jupiter.

The Pioneers refined all the Earth-based predictions. Jupiter's magnetic field is now known to be over 10 times as strong as Earth's field, with the...

 


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Figure 6-8. Jupiter's magnetosphere is differently shaped from that of Earth and has three distinct regions.

Figure 6-8. Jupiter's magnetosphere is differently shaped from that of Earth and has three distinct regions.

 

...total energy in the field being some 20,000 times that in the Earth's field.

The closer and highly inclined trajectory of Pioneer 11 produced a more precise and more detailed definition of the magnetic field of Jupiter. It was found to be 5 percent greater than that estimated from the Pioneer 10 measurements; the dipole moment produces a field which is a measure of source strength of 4.2 Gauss at the visible surface.

Jupiter's field is tilted almost 11 degrees to the planet's axis of rotation, and the center of the field does not coincide with the center of Jupiter but is offset from the spin axis twice as much as Earth's field is offset. Because of this offset the strength of the field emerging from the cloud tops of Jupiter is quite variable from 14 to 11 Gauss respectively in the north and south polar regions, compared with Earth's polar field of 0.5 Gauss. The poles of Jupiter's field are reversed compared with those of the Earth. A north-seeking compass would point south on Jupiter.

Closer than about three Jupiter radii the magnetic field appears to be more complex than a simple dipole field. These complexities may arise from complex circulation patterns within the metallic hydrogen bulk of the planet.

[106] The measurements made by Pioneer 11 revealed that the main magnetic field is somewhat more complex than the Earth's magnetic field. The magnetometer results fit a model in which the quadrupole and octupole moments are at least 20 percent of the dipole moment compared with only about 11 percent for the Earth.

The field is not symmetrical and it has been suggested that this distorts the motion of trapped particles forming the radiation belts and may be the cause of the periodic escape of relativistic electrons from Jupiter into interplanetary space. The concentration of field lines around the strong north pole is also speculated as playing an important role in the modulation of the decametric radiation by the satellite lo. The primary source of decametric radiation might be sporadic precipitation of particles into the northern hemisphere along the flux tube to lo.

The somewhat larger quadrupole and octupole moments present in the magnetic field of Jupiter could have significant implications about the interior of the planet, assuming that the field is generated by a dynamo as generally accepted today. The higher field strength compared to Earth would imply that the dynamo-producing core of Jupiter, i.e., the metallic hydrogen, is responsible for the field rather than a small core of metals and silicates. To produce larger quadrupole and octupole effects the internal core would have to be proportionately larger for Jupiter than for Earth. This could not be a core like that of the Earth.

Energetic particles are trapped in the magnetosphere of Jupiter and produce radiation belts as in Earth's magnetosphere (Figure 6-9). These radiation belts were known before Pioneer 10's flight because of the radio waves generated by them and received at Earth. Such radiation belts require that a planet should have a magnetic field. In fact, their presence led scientists to predict that Jupiter had a magnetic field, although its strength was uncertain before the Pioneer mission made direct measurements within the field.

Because of the tilted magnetic field, the radiation belts of Jupiter are also tilted and wobble up and down in the surrounding space as Jupiter rotates on its axis. So a spacecraft passing the planet moves in and out of the belts due to this wobble.

 


Figure 6-9. Radiation belts consist of particles trapped in Jupiter's magnetic field, and the satellites sweep up many of the charged particles.

Figure 6-9. Radiation belts consist of particles trapped in Jupiter's magnetic field, and the satellites sweep up many of the charged particles.

 

[107] The inner belt, consisting of a wide spectrum of energies of electrons and protons, forms a doughnut around the planet, corresponding generally with the classical dipole magnetic field shape extending to about 10 Jupiter radii. Prior to the Pioneer observations it was generally assumed that the trapped radiation originated from the solar wind. Pioneer 11 experiments, however, led to the conclusion that the particles in this inner magnetosphere may not be captured from the solar wind but instead originate from Jupiter.

An outer belt is also spread widely, but with its most energetic particles-high energy electrons concentrated into a flatter area. This outer belt extends to at least 100 planetary radii parallel to the planet's rotational equatorial plane.

Peak intensities of electrons in the belts, as measured by Pioneer 10, were 10,000 times greater than Earth's maximum. Protons were several thousand times as intense as Earth's belts. The inner radiation belts of Jupiter, as measured by Pioneer 10, had the highest radiation intensity so far measured; comparable to radiation intensities following an explosion of a nuclear device in the upper atmosphere.

Pioneer 11 confirmed these high intensities. In the inner region of the magnetosphere high energy protons exceeding 35 MeV appear to peak in two shells; the outer shell was detected at 3.5 Jovian radii by Pioneer 10, and confirmed by Pioneer 11, and an inner shell, discovered by Pioneer 11, has a peak at 1.78 radii of Jupiter. Pioneer 11 also found that there is a greater flux of energetic particles at high Jovian latitudes than would have been expected from the measurements made by Pioneer 10. It also discovered that the flux of energetic particles peaks on either side of the dipole magnetic equator.

An important discovery of Pioneer 10 was that Jupiter releases bursts of very energetic electrons into interplanetary space. Pioneer 11 also detected the electrons and discovered that they were in phase with the observations made previously by Pioneer 10. Two days before Pioneer 11 entered the magnetosphere of Jupiter the spacecraft's instruments began to detect bursts of low energy protons. Both electrons and protons are emitted in phase with the synodic rotation of Jupiter.

Jupiter has thus been found to be a new source of energetic particles in interplanetary space which scientists can use to study particle propagation in the solar wind. Figure 6-10 shows the increasing rate of energetic electron events derived from the helium vector magnetometer results as Pioneer 10 approached Jupiter while Pioneer 11, still far from the planet, saw many fewer electron events. These events are observed when the interplanetary magnetic field provides good connection between Jupiter and the spacecraft. Large amplitude waves in the magnetic field have been found associated with the electrons in interplanetary space. They are probably generated by the electrons. Figure 6-11 shows diagrammatically the relation of the spacecraft, Jupiter, the interplanetary magnetic field, and the magnetic waves during a Jovian interplanetary electron event. Since cosmic rays are probably confined to the galaxy by similar self-generated waves, this observation by the Pioneers has significance for galactic as well as interplanetary physics.

In addition to 10-hour variations in the electron intensity and spectrum observed inside Jupiter's magnetosphere, similar variations were observed as far as 150 million km (93 million mi.) from the planet. These variations are in phase with those observed inside Jupiter's magnetosphere as shown in Figure 6-12. Both inside and outside of the magnetosphere the variations appear to be locked in phase; i.e., they occur regularly every 10 hours at precisely the same time irrespective of distance from the planet.

Comparison of the timing of the 10 hour intensity and spectral variations observed in the energetic electron flux in Jupiter's outer magnetosphere and in interplanetary space has caused scientists to reexamine the idea that the variations are caused by the flapping of the outer magnetosphere and equatorial current sheet in the course of Jupiter's rotation. Instead, the data now suggest that the variations take place simultaneously in the whole outer magnetosphere, so that Jupiter...

 


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Figure 6-10. Comparisons of the number of energetic electron events observed by Pioneer 10 and Pioneer 11 during the encounter of Pioneer 10.

Figure 6-10. Comparisons of the number of energetic electron events observed by Pioneer 10 and Pioneer 11 during the encounter of Pioneer 10.

 

...resembles a blinking beacon more than a twirling floppy hat as originally suggested from the Pioneer 10 data.

The results of Pioneer 11 suggest that the model of Jupiter's trapped radiation in the middle and outer magnetosphere is best represented by a disc with the magnetic field lines near the magnetic equator being closed and hence capable of trapping charged particles out to a radius of 100 times that of Jupiter.

It is also now clear that the magnetosphere is quite blunted on the sunward side of the planet. No information was available from either Pioneer 10 or 11 on the shape on the anti-solar side of Jupiter. Pioneer 10 did, however, encounter phenomena suggestive of a magnetic tail of Jupiter extending beyond the orbit of Saturn.

A significant new finding from the Pioneer 11 observations is that there is a net streaming of both electrons and protons away from the planet along high latitude field lines. The electrons were at energies less than 40 keV and less than 560 keV, and the protons between 0.61 and 3.41 MeV.

A recirculation of energetic particles within the Jovian magnetosphere thus emerges as a dynamic feature of Jupiter. It also suggests that the emission of energetic particles into interplanetary space as observed by both Pioneers may take place from the poles and other latitudes rather than exclusively from the equatorial regions of the magnetosphere.

 


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Figure 6-11. Relation of the spacecraft, Jupiter and its magnetosphere, the interplanetary magnetic field and magnetic waves during a Jovian interplanetary electron event.

Figure 6-11. Relation of the spacecraft, Jupiter and its magnetosphere, the interplanetary magnetic field and magnetic waves during a Jovian interplanetary electron event.

 


Figure 6-12. Both inside and outside of Jupiter's magnetosphere, the 10-hour modulation of electron intensity appears to be in phase. BS is the bow shock. MP is the magnetopause.

Figure 6-12. Both inside and outside of Jupiter's magnetosphere, the 10-hour modulation of electron intensity appears to be in phase. BS is the bow shock. MP is the magnetopause.

 

[110] Particles may be transported from low to high Iatitudes without significant changes to their energy. Particles in the outer magnetodisc diffusing inwards toward Jupiter are eventually squeezed into a pancake-shaped volume in the inner magnetosphere. Apparently the particles interact with plasma waves generated inside the magnetosphere which cause the particles to escape from the pancake-shaped magnetodisc and reach low altitudes. It is at these low altitudes that they can be transported to high latitudes without significant change in energy. The particles are thereby injected onto high latitude field lines and provide the outward streaming high latitude flux observed by Pioneer 11 (Figure 6-13). This dynamic recirculation may explain the otherwise baffling problem of the presence of particles with megavolt energies in the region of the outer magnetosphere including its boundary.

 


Figure 6-13. Energetic particles appear to be circulating in the magnetosphere and ejected at high latitudes.

Figure 6-13. Energetic particles appear to be circulating in the magnetosphere and ejected at high latitudes.

 

[111] Also this process may explain why the 10-hour periodicity occurs rather than a 5-hour periodicity that would be expected if the energetic particles from Jupiter observed in interplanetary space were squirted from the equatorial regions. The interplanetary magnetic field at a particular point in space would be expected to connect to either the north or south polar regions of Jupiter at any given time. Energetic particles originating from these regions by the circulation process would be modulated by planetary rotation so as to show a 10-hour periodicity. Thus the process of cross field diffusion and escape of particles from the polar regions may account for the 10-hour periodicity observed by the Pioneer spacecraft. However, a 5-hour periodicity could have been missed by both Pioneer spacecraft because of their trajectories.

The dynamics of the Jovian magnetosphere with its charged particles appear to differ in important ways from those of the Earth. First the presence of substantial intensities of electrons having energies greater than 20 MeV in the outer magnetosphere cannot be explained by trapping of particles from the solar wind. Such solar wind particles could only reach about 1 keV. The corotation of energetic particles with Jupiter persists out to the magnetopause, whereas for the Earth corotation terminates at the outer boundary of the plasmasphere, far inside the magnetopause. The inner satellites, Amalthea, lo, Europa, and Ganymede, produce a fluctuating and complex structure of energetic particles. By contrast, the Earth's Moon is far outside Earth's magnetosphere.

It seems likely that the capture of solar wind particles may be a relatively minor feature of the dynamics of the Jovian magnetosphere, whereas the internal acceleration of particles internally available within the magnetosphere may be the dominant process. Nevertheless, whether or not the solar wind particles are captured within the system, the solar wind is essential to establishing the physical conditions under which transfer of rotational energy from Jupiter to the charged particles can take place. The flow of the solar wind past Jupiter generates the axially asymmetric and non-rotating situation that is essential for the development of the Jovian magnetosphere.

While Earth's Moon is far beyond Earth's radiation belts, the large satellites of Jupiter and the innermost known satellite, Amalthea, are immersed in the Jovian belts. Consequently, the satellites sweep up particles from the belts and remove high energy particles to reduce total radiation near Jupiter by as much as 100 times. By far the largest number of particles is removed by lo. Additionally, this satellite is known to accelerate particles and to induce, in some way, the emission of decametric radio waves. Within the orbit of Amalthea the radiation environment is extremely complex and the flux of energetic particles varies from place to place around Jupiter. There appear to be nodes of concentrated particles and no single maximum. Indeed, the effect of Amalthea may be what causes the flux of energetic particles to stop increasing closer to Jupiter. However, there is the possibility that other effects may be controlling the number of particles close in to the planet as well as Amalthea's sweeping action. The offset of the magnetic field creates a particle-free region between the cloud surface of the planet and one-tenth of a radii above it analogous to an eccentric cam action.

Pioneer 11 measured large reductions in electron flux for energies below 560 keV and in proton flux for energies around 2.1 MeV as the spacecraft crossed the orbit of lo. Smaller effects were observed at the orbit of Amalthea, and only a rather feeble effect was seen at the orbit of Europa. However, near the orbit of Ganymede, Pioneer 11 detected strong transient anisotropic bursts of 1 MeV protons. One sequence of one-minute bursts continued for several hours. These particles appear to be locally accelerated.

The Pioneer 11 spacecraft discovered a high electron current flow at the orbit of Ganymede. Such an increase in electron flow had not been observed at the other passages of the Pioneers through satellite orbits. Near the Io flux tube the magnetic field line of Jupiter extending to Io along which scientists had speculated that large currents should flow- Pioneer 11 detected an increase of about [112] ten times the flux of electrons with energies above 0.46 MeV.

The picture of Jupiter emerges as an enormous spinning magnetosphere buffeted by the solar wind, a magnetosphere that is continually stirred and mixed by the Galilean satellites and Amalthea, a magnetosphere in which processes are at work very different from those taking place in the magnetosphere of Earth. As a result of the encounter of Pioneer 11 in which the total electron dosage was less than that experienced by Pioneer 10 because of the highly inclined trajectory of the second spacecraft even though approaching much closer to Jupiter, spacecraft trajectories can now be planned to pass quickly through the plane of intense radiation. Thus the practicality of the gravity-assist slingshot technique to explore the outer Solar System has been demonstrated and Pioneer 11, now renamed Pioneer Saturn, is on its way to the next outer gas giant with virtually no damage to its electronics or scientific instruments from its close approach to Jupiter.

The Pioneers also permitted a close look at the planet Jupiter itself as well as the environment surrounding it. These close looks were made possible by the spin-scan imaging technique, the infrared and ultraviolet experiments, and the radio occultation experiment. As a result, astronomers have been able to revise theories about the internal composition and the meteorology and atmosphere of the giant planet. The spin-scan images are discussed in detail in Chapters 8 and 9, but it is appropriate here to summarize the current theories of Jupiter which have been strengthened by or have evolved from the Pioneer 10 and 11 results.

Jupiter appears to be almost entirely fluid, with possibly only a very small solid core (Figure 6-14). This liquid interior seethes with internal heat energy being transferred from deep within the planet to its outer regions.

Jupiter's center may be at a temperature of 30,000° C (54,000° F); heat from continued gravitational contraction and partly residual primordial heat. Since the temperature at the cloud....

 


Figure 6-14. Pioneer 10 confirms models of Jupiter that suggest the planet is nearly all liquid with a very small core and a deep atmosphere.

Figure 6-14. Pioneer 10 confirms models of Jupiter that suggest the planet is nearly all liquid with a very small core and a deep atmosphere.

 

[113] ...tops of Jupiter is around -123° C (-190° F), there is a large range of temperatures within the planet and millions of cubic miles of the atmosphere could be at room temperature.

Atop the main liquid bulk of the planet is an even more turbulent atmosphere, possibly 970 km (600 mi.) thick. The top regions of this atmosphere produce clouds which are the visible surface seen from Earth. A transparent atmosphere extends above the visible surface and ultimately leads to a multi-layered ionosphere of highly rarefied, electrically charged gas.

Jupiter has convective circulation patterns, but the rapid rotation and the flow of internal energy outwards makes the weather patterns very different from Earth's.

Measurements of the density distribution within Jupiter from the paths of the Pioneers as they flew by imply that the planet is largely liquid; it has no concentrations of mass and no detectable crust or solid surface. But Jupiter could still possess a small rocky core of a few Earth masses consisting of iron and silicates. The composition of Jupiter is not precisely like that of the Sun since there is a five-fold enhancement of heavy materials probably in the form of silicates and the ices of ammonia, methane and water. Scientists cannot yet define how these heavier materials are distributed throughout the planet.

Jupiter is probably 87 percent hydrogen, and this hydrogen is most likely liquid, not solid, at the high internal temperatures of Jupiter, despite the high internal pressures. However, the pressure within Jupiter at about 24,000 km (15,000 mi.) below the visible cloud tops is sufficient to convert liquid hydrogen into a metallic form which more readily conducts heat and electricity.

Temperatures and pressures are enormously high in the interior of Jupiter. At 970 km (600 mi.) below the cloud tops the temperature is probably about 2000° C (3600° F). At 2900 km (1800 mi.), the temperature is believed to be 6000° C (11,000° F). At 24,000 km (15,000 mi.), the temperature may reach 11,000° C (20,000° F), and the pressure three million Earth atmospheres. It is about this level that the liquid hydrogen should turn into liquid metallic hydrogen.

Jupiter also consists of at least 12 percent helium. This helium might theoretically be soluble in liquid hydrogen. It is speculated, however. that if conditions are not just right, the helium might be insoluble within the hydrogen and form a 'sea' around the central core of Jupiter on top of which the liquid metallic hydrogen would float. There is, however, no adequate theory yet on the miscibility of metallic hydrogen and helium within a planet such as Jupiter. There might be precipitation of helium in the molecular hydrogen which would be important to layering and convective processes within the planet. In turn these could affect the magnetic field. Additionally there is the question as to whether rocks might dissolve in a hydrogen/ helium mixture at high temperature. This could prevent the formation of a discrete rocky core or disperse such a core that had already formed early in the planet's history.

The internal structure of Jupiter still remains somewhat indistinct.

The seething activity in the metallic hydrogen of Jupiter is thought to be evidenced by the complex magnetic field of the planet. Hydrogen moving up from the center of Jupiter, like water coming to a boil in a saucepan, would produce eddy currents that give rise to the magnetic field through rotation of the planet.

Somewhere around 970 km (600 mi.) below the cloud tops, where the pressure is low enough for the liquid hydrogen to become a gas the atmosphere of Jupiter begins. It is unlikely, however. that there is a sharp transition surface similar to the surface of an ocean. Rather, there is most probably a gradual change through a mixture of gas and liquid. But the top 970 km (600 mi.) of the planet, where there is no longer hydrogen in liquid form, is defined as the atmosphere of Jupiter.

Jupiter's atmosphere accounts for about 1 percent of the mass of the planet. It is predominantly hydrogen (about 85 percent) with nearly 15 percent helium and less than I percent of other gases. This is the same as the proportions of [114] elements found in the Sun. Although helium was believed to be present on Jupiter, the gas was not positively identified there until Pioneer 10 made its experiments.

Jupiter's atmosphere also has small amounts of ammonia and methane and traces of deuterium, acetylene, ethane and phosphine. In recent years water vapor has been detected in small quantities and also carbon monoxide, hydrogen cyanide, and germane. Several of the trace gases have been discovered, and continue to be discovered, through the use of telescopes mounted on high flying aircraft that overcome some of the masking absorptions of the Earth's atmosphere.

ln the regions of the atmosphere, 32 km (20 mi.) or so above and below the cloud tops, solar heat as well as the internal heat from Jupiter flowing outwards affects circulation. Jupiter's clouds form in the atmosphere by condensation as on Earth. But Jupiter's clouds appear to be of ammonia and ammonia compounds as well as water. The topmost clouds are thought to be of ammonia crystals with water clouds confined to lower levels.

 


Figure 6-15. The temperature profile in the atmosphere of Jupiter and the location of various cloud layers are shown in this diagram from data of A. Ingersoll, Caltech.

Figure 6-15. The temperature profile in the atmosphere of Jupiter and the location of various cloud layers are shown in this diagram from data of A. Ingersoll, Caltech.

 

[115] An inversion layer 35 km (22 mi.) above the visible clouds is thought to be caused by a layer of aerosols and hydrocarbons such as ethane and acetylene. This is a layer where sunlight is absorbed and adds heat to the cooling atmosphere. Methane, too, would absorb sunlight and contribute to the inversion layer.

Pioneer I O's occultation experiment produced results for the temperature of the Jovian atmosphere in conflict with ground based observations. And the data for Pioneer 11 were consistent with those from Pioneer 10. They were finally matched with the ground based observations by taking into account the great oblateness, or spin flattening, of Jupiter and its effect upon the path of the radio waves through the Jovian atmosphere. For three measurements entry and exit of Pioneer 10, and exit of Pioneer 11 the occultation data are quite consistent. They show a temperature inversion between the 10 and 100 millibar levels, with temperatures between -133 and - 113° C (-207 and -171° F) at the 10 millibar level and -183 to -163° C (-297 to -261° F) at 100 millibars. At the 0.001 millibar level, the temperature of the Jovian atmosphere, determined by an occultation of Beta Scorpio, is about -103° C (-153° F); at the cloud tops, however, the temperature is about -148° C ( -234° F) (Figure 6-15).

The Pioneer observations also show that the poles and the equatorial regions of Jupiter have effectively the same temperature; the temperature is also the same on north and south hemispheres and the day and night sides. Also, because the axis of Jupiter is inclined by only a few degrees, the planet does not have seasons like those of the Earth.

Because the Sun's radiation falls more concentrated per unit area of the equatorial regions it would be expected that the equator would be warmer than the poles as on Earth and other planets. Two theories have been proposed to account for the even distribution of temperature measured by infrared radiation from Jupiter. The first says the circulation within the atmosphere should be very efficient to redistribute the solar

heat. The second suggests that the heat flux from inside Jupiter is sufficiently greater at the poles to balance the lesser solar input there. Since there is no equator to pole atmospheric flow pattern observed on Jupiter, the second theory seems more likely to tit conditions on the giant planet. It is believed that convection is so effective over the whole planet that it eliminates any temperature differences due to the solar input variations with latitude. Thus, at the poles, where the cloud temperatures would expect to fall, convection speeds up from the interior to bring up heat and keep the temperature constant. At the equator, where the clouds are warmed more by the Sun, the amount of convection is reduced. Thus the planet acts as though governed by a huge thermostat.

It has been speculated that spots on Jupiter including the Great Red Spot, are most probably large hurricane-type features consisting of groups of persistent rising air masses like gigantic thunderstorms (Figure 6-16). It is no longer believed that the Great Red Spot is a column of gas anchored to some feature on a hypothetical surface of Jupiter. The core of the planet is now believed to be much too small to produce effects that would extend to the visible surface of the clouds; and the Pioneer spacecraft revealed no noticeable density differences that could be interpreted as being caused by the Red Spot extending toward a core.

Fundamental questions such as what makes the spot red and why has it lasted so long still remain unanswered though there are new speculative theories. Theories about the cause of the spot that it is the upper atmospheric manifestation of a surface feature or a floating island-are giving way to hydrodynamic explanations. Even the concept of it being the Jovian equivalent of a hurricane is being doubted. Equations describing the atmospheric flow on a rapidly rotating planet with an internal heat source can now be solved by the newest computers. Several scientists have developed mathematical models to explain the Great Red Spot. Whether these new hydrodynamic solutions do in fact apply to the real red spot must await careful comparison of the predictions of the spot's....

 


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Figure 6-16. The Great Red Spot is probably a hurricane-like almost permanent feature consisting of a great system of thunderstorms rising several kilometers above the topmost clouds of Jupiter.

Figure 6-16. The Great Red Spot is probably a hurricane-like almost permanent feature consisting of a great system of thunderstorms rising several kilometers above the topmost clouds of Jupiter.


Figure 6-17. A northern red spot on Jupiter, recorded in new detail by Pioneer, lends credence to the view that the red spots are purely atmospheric phenomena.

Figure 6-17. A northern red spot on Jupiter, recorded in new detail by Pioneer, lends credence to the view that the red spots are purely atmospheric phenomena.


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Figure 6-18. Color contours of the Great Red Spot on the terminator show bluing of the light by atmospheric scattering. (Figure courtesy Lyn Doose. )

Figure 6-18. Color contours of the Great Red Spot on the terminator show bluing of the light by atmospheric scattering. (Figure courtesy Lyn Doose. )

 

....behavior and characteristics with further observations and analysis of the Pioneer pictures.

One of the most significant images from Pioneer 10 showed a similar spot, though much smaller, in the Northern Hemisphere at the same latitude as the Great Red Spot (Figure 6-17). It has the same shape and structure and implies that the red spots are meteorological features in the atmosphere.

The Great Red Spot appears to rotate counterclockwise as seen from above, it is anticyclonic and behaves as an ascending mass of gas flowing out at the level of its top which pokes several miles above the topmost cloud layers.

By looking at sunlight reflected off a cloud, it is difficult to tell even on Earth, what is under the cloud. But we can determine something from the reflected light, about the size, distribution, and refractive index of the droplets making up the cloud. There is no haze over the Red Spot as it is carried by the rotation of Jupiter across the limb. At the terminator, the Red Spot shows bluing of the reflected light, where there is scattering of the sunlight into space (Figure 6-18).

 


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Figure 6-19. Pioneer 11 obtained unique views of the polar regions of Jupiter, views which are impossible from the Earth.

.

Figure 6-19. Pioneer 11 obtained unique views of the polar regions of Jupiter, views which are impossible from the Earth.

Figure 6-19. Pioneer 11 obtained unique views of the polar regions of Jupiter, views which are impossible from the Earth. They showed that the atmospheric patterns are very different in the polar regions from those in the equatorial and temperate regions of the planet.

 

Scientists speculate that the red color of the spot may be a result of phosphine being carried to great heights and broken down by solar ultraviolet to produce red phosphorous. Very high clouds would thus be red on Jupiter.

The views of the north polar regions of Jupiter (Figure 6-19) were unique in that such views of the planet cannot be obtained from the Earth. North of the North Temperate Belt, Pioneer's pictures show that the dark belts and light zones characteristic of the equatorial regions are successively less organized. The banded structure changes into oval and circular features within 10 to 15 degrees of the pole.

The details are greater in the red images of the polar regions, thereby suggesting that the atmosphere is thicker above the polar clouds than over the temperate and equatorial clouds of the planet.

Photopolarimetry has been used to estimate the optical depth of the atmosphere above the cloud tops. It appears to be three times as great above 60° latitude than in the equatorial zone. But the effects may arise from a thin high cloud layer or an unknown absorption in the upper atmosphere.

We are just beginning to understand the atmospheric dynamics of cloudy planets, and the Pioneer observations of Jupiter add considerably to our basic knowledge, by providing information on very deep atmospheres in rapid rotation without any solid surface interactions with the atmosphere. It also provides information on atmospheres driven mainly by heat from below rather than from the Sun.

Pioneer results seem to confirm earlier theoretical deductions that the Red Spot and the light colored zones are regions of well developed clouds, swirling anti-cyclones and rising air masses. The darker belts, by contrast, are cyclonic, sinking masses of air leading to depressed clouds. The ways in which the belts and zones scatter sunlight reflected from them are very different. It is speculated that the belts may appear dark because of [119] dark aerosols suspended in the gaseous atmosphere there. On Jupiter the familiar cyclones and anticyclones of Earth are stretched into linear or hookshaped features on the rapidly rotating planet, with extremely turbulent areas separating adjacent bands of different velocities; areas in which there are many examples of classical von Karman vortices.

Whereas storm systems on Earth last for several days or for several weeks, as a moving system such as a hurricane, such storm systems on Jupiter last for very long periods. The Great Red Spot, for example, has been observed for centuries. On Earth, there are strong interactions between the atmospheric systems and the land masses over which they travel. These tend to break up the atmospheric system. In addition, the Earth systems are powered by solar heat concentrated in the tropics during the daytime. Thus, they tend to break up when they move away from the tropics and into the night hemisphere of Earth. Again, Jupiter is different since its storms are powered mainly by internal heat flow which is more evenly distributed planetwide and over the day and night hemispheres. It is because of the internal heat source that Jupiter weather systems can last for long periods of time.

Some of the bright zones on Jupiter (Figure 6-20) may be analogous to the tropical convergences on Earth which show up plainly on satellite photographs as bands of thunderstorms, a few degrees north and south of the equator. They are...

 


Figure 6-20. Some of the bright zones of Jupiter may be analogous to the tropical convergences on Earth weather patterns around the planet.

Figure 6-20. Some of the bright zones of Jupiter may be analogous to the tropical convergences on Earth weather patterns around the planet.

 

[120] ...caused on Earth by the trade winds, blowing toward the equator, and rising moist air in the tropics. The consequent thunderstorms spread their tops into cirrus clouds which then flow back toward the poles. Similarly, on Jupiter, rising air masses may produce great masses of cumulus clouds which spread into anvil shapes and give rise to the bright bands of the north tropical and south tropical zones.

A problem still not resolved is why, when ammonia and water are both colorless when condensed, Jupiter displays bands of colored clouds and the red spots. Certain ammonia compounds produce colors like those on Jupiter given sufficient exposure to ultraviolet radiation, and sufficient solar ultraviolet radiation does penetrate to the cloud levels. It may possibly be that carbon compounds or traces of sulfur and phosphorous all believed to be present in primordial material -supply some of the color. Only traces would be needed to react in sunlight and produce the types of colors seen on Jupiter. lt could very well be that because the gas of the Great Red Spot rises so high, it is subject to irradiation by solar ultraviolet which triggers a different set of photochemical reactions to deepen the color.

Since solar ultraviolet radiation penetrates to lower cloud levels, i.e., the belts, the Great Red Spot may be due to a different type of chemical reaction, temperature, or longer exposure to ultraviolet radiation because its gases experience less mixing than those in the belts.

Another possible cause of colors on Jupiter could be the presence of free radicals. At very low temperatures such as experienced in the higher cloud layers, chemical compounds can exist with some of their normal complement of atoms missing and still be relatively stable. They are called free radicals and are generally highly colored.

Limb darkening of Jupiter shows that the clouds of the planet consist of a thin upper layer which is semi-transparent to red light above a more dense lower layer. The particles of Jupiter's upper clouds are of much smaller dimensions than particles in Earth's clouds.

The precise modeling of cloud layers of Jupiter is still in progress. Generally, two cloud layers appear to be present; one a thick, low deck above which there is a gaseous atmosphere; two a thin high layer topped by a layer or layers of aerosols. The Jovian cloud particles are not spherical (unlike the sulfuric acid droplets in the Venus atmosphere that permit precise numerical analysis with existing theory). Instead the Jovian particles are irregular and most probably larger than the wavelength of light. At the poles, the clouds seem to be low. But alternatively the upper cloud layer may be diffuse with many aerosols suspended in the upper atmosphere.

The pictures of Jupiter revealed several surprises about the clouds. The detail cloud structures in intermediate latitudes were unexpected. The billows and whirls near the edges of belts and zones confirmed that there are rapid changes in wind direction and wind speeds there. Motions in latitude as well as in longitude seem to be evidenced by trends and slants in the North Tropical Zone for example. The plume in the equatorial zone was revealed in remarkable detail which provides structural information important to understanding these common cloud forms of the equatorial zone.

Infrared observations of Jupiter have been made from the ground at wavelengths of 5 micrometers where there is a window of transparency in both the Earth's atmosphere and that of Jupiter thereby permitting a look deep into the Jovian atmosphere. Maps of Jupiter at this wavelength made by James A. Westphal at the Hale Observatories show belts and zones very much the same as photographs of Jupiter in visible light. But the darker visible belts are light (hotter) in the infrared pictures and the light visible zones are dark (cooler) (Figure 6-21). The infrared radiation comes from deep within the atmosphere and shows that the dark visible belts are lower, or thinner. hotter clouds, while the bright visible features are high, or thicker, cooler clouds. There is also very close correlation between infrared maps of the dark, bluish-gray regions which are interpreted as...


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Figure 6-21. Ground-based infrared maps of Jupiter show correlation between infrared sources and the dark zones of the planet seen on the Pioneer images. (Photo: Hale Observatories )

Figure 6-21. Ground-based infrared maps of Jupiter show correlation between infrared sources and the dark zones of the planet seen on the Pioneer images. (Photo: Hale Observatories )

 

...dark holes in the clouds. These show as regions of increased infrared radiation. The 5 micrometer pictures also correlate very well with the Pioneer pictures of visible features; the prominent plume and various cells and wave effects are clearly the same. The Pioneer spacecraft also made infrared maps of Jupiter, but at 20 and 40 micrometers where, although there is less detail because of less penetration and less temperature contrast, the planet emits more infrared radiation than at 5 micrometers (Figure 6-22). These maps also provide confirmation of the high and low clouds and provide information on the general heat balance of the planet. They confirm that Jupiter emits more heat than it receives from the Sun.

 


Figure 6-22. An infrared map of Jupiter from Pioneer 11 provides information about the heat balance and shows that Jupiter emits 1.9 times as much heat energy as it receives from the Sun.

Figure 6-22. An infrared map of Jupiter from Pioneer 11 provides information about the heat balance and shows that Jupiter emits 1.9 times as much heat energy as it receives from the Sun.

 

[122] In spite of the loss of some of the data covering the northern hemisphere of Jupiter when radiation affected the instrument, the infrared radiometer carried by Pioneer 11 provided two infrared spinscan images of the planet. A complete image was centered at 41°S and a partial image was centered at 52° N latitude on Jupiter. The ratio of total thermal energy to absorbed solar energy was revised to 1.9 ± 0.2 compared with previous estimates of 2.5 ± 0.5. The tact that both Pioneer 10 and Pioneer 11 data yield this result adds confidence in the new value.

Thus, Jupiter does not appear to be emitting as much internal heat as was once thought; about 24 percent less than had been assumed from Earthbased observations.

Jupiter's ionosphere rises 4000 km (2500 mi.) above the visible surface. It is ten times as thick and five times as hot as was predicted. Also, the ionosphere has at least five sharply defined layers of different density, similar to Earth's ionospheric layers that permit long range radio communication around Earth by returning certain radio waves to the ground.

The determination that Jupiter has a warm, extended, hydrogen rich atmosphere has important implications for further exploration of the giant planet.

Prior to measurements by the two Pioneers, it was generally considered that the heating of an entry probe into Jupiter's atmosphere would be greater than could be overcome by present-day technology. Now the new determinations of the Jovian atmosphere suggest that a probe can be made to survive entry into the Jovian atmosphere and measure directly its characteristics and constituents.

Enough has been confirmed or found out about Jupiter by the Pioneers to encourage further exploration. These two spacecraft have also demonstrated that such exploration is quite within the capabilities of present space technology which offers the opportunity now to sample directly what may be primordial material of the Solar System; thus, dipping back four and a half billion years in time.


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