9. Jupiter: a giant primitive planet

Fundamentals

Jupiterís orbital radius is 5.2 times the radius of the Earthís orbit, so the planetís distance from Earth changes relatively little in the course of a year. As a consequence, its apparent size and brightness are fairly constant, unlike the behavior of Mars and Venus. When these nearby planets are on the same side of the Sun as the Earth, they appear much bigger and brighter than when they move to the opposite side of the Sun. Jupiter is a true monarch of the planets, the largest planet in the solar system with a radius of about 11 times that of the Earth. The giant is so large that it could contain more than 1,300 Earth-sized planets inside its volume. Yet, Jupiter is only 318 times as massive as our planet. So Jupiter must be composed of something lighter than the rock and iron that constitute the Earth.

If we divide the mass by the volume, we find a mean density of 1,330 kilograms per cubic meter, only about one quarter the mean mass density of the Earth. In fact, the mass density of Jupiter is only slightly greater than that of water, at 1,000 kilograms per cubic meter, and this implies that Jupiter, like the Sun, is composed primarily of hydrogen. No other element is light enough to account for the low density of the planet.

Physical properties of Jupitera

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Mass1.8992 x 1027 kilograms = 317.894 ME
Equatorial radius at one bar7.1492 x 107 meters = 11.19 RE
Polar radius at one bar6.6854 x 107 meters
Mean mass density1,330 kilograms per cubic meter
Rotation period9.9249 hours = 9 hours 55 minutes 29.7 seconds
Orbital period11.86 Earth years
Mean distance from Sun7.7833 x 1011 meters = 5.203 AU
Age4.6 x 109 years
Atmosphere86.4 percent molecular hydrogen, 13.6 percent helium
Energy balance1.67 Ī 0.08
Effective temperature124.4 degrees kelvin
Temperature at one bar level165 degrees kelvin
Central temperature17,000 degrees kelvin
Magnetic dipole moment 20,000 DE
Equatorial magnetic field strength4.28 x 10-9 tesla or 14.03 BE
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a The symbols ME, RE, DE and BE denote respectively the mass, radius, magnetic dipole moment, and magnetic field strength of the Earth.

Stormy weather

Wind-blown zones and belts

Fig. .. 

The only features we can see on Jupiter are multi-colored clouds drawn into adjacent bands or stripes by the planetís rapid rotation. There are alternate dark bands, called belts, and light ones known as zones. The zones and belts surround the planet, running parallel to the equator at different speeds.

The giant planetís belts and zones are the sites of counterflowing winds that are sometimes called zonal, or east-west, jets. Where the Earth has just one westward air current at low latitudes - a trade wind, and one nearly eastward current at mid-latitudes Ė a jet stream, at the cloud top level Jupiter has five or six of these alternating jet streams in each hemisphere. Jupiter has five or six of these alternating set streams in each hemisphere.

Storm clouds

Fig. .. 

The colorful spots and stripes that dominate Jupiterís face mark patterns of stormy weather, as clouds billow, churn and seethe. Red, white and brown spots stare out of Jupiterís atmosphere like gigantic eyes. Huge storms larger than the Earth in size swirl across the planet, while smaller eddies chase each other whirling and rolling about.

Fig. .. 

Jupiterís famous Great Red Spot is essentially a huge weather system, with an east-west dimension greater than Earthís diameter. Because of rapidly increasing pressure with depth it cannot extend deeply into the planet. It is simply an enormous, shallow eddy trapped between counter-flowing jets, so large that the strong prevailing winds are forced to flow around it. The winds, in turn, funnel smaller eddies toward the Red Spot, helping to roll it around.

Cloud layers and colors

Fig. .. 

If we could descend through Jupiterís thin cloud layer, we would find that the temperature and pressure increase with depth. As in any planetary atmosphere, the atoms and molecules collide more frequently in the increasingly compressed, denser regions of Jupiterís atmosphere so the pressure and temperature increase there. At the cloud tops the temperature is a freezing 114 degrees kelvin and the atmospheric pressure is about 0.1 bar, or one tenth that of the Earthís air at sea level. In slightly deeper layers, about 130 thousand meters down, the temperature rises to a balmy 300 degrees kelvin, well above the freezing point of water, at 273 degrees kelvin. In these warmer regions, the pressure is comparable to the air pressure at the surface of the Earth, leading to speculations that, if verified mixing were small, living things might reside there. And above them it is cold enough to freeze various gases into ice to form the clouds.

Fig. .. 

The visible cloud tops of Jupiter consist of ammonia ice crystals, which condense out of the atmosphere at the very low temperatures and pressures there. They create graceful white clouds that probably make up the cold, light-colored zones observed from Earth. This is consistent with spectroscopic measurements of abundant ammonia at the cloud tops of Jupiter.

Lightning bolts in wet spots

Ancient mythology was close to the mark when it designated Jupiter as master of the rains, hurling thunderbolts at those who displeased him. Lightning flashes were discovered in Voyager 1 and 2 images of the dark night side of Jupiter, apparently illuminating the clouds in massive thunderstorms, and the lightning was confirmed by instruments aboard the Galileo spacecraft. Both missions showed that the lightning is concentrated near oppositely directed winds where storm clouds are found.

Fig. .. 

How deep the lightning occurs can be estimated from its diameter. The larger the flash, the deeper the lightning discharge. The observed sizes of Jupiterís lightning flashes suggest that they originate from layers in the atmosphere where water clouds are expected to form, at about 100 thousand meters down. Only water could condense at these depths. When the Galileo cameras followed the night-side lightning sources into the day side, they confirmed that the lightning originates in deep moist clouds.

Plunging into a dry and windy spot

A pioneering descent into Jupiterís atmosphere took place on 7 December 1995 when a 339-kilogram probe was dropped from the Galileo spacecraft on a suicide plunge into the planet. Scientists had expected that the Galileo Probe would pass through three cloud layers, composed of different chemicals that condense from tenuous gases at successively higher and colder levels. Below the bottom, water-cloud layer, which for Jupiter is formed at the 5-bar level, the atmosphere was expected to be well stirred, and therefore more representative of the planetís uniform, global composition. But contrary to expectations, the clouds were not where everyone thought they would be. It was apparently a clear day at the probeís entry point. When the capsule plummeted into the maelstrom, its instruments saw almost no evidence for clouds. Moreover, the planet was a lot drier than anticipated, at least in the vicinity of the probe-entry site. Extrapolating from the Sunís makeup, researchers had expected at least five or ten times as much water, under the assumption that Jupiter coalesced out of similar material with the same proportion of oxygen, which in the outer atmosphere of Jupiter should all be in the form of water molecules. Far less lightening was also detected during the probeís hour-long descent, supporting the conclusion that this part of the upper atmosphere contains little water.

Fig. .. 

The missing clouds and water might be explained if the probe descended into an unusually clear spot of dry, downwelling air and reduced cloud cover. In fact, observations from Earth-based telescopes indicated that the entry site was a region where internal shines through a gap in the clouds, and dry, wrung-out air from high altitudes might have been forced downward in there. Jupiter has several of these clear hot spots, that alternate with cloudy places in a band near the planetís equator.

Beneath Jupiterís clouds

Educated guesses about Jupiterís internal constitution

We cannot see beneath the clouds of Jupiter, but we can use external measurements to constrain its internal properties. As an example, we now know that the giant planet emits its own heat radiation, which means that it is hot inside. Since Jupiter and the Sun originated from similar material at the same time, a good initial assumption is that they have the same ingredients with similar proportions. The planetís low average mass density indicates that it is in fact composed largely of hydrogen and helium, just as the Sun is. The planetís oblate shape and rapid rotation also tell us something about the way it is constructed inside. Due to the enormous pressures inside Jupiter, most of the planetís hydrogen is compressed into a liquid metallic form, which has been created in the terrestrial laboratory and helps account for the giantís strong magnetic field. All of these constraints have been pieced together to make a picture of Jupiterís invisible interior.

An incandescent globe

With the advent of ground-based infrared measurements of the planets, pioneered by Frank J. Low and his colleagues in the 1960s, astronomers were surprised to discover in 1969 that the giant planet is an incandescent globe with its own internal source of heat. This result was confirmed in greater detail with instruments aboard the Voyager 1 and 2 spacecraft, that determined precisely how much infrared heat radiation was emerging from inside the planet. They showed that Jupiter is radiating 1.67 times as much energy as the atmosphere absorbs from incoming sunlight. In other words, the giant planet radiates nearly twice as much energy as it receives from the Sun, and almost half of the total energy that Jupiter loses must come from its interior. That essentially meant that the planet had to be unexpectedly hot inside.

Ingredients at formation

According to the widely accepted nebular hypothesis, the Sun and planets formed together during the collapse of a rotating interstellar cloud called the solar nebula. Most of it fell into the center, until it became hot enough to ignite the Sunís nuclear fires. Further out, the planets formed out of a whirling disk of the same material. If the nebular hypothesis is correct, and the whole solar system originated at the same time, then you might expect Jupiter to have a similar chemical composition to the Sun. To a first approximation, the abundance of the elements in the giant planet does indeed mimic that of the Sun, with a predominance of the lightest element hydrogen. It is the most abundant element in most stars, in interstellar space, and in the entire Universe. The second most abundant element in both Jupiter and the Sun is helium, and hydrogen and helium together account for the low mean mass density of both objects, at 1,330 and 1,409 kilograms per cubic meter respectively.

An equatorial bulge

Observations with even a small telescope show that Jupiter is not a sphere. It has a perceptible bulge around its equatorial middle and is flattened at the poles. This elongated oblate shape is caused by Jupiterís rapid spin. The outward force of rotation opposes the inward gravitational force, and this reduces the pull of gravity in the direction of spin. Since this effect is most pronounced at the equator, and least at the poles, the planet expands into an oblate shape that is elongated along the equator. The same thing happens to all the giant planets, and even to the solid Earth.

The amount of Jupiterís non-spherical extension depends on both its rate of rotation and the internal distribution of its material. The faster the spin, the more the outward push and the greater the elongation. And given its rotation, with the rapid period of 9.9249 hours, the size of the equatorial bulge depends on how Jupiterís mass is distributed inside. The more massive the planetís dense core, the smaller the equatorial bulge.

If Jupiter were a perfectly spherical planet, it would act as if all its mass was concentrated in a single central point and the motions of natural satellites or spacecraft would not depend on their orientation with respect to the planetís equator. In contrast, an oblate planet produces an extra force that tugs the moving object toward its equatorial bulge, and also toward any internal core. When combined with the known mass, volume and rotation rate of Jupiter, observations of these effects indicate that Jupiter has a dense core containing up to 12 Earth masses. Such a central object, presumably composed of non-gaseous rock and ice, was apparently required to initiate the accumulation of the giant planetís extensive hydrogen shell, which now compresses the core to high temperatures and pressures

Enormous pressures and strange matter

Fig. .. 

To understand the internal constitution of Jupiter, we need to know what happens to its most abundant ingredient, hydrogen, as the pressure increases. At the low pressure in the outer, visible parts of Jupiter, hydrogen forms a molecular gas, but this atmosphere is just a thin veneer. In proportion, the outer layer of gaseous hydrogen molecules resembles the skin of an apple. Deeper down, where the pressures and temperatures are higher, the hydrogen is liquefied. Indeed the planet is almost entirely liquid. It is mostly just a vast, global sea of liquid hydrogen.

Most of Jupiter is in the form of liquid metallic hydrogen. And underneath it all is a relatively small core of molten rock and ice. This relatively little core is up to twelve times heavier than the Earth. Theoretical considerations suggest that Jupiterís hydrogen probably accumulated around such a massive, pre-existing central object.

Electrical currents, driven by Jupiterís fast rotation within its liquid metallic shell, apparently generate the planetís strong magnetic field, in much the same way that electricity in the Earthís molten metallic core produces our planetís magnetism. Jupiterís magnetic field is much more powerful than Earthís magnetism, with a magnetic moment that is 20,000 times as large and a cloud-top strength that is about 14 times Earthís surface magnetic field strength. The greater strength of Jupiterís magnetism could be attributed to the planetís faster rotation, more extensive metallic region, and the relative proximity of the internal electrical currents to the cloud tops. By way of comparison, Earthís magnetic field is produced within a much smaller metallic core, which extends only half way to the surface.

The Galilean satellites

Introduction to the largest moons

Fig. .. 

When the two Voyager spacecraft flew past Jupiter in 1979, they got only a brief look at the Galilean satellites. However, it was time enough for their cameras to discover active volcanoes on Io, smooth ice plains on Europa, grooved terrain on Ganymede, and the crater-pocked surface of Callisto. The incredible complexity and rich diversity of their surfaces, which rival those of the terrestrial planets, are only visible by close-up scrutiny from nearby spacecraft. Ground-based telescopes provide only a blurred view of the tiny, distant moons.

Fig. .. 

Scientistís have created three-layer models for the interiors of Io, Europa and Ganymede, based on the Galileo spacecraftís gravity and magnetic data and constrained by the satellitesí surface properties and overall mass density. They all have a large metallic core, a rocky silicate mantle, and an outer layer of either water ice, for Europa and Ganymede, or rock, for Io. In contrast, Callisto is a relatively uniform mixture of ice and rock.

Io: a world turned inside out

Fig. .. 

The innermost Galilean satellite, Io, has a radius and density that are nearly identical to those of our Moon, but contrary to expectation, there are no impact craters on Io. The dramatic landscape is instead richly colored by hot flowing lava and littered with the deposits of volcanic eruptions. The active volcanoes emit a steady flow of lava that fills in and erases impact craters so fast that not a single one is left

Sulfur and sulfur dioxide give rise to Ioís colorful appearance. Its red and yellow hues are attributed to different forms of sulfur, probably formed at different temperatures. Volcanic plumes of sulfur dioxide gas fall and freeze onto the surface, forming white deposits that were first detected by ground-based infrared spectroscopy in the 1970s.

Fig. .. 

Whereas our Moon has been geologically inactive for eons, Io is the most volcanically active body in the solar system. It exhibits gigantic lava flows, fuming lava lakes, and high-temperature eruptions that make Danteís Inferno seem like another day in paradise. Scientists estimate that Io has about 300 active volcanoes, and the hotspots of at least 100 of them have been observed.

Fig. .. 

The cameras aboard the Voyager 1 spacecraft discovered nine active volcanoes during its flyby in 1979, and the most active volcanoes, such as Prometheus, Loki and Peli, were observed from the Galileo spacecraft two decades later. Prometheus is the ďOld FaithfulĒ of Ioís many volcanoes, remaining active every time it has been observed. Loki is the most powerful volcano in the solar system, consistently putting out more heat than all of Earthís active volcanoes combined. And Pele, the first volcano to be seen in eruption on Io, has repeated the performance for Galileo and the Hubble Space Telescope. Like other volcanic centers on Io, these active volcanoes have been named for gods of fire, the Sun, thunder and lightning.

Fig. .. 

Plumes of volcanic gas erupt from Ioís active volcanic vents, rising up to half a thousands kilometers above the surface. They spread out in graceful, fountain-like trajectories, depositing circular rings of material about a million meters in diameter. Instruments aboard Galileo have practically smelled the hot, sulfurous breath of the eruptions, monitoring the sulfur dioxide gas as it rises, cools and falls. Diatomic sulfur, consisting of two sulfur atoms joined in pairs, has also been detected gushing out of the active volcanoes by instruments on the Hubble Space Telescope.

Ioís tides of rock

What is keeping Io hot inside, warming up its interior, melting its rocks, and energizing its volcanoes? The heat released during the moonís formation and subsequent radioactive heating of its interior should have been lost to space long ago, just as our Moon has lost the internal heat of its youth and become an inert ball of rock. Unlike the Earth, whose volcanoes are energized by heat from radioactivity and friction due to mass motion, it is the dominant tidal distortions, created by massive Jupiter and its other moons, which sustain Ioís molten state.

Just as the gravitational force of the Moon pulls on the Earthís oceans, raising tides of water, the gravitational force of massive Jupiter creates tides in the rocks of Io. Since the pull of gravity is greatest on the closest side to Jupiter, and least on the farthest side, Ioís solid rocks are drawn into an elongated shape. But this tidal distortion does not melt the rocks by itself. If Io remained in a circular orbit, one side of the moon would always face Jupiter, its tidal bulges would not change in height, and no heat would be generated.

The three Galilean satellites Io, Europa and Ganymede resonate with each other in a unique orbital dance, known as the Laplace resonance, in which Io moves four times around Jupiter for each time Europa completes two circuits and Ganymede one. This congruence allows small forces to accumulate into larger ones. The resultant gravitational tug-of-war between Jupiter and the satellites distorts the circular orbits of all three moons into more oblong elliptical ones. The effect is greatest for Io, which revolves nearest to Jupiter, but there is a noticeable consequence for Europa and perhaps even Ganymede.

Fig. .. 

During each lap around its slightly eccentric orbit, Io moves closer to Jupiter and further away, wobbling back and forth slightly as seen from Jupiter. The strong gravitational forces of the planet squeeze and stretch Io rhythmically, as the solid body tides rise and fall. Friction caused by this flexing action heats the material in much the same way that a paper clip heats up when rapidly bent back and forth. This tidal heating melts Ioís interior rocks and produces volcanoes at its surface.

Magnetic connections with Io

Fig. .. 

Earth-based observations in the 1970s revealed a vast cloud of sodium atoms that envelops Io, forming an extended atmosphere that is nearly as big as Jupiter. The sodium cloud stretches backward and forward along Ioís orbit, until the sodium atoms become ionized and no longer emit the light that makes them visible. The neutral, or unionized, sodium atoms have probably been chipped off the surface of Io by the persistent hail of high-energy particles found near the giant planet.

The volcanoes on Io provide the raw material for the satelliteís tenuous atmosphere of sulfur dioxide, designated SO2, that gathers above the erupting vents like localized umbrellas. The volcanic plumes are like fountains, with eruptions that arch gracefully back to Ioís surface, and the gas is not propelled with sufficient velocity to escape the satelliteís gravitational pull. Nevertheless, atoms of sulfur, S, and oxygen, O, can escape from Io once they are ionized by exposure to radiation form the Sun or from the hail of energetic particles in Ioís vicinity. These ions have been detected from the Voyager and Galileo spacecraft by their ultraviolet glow.

Fig. .. 

Since charged particles cannot cross magnetic field lines, Jupiterís spinning magnetic field confines and directs the sulfur and oxygen ions into a doughnut-shaped ring known as the plasma torus. As the giant planet rotates, it sweeps its magnetic field past Io, stripping off about a ton, or 1,000 kilograms, of sulfur and oxygen ions every second. This material is lost from Io forever, and is continuously replenished by its volcanic activity, albeit indirectly through subsequent ionization.

Europaís bright, smooth icy complexion and young face

The smallest, and yet brightest, of the Galilean satellites, Europa, has a density comparable to that of rock, but its surface is as bright and white as ice. In fact, it is water ice! With surface temperatures of 110 degrees kelvin or less, the water ice on Europa is frozen as hard and solid as granite. Europaís surface is nearly devoid of impact craters, and there are no mountains or valleys on its bright smooth surface. No features extend as high as 100 meters, making Europa the smoothest body in the solar systems.

Long cracks, ice rafts and dark places on Europa

Fig. .. 

A veined, spidery network of long dark streaks marks Europaís young face, suggesting great inner turmoil. The fine lines run for millions of meters, intersecting in spider-web patterns. They give Europa a broken appearance that resembles a cracked mirror or an automobile window that has been shattered in some colossal accident. The dark lines are most likely deep fractures formed when that part of the ice cracked open, separated, and filled with darker, warm material seeping and oozing up from below. Dirty liquid water or warm dark ice has apparently welled up and frozen in the long cracks, producing the lacework of dark streaks.

Fig. .. 

As two adjacent pieces of ice pull apart slightly, warm soft ice might push up and freeze to form long ridges that parallel the cracks. Other ridges may have originated when the sides were pushed together, closing the crack and crumpling its edges to form a ridge.

The surface of Io is fragmented everywhere, as if pieces of ice have broken apart, drifted away and then frozen again in slightly different places. Large blocks of ice have floated like rafts across the moonís surface, shifting away from one another like moving pieces of a jigsaw puzzle. Some of them are tilted; others rotated out of place, like plastic toys bobbing in a bathtub. This shows that the ice-rich crust has been or still is lubricated from below by either slushy ice or maybe even liquid water.

Fig. .. 

Explosive ice-spewing volcanoes and geysers may erupt from the buried seas, reshaping the chaotic surface of the frozen moon and leaving dark scars behind. Extended dark regions may, for example, have formed when the underground ocean melted through Europaís icy shell, exposing darker material underneath, or when upwelling blobs of dark, warm ice broke through the colder near-surface ice.

Europaís underground sea of melted ice

Tidal distortions could explain how water ice has melted in the frigid environment near Europa. The satellite has a slightly eccentric orbit due to gravitational interactions with Io and Ganymede, which revolve closer and farther away from Jupiter than Europa. Over the course of one trip around Europaís elongated path, Jupiterís strong gravity stretches and compresses the satellite, in a process called tidal flexing. Frictional heat associated with similar tidal flexing melted the rocks inside Io, and it operates on Europa as well Ė to a smaller extent since Europa is further from Jupiter. But the warmth generated by tidal heating may have been or may still be enough to soften or liquefy some portion of Europaís icy covering, perhaps sustaining a subsurface ocean of liquid water.

Magnetic measurements from the Galileo spacecraft provide more evidence for an otherworldly ocean inside Europa. The satelliteís magnetism changes direction as Jupiterís magnetic field sweeps by in different orientations to the satellite, owing to the tilt between the planetís rotation axis and magnetic axis. This means that the magnetic field at Europa is not generated in a core, but is instead induced by the passage of Jupiterís field in an electrically conducting liquid, such as salt water, beneath the ice. Although this evidence for a subsurface liquid ocean is indirect, it is the only indication that buried water is there now, rather than in the geological past.

Cratered, wrinkled Ganymede

Ganymede, the largest moon in the solar system, has a radius that exceeds that of the planet Mercury, but the satelliteís density is so low that it must contain substantial quantities of liquid water or water ice. Its icy surface has experienced a violent history involving crustal fractures, mountain building and volcanoes of ice.

Fig. .. 

Bright regions on Ganymedeís surface contain sets of parallel ridges and valleys, termed grooved terrain, which looks like the swath of a giantís rake. The grooved terrain was most likely formed when the moonís water-ice crust expanded and stretched, cracking and rifting open as it was pulled apart. The crustal expansion might have happened when the satelliteís rocks melted and moved into its interior while its water migrated to the top where they froze.

Fig. .. 

Sets of intersecting mountain ridges overlap and twist into each other. Some of the ridges cut across craters, while craters appear on other ridges. Ganymede evidently experienced several epochs of mountain building. These crustal deformations may have continued for a billion years.

Fig. .. 

Water-ice volcanism played a role in creating the bright terrain on Ganymede. Prominent depressions were apparently flooded with liquid water or icy slush, and then froze into bright smooth bands that now cover much of the moon. Craters found in these areas indicate that this also happened early in the satelliteís early history, at least a billion years ago.

Darker regions on Ganymede are older and more heavily cratered. Some of these large polygonal blocks rise about a thousand meters above the bright, grooved terrain, and look as if they have moved sideways for tens of thousand of meters along the moonís surface.

Ganymede, a moon with its own magnetic field

One of the major surprises of the Galileo mission was the discovery that Ganymede has its own intrinsic magnetic field. The moon is generating a magnetic dipole similar to those of most planets, and roughly a thousandth of the strength of Earthís. No other satellite now has such a magnetic field, but our Moon might have had one in the distant past.

Callisto, an ancient, battered world

Fig. .. 

Remotest of the Galilean moons, Callisto has had a much more sedate and peaceful history than the other large satellites of Jupiter, with little sign of internal activity. It is a primitive world whose surface of ice and rock is the most heavily cratered in the solar system. Unlike nearby Ganymede, the moon Callisto has no grooved terrain or lanes of bright material, and it exhibits no signs of icy volcanism. So, Callisto is a long dead world unaltered by resurfacing since it formed and ancient impacts molded its face, a fossil remnant of the origin of planets and their moons. In fact, with a surface age of about 4 billion years, Callisto has the oldest landscape in the solar system.

Fig. .. 

Yet, when seen close up by the Galileo spacecraft there are indications of subdued, youthful activity on Callistoís surface. It is blanketed nearly everywhere by fine, mobile dark material, interrupted only where bright crater rims poke up through it. Small impact craters are mostly absent, and those that are found sometimes appear worn down and eroded. Thus, the smaller craters seem to have been filled in and degraded over time, perhaps by the dark blanket of debris that might have been thrown out by the larger impacts. Ice flows may have alternatively deformed and leveled many craters, because ice, which is rigid to sharp impact, can flow gradually over long periods of time, as glaciers do on Earth. The lack of small craters on Callisto might also be explained if the ancient population of impacting objects near the remote satellite had relatively few small objects when compared to the population near the Moon and Mercury.

Jupiterís mere wisp of a ring

Fig. .. 

In 1979 Ė after much debate about the likelihood of finding a ring around Jupiter Ė a search was carried out with a camera on Voyager 1, and a narrow faint belt of material was found encircling the planet in its equatorial plane near the same distance that the energetic particles had disappeared. The ring was not previously observed from Earth because it was too faint and close to the bright planet. Since its discovery, Jupiterís main ring has been detected by Earth-based telescopes sensing infrared radiation, and fully confirmed by the inquisitive eyes of the Galileo spacecraft.

The outer edge of the main ring lies just inside the orbit of the tiny moon Adastea, just 15 thousand meters in size and too small to be seen from Earth. It was discovered by the Voyager spacecraft, as was another tiny moon, named Metis, which is embedded near the bright midpoint of the main ring. The dust generated by meteorite impact on Adastea and Metis can easily escape the small gravity of these moons, accounting for the dense accumulation of particles in the main ring. Some of the microscopic particles are small enough for electromagnetic forces to overpower the effects of Jupiterís gravity, pumping them into the inner halo that is seen above and below the main ring.

Fig. .. 

(Summary Diagram)