The three large white ovals are the longest-lived features in Jupiter’s atmosphere, after the Great Red Spot. Like the Red Spot, they are anticyclonic, or high-pressure, regions. During Voyager 2 encounter, one of the ovals was just south of the Red Spot. This picture shows the other two ovals as they looked in early July. The clouds show very similar internal structures. To the east of each of them, recirculating currents are clearly seen. [P-21754C]
In this frame, a similar structure is seen to the west of the cloud.
A sequence of pictures of Jupiter, taken once per rotation (about ten hours), can be used to construct a time-lapse movie of the circulation of the Jovian atmosphere. These frames are from the Voyager 1 “Blue Movie” of the Great Red Spot region. Every odd Jovian rotation is shown, so the 24 frames correspond to 48 Jupiter days, or about 20 Earth days. White spots can be seen entering the Red Spot from the upper right and being carried around by its six-day rotation until they are ejected toward the lower right. Above the Red Spot, the flow is toward the right; below it, toward the left. The rotation of the Spot is counterclockwise, or anticyclonic. [260-449]
At different latitudes, the strong prevailing zonal winds produce different apparent rotation rates. Plotted here are the horizontal velocities measured from a pair of Voyager images taken one rotation (about ten hours) apart. Also shown for comparison are older ground-based measurements obtained from careful timings of the apparent rotation rate at different latitudes. The excellent agreement of the two plots indicates the stability of the zonal winds. Also, the wind pattern shows much greater symmetry between northern and southern hemispheres than do the more superficial cloud patterns.
“At first, Voyager seemed to do nothing but emphasize the chaos, not the order in Jupiter’s atmosphere,” Andy Ingersoll stated. “There are turbulent regions in which individual little spots seem to change every Jovian rotation. And the whole texture in certain turbulent regions is unrecognizable in one earth day.” With the Voyager spacecraft, more detail could be seen than ever before; in addition, changes could be observed on a small timescale, as they happened. “It became much more of a mystery how large-scale order could exist in the face of all this small-scale chaos. But I think we are beginning to see the order underneath. What we are looking at when we observe Jupiter are minute cloud particles representing only a small fraction of the mass of the atmosphere.” The large-scale order the scientists had found was a regular alternation of eastward and westward jets. “If we take all the measurements from Earth-based observations over the last 75 years, we find that every current that has ever been seen from the Earth over 75 years is visible in one ten-hour rotation. They’re all there—they were just invisible.” The ever-changing appearance was dancing above a regular, almost stationary pattern of alternating flows, which may come from deep within Jupiter’s atmosphere. Why this alternating pattern persists remains a mystery. Even if the underlying pattern can be thought of as a sort of Jovian climate, it still does not explain the mechanics of “the minor sideshow”—the changing weather patterns. Analysis of the Voyager pictures is sure to keep planetary meteorologists busy for many years to come.
Lights in the Night Sky
Toward the end of the first encounter period, Voyager 1 flew behind Jupiter, and the spacecraft’s wide-angle camera scanned the northern hemisphere on the nightside of the planet, searching for aurorae and lightning bolts. The most impressive darkside feature found was a tremendous aurora in the north polar region. But this was not the first time Jovian aurorae had been detected. Very-high-energy auroral emissions resulting from ultraviolet glows of atomic and molecular hydrogen had been detected prior to encounter on the bright side of Jupiter by the ultraviolet spectrometer. The ultraviolet observations indicate that atmospheric temperatures in the auroral regions are at least 1000 K. In both the visible and the ultraviolet spectra, the aurorae are confined to the polar regions and result from charged magnetospheric particles striking the upper atmosphere. The ultraviolet aurorae are created when high-energy particles from the Io plasma torus spiral in toward Jupiter on magnetic field lines.
Several meteor trails were also evident in the darkside pictures of Jupiter’s atmosphere. Traveling at roughly 60 kilometers per second as they entered, these fireballs brightened quickly and seemed to survive for about 1000 kilometers before they died.
Clusters of lightning bolts—indicative of electrical storms—were also discovered on Jupiter’s nightside. This particular phenomenon does not seem to depend on latitude. The Voyager 1 photograph that captured the huge Jovian aurora also caught the electrical discharges of 19 superbolts of lightning, and Voyager 2 photographs located eight additional flashes. Radio emission (whistlers) from lightning discharges were also detected by the Voyager radio astronomy receivers and the plasma wave instrument.
The night side of Jupiter is not dull. A large aurora (northern light) arcs across the northern horizon, while farther south about twenty large bolts of lightning illuminate electrical storms in the clouds. Similar pictures also revealed fireballs, or large meteors, burning up in the atmosphere of Jupiter. [P-21283B/W]
Magnetic Field
Deep in the interior of Jupiter, the pressures are so great that hydrogen becomes an electrical conductor, like a metal. Currents driven by the rapid rotation of the planet are thought to flow in this metallic core. The result is a magnetic field that penetrates the space around Jupiter.
Direct measurements of the Jovian magnetic field were first made by the Pioneers, and Voyager results generally confirm the initial findings. The strength of the Jovian field is about 4000 times greater than that of the Earth. The dipolar axis is not at the center of Jupiter, but offset by about 10 000 kilometers and tipped by 11 degrees from the axis of rotation. Each time the planet spins, the field wobbles up and down, carrying with it the trapped plasma of the radiation belts. The Voyager particles and fields instruments concentrated not on the planetary magnetic field but on the processes taking place in the magnetosphere.
A probe of the Jovian atmosphere is obtained each time a spacecraft passes behind the planet as seen from the Earth. Passage through the ionosphere and atmosphere alters the phase of the radio telemetry signal, and subsequent computer analysis allows members of the Radio Science Team to reconstruct the profile of the atmosphere. Shown here is the atmospheric temperature as a function of pressure as derived from the Voyager 1 X-band occultation data, corresponding to a point at latitude 12°S, longitude 63°. The two curves represent extreme interpretations of the same data; the best fit lies somewhere between. Accuracy is high at greater depths but poor at levels above a pressure of about 0.03 bar. Clearly shown is the temperature minimum near 0.1 bar and the steady increase of temperature with depth as the radio beam probed toward the cloud level near 1.0 bar.
At a wavelength near 5 micrometers, the primary gases in the atmosphere of Jupiter are particularly transparent, and the infrared radiation from the planet comes from relatively great depths. At these depths, it is possible to see evidence of gases such as water vapor that condense at higher altitudes where the temperatures are lower. This IRIS spectrum in the 5-micrometer spectral region shows features identified with water (H₂O), germane (GeH₄), and deuterated methane (CH₃D), as well as the more easily detected ammonia (NH₃).
- Wavelength (µm)
- H₂O
- CH₃D
- GeH₄
- NH₃
- Voyager 1
- Voyager 2
- Wave number (cm⁻¹)
- Voyager 1 brightness temperature (K)
- Voyager 2 brightness temperature (K)
The structure of the Jovian atmosphere can be derived from infrared spectra as well as from the radio occultation data. This profile of temperature as a function of pressure covers the same range of altitudes as does the preceding figure and is in good agreement. Both profiles locate a temperature minimum of about 110 K near a pressure of 0.1 bar (1000 mb = 1 bar = 1 atm). The IRIS data also show variation of structure with position, including a cooler minimum temperature (about 100 K) over the Great Red Spot.
The structure of the atmosphere can be inferred from IRIS spectra at many locations over the disk of Jupiter. Scientists are beginning to assemble this vast amount of information into maps that show the temperatures at a given pressure. The temperature contours are labeled in degrees Kelvin. The banded structure, with higher temperatures near the dark equatorial belt, is most clearly evident at the lower altitude. Surprisingly, the cool region associated with the Great Red Spot (latitude 23°S) is more apparent at high altitude.
The observed temperature at a depth near the cloud tops (0.8 bar).
The observed temperature at an altitude about 30 kilometers higher (0.15 bar).
The Magnetosphere
Giant Jupiter has an enormous realm—from the size of its satellite system to its tremendous aurorae and superbolts of lightning, to the huge planet-sized cloud features that surround its atmosphere. The most gargantuan Jovian feature is its magnetosphere, which envelopes the satellites and constantly changes in size, pumping in and out at the whim of the solar wind. The Pioneer and Voyager spacecraft provided four cuts through this dynamic region, showing that its borders in the upwind solar direction lie between 50 RJ and 100 RJ from Jupiter. Downwind, away from the Sun, the magnetosphere extends much farther; some scientists postulate that a magnetotail may reach as far as the orbit of Saturn.
Charged particles in the magnetosphere are subject to powerful forces. Tightly embedded in Jupiter, the magnetic field spins with a ten-hour period as the planet rotates. The particles are caught in the spinning field and accelerated to high speeds. The result is a co-rotating plasma in the magnetic equator of Jupiter, extending outward to at least 20 RJ. Beyond this distance, the flow breaks up and the magnetosphere is more unstable. Within the co-rotation region, the spinning plasma sets up a powerful electric current girdling the planet.
Charged particles can be accelerated in the magnetosphere to high energies, corresponding to speeds tens of thousands of kilometers per second. Some of these particle streams escape from the inner parts of the magnetosphere and can penetrate the magnetopause and be ejected from the Jovian system. On Voyager 1, the low energy charged particle instrument began detecting these streams of “hot” plasma on 22 January, when Voyager 1 was still 600 RJ (almost 50 million kilometers) from the planet. Voyager 2 first detected Jovian particles at an even greater distance, 800 RJ. Hydrogen and helium ions (protons and alpha particles) dominate the magnetosphere at great distances from Jupiter, but increasing amounts of sulfur and oxygen appeared as the spacecraft crossed the magnetopause. The heavier ions presumably originate from Io.
In the inner magnetosphere, the Galilean satellites have a powerful influence on the populations of fast-moving particles. During the Voyager 1 encounter, the primary effect was observed at Io, where the satellite apparently sweeps up energetic electrons. In the million-volt energy range, these particles are depleted near Io, with peaks observed both inside and outside the satellite’s orbit. Voyager 2 passed close to Ganymede, and here also major effects were seen, with the satellite apparently absorbing electrons. In the wake created by the motion of the co-rotating plasma past Ganymede, the particle populations showed large and complex variations.
The magnetosphere of Jupiter can be “seen” from Earth by its emissions at radio wavelengths. The recent development of imaging radio telescopes in Great Britain, The Netherlands, and the United States allows frequent mapping of the large-scale features of the innermost magnetosphere, inside the orbit of Io. These six images showing one rotation of Jupiter were obtained near the time of the Voyager 1 flyby by Imke de Pater at the Westerbork Radio Observatory near Leiden. The brightest emission is shown by dark red or black. The size of the planet is indicated by a dotted white circle. The tilt of the magnetosphere relative to the rotation axis of the planet can be seen by the wobble of the magnetosphere as Jupiter rotates.
Just inside the Jovian magnetosphere is the “hot spot” of the solar system: a 300-400 million degree plasma detected by Voyager 1 while it was still about 5 million kilometers from Jupiter. T. P. Armstrong commented, “Even the interior of the Sun is estimated to be less than 20 million degrees.” S. M. Krimigis added that the temperature of this plasma is “the highest yet measured anywhere in the solar system.” Fortunately for Voyager, this region of incredibly hot plasma is also one of the solar system’s best vacuums. The spacecraft was in little danger because the bow shock protects this region from the solar wind, and most of the particles in Jupiter’s magnetosphere are held in much closer to the planet.
Each Voyager passed through the boundaries of the magnetosphere—the bow shock (BS) and the magnetopause (MP)—on both the inbound and the outbound legs of its passage through the Jovian system. In this diagram, the heavy solid line represents the spacecraft trajectory, as seen looking down from the north. Also shown are the positions of the bow shock in March and of the magnetosphere in both March and July.
Several regions of plasma (charged particles) make up the Jovian magnetosphere. These sketches, based on Voyager data, show the magnetosphere as viewed from above (a) and as seen from the Jovian equatorial plane (b). Most of the plasma co-rotates with the planet and is confined near the magnetic equator, where it forms a broad plasma sheet about 100 RJ across.
The very hot plasma in the outer magnetosphere discovered by Voyager is thought to play an important role in establishing the size of the Jovian magnetosphere. Although the density is low, only about one charged particle per hundred cubic centimeters, this plasma actually carries a great deal of energy because of the high speed of the particles. It is this plasma pressure, rather than the magnetic field pressure, that appears to hold off the pressure of the solar wind. However, the balance between hot plasma inside the magnetopause and the solar wind outside is not very stable. The Voyager experimenters suggest that a small change in solar wind pressure can cause the boundary to become suddenly unstable. A large quantity of the hot plasma can then be lost, producing the bursts seen at large distances and permitting a sudden collapse of the outer magnetosphere. Continued injection of hot plasma from within would then reinflate the magnetosphere, which would expand like a balloon until another instability developed. Processes of this sort may be the cause of the rapidly varying magnetospheric boundaries observed by both Voyager spacecraft.
The rings of Jupiter are best seen when looking nearly in the direction of the Sun, since the small particles that comprise them are good forward scatterers of sunlight. This mosaic is of Voyager 2 images (two wide angle and four narrow angle) obtained from a perspective behind the planet and inside the shadow of Jupiter. The spacecraft was 2 degrees below the equator of Jupiter and 1.5 million kilometers from the rings. The shadow of the planet can be seen to obscure the near segment of the ring near the edge of the planet. The brightest region of the ring is about 1.8 RJ from the center of Jupiter. [260-678B]
Rings of Jupiter
One of the spectacular discoveries of Voyager was the existence of a ring system of small particles circling Jupiter. Saturn and Uranus were known to have rings, but none had been seen before at Jupiter.
As revealed by the Voyager cameras, the rings extend outward from the upper atmosphere to a distance of 53 000 kilometers above the cloud tops, 1.8 RJ from the center of the planet. The main rings, however, are much narrower, spanning from 47 000 to 53 000 kilometers above Jupiter. There are two main rings, a 5000-kilometer-wide segment, and a brighter, outer 800-kilometer segment. The thickness of the rings is unknown, except that it is certainly less than 30 kilometers, and probably under 1 kilometer.
Structure within the ring can be seen in the best Voyager 2 images, taken about 27 hours after closest approach to Jupiter. This enlarged portion of a wide-angle picture taken with a clear filter shows a bright core about 800 kilometers across with a dimmer region a few thousand kilometers across on the inside, and a narrow dim region on the outside. [260-674]
The rings of Jupiter are quite tenuous, which explains why they are invisible from Earth [although they have been detected from Earth since Voyager]. Seen face on, the brightest part of the ring blocks less than one part in ten thousand of the light passing through, making it essentially transparent. In fact, the ring does not even offer much resistance to a spacecraft; Pioneer 11 traversed the ring in 1974 with no obvious ill consequences. Apparently the individual particles that make up the ring are widely dispersed. They can be seen only when the rings are viewed nearly edge-on, or toward the Sun, where they show up well in forward scattered light. It is this extra brilliance when backlit that created the excellent photos taken by Voyager 2 from inside the shadow of Jupiter.
The individual ring particles are probably dark, rocky fragments that are very small—essentially dust grains. They move around Jupiter in individual orbits, circling the planet in 5 - 7 hours. Scientists postulate that such orbits are not stable and that the particles fall slowly in toward Jupiter. Apparently the rings are constantly renewed from some source, which may be the satellite Adrastea (J14), discovered by Voyager 2. There has also been speculation that Adrastea may influence the ring structure by sweeping particles out of the ring. At present the rings of Jupiter remain mysterious. They are clearly very different from the rings of Saturn and Uranus, and reaching an understanding of their origins and dynamics presents many challenges to planetary scientists.
The four Galilean satellites of Jupiter are planet-like worlds, revealed by Voyager to be as diverse and fascinating as the terrestrial planets Mercury, Venus, Earth, and Mars. In this Voyager 1 composite, all four are shown in their correct relative size, as they would appear from a distance of about 1 million kilometers. Relative color and reflectivity are also approximately preserved, although it is not possible to show on a single print the full range of brightness from the dark rocky surface of Callisto to the brilliant white of Europa or orange of Io. [260-499C]
Io (longitude 140°).
Europa (longitude 300°).
Callisto (longitude 350°).
Ganymede (longitude 320°).
CHAPTER 9
FOUR NEW WORLDS
Jupiter’s Satellite System
In a sense, the Voyager Mission revealed a new planetary system. Astronomers had long been fascinated by the large Galilean satellites of Jupiter, but they had only looked from afar, watching the dancing points of light in their telescopes, and, occasionally, as the atmosphere steadied, seeing these points resolve themselves into tiny disks before dissolving again in the turbulence of the terrestrial atmosphere. Much had been learned from telescopic studies, but not until the Voyager flights had we truly seen the Galilean satellites. The historic hours as Voyager 1 cruised past each satellite on March 5 and 6, 1979, fundamentally altered our perspective. Four new worlds were revealed, as diverse and fascinating as the more familiar terrestrial planets. Although not yet household words, the names Io, Europa, Ganymede and Callisto have now been added to Mercury, Venus, Moon, and Mars in the lexicon of important “Earth-sized” bodies in the solar system.
Jupiter has fifteen known satellites, counting the two new satellites discovered by Voyager. These moons vary greatly in size, composition, and orbit. The four outermost satellites, Sinope, Pasiphae, Carme, and Ananke, circle the planet in retrograde orbits of high inclination; their distances from Jupiter vary between 20 and 24 million kilometers (290 RJ to 333 RJ). These small bodies, none more than 50 kilometers in diameter, require nearly two years for each orbit of Jupiter. It is possible that they are captured asteroids, but so little is known about them that astronomers cannot tell if their surface properties resemble those of asteroids, or if these four satellites are even similar to each other.
The next group of Jovian satellites consists of four small difficult-to-observe objects. These are Lysithea, Elara, Himalia, and Leda, the latter discovered by Charles Kowal of Hale Observatories in 1974. They have similar orbits, varying in distance from Jupiter between 11 and 12 million kilometers (about 160 RJ). Like the outer group, these satellites have orbits of high inclination; unlike the outer group, they move in the proper, prograde direction around Jupiter. The largest, Himalia (170 kilometers in diameter) and Elara (80 kilometers diameter), are known to be very dark, rocky objects, and it seems probable that the others are similar. It is unlikely that the census of the outer groups of irregular satellites is complete, and new satellites less than 10 kilometers in diameter will probably be discovered.
The Jovian system is dominated, of course, by the large Galilean satellites, which vary in size from just smaller than the Moon (Europa) to nearly as large as Mars (Ganymede). These satellites are in regular, nearly circular orbits in the same plane as the equator of Jupiter, and all four lie within the inner magnetosphere of Jupiter, where they interact strongly with energetic charged particles and plasma. Most of this chapter will be devoted to a discussion of these fascinating worlds.
We now know of three additional small satellites inside the orbit of Io, orbiting close to Jupiter. The first, Amalthea, was discovered in 1892; it orbits Jupiter in just twelve hours at a distance of 181 000 kilometers (2.55 RJ). A smaller object, Adrastea (officially 1979J1 for the first new satellite of Jupiter discovered in 1979), is much closer, at 134 000 kilometers (1.76 RJ). As described in Chapter 7, it skirts the outer edge of the ring, circling Jupiter in just over seven hours. The inner satellite moves faster than Jupiter’s rotation; seen from the planet, it would rise in the west and set in the east. Both Amalthea and Adrastea are buried deep within the inner magnetosphere where they are continually bombarded by energetic electrons, protons, and ions. Depletion of the Jovian radiation belt particles was observed at the orbits of both satellites by Pioneer 11, which went much closer to Jupiter than the Voyagers, testifying to the intensity of the interaction between these objects and their surroundings.
Callisto was revealed by the Voyager cameras to be a heavily cratered and hence geologically inactive world. This mosaic of Voyager 1 images, obtained on March 6 from a distance of about 400 000 kilometers, shows surface detail as small as 10 kilometers across. The prominent old impact feature Valhalla has a central bright spot about 600 kilometers across, probably representing the original impact basin. The concentric bright rings extend outward about 1500 kilometers from the impact center. [260-450]
| SATELLITES OF JUPITER | |||||
|---|---|---|---|---|---|
| Distance From Jupiter | |||||
| Name | 10³ kilometers | Jupiter Radii | Period (days) | Year of Discovery | |
| Adrastea | J14 | 134 | 1.76 | 0.30 | 1979 |
| Amalthea | J5 | 181 | 2.55 | 0.49 | 1892 |
| 1979J2 | J15 | 222 | 3.11 | 0.67 | 1980 |
| Io | J1 | 422 | 5.95 | 1.77 | 1610 |
| Europa | J2 | 671 | 9.47 | 3.55 | 1610 |
| Ganymede | J3 | 1070 | 15.10 | 7.15 | 1610 |
| Callisto | J4 | 1880 | 26.60 | 16.70 | 1610 |
| Leda | J13 | 11 110 | 156 | 240 | 1974 |
| Himalia | J6 | 11 470 | 161 | 251 | 1904 |
| Lysithea | J10 | 11 710 | 164 | 260 | 1938 |
| Elara | J7 | 11 740 | 165 | 260 | 1904 |
| Ananke | J12 | 20 700 | 291 | 617 | 1951 |
| Carme | J11 | 22 350 | 314 | 692 | 1938 |
| Pasiphae | J8 | 23 300 | 327 | 735 | 1908 |
| Sinope | J9 | 23 700 | 333 | 758 | 1914 |
The state of the interiors of the Galilean satellites can be judged from their sizes and densities. These cross-sectional views represent the best guess following the Voyager flybys as to the composition and structure of the objects. Io, with a density equal to that of the Moon and a long history of volcanic activity, is a dry, rocky object. Europa is less dense, and it probably has a global ocean of ice as much as 100 kilometers thick over a rocky interior. Ganymede and Callisto both have densities near 2 grams per cubic centimeter, suggesting a composition about half water and half rock. There is probably a rocky core surrounded by an icy mantle.
- Io
- Active volcanoes
- Sulfur and frozen SO₂
- Molten silicate interior
- Europa
- Global fracture patterns
- Ice crust
- Rocky interior
- Ganymede
- Fresh craters expose ice
- Young grooved terrain with intricate fracture
- Old, dark cratered areas
- Ice crust
- Water or ice mantle
- Callisto
- Large basins reduced by ice flow
- Fresh craters expose ice
- Ice/rock crust
- Water or ice mantle
- Moon
- Mercury
Long after the flybys of Jupiter, continued analysis of Voyager images revealed another new satellite, Jupiter’s fifteenth. Initially designated 1979J2, the unexpected new satellite orbits the planet at 3.17 RJ, between Io and Amalthea. Stephen Synnott of the JPL Optical Navigation Team discovered the satellite on pictures taken during the Voyager 1 events on March 5, 1979, while searching for additional images of satellite 1979J1. It is about 75 kilometers in diameter, but nothing else is known about its physical properties.
Together, the 15 satellites circling giant Jupiter form a mini-solar system. Perhaps the outer, irregular satellites were captured or resulted from the catastrophic collisions of one or more larger satellites with passing asteroids. The inner seven satellites constitute a coherent system, almost certainly formed together with Jupiter and sharing a common 4.5-billion-year history. They are fascinating as individual worlds, and also as brothers and sisters, and the study of their interrelationships undoubtedly will provide insights into the general problems of planetary formation and evolution.
| SIZES AND DENSITIES OF THE GALILEAN SATELLITES | ||
|---|---|---|
| Name | Diameter (kilometers) | Density (grams per cubic centimeter) |
| Io | 3640 | 3.5 |
| Europa | 3130 | 3.0 |
| Ganymede | 5270 | 1.9 |
| Callisto | 4840 | 1.8 |
Callisto
Callisto is the least active geologically of the Galilean satellites. Basically a dead world, it bears the scars of innumerable meteoric impacts, with virtually no sign of major internal activity. Callisto is a world of craters, and to understand it we must explore the role that cratering plays in molding planetary surfaces.
The space between the planets is filled with debris, ranging from the larger asteroids, hundreds of kilometers in size, down to microscopic grains of dust. Inevitably, each planet collides with some of these fragments. The smaller particles do little damage; in the case of a planet with an atmosphere, like Earth, they burn up as meteors before reaching the surface, whereas on an airless planet, they erode the surface by sandblasting the exposed rock. The larger impacts are another matter, and the craters they produce can be the dominant features on the surface of a planet.
Voyagers 1 and 2 photographed most of the surface of Callisto at resolutions of a few kilometers or better. Shown here is a preliminary shaded relief map. Additional measurements will improve the accuracy of the coordinate system. [260-672]
Callisto is a world of craters, as is well shown in this Voyager 2 photomosaic taken from a distance of 400 000 kilometers. Craters about 100 kilometers in diameter cover the surface uniformly. Many have bright rims, perhaps composed of exposed water-ice. There are very few craters larger than 150 kilometers in diameter, however, indicating that the scars of very large impacts do not survive on the surface of Callisto. [P-21746B/W]
The concentric rings surrounding Valhalla are perhaps the most distinctive geological feature on Callisto. This Voyager 1 close-up shows a segment of the ridged terrain. The presence of superposed impact craters shows that the rings formed early in Callisto’s history; however, the density of craters is less here than on other parts of the satellite, where the surface is older. [P-22194]
We who live on Earth tend not to realize the importance of cratering, for the simple reason that our planet has very few craters, and these are frequently of volcanic rather than meteoric origin. Why are we so favored? Is there an invisible shield to protect us from the cosmic shooting gallery? Clearly not; the Earth has experienced just as many cratering impacts as has the Moon or other planets. The difference is not that craters are formed less often, but that the great geological activity of Earth—erosion, volcanism, mountain building, continental drift, etc.—erases craters as fast as they are formed. On the average, a 10-kilometer-wide crater is formed on Earth about once every million years, but all those older than a few million years have been eroded away, filled in, or crushed beyond recognition by crustal motion.
If a planet lacks great internal geologic forces, large craters can survive almost indefinitely. Such is the case for the Moon. Most of the volcanism and other activity on the Moon ceased 3½ billion years ago, as the dating of lunar samples obtained by the Apollo astronauts showed. Since that time, the lunar surface has been passively accumulating impact scars. The longer any particular surface area has been exposed, the more densely packed are the craters. Thus crater density is the first thing a planetary geologist looks for in photos of a new world. Craters are the touchstone of this field, revealing the degree of internal activity and allowing the determination of the relative ages of different surface units.
On Callisto the density of craters is very high. In some places they are packed as closely as one can imagine, particularly for craters several tens of kilometers in diameter. Although no one knows the exact rate of formation of impact craters on the Jovian satellites, geologists on the Voyager Imaging Team estimate that it would require several billion years to accumulate the number of craters found on Callisto. They therefore conclude that Callisto has been geologically inactive almost since the time of its formation.
Although superficially similar to the heavily cratered surfaces of the Moon and Mercury, Callisto is far from identical to these rocky worlds. One of the most obvious differences is a lack of craters larger than about 150 kilometers on Callisto, together with a tendency for large craters to have much shallower depths. Apparently the ice-rock composition of Callisto alters the ability of the crust to retain large craters. Geologists speculate that the ice flows over many millions of years, filling in crater floors and gradually obliterating the largest craters. There is also a conspicuous absence of mountains on Callisto, again suggestive of a weak, icy crust.
The IRIS instrument measured the temperature of spots on the surface of Callisto as each Voyager sped past. The measurements shown here were all made at equatorial latitudes (between -10° and 25°). Shown are very low predawn temperatures (-190° C) followed by an increase to a noon-time maximum of about -120° C, and then a drop again as the Sun sets. [260-735]
The most prominent features in the Voyager pictures are the ghost remnants of what must have been immense impact basins. The largest of these, the “bullseye” of the Voyager 1 images, has been named Valhalla, for the home of the Norse gods. These ghost basins have lost nearly all their vertical relief. What remains is a central, light-colored zone (probably the location of the original crater), surrounded by numerous concentric rings of subdued, bright ridges. Such features had never been seen before on any planet, and they appear to be the characteristic geologic feature of an ice-rock planet.
Little is known about the composition of Callisto’s surface, the material from which sunlight is reflected. It appears to be primarily dark rock or soil, but it lacks diagnostic spectral features, except for one infrared band due to water molecules bound in the soil. The many lighter spots and arcs that outline craters in the high-resolution pictures may be regions in which the ice is showing through, but these cover only a very small fraction of the exposed surface. (It should be noted that, although Callisto is the darkest of the Galilean satellites, the term “dark” is relative, for even Callisto is brighter than Earth’s moon.)
The daytime surface temperature of Callisto, observed both from the ground and by Voyager, is about -118° C. The Voyager infrared interferometer spectrometer also determined the minimum temperature, reached just before dawn, of -193° C. No atmosphere is expected at these cold temperatures, and none was seen.
Analysis of Voyager images provided an improved diameter for Callisto of 4840 kilometers, yielding an average density of 1.8 grams per cubic centimeter. As noted previously, it is this low density that leads to the conclusion that ice or water is an important component of the interior of Callisto. The ice has never been detected directly, but the peculiar nature of the craters seen by Voyager adds strong circumstantial support to this conclusion.
Callisto, with its heavy cratering, is the most familiar-looking of the Galilean satellites; if all of them had turned out to be as geologically dead as Callisto, planetary geologists would certainly have been disappointed. However, each satellite, progressing in toward Jupiter, presents increasing evidence of internal activity.
Ganymede
The largest of the Galilean satellites (5270 kilometers in diameter), Ganymede was expected to be similar to Callisto in many ways. Both have low densities (for Ganymede, 1.9 grams per cubic centimeter), indicating a bulk composition of about half rocky materials and half water. In addition, their diameters differ by only eight percent, and both are far enough from Jupiter to escape the severe pounding Io receives from magnetospheric charged particles. Thus it was with great interest that Voyager scientists looked at the differences that emerged between these two satellites.
Shaded relief map of Ganymede. [260-673]
The surface of Ganymede as revealed by the Voyager cameras is one of great diversity, indicating differing periods of geologic activity. At one extreme there are numerous dark areas that resemble the surface of Callisto in both albedo (reflectivity) and crater density. The largest of these, Regio Galileo, stretches from the equator to latitude +45° and is 4000 kilometers across, nearly as large as the continental United States. This ancient terrain even preserves the remnants of a Callisto-type impact basin in the form of a system of parallel, curving, subdued ridges about 10 kilometers wide, 100 meters high, and spaced about 50 kilometers apart. The central part of this ghost basin is missing, however; it was presumably destroyed by subsequent geologic activity.
Other regions of the surface of Ganymede are clearly the product of intense internal geologic activity. Generally, these regions are of higher albedo and consist of many straight parallel lines of mountains and valleys. The Voyager geologists call these the grooved terrain because of their appearance from a great distance. Typically, these mountain ridges are 10-15 kilometers across and about 1000 meters high, similar in scale to some sections of the Appalachian Mountains in the Eastern United States. No higher relief exists, presumably for the same reason it is absent on Callisto. In many places the grooved terrain forms between areas of the older, darker surface, giving the appearance of mountains extruded between separating plates of ancient crust. In other areas the relationships are much more complex, with curved systems of grooves and ridges overlying each other, displaying intricate crosscutting relationships. Apparently Ganymede has experienced a series of mountain-building events.
The grooved terrains show a substantial range in age, as indicated by the crater densities. The oldest have nearly the same density as the ancient, dark plains, suggesting that formation of the grooved terrain began early, perhaps 4 billion years ago. The youngest grooved terrain has only about one-tenth as many craters, but this is still as many as are seen in the 3.5-billion-year-old lunar plains. The Voyager geologists believe that even in these areas geologic activity ceased billions of years ago.
The hemisphere of Ganymede that faces away from the Sun displays a great variety of terrain. In this Voyager 2 mosaic, photographed at a range of 300 000 kilometers, the ancient dark area of Regio Galileo lies at the upper left. Below it, the lighter grooved terrain forms bands of varying width, separating older surface units. On the right edge, a prominent crater ray system is probably caused by water-ice splashed out in a relatively recent impact. [260-671]
The sinuous nature of some of the narrower Ganymede groove systems can be seen in this oblique view, obtained on March 5 by Voyager 1. The area shown is about the size of California, with features visible as small as 5 kilometers across. The ridges appear to be the result of deformation of the crust of Ganymede. [P-21235]
Other types of surfaces are seen on Ganymede. Some regions are lightly cratered and smooth, with no indication of mountain building. In one place, there is a rough mountainous area that looks more like the jumbled lunar mountains than the long ridges and valleys of the rest of Ganymede. Many of the larger craters are distinguished by brilliant white halos and rays that suggest that impacts may have splashed large quantities of water or ice over the surface.
Many of the geologic features seen on Ganymede appear to have been caused by breaking, faulting, or spreading of the crust. In a few cases, there even seem to be indications of transverse, or sideways, motion along faults. This evidence is extremely exciting to geologists, since similar crustal motion on Earth is associated with the drift of continental plates, drawn by convection currents deep in the mantle. Such activity has never been seen before on another planet.
Astronomers on Earth had known since 1971 that about half the surface of Ganymede was covered with exposed water ice and about half with darker rock. An examination of the albedo variations in the Voyager pictures suggests that the ice is exposed near large craters and, to a lesser extent, in the grooved terrain, but no direct measurements were made by Voyager of the composition of different parts of the surface.
The presence of ice on the surface suggested to many astronomers that Ganymede might have a very tenuous atmosphere of water vapor or oxygen, which might be released by the breakdown of water vapor by sunlight. During the Voyager 1 flyby, a sensitive test for an atmosphere was made by the ultraviolet instrument from observations of the star Kappa Centauri as it was occulted by Ganymede. No dimming of the starlight was seen, yielding an upper limit for the surface pressure of the gases oxygen, water vapor, or carbon dioxide of 10⁻¹¹ bar, or one hundred-billionth the atmospheric pressure at Earth.
The differences between the geologic histories of Ganymede and Callisto are surprisingly large. No one knows the reason. Perhaps only a small increase in internal temperature is necessary to initiate geologic activity in an icy planet, and for some reason Ganymede crossed this threshold for a part of its history, whereas Callisto did not.