Part 2

The largest aurora ever observed, nearly 29,000 kilometers (18,000 miles) long, appears in this Voyager 1 photograph, taken on the dark side of Jupiter six hours after closest encounter. The auroral lights are brighter than any northern lights seen on Earth. Jupiter’s north pole is approximately midway along the auroral arc. This timed exposure of the aurora also shows what appear to be lightning storms several thousand kilometers below the aurora. The strength of the lightning bolts is comparable to that of superbolts seen near cloud tops above Earth. Lightning had been suspected to exist on Jupiter, but at lower levels in the atmosphere.]

The first evidence of a ring around Jupiter is seen in this photograph taken by Voyager 1. This photograph was part of a sequence planned to search for such rings around Jupiter. The multiple image of the extremely thin, faint ring appears as a broad light band crossing the center of the picture. This multiple image and the elongated, wavy motion of the background stars are due to the 11-minute, 12-second exposure and the very slow natural oscillation of the spacecraft. The ring, which is in Jupiter’s equatorial plane, is invisible from Earth because of its thinness and transparency and because of Jupiter’s brightness. The black dots in the picture are calibration points in the camera.]

In this four-picture mosaic, the arms of the ring curving toward the spacecraft (on the near side of the planet) are cut off by the planet’s shadow. Scientists estimate that the distance from the Jovian cloud tops to the outer edge of the ring is 55,000 kilometers (35,000 miles).]

In this picture, which is composed of six images, there is evidence of structure within the ring, but the spacecraft motion during these long exposures obscured the highest resolution detail. However, there is speculation that the ring width, estimated at 6000 kilometers (4000 miles), contains more than one ring.]

This photograph is an enlargement of the isolated left frame in the first picture and reveals a density gradient of very small particles extending inward from the ring. The thickness of the ring has been estimated at less than one kilometer (0.6 mile) although the ring appears about 30 kilometers (19 miles) thick in the image, due to camera motion and finite resolution. Composition of the low-albedo (dark) particles is not known, but particle size probably ranges from microscopic to at most a few meters in diameter. If collected together to form a single body, the total mass of the Jovian rings would form an object with a diameter less than twice that of tiny Amalthea.]

Jupiter and two of its planet-sized satellites, Io at left and Europa at right, are visible in this Voyager 1 picture. Jupiter’s four largest satellites—Io, Europa, Ganymede and Callisto—were discovered in 1610 by Galileo Galilei. The two outer Galilean satellites are Ganymede and Callisto, not shown in this picture. All four satellites probably formed about four billion years ago but their surfaces vary in age tremendously. Io and Europa have younger, more active surfaces than Ganymede and Callisto. Like our Moon, the satellites keep the same face toward Jupiter. In this picture, the sides of the satellites that always face away from the planet are visible.]

Amalthea was observed end-on in the Voyager 2 picture, which has been computer-processed to enhance the image.]

Io, Jupiter’s innermost Galilean satellite, displays great diversity in color and brightness. This Voyager 1 four-picture mosaic shows Io’s complex coloration of red-orange, black, and white regions, and the two major topographic features: volcanic regions, the most prominent of which is the “hoofprint” (volcanic deposition feature) in the center-right, and the intervolcanic plains that are relatively featureless. Io’s vivid coloring is probably due to its composition of sulfur-rich materials that have been brought to the surface by volcanic activity.]

The bright area at the upper right in this Voyager 1 picture of Io appears to be a caldera (collapsed volcano) that is venting clouds of gases. The clouds may condense to form extremely fine particles that scatter light and appear blue. Because the infrared spectrometer discovered sulfur dioxide on Io, scientists believe this gas may be the main component of the clouds. Sulfur dioxide clouds would rapidly freeze and snow back to the surface. It is also possible that dark areas in the floors of the calderas are pools of encrusted liquid sulfur.]

Evidence of erosion in Io’s southern polar region is visible in this Voyager 1 high-resolution image. The picture has been computer-enhanced to bring out surface detail while suppressing bright markings. A depressed segment of the crust, bounded by faults, is seen near the terminator in the upper right portion of the image. At the lower center are complicated scarps (slopes) and portions of isolated elevated terrain that geologists interpret as “islands” left behind as the scarps eroded. Scientists speculate that sulfur dioxide (as a subsurface liquid) may be a determinant in the creation of these features.]

Io’s surface, less than ten million years old, is quite young compared to the other Galilean satellites and to other terrestrial bodies, such as Mercury and the Moon. The surface is composed of large amounts of sulfur and sulfur dioxide frost, both of which account for most of the surface color. This picture was taken by Voyager 1. Material deposited by the volcano (see following pictures) can be seen as a white ring near the center of Io.]

In this picture, the plume visible on the right edge extends more than 100 kilometers (60 miles) above the surface.]

The same volcano is shown in this picture, photographed one hour and 52 minutes earlier.]

Special color reconstruction by means of ultraviolet, blue, green, and orange filters allowed scientists to study the amount of gas and dust and the size of the dust particles that erupted from the volcano on Io shown in this Voyager 1 image. The region that is brighter in the ultraviolet (blue area) is about 210 kilometers (130 miles) high, over twice the height of the denser, bright yellow core. The vent area is visible on page 18 as a dark ring in the upper left region of Io.]

Of the eight active volcanoes discovered on Io by Voyager 1, six of the seven volcanoes sighted by Voyager 2 were still active. The giant volcano observed by Voyager 1 over the “hoofprint” region (see page 18) had become inactive. Scientists, therefore, believe that the satellite is undergoing continuous volcanic activity, making Io’s surface the most active in the solar system. This Voyager 2 photograph, which shows three active volcanoes, was one of the last of an extensive sequence of “volcano watch” pictures planned as a result of Voyager 1’s volcano discovery. The black dots are calibration points on the camera.]

Europa, approximately the same size and density as our Moon, is the brightest Galilean satellite. The surface displays a complex array of streaks, indicating that the crust has been fractured. In contrast to its icy neighbors Ganymede and Callisto, Europa has very few impact craters. The relative absence of features and low topography indicate that the crust is young and probably warm a few kilometers below the surface. The warmth is probably due to a combination of radioactive and tidal heating. The tidal heating within Europa is estimated to be ten percent that of the stronger tidal heating effect within Io. The regions that appear blue in this Voyager 2 image are actually white.]

Europa’s surface is probably a thin ice crust overlying water or softer ice (slush) about 100 kilometers (60 miles) thick that covers a silicate interior. The tectonic processes on Europa’s surface create patterns that are drastically different from the fault systems seen on Ganymede’s surface, where pieces of the crust have moved relative to each other. On Europa, the crust evidently fractures, but the pieces remain roughly in their original position. This Voyager 2 picture is composed of three images.]

Long linear fractures or faults which crisscross Europa’s surface in various directions are over 1000 kilometers (600 miles) long in some places. Large fractures are 200 to 300 kilometers (125 to 185 miles) wide, wider than the crust is thick. Also visible are somewhat darker mottled regions that appear to have a slightly pitted appearance. No large craters (more than five kilometers in diameter) are identifiable in this Voyager 2 picture, indicating that this satellite has a very young surface relative to Ganymede and Callisto, although perhaps not as young as Io’s surface. Scientists believe that the surface is a thin ice crust overlying water or softer ice and that the fracture systems are breaks in the crust. Resurfacing processes, such as the production of fresh ice or snow along the cracks and cold glacier-like flows, have probably removed evidence of impact events (cratering). Europa, therefore, appears to have many properties similar to Ganymede and Io.]

Complex narrow ridges, seen as curved bright streaks 5 to 10 kilometers (3 to 6 miles) wide and typically 100 kilometers (60 miles) long, characterize the surface topography of this view of Europa. The dark bands also visible in this Voyager 2 photo are 20 to 40 kilometers (12 to 25 miles) wide and up to thousands of kilometers long. The fractures on the icy surface are filled with material from beneath, probably as a result of internal tidal flexing which continually heats the thin outer ice crust. A few features are suggestive of degraded impact craters.]

Ganymede, Jupiter’s largest satellite, is about one and one-half times the size of our Moon but only about half as dense and is composed of about 50 percent water or ice and the rest rock. The bright surface of Ganymede is a complex montage of ancient, relatively dark and cratered terrain, grooved terrain that resulted from a dramatic history of tectonic movement in the icy crust, and bright young ray craters that expose fresh ice. This photograph was taken by Voyager 1.]

The dark, cratered, circular feature in this Voyager 2 photograph is about 3200 kilometers (2000 miles) in diameter and is on the side of Ganymede opposite to that shown in the previous picture. This region is apparently the largest piece of ancient, heavily cratered crust left on Ganymede. The light branching bands are ridged and grooved terrain which are younger than the more heavily cratered dark regions. Despite the dramatic surface appearance, Ganymede is relatively devoid of topographic relief due to the consequences of glacier-like “creep” in the icy crust.]

Several different types of terrain common to Ganymede’s surface are visible in this Voyager 2 picture. The boundary of the largest region of dark ancient terrain (also shown in the previous photo) can be seen to the right, revealing the light linear features that may be the remains of shock rings from an ancient impact. The broad light regions are the typical grooved structures contained within the light regions on Ganymede. On the lower left is another example of what might be evidence of large-scale lateral faulting in the crust; the band appears to be offset by a linear feature perpendicular to it. These are the first clear examples of lateral faulting seen on any planet other than Earth.]

This color reconstruction of part of Ganymede’s northern hemisphere, taken by Voyager 2, encompasses an area about 1300 kilometers (800 miles) across. It shows part of a dark, densely cratered region that contains numerous craters, many with central peaks. The large bright circular features have little relief and are probably the remnants of old, large craters that have been annealed by the flow of icy material near the surface. The gradually curving lines that press through the dark region suggest the presence of a large impact basin to the southwest, which has been obliterated by the subsequent formation of younger grooved terrain.]

A broad, north-south strip of grooved terrain on Ganymede, offset by a traversing fault in the upper part of the picture, is shown in this Voyager 1 photograph. There are several other perpendicular fault lines farther down on the fault. Within the major light stripes, the more closely spaced, shallow grooves run parallel to the boundaries of the stripes. The larger striped features divide the cratered terrain into isolated polygons several hundred to about 1000 kilometers (600 miles) across.]

The grooved terrain at higher resolution emphasizes numerous interwoven linear features in this Voyager 1 picture, near the terminator on Ganymede. This suggests an early period in Ganymede’s history when the crust was active and mobile, resembling Earth’s plate tectonics in some ways. The causes of the extreme differences in crustal evolution between Callisto and Ganymede are under investigation. Combinations of radioactive heating and a greater degree of tidal heating for Ganymede are possibilities.]

This mosaic of Ganymede, composed of photographs taken by Voyager 2, shows numerous impact craters, many with bright ray systems. The rough terrain at the lower right is the outer portion of a large, fresh impact basin that postdates most of the other terrain. The dark patches of heavily cratered terrain (right center) are probably ancient mixtures of ice and rock formed prior to the grooved terrain. The large rayed crater at the upper center is about 150 kilometers (95 miles) in diameter.]

Curved troughs and ridges in this high-resolution Voyager 2 photograph of Ganymede are the distinctive characteristics of an enormous, ancient impact basin. The basin itself has been eroded by later geologic processes; only the shock ring features are preserved on the ancient surface. Near the bottom of the picture these curved markings are perforated with the younger, grooved terrain.]

Callisto, only slightly smaller than Ganymede, has the lowest density of all the Galilean satellites, implying that it has large amounts of water in its bulk composition. Its surface is darker than the other Galilean satellites, although it is still twice as bright as our Moon. This Voyager 2 image shows Callisto to have the most heavily cratered and, therefore, the oldest surface of the Galilean satellites, probably dating back to the period of heavy meteoritic bombardment ending about four billion years ago.]