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Galileo has provided high-resolution images of the satellites of Jupiter that change our thinking about this miniature solar system.
by James W. Head, III, Brown University, Providence, R.I.
In the last 35 years, we have seen an unparalleled increase in the knowledge of our Solar System, as spacecraft have visited every planet except Pluto and beamed data back as to the nature of planet and satellite interiors, surfaces, atmospheres, and interplanetary space. As a better picture of the nature and evolution o f our Solar System emerges, imagine having the opportunity to compare a miniature solar system to the larger one! Such a miniature version exists in the form of the Galilean satellites arrayed in orbit around Jupiter.
Galileo Galilei discovered these four moons, Io, Europa, Ganymede, and Callisto, in 1610, shortly after he built his first telescope. As the Voyager 1 and 2 spacecraft journeyed through the outer Solar System almost 20 years ago, they returned intrigui ng views of these varied and complex Moon and planet-sized bodies and provided an impetus to return to Jupiter at a later date to study the system in more detail.
Io, the innermost, is the size and density of the Earth's Moon and is constantly volcanically active, as it is caught between the tidal forces of giant Jupiter and the next moon Europa. Europa also has a size and density simila r to the Earth's Moon, but it is covered with a layer of highly fractured water ice and possibly a liquid water ocean below. From here outward, the Galilean satellites change character from moons that resemble the inner silicate-rich planets to bodies tha t are more similar in density and composition to objects in the outer Solar System.
Ganymede is larger than the planet Mercury, about 1/3 the diameter of Earth, but its density is less than 2 grams per cubic centimeter. Its surface is composed 1/2 of ancient heavily cratered dark terrain and 1/2 of younger bright terrain that has been heavily tectonically modified. Both of these surfaces are dominated by water ice with the older terrain having more darkening agents.
Callisto is similar to Ganymede in size and density, but its surface is almost completely dark terrain that is even more heavily cratered than that of Ganymede. These basic observations raise many questions about the formation and geologic history of these satellites and the system as a whole. The Galileo Mission was designed to obtain high-resolution images of key terrain relationships and to fill gaps in coverage from the Voyager flybys in order to address these questions.
For several years now, scientists from around the world have been involved in the planning, data acquisition, and analysis of images from the NASA Galileo spacecraft, as it has travelled through the Solar System on its way to its primary target, Jupit er and its satellites. Launched October 18, 1989, the Galileo spacecraft initially headed toward Venus to obtain a gravity assist to increase its velocity, returned to the Earth-Moon system in December 1990, swung through the asteroid belt, passed close b y the asteroid Gaspra, circled around the Earth-Moon system again in December 1992 for yet another gravity assist, passed through the asteroid belt again to obtain images of the Rhode Island-sized asteroid Ida, and finally, in December 1995, deployed a pr obe into Jupiter's atmosphere and entered orbit around the giant planet. At that point, an almost two-year-long primary mission began of exploring the Galilean satellites and atmosphere of Jupiter. Although extremely ungainly looking, the beauty of the Ga lileo spacecraft comes from its multiple instruments, which permit simultaneous observations of surface morphology, texture, composition, interior structure, magnetic fields, and inter-satellite space. Such observations have revolutionized our view of the se individual bodies and their relationships.
How do we obtain data from these instruments to reach new conclusions? In my case, involvement in the Galileo mission began in the late 1970s when we wrote a successful proposal to be included on the Galileo Solid State Imaging (SSI) Team, a group consisting of geoscientists and atmospheric scientists from several institutions around the world and headed by Team Leader Mike Belton of the National Optical Astronomy Observatories. Working with the SSI team, inves tigators on other experiments on the spacecraft, and scientists and engineers at NASA's Jet Propulsion Laboratory, Brown University faculty, graduate, and undergraduate students through the years have helped define the scientific objectives, plan the miss ion sequences, and analyze the results of the several satellite encounters.
This process is repeated in many participating institutions across the world, and involves many hours of hard work, teleconferences, meetings, decisions, and compromises, and of course, excitement and occasional disappointment. Several women and men who were students at Brown have graduated and now work at the Jet Propulsion Laboratory planning sequences for instruments and participating in the data analysis. The many years of hard work have paid off in the return of very excit ing data and new discoveries that we have shared with the world by placing it on the World Wide Web (see URL http://www.jpl.nasa.gov/galileo/ and http://www.jpl.nasa.gov/galileo/sepo/).
The Galileo spacecraft is now another "satellite" in orbit around Jupiter, the largest planet in the Solar System, and on June 27, 1996, it came within a few hundred kilometers of Ganymede to begin the first of 11 "close" encounters with Jupiter's Ga lilean satellites (see cover photo and description on back page). Observations are made both from these close encounters, in order to get very high-resolution data, and also from some distance, to fill in regional gaps in photographic coverage, and to loo k for changes in the surfaces.
At first glance, the innermost satellite Io looks like an incompletely cooked pepperoni pizza. At the time Voyager discovered active volcanic plumes on Io, observations also indicated that there were no impact craters, suggesti ng that the satellite was being resurfaced by volcanic deposits so rapidly that impact craters were quickly covered up. Monitoring by Earth-based telescopes since Voyager indicated that volcanic activity was operating continuously and the Galileo spacecra ft now serves as a "volcano observatory in orbit," documenting the changes that have occurred on Io in the years since the Voyager flybys. Galileo images reveal the detailed morphology of radical changes and new deposits on the volcano Ra Patera. Dark mat erials, previously confined to a summit caldera, have overflowed the caldera walls to produce flows to the south and southeast. Surrounding the dark materials are new bright deposits covering an area of about 40,000 square kilometers (the size of New Jers ey)!
The morphology of the bright materials suggests emplacement as lava flows rather than pyroclastics. The colors of the flows match those of sulfur plus sulfur dioxide frost. Observations by other instruments reveal that the flow s have very high temperatures, showing that these deposits are silicates (like Earth lava flows) coated by ubiquitous sulfur deposits. Galileo images have also shown that the Ra Patera plume glows in the dark, perhaps due to the fluorescence of sulfur and oxygen ions created by energetic particles in the Jovain magnetosphere breaking apart the sulfur dioxide molecules.
At the site of Pele, one of the most famous volcanic plumes, the radial dark features close to the vent show little change since Voyager, but significant changes in the shape of the deposits are noted further from the vent. In addition to changes in the volcanoes known from Voyager, a new blue-colored volcanic plume extending about 100 kilometers into space was observed. Volcanic eruptions on Earth cannot throw materials to such high altitudes. The blue color of the plume is co nsistent with the presence of sulfur dioxide gas and "snow" condensing from the gas as the plume expands and cools.
This "volcano monitoring" phase is proving invaluable to documenting the nature of Io's volcanism and how it varies in time and space. Careful tracking of how the gravity field of each Galilean satellite perturbs Galileo's orbi t yields information about the satellite's interior structure, and in Io's case, revealed the presence of an iron core. Following the end of the nominal Galileo Mission (December 1997), the Galileo Europa Mission plans to obtain very high-resolution (10B1 00 meter) images of volcanic features on Io. Together these data will provide a new understanding of this most active planetary body in the Solar System, and how tidal interaction can provide enough energy to melt a planet many times over!
Europa, similar in size and density to the Earth's Moon and Io, is entirely different! Galileo data show that Europa has an outer layer about 100 kilometers thick that is composed largely of water. Major questions are how old t his surface is and whether there is a liquid water ocean below the ice surface today. Recently, NASA scientists reported detecting signs of possible fossil life in a meteorite blasted off Mars and delivered to Earth. This has raised interest in the possib ility that a liquid water ocean below the surface of Europa might have some of the ingredients for life (e.g., liquid water, heat, and organic material delivered by meteorites or derived from the interior).
Images of parts of Europa (cover photo) show a cracked and mobile surface, as if thin plates of "sea-ice" had separated and drifted apart, causing liquid water to well up and freeze, creating dark patches in between the plates. In other places, the surface is dominated by intersecting lines of ridges and troughs that give the appearance of a giant "ball of string." The density of craters on these surfaces is very low, and although the flux of impacting bodies is not precisely k nown in the Jupiter system, most scientists think that the surface is very, very young geologically. In other places on the surface, 7 to 15 kilometer diameter pits, spots, and domes disrupt the cross-cutting ridges, suggesting that in places the lower pa rt of the ice layer is upwelling in blobs (like those rising in a lava lamp) that push up and deform the surface.
One interpretation of these features is consistent with an ice layer several tens of kilometers thick overlying a liquid water ocean. The energy required for this melting and continuous deformation of the surface comes largely from tidal interaction of Europa with Io and Jupiter. This energy results in the interior and surface activity lasting much longer than it would have if Europa had evolved in isolation.
On Ganymede, flybys revealed the presence of a magnetic field and a layered internal structure consisting of an iron core, a silicate mantle, and an 800-kilometer thick layer of ice on the top. Thus, the core and mantle of Gany mede are about the same size as Io (1800 kilometers), but Ganymede has an additional thick ice layer on top! Galileo has obtained high-resolution images (15 to 20 times better than Voyager) of both the half of the satellite covered by ancient, heavily cra tered, dark terrain and the half comprising the younger, heavily tectonically modified, bright terrain (cover photo). The dark terrain shows abundant impact craters, testifying to the great age of the terrain, dating back several billion years. Deep furro ws in the ancient crust of dirty water ice appear to be ancient impact basin rings. The dark material in part is related to accumulation of many meteorite fragments that formed on Ganymede.
No conclusive evidence for volcanism in the dark terrain has yet been observed, but water vapor appears to be migrating from sunlit slopes and deposited in "cold traps" in north-facing slopes, such as those seen inside impact c raters. In Voyager images of bright terrain, linear bright and dark bands can be seen, but their detailed structure and origin are not clear. In the Galileo images, each band is now seen to be composed of many smaller ridges, the structure and shape of wh ich permit planetary geologists to determine their origin.
Initial analysis suggests that stretching of the ice at depth is causing fractures to form in the near-surface ice, creating lanes of bookshelf-like rotated fault blocks. Evidence is also seen for shear displacement, such as th at observed in the San Andreas fault on Earth. In addition, some bright terrain that appeared to be relatively young in Voyager data was revealed to be more heavily cratered, and thus older, inverting the sequence in several areas. This new sequence of ev ents shows that many broad linear groove lanes (cover photo) appear to be formed as a result of several tectonic events influencing much longer and wider areas at one time than previously thought from Voyager data.
On a global scale, Callisto's dark terrain is heavily cratered (cover photo), indicating the great age of its surface. The lack of evidence for modification of the dark terrain by younger bright terrain, such as seen on Ganymed e, led to the prediction that Callisto would be an ancient, little-modified planetary body. Indeed, Galileo flybys suggest that Callisto is not differentiated internally into a core, mantle, and crust! It was thus further anticipated that the surface of C allisto would be heavily cratered at small scales, providing a record of the nature of cratering and impactors since the early history of the Jovian system.
Instead, as is often the case with exploration, something entirely different was seen. There is a surprising lack of small craters in many places on Callisto, suggesting that some processes have obliterated these and other smal l-scale features. For example, downslope movement of ice-rich debris could bury small craters, and the sublimation (ice going directly to gas and escaping, leaving a fine grained rubble) of some components could lead to the obliteration of small craters. Galileo images reveal evidence for landslides inside the floors of some craters and several sinuous valleys associated with the pervasive local smoothing, suggesting sublimation.
Also observed on Callisto were chains of impact craters believed to result from impacts of split objects, similar to the fragments of Comet Shoemaker-Levy 9, which smashed into Jupiter's atmosphere in 1994. Thus, Callisto appe ars to be one of the most primitive bodies in the Solar System, but its surface is being modified by some as yet poorly understood process! Why is Ganymede so different from Callisto? Some scientists think that Ganymede fell under the influence of tidal d eformation from time to time, causing large linear deformation features (groove lanes) and melting and differentiation of the interior, while Callisto did not.
The Galileo Mission has discovered that this miniature solar system contains what appears to be a completely undifferentiated planetary body (Callisto) together with two bodies that are the youngest and most active in the Solar System (Io and Europa).
Ganymede may be a link between these two extremes. Further analysis of the Galileo data and information to be obtained from Galileo's Europa Mission will help complement and test this exciting emerging picture.
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