Space Science in the Twenty-First Century

Imperatives for the Decades 1995 to 2015

Planetary and Lunar Exploration

3

Status of Planetary Science in 1995

OVERVIEW

This chapter begins with two sections summarizing the accomplishments of solar system exploration over the three decades from 1965 to 1995, and the expected scientific questions as of the end of that period. The remaining sections constitute a much more detailed status report for individual classes of objects, with further discussions of open questions. All this material supports, and leads to, the program of missions for the period 1995 to 2015 presented in Chapter 4.

State of Planetary Exploration as of 1995

Among the high points already attained or anticipated for the first three decades off planetary study ending in 1995 are:

Applications of these results to the study of planetary origin and evolution include:

In supporting future investigations, an essential contribution will be made by theorists who endeavor to model the natural evolution of gas-dust disks into stars and their associated planetary bodies. Theoretical investigations of the early stages of this evolution begin with numerical and analytic modeling of star formation, in particular, the conditions under which single stars like the Sun can form. Study of the later stages of this evolution emphasizes modeling the manner and time scale for the accumulation of dust into planetesimals, and the subsequent accumulation of these planetesimals into planetary cores of silicates, metal, and ices. In the case of at least Jupiter and Saturn, the final stage of formation involved the gravitational capture of massive envelopes from the gas of the disk.

Between now and 1995 we can expect that continuing progress will be made in this field, most likely without the help of crucial observations or sudden theoretical breakthroughs. However, in the absence of a new generation of observational facilities that permit higher resolution imaging of other protostellar systems, it is quite possible that in the next decade we will not address the first-order questions required to make substantive progress. On the other hand, we can look forward to a significant refinement and enhancement of theoretical understanding concerning many aspects of nebular evolution. Much of this progress in theoretical understanding is contingent upon the availability of computational resources of continually greater power.

If the first asteroidal flybys occur during the next decade, we can expect to begin to be able to place the great wealth of meteoritical data into a planetological context. We can also expect that basic information regarding early solar system history will continue to flow from laboratory study of meteorites and stratospheric collection of interstellar particles. In this connection, it should be pointed out that, to a large extent, the current laboratory instrumentation used in this work was obtained during lunar sample analysis during the 1960s and early 1970s, and that attention must be given to modernizing the laboratories in which this work is done.

Scientific Questions as of 1995

Fundamental questions in planetary science will remain much the same in 1995 as they are today, but new knowledge and new capabilities will alter our view of how to approach them. First, the reconnaissance and exploration of the solar system will by no means be completed. Saturn and Titan are already ripe for in situ investigation and study of interactions among the magnetosphere, rings, and other satellites. Investigation of comets and asteroids will have begun, but intensive study and exploration of the wide diversity of asteroids will remain. In this area we will want to know the following: the overall structure of the asteroid belt and its radial variations of composition and physical characteristics, which are expected to reveal clues about the structure of the protoplanetary nebula; the mechanisms that powered the evolution of differentiated asteroids; and the chemical composition and physical character of comet nuclei, in order to determine under what conditions these most primitive planetesimals formed.

Internal structure of terrestrial bodies is a broad field for which, apart from the Earth, we still will have only the limited data for the Moon from Apollo, and the even more limited data for Mars from Viking. Even such basic information as crustal thickness will still be lacking. The absolute history of planetary bodies will not be understood without an unambiguous chronology based on radioactive clocks. For example, it is suspected that the martian channels and volcanoes were formed over a protracted period, even though the time scale is based only on crater counts and is very uncertain. There is little prospect of obtaining dates by other means than laboratory analysis of returned samples. Such samples remain valuable long after their acquisition and return to Earth: improved techniques can (and do for the Moon) continue to be applied to the original samples.

Only one side of Mercury will have been imaged from spacecraft, but all the other terrestrial planets are known to be asymmetric in the distribution of geological provinces. While the Galilean satellites of Jupiter will have been studied in some detail, only the most rudimentary reconnaissance will have been made of the other outer planet satellites. Only single flybys of Saturn, Uranus, and Neptune will have taken place, and the Pluto system will remain unvisited.

Our ideas about the origin of this solar system lead us to believe that planet-forming processes occur commonly during star formation. We will want to determine the prevalence and the properties of planetary systems around other stars accurately enough to compare them with one another, as well as with our own system. We will want to carry on detailed studies of protostars in order to ascertain the physical character of their accretion disks, thought to be the sites of planet formation.

It seems likely that Earth is the only site of organic life in the solar system, but there is no dearth of organic molecules on or in such objects as meteorites, Titan, the jovian planets, and giant molecular clouds located in other parts of the galaxy. Mars, formerly the object of greatest interest, is now seen to be the site of destruction of organic compounds by an intensely oxidizing atmosphere and soil. Conditions, however, may have been more benign in the remote past. There is still much to be learned about the origin of life by study of the objects mentioned above, and perhaps others such as comets. If other planetary systems exist, they may be seats of organic evolution.

PLANETARY GEOSCIENCES

During a relatively short period of time, studies of planets made by earth-based telescopes have advanced to detailed in situ measurements from. spacecraft of the planets' surfaces and atmospheres. A complex view of the planets and their satellites continues to emerge.

In late 1962, Mariner 2—the first interplanetary spacecraft—flew by Venus: the journey of Voyager 2 is still in progress. The 203-kg Mariner 2 had only six instruments, whereas the 818-kg Voyager 2 has two color TV cameras and ten other advanced instruments. These two spacecraft represent the simple beginning and the sophisticated continuation of solar system exploration.

In the early years of exploration, missions were selected more by technical feasibility than by scientific priority. So little was known that any mission greatly increased our knowledge. Now, comparative study of the planets is a significant scientific endeavor. Great advances in understanding the origin and evolution of the planets and properties of the solar system will come from comparisons of all planetary objects. Common features such as atmospheres, magnetic fields, and geologic processes can be understood best by such comparison. In turn, these comparative planetary studies provide insight about the history and evolution of the Earth. Nevertheless, exploration has shown that each planet is unique and interesting in its own right.

Scientific Objectives for Planetary Geosciences

The following topics in planetary geosciences contribute to an understanding of the solar system: formation; interior structure, dynamics, and physical state; crustal evolution; and planet morphology and surface processes. These topics, and the measurement objectives for them, are discussed below.

Formation

One key to understanding the formation of the planets is the determination of their chemical and isotopic compositions and the timing of their accretion. The results can be compared for all the planets, satellites, and meteorites in order to place constraints on models of chemical differentiation as a function of heliocentric or planetocentric distance. The results also shed light on the potential for heat sources—important for considerations of internal activity—and to assess models of planetary accretion.

Interior Structure, Dynamics, and Physical State

Measurements of the seismic behavior of planets, the strength and nature of their magnetic and gravity fields, and the heat flow from their interior are critical for determining the characteristics of planetary interiors. When combined with knowledge of mass and composition, the results permit assessment of the nature of possible interior differentiation (core/mantle/crust and the possibilities for an internal dynamo.

Crustal Evolution

A principal objective in planetary exploration is the determination of the age, composition, and distribution of crustal materials, including volatiles. The results allow refinement of models relating to planetary accretion, differentiation, and degassing. In addition, such determinations allow assessment of the style and timing of volcanism and tectonism and their relation to other geological events, as well as the role of volcanism in the evolution of possible atmospheres.

Planet Morphology and Surface Processes

The types and distributions of landforms and other geological units on planetary surfaces can be determined through geological mapping using remote-sensing data. The results allow assessment of the processes, such as volcanism and tectonism, that have led to the formation and modification of planetary surfaces. Some landforms, such as dunes and valleys, are indicative of processes associated with wind and water, and thus contribute to models of atmospheric evolution. Assessments must therefore be made of the distribution and exchange of volatiles among the crust, regolith, poles, and atmosphere. Knowledge of the geological processes—volcanism, tectonism, impact cratering, and surficial modifications—can be combined with relative and radiometric age determinations of the features associated with those processes to derive geological histories of the planetary surfaces.

An important aspect of comparative planetology relates to the origin and evolution of life. Knowledge of the geological environments permits assessment of the likelihood for the evolution and sustenance of organic life, at least in comparison to Earth. The images of Earth taken from space with its thin skin of oceans and clouds help us begin to appreciate the uniqueness of our planet and the fragile balance that makes life here possible.

An additional impetus for planetary exploration is the potential for using space resources. In a period when natural resources are being depleted rapidly on Earth, no detailed assessment has been made of the resources that exist in space. The Moon and asteroids may hold significant potential as sources of metals and minerals for utilization in space. The initial utilization of such resources may be to support space missions that would travel farther, into space, or permanent bases on the Moon or Mars.

Measurement Objectives

The goals outlined above guide the definition of a set of general scientific objectives as follows:

The Inner Solar System

The inner planets—Mercury, Venus, Earth and its Moon, and Mars—range from 0.4 to 1.5 AU in distance from the Sun and are smaller and denser than the outer planets. These terrestrial planets are composed chiefly of rock and metal, are poor in volatiles, and have few satellites. Their densities range from 5.4 g/cm³ for Mercury to 3.9 g/cm³ for Mars. The variation in density with solar distance has been discussed in the context of a thermodynamic model for the proto-solar nebula in which temperature and pressure decrease with distance from the nebular center and control the chemistry of condensed material. However, it may be that primary differences in planetary density are due to accidental variations in fractionation and reaggregation from collisions. After their formation, all inner planet surfaces were significantly modified by a wide variety of internal and external processes. Nevertheless, each planet has followed its own evolutionary path.

By exploring this diverse family of planets and by comparing their features with those of the Earth, we seek to characterize the evolution of the inner solar system and the causes of the unique aspects of each planet. We also seek to gain insights into the history, as well as the future, of Earth and the life that has evolved on it. Further insights into the terrestrial planets will come from study of the large satellites of the outer solar system as well.

The Moon

Apollo yielded an enormous advance in the understanding of planets by providing samples of the Moon and a wealth of other information. The time scale of the Moon's evolution has been established, and several first-order questions have been answered. Equally important, a basis has been established for interpreting the evolution of other planetary bodies, including the Earth. During the accretionary phase of continuous planetesimal in-fall, the Moon appears to have melted to depths of at least a few hundred kilometers. The ancient crust developed during this maelstrom, with segments repeatedly fragmented and reincorporated into the evolving magmas until a thickness was established that could withstand the waning bombardment.

The larger craters on the Moon record a period of intense bombardment that ended about 3.7 billion years ago, a phenomenon that presumably affected all of the inner planets at about the same time. This bombardment provides a chronological reference, accurately measured in the case of the Moon by radioisotope dating techniques, that is the basis for constructing the geologic history of Mars, Mercury, and (presumably) Venus.

The evolution of the crust of the Moon is known from remote sensing, from instrument data provided by landers, and from study of returned samples. Remote-sensing data show that two major provinces constitute the lunar crust: young (sparsely cratered), low-albedo mare terrains, and old (heavily cratered), high-albedo highlands. The oldest reliably dated rocks on the Moon (from the highlands) are radiometrically dated at about 4.5 billion years old. The youngest mare basaltic lava flows are estimated to be about 2.3 billion years old. The lunar highlands appear to be the result of differentiation at 4.5 billion years and consist of minerals that floated in the melt. However, there is controversy as to whether the upper crust of the Moon was generated in a "magma ocean," whether the whole planet was molten, or whether local areas were successively molten over a long period of time.

Geophysical data show that the Moon had a strong magnetic field early in its history, but the field has since disappeared. Although most scientists consider the Moon to have a small, partly molten core, its presence is the subject of intense debate due to differences in interpretation of the seismic record.

Because all the data provided by Surveyor, Apollo, Soviet landers, and sample returns are gathered from the Earth-facing hemisphere, there is a need to obtain data from other areas of the Moon in order to establish a better understanding of the geochemical, geophysical, and geologic history. Since the Soviet Union's Luna 24 sample return in 1976, there have been no missions to the Moon. However, a lunar geoscience orbiter (LGO) may fly before 1995. During its one-year mission, LGO would map the global elemental and mineralogical surface composition, measure surface topography, and map the global gravity field. Although no specific date has been set for this mission, the United States, the Soviet Union, Japan, and the European Space Agency are considering LGO-type missions for the 1991 to 1995 period.

Mercury

The geology of Mercury is known primarily from data returned by the flybys of Mariner 10, in 1974. From photogeological studies and remote-sensing data we have determined that the evolution of the crust of Mercury resembles in many ways the crustal evolution of the Moon. Less than half of the surface was imaged by Mariner 10, however, and there may well be other terrains and processes not yet known. Moreover, most of the images are of moderate resolution, on the order of hundreds of meters, and details of the surface are very poorly known.

Most of the known surface of Mercury is heavily cratered and may consist of rocks similar to those of the lunar highlands—differentiated rocks high in silica and alumina. The dominant feature on Mercury is Caloris, an enormous multiringed impact basin. The Caloris basin is partly filled with smooth plains materials interpreted as flood lavas similar to the mare basalts on the Moon. However, the characteristic vents and lava flow fronts seen so clearly on the Moon and Mars are less well displayed on Mercury, possibly due to poor image resolution, and so this interpretation is more controversial.

Apparently, the silicate crust formed early on Mercury—as on the Moon—and heavy bombardment continued. Later flooding of mafic lava flows also occurred, but this may have declined around 3 billion years ago as it did on the Moon. Imaging at higher resolution and coverage for the other half of the planet may discover features indicative of other processes and much younger events. Mapping compositional distributions globally could reveal distinctive provinces and help to explain the high density of the planet.

Venus

Of the inner planets, we know the least about the geological evolution of Venus. Most of the data, for this "sister" planet of Earth have come from the U.S. Pioneer-Venus mission, along with Earth-based radar, and Soviet landers (Veneras 8 through 14, Vegas 1 and 2) and orbiters (Veneras 15 and 16).

The surface of Venus is hidden from view except by radar imaging systems and imaging from the surface. Earth-based radar and data from Pioneer-Venus provide an assessment of the gross topography. Venera-15 and -16 radar images have spatial resolution of about 1 to 2 km and cover part of the northern hemisphere, approximately one-quarter of the planet. Two-thirds of Venus is highland terrain, which includes plateaus higher and more extensive than those on Earth. In contrast, the lowland plains ("ocean basins") are only one-third as extensive and one-fifth as deep as the ocean basins on Earth. There are linear mountain belts around Ishtar Terra, the northern "continent," which include the highest mountains, known as Maxwell Montes. These mountains surmount Lakshmi Planum—a plateau that is twice as large and 1000 m higher than the Tibetan Plateau, the largest on Earth.

The Beta and Atla regions, first identified on earth-based radar images, are large shield volcanoes. Beta is composed of high-potassium basalt, while the lowland plains east of it are covered by tholeitic basalts similar to the ocean floor on the Earth and the mare lavas on the Moon. Feldspathic (high silica and alumina) rocks occur in the upland rolling plains that occupy most of the Venus surface. Both Beta and Atla have large gravity anomalies of approximately 135 mgals, corresponding to a compensation depth of more than 100 km, and thus requiring support by an upward flow of mantle material. The indirect discovery of abundant lightning in these areas has been interpreted as indicating the presence of active volcanoes. In general, there is a strong positive correlation between gravity and topography, suggesting that the dominant source is interaction of mantle convection with a surface layer—perhaps a global crust of tens of kilometers thickness. The long-wavelength gravity field of Venus thus contrasts sharply with the Earth, which is a mix of deep sources (hot spots), plate tectonic, and other effects.

The linear mountain belts may be due to tectonic compression; other belts of fractures may be due to crustal extension. Some circular fracture patterns, called coronae, are more than 600 km across. They may be volcanic-tectonic features, some form of impact-generated structure, or a result of both internal and external processes. Many small craters have been observed, which may originate in impacts or volcanism, although it is difficult to determine their origin with the resolution of present images.

The high deuterium/hydrogen ratio observed by Pioneer-Venus may indicate the loss of substantial water by photodissociation and hydrogen escape. The yellow-gray color of the surface rocks may indicate that the iron minerals in those rocks have been oxidized like those on the surface of Mars, and fine-grained dark deposits may be windblown fine material that partly covers the surface. There are also extensive rift zones that segment the crust. However, these tectonic features do not form an integrated planet-wide system of rifts and ridges, nor are there obvious continent marginal trenches. Apparently, Venus lacks these indicators of integrated plate tectonic motion; thus Venus must get rid of its internally generated heat largely through conduction, aided possibly by "hot spot" volcanism, rift tectonics, and rift volcanism. Venus seems to be closer to the "one plate" planets like the Moon and Mars rather than the multiplate Earth, but in volcanic and tectonic behavior it is more complex than Mars.

The Magellan (Venus Radar Mapper) mission scheduled to begin operation around 1990, will provide near-global images of the surface at 1-km or better resolution. This will enable assessment of the geological processes that have shaped the surface of Venus, and estimates of the ages and sequences of surface units, and internal processes. These images will also allow us to address questions regarding the former existence of liquid water (e.g., ocean shorelines, river channels), possible correlations of topography and gravity, and styles of tectonism and volcanism.

Mars

The martian surface is divided into two roughly equal hemispheres, the southern highlands and the northern lowlands. The southern highlands have elevations from 1 to 10 km above the planetary reference elevation. This region is dominated by large impact craters and several enormous impact basins, of which Hellas and Argyre are the largest. This ancient, heavily cratered terrain may be similar in some respects to the lunar terrae, formed 3.9 to 4.2 billion years ago. However, the martian highlands are partly covered by younger lavas and mantling deposits possibly of sedimentary origin. These highlands also show extensive reworking by wind and water. Although the composition of the highland rocks is not known, they probably include felsic rocks. The northern lowlands consist of young lava flows similar to the more mafic lunar mare lavas, and have been modified by aeolian, fluvial, and periglacial processes.

Two impressive volcanic provinces dominate Mars: the Tharsis region and the Elysium region. The Tharsis region includes Olympus Mons and three other very large (550 km across volcanoes surmounting the Tharsis "Rise"—an elevated area standing about 10 km high—plus many smaller volcanoes. Olympus Mons, 600 km in diameter and more than 26 km high, is one of the larger volcanoes in the solar system. The Elysium region also includes several large volcanoes, but appears to be older than the Tharsis region. Moreover, the morphologies of the Elysium volcanoes and the lava flows are different from Tharsis and may -indicate differences in the style of volcanism or differences in the composition of the magma at the time of eruption.

Although the Tharsis and Elysium regions are impressive, by far the greatest extent of volcanic rocks occur as various flood lavas and lava plains in the northern lowlands and the southern cratered terrain. Still other large volcanic deposits may represent high-silica eruptions and may include ash flow tuffs that have been heavily eroded by the wind.

The polar areas are covered by extensive, thinly layered deposits, probably of sedimentary origin, that are associated with the permanent and ephemeral ice deposits. The perennial ice caps may be water ice in the north and carbon dioxide in the south. In addition, a vast dune field surrounds the north polar region. Dunes also occur in isolated fields elsewhere on the planet, where they partly fill many of the craters.

Many parts of Mars have been extensively modified by faulting. Vanes Marineris is a canyon system more than 4000 km long that resulted from rifting and other tectonic processes. Extensive layered deposits are visible in the sides of the canyons and in mesas within the canyon system. Some of these deposits may have arisen in vast lakes that once filled the canyons. Other theories suggest that some of the layered deposits are lava flows, volcanic explosive deposits, or wind-laid sediments. Regardless of origin, the deposits may represent a large part of martian geologic history, just as the deposits in the Grand Canyon of Arizona represent a substantial fraction of Earth's history.

Some regions of Mars are dissected by large and small channels cut into young rocks, indicating that liquid water existed late in martian history. Other more degraded channels dissect ancient terrain and appear to have formed earlier in martian history. Thus, there may have been many periods in Mars' past when liquid water could exist on the surface.

Despite several successful missions to Mars, including the two Viking landers, many fundamental questions regarding its present state and geological history remain unanswered. The Mars Observer mission will return data in the early 1990s, providing global maps of the surface composition, details of the topography, and data on the lower atmosphere. However, questions about the interior (presence of a core, nature of the mantle, etc.) and of active surface processes will remain unanswered.

Internal Characteristics of the Inner Planets

All the inner planets, including the Moon, underwent significant early heating, melting, and differentiation, but the evolution of the Moon and Mercury terminated early as heat was lost rapidly due to their small size. Like Earth, Mars and Venus are presumably sufficiently large that they are losing internal heat slowly. Their heat sources have continued to operate over billions of years, manifested at their surface in the form of volcanic and tectonic features.

Earth's surface continues to evolve dynamically. Crustal material is continually created at mid-ocean ridges and destroyed beneath deep-sea trenches, as the plates that make up the Earth's crust move in more or less steady relative motion. The formation of mountain belts, the development of volcanic chains, and the driving force behind many large earthquakes are linked to these plate motions. Neither the Moon, Mercury, nor Mars shows global tectonics of such vigor; the surfaces of the Moon and Mercury are old and preserve a record of early heavy cratering and ancient volcanism. The surface of Mars also shows heavy cratering modified by wind and water erosion, but also demonstrates an extensive history of volcanism and tectonism. Data for Venus, although limited, show that this planet also underwent differentiation and has experienced tectonism and volcanism.

The collective study of the inner planets implies that all have been melted and internally differentiated, leading to core, mantle, and lithosphere. Earth's interior is known from seismic measurements to be layered, a product of global differentiation. At Earth's center is a metallic core, largely fluid and in convective motion, but with a small solid inner core. The core is surrounded by a mantle of ferro-magnesian silicates, mostly solid and in very slow convective motion. At the surface, the mantle is capped by a thin, rigid lithosphere of mostly igneous and metamorphic rocks, overlain by a veneer of volcanic rocks and sedimentary material. Each of the other terrestrial planets is thought to be similarly layered, but the evidence is limited.

Fundamental unresolved issues in inner planet studies are: (1) the nature of the convective motions that drive plate tectonics on Earth and the importance of such tectonic processes early in Earth's history, (2) the character of tectonism on other planets and satellites, and, (3) the causes of the major differences in evolutionary style.

Magnetic Fields of the Inner Planets

Earth has a substantial dipolar magnetic field of internal origin, evidently produced by the action of a hydro-magnetic dynamo sustained by motions in the fluid core of the rotating planet. Of the other inner planets, only Mercury has a magnetosphere comparable in character with that of Earth, though much smaller in size. The Moon shows evidence for an early complex magnetic history, now recorded in the remnant magnetism of lunar rocks and the lunar crust, but the origin of the field is not known. Slowly rotating Venus apparently has no internal magnetic field. The existence of a martian magnetic field is debatable because of scanty measurements; however, if a field exists, it is small. The wide differences in the nature of planetary magnetic fields are not understood but may be related to rotation rates and the nature of the core.

Characterizing the particle and field environment, including internal magnetic fields, is important for understanding the solar wind's interaction with a planet. Earth's field extends through a volume of space many times larger than the planetary volume, forming an umbrella that shields Earth from the interplanetary plasma; in contrast, the solar wind blasts the Moon directly. Although Venus does not have an internal field, there is a complex interaction with the solar wind that may be similar to that of comets.

Atmospheric-Climatic Connections of the Inner Planets

The geological record and measurements of planetary atmospheres provide clues to the evolution of surfaces and climate history. The Viking entry and lander measurements of nitrogen isotopes in the martian atmosphere indicate that large quantities of volatiles have been lost from Mars. It is inferred from this and other measurements that Mars has outgassed and subsequently lost to space or to permafrost cold traps the global equivalent of a depth of some tens of meters of water. Because of the planet's low atmospheric pressure, liquid water cannot exist on Mars at present. However, there is strong geologic evidence for the earlier presence of liquid water on Mars and the suggestion of a hydrologic cycle and thus a long, perhaps episodic history of free water on the martian surface.

NASA's Mars Observer, planned to gather data in the early 1990s, will shed light on key questions regarding the present and past interactions between the atmosphere and lithosphere. This mission is proposed to provide: (1) a global view of the distribution of elements and minerals on the martian surface, (2) the water vapor, clouds, and dust distribution in the atmosphere and the vertical temperature profile, and (3) the distribution of ice on or near the surface. In addition, the Mars Observer will obtain global radar altimetry data that are crucial to the integration of the image, gravity, thermal, and chemical information.

A Mars aeronomy mission, possibly flying at the same time as the Mars Observer, could explore the planet's upper atmosphere and interaction with the solar wind and answer long-standing questions about Mars' internal and external magnetic fields. In addition, such a mission could provide data on the net mass exchange between the atmosphere and the solar wind, and provide important clues regarding the history of Mars' present and past atmospheres by determination of the rates at which various volatile elements escape from the martian ionosphere.

Rocky Satellites

The Voyager results made possible the geological study of a host of new objects, ranging from relatively large, silicate bodies to small satellites composed predominantly of ice. Collectively, these objects show surfaces that have experienced impact, volcanic, and tectonic processes similar to the inner planets. In addition, they show processes not seen, or at least not fully appreciated, on the inner planets, including volcanism induced by tidal heating, sulfur-driven volcanic eruptions, deformation of surface features through slow flow of ice-rich crusts, and resurfacing through the eruptions of ice-rich materials.

Voyager results also caused a reassessment of any notion that internal planetary activity is simply correlated with planetary size. Larger objects contain more radioactive constituents relative to their surface area, and hence generate more heat to drive planetary tectonic activity. Thus, it was thought that small bodies would cool and freeze quickly and have short, simple histories of internal activity in comparison to large bodies. Although this seemed to be the case with the inner planets, the concept was drastically modified by Voyager observations. Io, a body the size of Earth's Moon, had nine volcanic eruptions in progress during the encounter, making it the most internally active body in the solar system. The heat to drive these volcanoes is likely derived predominantly from tidal stresses created by Jupiter and the nearby satellite Europa, rather than from radioactive decay, as on larger planetary bodies. Some small satellites of Saturn (notably Enceladus) and of Uranus (such as Miranda) show evidence of resurfacing and extensive tectonism. This is indicative of internal activity. On the other hand, some other larger and smaller satellites appear to have less vigorous or no internal activity.

Many of the questions raised by Voyagers 1 and 2 during their brief encounters with the Jupiter system will be addressed by the Galileo spacecraft scheduled to arrive at its destination in the mid-1990s. During its 20-month mission, Galileo will obtain better estimates of the chemical composition and physical state of the satellites, along with data on magnetic field and particle fluxes in the Jupiter system. The cratering record, the nature of volcanic processes on Io and possibly Europa, and the styles of resurfacing and tectonic processes on Europa and Ganymede will all be substantially better known after Galileo.

Studies of the outer planet icy satellites with their solid crusts and possibly mobile mantles represent an opportunity to address the fundamental problems of the physics, chemistry, and geology of deformed crusts. They will also allow us to study the internal constitution of bodies that differ radically from the inner planets. By 1995, data will be available for a range of bodies, from those with thoroughly deformed crusts to those that are minimally disturbed. With additional geophysical and geochemical data we may be able to verify the causes of the formation of the core, mantle, and lithospheres of these bodies. We may also be able to characterize the lithosphere, asthenosphere, and mantle, and their interactions to produce tectonism and volcanism in terrestrial bodies. Subtle differences in chemistry and phase are necessary to make the terrestrial systems function. Study of systems that involve water-ice crusts and liquid-water mantles may broaden our appreciation of these fundamental problems.

Satellites of Jupiter

The most important advance in understanding satellite evolution arose from the observations of the Jupiter system by Voyager. This understanding will be further advanced by the Galileo mission. The Galilean satellites of Jupiter form a well-ordered family of bodies that cover a wide range of internal dynamism. The inner two, Io and Europa, are about the size of Earth's Moon. Their densities, 3.5 and 3.2 g/cm³ respectively, indicate that they are probably composed primarily of silicate materials. The outer two, Ganymede and Callisto, are about the size of Mercury and appear to be mixtures of silicates and water, indicated by their densities of 2.0 and 1.8 g/cm³, respectively.

As discussed above, the discovery of active volcanoes on Io by Voyager makes it the most volcanically dynamic body known in the solar system. Internal heat appears to be generated by the tidal stresses in Io, and the eruptions, apparently driven by sulfur dioxide, reach heights of 250 km. In addition to the products of active volcanic explosions, the surface of Io is dominated by other volcanic features, including a variety of domes, calderas, collapsed depressions, and digitate lava flows that radiate from volcanic centers. The absence of impact craters at the resolution of Voyager images indicates that the surface of Io is very young. Evidently, resurfacing by lava flows and deposits from volcanic emissions is taking place very rapidly.

The composition of the surface material on Io is enigmatic. The presence of sulfur is indicated by spectral reflectance data and various "hot spots" having temperatures consistent with molten sulfur. However, pure sulfur has insufficient strength to form large, steep landforms. The presence of mountains as high as 8 km and scarps up to 1.5 km indicates the probable presence of silicate rocks.

The surface of Europa has very high albedo. This probably indicates an ice or ice-rock crust, which is often densely fractured. Europa is also thought to have a water substrate and a rocky interior. Only a few impact craters have been identified, and, like Io, the surface is considered to be very young.

Ganymede displays two fundamental surface units: an older, heavily cratered, dark terrain and a younger, brighter unit that has been extensively modified by fracturing and other tectonic processes. Impact craters exhibit both bright and dark ejecta; many of the craters have been deformed by viscous flow of the icy crust moving under its own weight. However, some craters retain their topographic expression, and it has been proposed that there was higher heat flow early in the history of Ganymede, which allowed viscous creep to occur at a high rate. Later cooling led to a more rigid crust.

The outermost Galilean satellite is Callisto. Its surface is very heavily cratered and includes several multiringed structures. Callisto does not appear to have experienced any tectonic deformation or volcanism.

Satellites of Saturn

The satellites of Saturn are more diverse and irregular in their crustal evolution than those of Jupiter. The most interesting is Titan. It has low density (1.9 g/cm³) and a thick atmosphere (1.5 bars) of predominantly nitrogen, plus methane and minor constituents. Clouds masked views of the surface from Voyager; consequently, little is known about the crustal evolution of Titan.

Enceladus is a particularly intriguing satellite of Saturn. It displays ancient, heavily cratered terrain, a crust that is broken by faults, and areas that have been resurfaced. Its density (1.2 g/cm³) is consistent with water ice, but substantial amounts of methane and other ices may also be present. Enceladus, like Io, may have been volcanically active as a result of tidal stresses, giving it a crust and mantle that may still be active.

The other Saturnian satellites are heavily cratered and are so cold and rigid that the craters retain their original topographic form and are not viscously deformed, as are the craters on Ganymede and Callisto in the Jupiter system. They exhibit different degrees of internal activity, varying from rift valleys, as on Tethys, to faulted and partially resurfaced crusts, as on Dione. Bright wispy zones, some of which are associated with faults, may reflect water frosts erupted from fissures.

Iapetus has a larger variation in albedo (dark to bright) than any other satellite in the solar system. It may be affected by dark material swept up from orbit, emplaced by impact, or deposited via internal activity. The current data are of highly inadequate resolution to resolve this question.

Satellites of Uranus and Neptune

The Voyager 2 encounters of the Uranus system in early 1986 returned the first images and other data on the nature of the satellites. Ground-based studies have already demonstrated that the five known satellites of Uranus are icy objects, perhaps similar in bulk composition to the satellites of Saturn, but with darker (dirtier) surfaces. All five satellites were imaged by Voyager 2 at resolutions of a few kilometers or better, but the mission emphasis was on the innermost known satellite, Miranda, which was encountered at close range, yielding subkilometer resolutions, as good as any obtained by Voyager of the satellites of Jupiter and Saturn.

The 1989 Voyager encounter with Neptune is being planned with that planet's largest satellite, Triton, as a prime target. The spacecraft will fly close to Triton and will provide an occultation as seen from Earth. The occultation is particularly important since Triton is known to have an atmosphere.

Ground-based telescopic studies have also revealed evidence of liquid nitrogen and perhaps hydrocarbons on the surface, as well as frozen methane, making this nearly lunar-sized object potentially one of the most interesting members of the satellite family.

ATMOSPHERES

Atmospheres of planets and moons exist in an enormous variety. The terrestrial objects Venus, Mars, and Titan have atmospheres somewhat similar to Earth's, even though they span several orders of magnitude in surface pressure. The jovian planets, including Jupiter, Saturn, Uranus, and Neptune, have extremely deep atmospheres of which we can expect to explore only the outermost skin. They are dominated by hydrogen and helium rather than the oxygen, nitrogen, and carbon dioxide of the terrestrial planets. Very tenuous atmospheres are found on Mercury, Io' comets, and probably a few of the icy satellites of the jovian planets; in many ways they resemble the outermost parts of the denser atmospheres, exhibiting phenomena such as escape and the presence of ionization.

Atmospheres are studied to determine their present state—their composition, structure, meteorology—and also to find clues to their origin and evolution. To first order, the gases in the jovian planets resemble those in the Sun, and are therefore taken to be of primary origin, with little change since their formation. Such gases are almost, but not quite, absent on the terrestrial planets; instead we find "volatiles" that could plausibly have accreted as components of solids. Later degassing and chemical alteration would then produce what is found today. For example, photolysis of ammonia and loss of hydrogen to space give nitrogen. Likewise, life on the Earth has converted carbon dioxide to oxygen, organic molecules, and buried carbon in the form of carbonate rocks. The traces of "primary" gases, neon, and heavier noble gases can only be measured from within the atmosphere or on a sample. In this area our present information is limited to Earth, Mars, Venus, and the parent bodies of certain meteorites. The most remarkable thing about these results is their diversity, which has so far resisted any attempts at an overall explanation. Nevertheless, it is important to know that primary atmospheres either never existed on bodies as large as the Earth or were almost entirely lost.

Earth, Mars, and Venus

The atmospheres of Earth, Mars, and Venus have all evolved markedly from their initial states. The other planets show that biological activity, which seems to have dominated on Earth, is not the only agent that can have a profound effect on a planetary atmosphere or surface.

Viking measurements of the Mars atmosphere showed that a large fraction of the original nitrogen has been lost. The abundance of water must have been enough at one time to cut large numbers of fluvial channels; current surface temperatures and pressures do not allow water in the liquid state. Mars may have lost an amount of water equivalent to a layer 10 m deep to space and to sinks below the surface, although some resides in the polar caps.

Even larger amounts are missing from Venus, as shown by the huge enrichment of deuterium in the atmosphere measured by Pioneer Venus. Depending on the exact style of loss, as much as an earth ocean of water could have been lost over the life of the planet. This could explain the immense underabundance of water on Venus relative to the Earth, since the planets are otherwise quite similar. However, the original amount is very poorly determined, and the site of the oxygen left behind by the escaping hydrogen has not been established. It is usual to assume that the loss took place early in Venus' history. However, this timing should be determined and any possible relation to a change in the tectonic style of the planet should be explored.

The history of Earth is similar to the histories of other terrestrial planets. Common aspects include early global differentiation of crust and core, outgassing and evolution of an atmosphere, and early bombardment of the surface by a heavy flux of meteoroids. Of course, Earth has many attributes not shared by any other planet. These include its oceans, its high oxygen abundance, its tectonic motions and the consequent complex history of crustal deformation, its life forms, and its development of a global magnetic field and magnetosphere.

Earth is the only planet that has large quantities of free water on its surface and in its atmosphere. The dynamics of Earth's oceans play a large, incompletely understood role in the regulation of the terrestrial climate. Earth is also unique in the large quantities of molecular oxygen in its atmosphere as the result of biological activity. Venus makes a startling contrast: it is covered by a dense global blanket of clouds composed of sulfuric acid droplets and has a thick, hot atmosphere of carbon dioxide. Cloud motions and tracking of descent probes and balloons indicate a global wind pattern with substantial dependence on height for the mean wind speed. Surface winds are mild, but 100 m/s winds blow at the cloud tops. Martian winds are variable, as on Earth, with annual episodes of high-velocity winds that often give rise to global dust storms. Mars also has marked seasons, with carbon dioxide cycling between the polar caps driving a major component of atmospheric circulation. Layered sedimentary deposits at the martian poles are evidence of long-term climatic changes, whose origins are poorly understood. On Earth, such climatic changes have given rise to the periodic ice ages. Earth is the only planet whose surface, atmosphere, and hydrosphere have provided an environment conducive to the development of life and the evolution of complex living organisms. These life forms have substantially influenced the chemistry of the atmosphere and the oceans and the major sedimentary rocks on Earth's surface.

The Viking mission showed the absence of detectable organic molecules on Mars. It also revealed that an intense ultraviolet flux from the Sun reaches the surface. This suggests that living organisms are not present on Mars now. Whether Mars was less hostile to the development of life during earlier times, when it may have had a denser atmosphere and flowing surface water, is still an open question. Other basic questions about Mars are the fate of all the missing water and the nature of the current hydrological cycle linking the polar caps, ground water and ice, and the atmosphere.

A rare group of meteorites, called SNC for the initials of places where the first examples were found, are widely believed to be samples from a martian lava flow. Most remarkably, they contain gases whose elemental and isotopic composition is substantially identical to that measured by the Viking landers, within the errors of the latter.

Study of the present state of an atmosphere is carried out both in situ and remotely by flybys and orbiters. Except for Mars, all the terrestrial bodies with substantial atmospheres exhibit a greenhouse effect: the surface temperatures are higher than they would be in the absence of an atmosphere. There is a great deal of interest in this phenomenon from the standpoint of past and future climates on the Earth, where we are in the midst of a grand experiment of the effect of massive injections of carbon dioxide from human activity. Clouds, smogs, and dust are interesting both in their own right and as tracers of atmospheric motions. Observed clouds include water and ice on Earth, ice and solid carbon dioxide on Mars, ammonia, methane, and perhaps liquid water or ice on the jovian planets. Smogs (produced photochemically rather than by condensation) are found on Earth, Venus, Titan, and at least some of the jovian planets. Dust is very important on Mars and is significant on the Earth as well.

Theoretical meteorology of the Earth is finely tuned to our observational knowledge. It has been extended to Mars with considerable success, but is very unsatisfactory with Venus and Jupiter. On Venus we observe winds near the cloud tops (about 65-km altitude blowing from the east at 100 m/s, and similar winds are thought to exist in the region of the ionosphere. Not only were these motions unpredicted, they still cannot be explained in any fundamental way now that they have been observed. On Jupiter and Saturn, similar velocities are seen, but with large shears between zones. These are just as far from being explained.

Progress in meteorological theory will rely on a combination of further theoretical and computational work, and more and different observations. A theory with true predictive power would be especially important in the area of climatic change. Improved short-term forecasting, however, is more likely to be obtained by still-more-detailed observations of the Earth itself.

The study of photochemistry and photoionization is much more readily transferable between planets. Mars and Venus are excellent Earth analogs. The dayside ionosphere of Venus is well understood, and the stratospheres exhibit similar phenomena. Limited data on the Mars ionosphere suggest that it is similar to that of Venus, but measurements from a long-lived orbiter are needed. Catalytic ozone destruction is much more important in the stratospheres of both Mars and Venus than on Earth. Both Venus and Earth have sulfate layers, but again this layer is much denser on Venus. Further study of these planets should continue to shed light on terrestrial pollution problems.

The nightside upper atmosphere and ionosphere of Venus are unexpectedly cold. The corresponding region of Mars is unexplored and may or may not be similar. Comparative study should cast light on this mysterious phenomenon.

Titan

Titan, the largest satellite of Saturn, is unique among satellites in having a dense atmosphere: predominantly nitrogen, but also containing a small amount of methane and possibly argon. The surface pressure is 1.5 bars. Since the surface gravity is only 135 cm/s², the amount of gas per unit area is nearly 11 times that of the Earth.

A nearly uniform orange haze hides the surface and any condensation clouds that might exist. This orange haze is likely composed of condensed hydrocarbons, nitrile compounds, polyacetylenes, and HCN. The precipitation of these compounds may form a deep layer lying on the surface, or they may be dissolved in a liquid ocean. Dense methane clouds probably lie some distance above the surface. This rich inventory of organics in a nitrogen medium provides a natural laboratory for the study of prebiotic organic chemistry, which may be relevant to the question of the origins of life on Earth.

The origin of nitrogen on Titan is a fundamental, unsolved question. One theory is that the nitrogen could be primordial, incorporated in a clathrate ice during accretion. Two other sources have also been suggested: photolysis of ammonia and high-temperature formation from ammonia during impacts. The other atmospheric constituents can be derived by photochemistry from a nitrogen-methane mixture, except for the traces of carbon monoxide and carbon dioxide. The carbon monoxide could be outgassing from the interior, or could be derived from the ice in incoming meteoric material, with the carbon coming from the atmospheric methane.

Titan is embedded in a torus of escaped gases, which includes atomic hydrogen, observed by Voyagers 1 and 2, and probably molecular hydrogen and nitrogen. Ionized torus material contributes to the plasma in Saturn's magnetosphere: impact by magnetospheric particles is an important loss process for the neutral torus. Voyager 1 passed through Titan's magnetospheric wake and observed a number of changes in the plasma and magnetic environment.

The existence and nature of the predicted ocean of ethane and nitrogen (or alternatives, such as lakes or puddles) should be studied, both remotely and in situ. There are important interactions of such an ocean with the atmosphere, for which it would be a source of volatiles and a sink of photochemical products. Of the volatiles, methane is a likely candidate to produce the dense cloud layers, whose presence is strongly suspected, but which have not been confirmed because they are hidden by the photochemical smog.

An atmospheric circulation, especially strong in the stratosphere, has been inferred from the temperature field observed by Voyager 1. Testing by direct wind measurements could tell us whether our understanding, based on Earth and Venus data, is good enough to encompass Titan as well. Finally, the hydrogen torus surrounding Titan's orbit has provoked speculation about the presence of other atoms and molecules in the torus, and the torus' interactions with the magnetospheric plasma. Also of interest is the possibility of sources for the torus other than Titan itself; Saturn is a possible candidate.

Io and the Plasma Torus

Io, Jupiter's innermost large satellite, exhibits such remarkable phenomena as volcanoes believed to be driven by sulfur dioxide, a tenuous atmosphere of sulfur dioxide, a persistent extended cloud of sodium atoms, and a plasma torus containing ions of sulfur and oxygen enveloping the orbit. In turn, many of these ions become energized and populate the magnetosphere, and probably drive the large escape rates that populate the torus itself. The energy source for the vulcanism is tidal heating by Jupiter and nearby satellites. Most of the other aspects are poorly understood at best, and some are totally mysterious. For example, although it seems clear that sulfur dioxide is passing through the atmosphere to the torus, there is no agreement on the density of the atmosphere.

Study of all these phenomena would be near the top of any priority list if it were not for the unbearable environment of the enveloping magnetosphere. Pioneer and Voyager instruments making a single pass have been damaged, and radiation damage is a primary concern for the Galileo spacecraft and its instruments. A prolonged stay near Io does not seem possible with current or easily foreseen technology.

The most burning question about Io's atmosphere is the actual quantity of sulfur dioxide and its variations with latitude and time of day. The amounts of other gases, such as oxygen, are also of great interest. A large pressure bulge is expected near the subsolar point, and some models predict strong winds blowing away from this region. Probably the most remarkable thing about this atmosphere is the huge escape fluxes of sulfur, oxygen, and sulfur dioxide that populate the torus, and that require replacement in a very short time. The mechanism itself is obscure, and there is the possibility of large variations with time. Among the atoms involved is sodium, which gives a bright glow easily observed from Earth. It must be a substantial component of the crust, but the chemical form is unknown.

Although the ionic composition of the torus is fairly well known, the energy sources are not fully established. Since this object drives much of the rest of the magnetosphere, it needs to be understood as well as possible. Another energetic phenomenon related to Io is the intense radio (decametric) bursts, which have been studied from Earth for decades but whose origin is still being discussed.

Jovian Planets

From their very low mean densities we know that Jupiter, Saturn, Uranus, and Neptune have extremely deep atmospheres, extending perhaps more than halfway to the centers. They also possess cores of 10 to 20 earth masses similar to large terrestrial planets, but possibly much richer in the ice-forming compounds water, methane, and ammonia. A large percentage of the atmospheres appears to be made up of these compounds; there is strong evidence for methane and ammonia, but much weaker indications of water. They also form cloud layers, perhaps more than one per planet, but the highest layer (ammonia on Jupiter and Saturn, methane on Uranus and Neptune tends to obscure the deeper ones. Particularly for Jupiter, the cloud patterns are arranged in bands parallel to the equator, instead of the cyclonic whirls found on Earth. Several simple organic molecules and a dark stratospheric smog, also seen on Titan, are believed to be produced photochemically from methane.

Jupiter, Saturn, and Neptune all have internal heat sources that are very large by terrestrial standards, and comparable to the heat they receive from the Sun. Uranus' source may be almost as large as Neptune's, but it cannot be resolved from the reradiated solar heat. Such a flux from the interior is expected to have profound effects on the structure and circulation of an atmosphere, but the details are not understood. The heat source itself is of major interest; it is probably a combination of residual heat from planetary formation and gravitational energy from continued differentiation and rainout of heavier elements, such as helium, due to immiscibility with hydrogen. The former heat source dominates on Jupiter and the latter on Saturn. Precise in situ measurements of the helium-to-hydrogen ratio in the atmospheres of Saturn, Uranus, and Neptune will contribute to an understanding of these processes.

The dynamical properties of the interior are closely coupled to the composition, structure, heat flow, and rotation of the planet. Convective flow of conducting matter generates an external magnetic field that reflects some of the properties of the internal flows, with changes in the flow producing secular changes in the magnetic field. These properties of planetary magnetic fields can be best determined from orbiting spacecraft with a small periapsis.

Differences in the bulk composition of the outer planets are related to differences in the temperature, pressure, and chemistry occurring at different radial locations in the solar nebula at the time of planetary formation and to the nature of the accretion and collapse process. Because the major species such as carbon, nitrogen, and oxygen are present mainly as methane, ammonia, and water, which are deficient in the upper atmosphere due to condensation into clouds, their abundances must be determined by probes below the cloud decks. On Uranus and Neptune, it is thought that the water cloud base will lie below 100 bars.

It is often presumed that the different-colored clouds of Jupiter are at different altitudes and indicate regions of upwelling and downwelling. However, there is little direct evidence that this is the case and there are other indications from observations of atmospheric scattering properties that are difficult to reconcile with such a model. Synoptic multiband observation with moderate spatial resolution would be valuable, as would in situ observations of cloud depths in different latitudinal regions.

Internal structure is related to the origin and evolution of the planets, since it depends on bulk composition, on the accretion process, and on subsequent evolution. It is also of interest for the insight it provides into the properties of matter at high temperatures and pressures. Knowledge of the bulk composition will be essential, and improved values for the higher gravitational moments, as can be derived from an orbiting spacecraft, would be useful. Laboratory and computer studies of high-pressure properties are also essential.

It is possible that the observed zonal wind flows are an atmospheric skin effect extending only as deep as sunlight penetrates (several bars). Alternatively, the zonal winds may reflect an internal flow pattern extending deep into the neutral atmosphere, driven by the internal heat source. In situ wind measurements with a simple atmospheric probe, and synoptic optical, infrared, and microwave observations from an orbiting spacecraft, may provide relevant information.

RINGS

Planetary rings, once thought to be unique to Saturn, have been observed around all the giant planets except Neptune. Even for Neptune, there is evidence for the existence of partial rings. The ring of Jupiter is optically thin and composed of dustlike small particles. Saturn's rings are broad, bright, and opaque, whereas the rings of Uranus are narrow and dark. They all lie predominantly within the Roche limit, where tidal forces would destroy a self-gravitating body, and also within the planetary magnetosphere.

The goals of the study of planetary rings include three major objectives. The first is to understand their composition, active processes, and origin. A second objective is to study their active processes as analogs of those that operate in other flattened, rotating, dynamic systems like galaxies, accretion disks, and our own solar system at an earlier stage. A third objective is to study the particles as remnants of an earlier stage of solar system evolution; they are not as primitive as the comets, but less processed than the larger planets and satellites in the outer solar system.

The best-studied ring system is Saturn's, which has been observed from the ground for centuries. The most detailed information on planetary rings is from Voyager spacecraft observations. Ground-based radar, photometry, and infrared spectroscopy have been complemented by spacecraft imaging, spectroscopy, and occultation. We now have a reasonably comprehensive inventory of the ring material surrounding Jupiter, Saturn, and Uranus, and a preliminary understanding of some important dynamic processes in each of these systems. Continuing theoretical modeling using existing data sets will focus questions about the physical processes that govern the morphology and stability of planetary rings.

The occurrence of rings around the massive planets is evidence of an evolutionary path parallel to planetary aggregation. The nonlinear, dynamical interactions responsible for forming and flattening planetary rings also operate in galaxies, planetary systems, and stellar accretion disks. Generally, rings consist of planetary material that accreted originally with the planet. This material was either never incorporated into larger bodies, or may have formed satellite bodies that were later broken up. In both Saturn's E-ring and the jovian ring, there is Voyager evidence for short particle lifetimes, arguing for continuous replenishment from satellite surfaces. A fundamental open question is the age of the Saturn ring system: the proximity of the inner satellites is inconsistent with the action throughout the age of the solar system of current dynamic processes, which push them away from the rings.

Saturn's rings are mostly water ice and emit thermal radiation in energy balance with incident sunlight. Radar, radio occultation, and spectrophotometric studies indicate that particles larger than 1 cm are responsible for most of the Saturn ring opacity. The composition and thermal state of Jupiter's and Uranus' rings are unknown, but interaction with magnetospheric ions at Jupiter may both heat and differentiate ring material. Remote sensing has a limited capability to characterize ring particle composition and size: by 1995, any spectroscopically active constituents should have been identified in both the solid and gas phases. In the subsequent period, a ring rendezvous mission could provide in situ analysis of particle and ring-atmosphere composition.

Ring particles display complex collective interactions. Voyager Saturn data show mass clustering into thousands of ring features. Density and bending waves, spokes, and even multiple strands have also been observed. Nine distinct, narrow rings have been identified at Uranus. Many of the Saturn structures are gravitationally induced by satellite orbital resonance; for example, the outer ring edges of Saturn's brighter ring occur at radial distances where particle orbital periods are commensurate with those of satellites. Small, close satellites "shepherd" the F-ring. A satisfactory explanation does not exist for the multitude of smaller-scale Saturn ring features, and explanations of the broad structure of Uranus' rings and Jupiter's ring are unconfirmed. Since ring particles may acquire net charge by either ultraviolet or charged particle irradiation, electrodynamic forces influence the motions of the smallest particles, probably producing the Saturn ring spokes and likely limiting the lifetime of the small particles in Jupiter's ring. A ring rendezvous could measure the plasma environment of Saturn's rings in the vicinity of a spoke feature.

The orbit pole of an inclined elliptical ring precesses in response to the higher moments of the planetary mass distribution. This effect has been observed for Uranus' epsilon ring, and has provided a value of J₂, the second gravitational moment, with a small uncertainty. By 1995, improved values for the even the multipole moments of giant planet gravity should be available from improved ring and satellite astrometry.

In the mid-1990s, we will have new observations of the jovian ring from the Galileo orbiter and observations of Saturn's rings (including stellar occultations), from both earth-based and e