Planetary and Lunar Exploration Space Science in the Twenty-First Century -- Imperatives for the Decades 1995 to 2015 (1988) / Chapter Skim
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3. Status of Planetary Science in 1995
3. Status of Planetary Science in 1995
Pages 19-87
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From page 19... ...
Mercury: Characterization of physiographic provinces for half the surface; discovery of a planetary magnetic field.
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. Moon: Determination of detailed geological history, chronology, and geochemistry of major geological provinces; detailed study of selected samples of surface material; investigation of cratering, regolith formation, and interaction of the surface with the solar wind for an airless body; discovery of remanent magnetic fields; seismic characterization; measurement of heat flow; determination of composition of the solar wind, both present and ancient.
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From page 21... ...
Applications of these results to the study of planetary origin and evolution include: ~ Establishment of the age of the solar system as 4.6 billion years by analysis of radioactive decay products in the Earth, meteorites, and lunar samples. ~ Dating of the late stages of accretion of the Moon (and presumably the other terrestrial planets)
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From page 22... ...
~ Discovery of the uniquely high levels of volcanic activity on To, and preliminary characterization of volcanism based on different physical-chemical systems than had been encountered in the terrestrial planets. In the Saturn system, resurfacing on Enceladus represents yet another example of such volcanic activity.
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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.
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From page 24... ...
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.
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From page 25... ...
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.
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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.
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Measurement Objectives The goals outlined above guide the definition of a set of general scientific objectives as follows: ~ Characterize the internal structure, dynamics, physical state, and bulk composition of the planet of interest; . Characterize the planet's chemical composition and mineralogy of surface materials on a regional and global scale;
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From page 28... ...
Further insights into the terrestrial planets will come from study of the large satellites of the outer solar system as well.
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From page 29... ...
The ancient crust developed during tints 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 fume.
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Apparently, the silicate crust formed early on Mercury as on the Moon and heavy borr bardment 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.
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From page 31... ...
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.
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From page 32... ...
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)
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From page 33... ...
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.
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From page 34... ...
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.
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From page 35... ...
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.
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From page 36... ...
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
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From page 37... ...
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, sulfurdriven volcanic eruptions, deformation of surface features through slow flow of ice-rich crusts, and resurfacing through the eruptions of ice-rich materials.
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From page 38... ...
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.
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From page 39... ...
Only a few impact craters have been identified, and, like To, the surface is considered to be very young. Ganymede displays two fundamental surface units: an older, heavily cratered, clerk terrain and a younger, brighter unit that has been extensively modified by fracturing and other tectonic processes.
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From page 40... ...
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.
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From page 41... ...
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.
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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.
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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.
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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 weli.
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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.
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From page 46... ...
lo and the Plasma Torus To, 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.
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From page 47... ...
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-form~ng compounds water, methane, and ammonia.
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From page 48... ...
Convective flow of conducting matter generates an external magnetic field that reflects some of the properties of the internal flows, with changes in the Sow producing secular changes in the magnetic field. These properties of planetary magnetic fields can be best determined from orbiting spacecraft with a small periapsis.
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We now have a reasonably comprehensive inventory of the ring material surrounding Jupiter, Saturn, and Uranus, and a preliminary understanding of some important dynarn~c 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.
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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.
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From page 51... ...
Among the dynamical processes believed to be important in Saturn's rings are direct collisions, gravitational scattering, diffusion and angular momentum flow, resonance interactions with various satellites and Saturn, electrodynamic processes, and possibly diffusional instabilities. The exact nature of these processes is not understood, however.
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The overall composition of the ring material is related to the origin of the ring, while any segregation of the material according to particle size or location involves dynamical processes that have either produced or maintained the segregation. Closely related to the composition Is the physical nature of the individual particles.
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We hope to clarify the relation between the ring particles and the small moons embedded in it, which appear to be supplying ring material. Since the life history and lifetime of the small jovian ring particles are strongly affected by the plasma environment, deeper understanding requires directly measuring that environment.
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The mass excess indicated is approximately 10 earth masses for each of the giant planets. This is a large excess relative to the planetary mass for Uranus and Neptune, and so can be obtained from even crude knowledge of the planetary parameters, but it is a much smaller fraction of the planetary mass for Jupiter and Saturn, so that the higher precision of the knowledge of the planetary parameters has been essential in those two cases.
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From page 55... ...
These are among the important questions that can be addressed through improved modeling, which, in turn, depends upon better measurements of envelope composition and gravitational moments. Another theoretical expectation is that the cores of the giant planets, whether of rock or ice composition, would dissolve into the overlying hydrogen at the central pressures of the planets, if given an opportunity to do so (unless the rocky material arrives in very large chunks)
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From page 56... ...
That will give us an improved understanding of the structure of these planets. However, hydrogen and helium make up only a relatively small part of the mass of these two planets, and so it is still highly desirable to obtain the best possible measurements of the higher gravitational moments to obtain the interior mass distribution, and to measure the envelope composition, the heat flow from the interior, and the magnetic moments and their time variations with considerable accuracy.
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From page 57... ...
The surface intensities of known planetary magnetic fields typically fall in the range of a few tenths of a gauss to 10 G at the planetary surfaces, although Mercury's magnetic field is weak in comparison to those of most of the other planets—a few thousandths of a gauss. A distorted magnetic field stores energy and exerts forces on the medium in which it is embedded.
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From page 58... ...
Moreover, detailed observations of a planetary magnetic field can yield important information about the characteristics of the generating fluid motions. Indeed, it was detailed studies of the temporal behavior of the geomagnetic field that provoked early ideas about the motion of fluids in Earth's core and that provided the earliest insights into the regeneration of magnetic fields in natural objects through the phenomenon of the hydromagnetic dynamo.
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From page 59... ...
Strictly speaking, the fluid motions in dynamos cannot generate magnetic fields from scratch. The dynamo process is one of field maintenance through regeneration and amplification.
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From page 60... ...
Because large-scale planetary magnetic fields result from correlated regenerative action at smaller scales, the fact that most fields are nearly centered in their respective planets is also not hard to understand. Moreover, deviations of planetary magnetic fields from the ~ideal" centered and axially aligned structure can be understood on the basis of random variations in the fluid motions, although other effects may also occur.
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From page 61... ...
The correct interpretation of its unusual spatial structure depends strongly on whether the field is in a stationary state, an oscillating state, or a transient configuration deviating markedly from a more regular average structure. ~ Earth's Magnetic Field For obvious reasons, Earth's magnetic field is the best studied of all cosmical magnetic fields.
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From page 62... ...
Studying Planetary Magnetic Field It is probably safe to say that the fundamental basis of magnetic field generation in planets is understood. The physical and mathematical machinery of Maxwell's equations and Newton's laws has led to a dynamo theory of magnetic field generation that accounts for basic aspects of the existence and behavior of known magnetic fields.
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From page 63... ...
It is especially important to ascertain the spatial structures and time dependence of the geomagnetic field on both large and intermediate scales, and over both short and geological times. Measurements of the temporal variations of other planetary magnetic fields are also of great importance.
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PRIMITIVE BODIES AND THE ORIGIN OF THE SOLAR SYSTEM A major goal of space science is understanding how the Sun and planets were formed. Progress toward achieving this goal involves synthesis of knowledge obtained from many sources, including missions to planetary bodies, astronomical observations from earth-based and earth-orbiting observatories, and geological, geophysical, and geochem~cal studies of the Earth itself.
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From page 65... ...
The detailed chemical and isotopic compositions of the various planetary bodies including satellites, comets, and asteroids are particularly relevant because of their bearing on our understanding of processes and thermodynamics within the solar nebula. Sometimes these planetary observations match in
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Detailed laboratory chemical, petrographic, and isotopic investigations of this material continue to provide a wealth of information regarding conditions that prevailed in the primordial solar nebula, as well as of processes that occurred during the formation and subsequent history of these small planets. Finally, theorists endeavor to model the natural evolution clef gas-dust disks into stars and their associated planetary bodies.
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From page 67... ...
There are very good, if not compelling, reasons for believing that these cores formed before the final accumulation of the terrestrial planets. However, at least in their most simple form, present theories of planetary accumulation lead to the opposite conclusion, that the accumulation of the outer planets required much more time than was required for the growth of the terrestrial planets.
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From page 68... ...
One of the possible models for formation of the terrestrial planets, and perhaps for the cores of the outer planets as well, involves the formation of small solid bodies called planetesimals. Progress in understanding this pivotal phase, especially in the context of specific nebular models, is central to gaining an overall understanding of the planet formation problem.
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From page 69... ...
The temporal aspects of this topic are of particular importance; for example, if Jupiter formed before the other planets it may have controlled to some extent the formation of the other planets. If, on the other hand, the terrestrial planets formed before Jupiter, the details of their formation are likely to differ substantially from those associated with formation in the presence of a massive object like Jupiter.
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From page 70... ...
At the present time there is no unambiguous evidence for the existence of another planetary system, let alone detailed information concerning statistical properties of planetary systems in general, or of structural details of specific planetary systems. IRAS and some ground-based instruments have detected discrete dusty structures associated with some nearby main-sequence stars, where the dust particles are some 2 orders of magnitude larger than typical interstellar dust particles (~100 Em versus ~0.2 ,um)
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For example: What is the frequency of occurrence of planetary systems as a function of spectral type of their central star? What are the masses of the planetary bodies relative to that of their central star?
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From page 72... ...
. The previous questions focus on detection of the products of the disk evolution, the planetary bodies.
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Asteroid, SmaB Satellites, and Meteorites General Characteristics The asteroids are small bodies that orbit the Sun, for the most part at distances of 2 to 3.5 AU. The total mass of material in the asteroid belt is less than 0.001 earth masses.
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It is quite possible that some S asteroids are differentiated bodies, whereas others are similar or identical to ordinary chondrites in composition. Some asteroidal reflectance spectra clearly indicate differentiation, such as E (enstatite)
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An uncertain estimate of the relative proportions, based on interpretation of observations and theoretical reasoning, is that at least 20 percent of these bodies are derived from the asteroid belt, and that at least 10 percent are of cometary origin. The Earth-approaching bodies of asteroidal origin have a close kinship to meteorites, and many meteorites are probably collision fragments of these bodies.
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From page 76... ...
In addition, missions will map the surface in several spectral regions to provide mineralogical information. The observations should be of significant help in the interpretation of data from ground-based spectrophotometric observations limited to integral whole-disk reflectance spectra.
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From page 77... ...
Many asteroids have been grouped according to spectral refiectance features, and many of these features have been related to known meteorite types. One difficulty in the link between asteroids ant} meteorites is that the reflectance spectra of the most common meteorites, the ordinary chondrites, are not precisely matched by the main-belt asteroid spectral reflectance groups.
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magnetic fields that were present before the asteroids accumulated. Several of the carbonaceous chondrites have been found to contain amino acids and other complicated organic molecules, which are clearly of extraterrestrial origin.
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From page 79... ...
When combined with higher resolution spectral and imaging data, this affords an opportunity to overcome the apparent irritation to surficial composition. This may facilitate addressing such questions as the fragmentation history and internal structure of differentiated asteroids, and the identification of specific asteroids as sources of particular meteorite classes.
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From page 80... ...
Several schools of thought hold that comet-like objects were among the fundamental building blocks of some larger planetary bodies. As a result of their small sizes and their large average distances from the Sun, evolutionary processes that differentiated the planets are thought to have been insignificant for many comets.
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From page 81... ...
All of these phenomena result from the gas and dust that emanate from the nucleus; the nucleus itself is small, of the order of 1 to 10 km in diameter for typical comets. The ice and snow in comet nuclei are composed of condensed gases and other volatile materials, including water and probably carbon monoxide, carbon dioxide, HCN, CH3CN, and unidentified complex organic molecules.
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From page 82... ...
Carbon Is a good example. Bodies within the solar system, including the Sun, Moon, terrestrial planets, meteorites, and Jupiter, exhibit a common value of about 90 for the i2C/~3C isotopic ratio.
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From page 83... ...
The United States will carry out a comet rendezvous mission by the middle of the l990s, which will follow a comet through most of its inner-solar-system passage. Analyses of cometary gas and dust, energetic particles, and magnetic fields and plasmas, as well as detailed investigations of the structure and gross composition of the comet nucleus will be carried out by the comet rendezvous mission.
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From page 84... ...
We must understand the physical processes that determine the loss of material from the cometary nucleus and the resulting short-term evolution of its structure, and those that produce the elaborate extended coma and tail of the comets. Some of these same physical processes determine the nongravitational evolution of cometary orbits, that is, a Rocket effect" caused by the asymmetrical emission of gases from the nucleus.
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From page 85... ...
This extrapolation back to the time of the origin of the solar system is also needed to interpret the extent to which comet-like objects contributed to the formation of the giant planets and the volatile inventories of the terrestrial planets. Comet Measurements and Technical Requirements Addressing the scientific questions cited above will require detailed investigations of the comet's nucleus and the emitted dust, as well as the gas, plasmas, and fields in the comet.
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From page 86... ...
The structure of the large-scale cometary phenomena should be deterrn~ned. Measurements of the electromagnetic fields, plasma, energetic particles, and neutral gas should be made in a volume surrounding the nucleus and encompassing the upstream coma and solar wind interaction region, as well as a significant volume of the tail.
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From page 87... ...
87 and its more quiescent times. Return of high-~ntegrity samples of cometary nucleus material to Earth for detailed analysm in terrestrial laboratories will be essential to realize the overall goals of comet science, as well as of solar system science in general.
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